Dr. Peter M. Graham

Abstract

Vaska’s complex, Ir(Cl)[P(C₆H₅)₃]₂(CO), reacts with O₂ at room temperature to form Ir(Cl)[P(C₆H₅)₃]₂(CO)(O₂) at a ratio of 3.11:1 reactant to product and with HCl to form Ir(Cl)₂[P(C₆H₅)₃]₂(CO)H at 47.7% yield. The CO stretch on the IR spectra of these compounds is found at a lower frequency than that of the CO stretch from Vaska’s complex because the additional ligands lead to increased π back-bonding. In comparison to the ³¹P NMR spectrum of Ir(Cl)[P(C₆H₅)₃]₂(CO), the signal given by the ³¹P NMR spectrum of Ir(Cl)₂[P(C₆H₅)₃]₂(CO)H is shifted upfield because of additional electronegative ligands added to the metal, which draw in electron density and deshield the phosphorus molecules. Finally, the ¹H NMR spectra of Ir(Cl)[P(C₆H₅)₃]₂(CO) and Ir(Cl)₂[P(C₆H₅)₃]₂(CO)H are nearly identical aside from a signal given off by Ir(Cl)₂[P(C₆H₅)₃]₂(CO)H at δ -15.36 representative of the Ir-H addition. This signal is seen as a triplet of quartets due to coupling with Ir and P.

Introduction

Vaska’s complex, Ir(Cl)[P(C₆H₅)₃]₂(CO), is a versatile complex because of its ability to bind other additional ligands.¹ Whereas most complexes contain 18 electrons and are considered to be saturated, Vaska’s complex contains only 16 electrons and is thus able to add certain two-electron donors to reach 18 valence electrons. It can also undergo oxidative addition in which the Ir¹ center inserts into the σ bond of certain molecules, and the oxidation state of Ir is increased while reaching 18 electrons. Oxygenation and hydrochlorination of Ir(Cl)[P(C₆H₅)₃]₂(CO) proceed in the following manners:

Scheme 1

Scheme 2

These oxidative additions are of interest because the relative ease of identifying their products. The additional ligands will both increase π back-bonding to the metal and draw in electron density. The effects of these two characteristics are apparent through IR and ³¹P NMR spectroscopy, as a weakened CO bond will cause a lowering of CO stretch frequency in an IR spectrum, and deshielded phosphorus will be shifted upfield in a ³¹P NMR spectrum.

Experimental

All syntheses were carried out in air and the reagents and solvents were purchased from commercial sources and used as received unless otherwise noted. The synthesis of Ir(Cl)[P(C₆H₅)₃]₂(CO)(O₂) (2) and Ir(Cl)₂[P(C₆H₅)₃]₂(CO)H (3) were based on reports published previously.¹

Ir(Cl)[P(C₆H₅)₃]₂(CO) (1). The ¹H NMR, ³¹P NMR, and IR spectra of (1) were taken by Dr. Graham. This compound was not synthesized but purchased commercially. ¹H NMR (CDCl₃): δ 7.25-7.97 (3 H, series of signals, -C₆H₅). ³¹P NMR (CDCl₃): δ 24.5 (s, -P(C₆H₅)₃). FTIR (ATR) ν_(CO) 1951 cm^-1 (s, C-O linkage).

Ir(Cl)[P(C₆H₅)]₂(CO)(O₂) (2). A stir bar, 1 (0.010 g, 1.28 x 10^-5 mol), and toluene (10 mL) were subsequently added to a 25 mL single neck round bottom flask. The flask was covered with a septum and the vessel was degassed with O₂ for 3 minutes. The solution was then stirred at room temperature at moderate speed for 1 h. The septum was taken of the flask and the solvent was removed via rotary evaporation so that only a few mL of solution remained. One drop of this solution was placed onto the ATR and allowed to dry before taking the IR spectrum of the complex. FTIR (ATR) ν_(CO) 1952 cm^-1 (m, C-O linkage), ν_(CO) 2000 cm^-1 (m, C-O linkage).

Ir(Cl)₂[P(C₆H₅)₃]₂(CO)H (3). 1 (0.032 g, 4.10 x 10^-5 mol), THF (10 mL), HCl (concentrated, 5 drops), and Et₂O (50 mL) were subsequently added to a 250 mL Erlenmeyer flask. The solution was swirled around for about 5 minutes to allow a whitish precipitate to form. The solution was filtered using a small frit and the precipitate was vacuum dried. The product was determined to be 3 (0.016 g, 1.96 x 10^-5 mol, 47.7% yield based on the amount of 1 used). An extension of the ¹H NMR spectra of this substance was given out by Dr. Graham. ¹H NMR (CDCl₃): δ -15.36 (tq, J_Ir, J_P, Ir-H), δ 7.24-7.91 (3 H, series of signals, -C₆H₅). ³¹P NMR (CDCl₃): δ -1.74 (s, -P(C₆H₅)₃). FTIR (ATR) ν_(CO) 1951 cm^-1 (s, C-O linkage), ν_(CO) 2021 cm^-1 (s, C-O linkage).

Results

The reaction of 1 with O₂ was not directly measured for percent yield, but could indirectly be measured via the IR spectrum of the product 2. The stretch at 1952 cm^-1 was telling of 1 and the stretch at 2000 cm^-1 was indicative of 2. The ratio of the area of these peaks was 3.11:1, reactant to product. The reaction of 2 with HCl resulted 0.016 g of product, which was determined to be 3. This translated to 1.96 x 10^-5 mol and a 47.7% yield based on the amount of 1 used. The reactants and products reacted in 1:1 ratios in both instances. The IR spectrum of 1 was used to differentiate the two stretches in the 1950 to 2000 cm^-1 region on the IR spectra of 2 and 3. The stretch around 1950 cm^-1 could be identified as the CO stretch from 1, and the stretches around 2000 cm^-1 were from the CO stretches of 2 and 3.

The ¹H NMR spectrum of 1 showed a series of signals from δ 7.25-7.97 which was representative of protons attached to the phenyl rings. The ¹H NMR spectrum of 3 had these same phenyl signals, but additionally contained a signal from the hydride at δ -15.36 that was coupled with Ir and P into a triplet of quartets. The ³¹P NMR spectrum of 1 gave a singlet peak at δ 24.5 representative of the two identical phosphorus molecules on the compound. The ³¹P NMR spectrum of 3 displayed its singlet peak shifted upfield to δ -1.74.

Discussion

The ratio of formation of 2 seems reasonable, but it is unable to be determined whether or not that is a high or low ratio given the reaction time. If the solution was given more time to react, perhaps more product would have formed. If the flask was degassed with O₂ for a longer amount of time, it is likely that would also favor the formation of more product, as additional oxygen would increase the interactions with 1 to form 2. In retrospect, the solution could have been stirred at more rigorous speed, as that would also likely increase the interactions between 1 and O₂. The percent yield of 3 was close to 50%, which seems fairly good, though not all of the product that was weighed out actually was 3. It can be seen on the IR spectrum of the product that a significant amount of 1 remained, due to the stretch visible around 1950 cm^-1.

The IR spectra for 1, 2, and 3 are all very similar. The IR spectrum of 1 shows a single CO stretch at 1951 cm^-1. The IR spectra of 2 and 3 also show stretches around 1950 cm^-1, which suggests that those products obtained contained unreacted 1. The IR spectra of 2 and 3 also contain a second CO stretch around 2000 cm^-1. These stretches are representative of desired product. The lowered frequency is due to increased π back-bonding from the addition of O₂ and HCl to the Ir in 1 in each case. This strengthens the Ir-C bond and weakens the C-O bond causing the shifts in frequency.¹

The ¹H NMR spectra of 1 and 3 are nearly identical save for the signal given off by the Ir-H bond by 3 at δ -15.36. This signal is first coupled with phosphorus, which has a spin of 1/2.¹ There are two phosphorus, so using the equation 2nI + 1, the value of 3 is obtained. Because the spin is 1/2, this means that 3 peaks will be observed in a 1:2:1 ratio. The hydrogen is then coupled to iridium, which has a spin of 3/2.² There is only one iridium, so using the equation 2nI + 1, the value of 4 is obtained. Because the spin is 3/2, this means that the 4 peaks will be observed in a 1:1:1:1 ratio.³ This explains the appearance of the hydride signal.

The ³¹P NMR spectrum of 3 has a singlet peak shifted upfield from that of the ³¹P NMR spectrum of 1. This is due to the electronegativity of the hydrogen and chlorine added to the metal, which draw electron density away from the phosphorus molecules leaving them less shielded. The oxidation state of Ir in complex 1 is +1 while being +3 in complexes 2 and 3. Its electron count in 1 is 16 electrons, while its electron count in 2 and 3 is 18 electrons.

Conclusion

The main purposes of the experiments were to synthesize 2 and 3, confirm their structures by comparing their various spectra to those of 1, and to determine their percent yields or reactant to product ratios. The ratio of reactant to product for 2 was 3.11:1 which was obtained from the ratio of the areas of the two CO stretches on its IR spectrum representative of reactant and product. The percent yield of 3 was calculated to 47.7%, but was in reality lower because of CO stretches on its IR spectrum representative of 1. The visibility of this stretch means the reaction did not go to completion.

The structures of 2 and 3 were somewhat validated by their IR spectra, which gave CO stretches around 2000 cm^-1. It was expected to see CO stretches for those compounds in this area because the additional ligands would cause increased π back-bonding, making the C-O bond weaker and thus lowering the frequency.¹ A series of peaks seen on both the ¹H NMR spectra of 1 and 3 around δ 7.24-7.91 is suggestive of phenyl groups. The only notable difference between the ¹H NMR spectra of 1 and 3 is the addition of a triplet of quartets at δ -15.36 for compound 3. The splitting of the peaks is due to coupling of the proton from the Ir-H bond to Ir and to the two P. The P split the signal into triplets of 1:2:1 ratio and the Ir split those signals into quartets of 1:1:1:1 ratio. Finally, the comparison of the 1 and 3 ³¹P NMR spectra seem to confirm the identity of 3 as the singlet seen in the spectrum of 1 is shifted upfield in the spectrum of 3. This is due to the addition of H and Cl to the compound, which are electronegative and draw electron density away from the P, leaving it deshielded.

References

(1) Angelici, R. J.; Girolami, G. S.; Rachufuss T. B. Synthesis and Technique in Inorganic Chemistry: A Laboratory Manual; University Science Books: Sausilito, CA, 1999; pp 189-195, 259.

(2) http://www.webelements.com/iridium/nmr.html

(3) http://en.wikipedia.org/wiki/NMR_spectroscopy

Abstract

The reaction of RuCl₂(PPh₃)₂, THF, and diphenylpropargyl alcohol under reflux yields C₅₁H₄₀P₂Cl₂Ru in 46% yield. ¹H NMR spectroscopy of C₅₁H₄₀P₂Cl₂Ru shows a series of overlapping peaks at δ 7.3-7.8. C₅₁H₄₀P₂Cl₂Ru can then react with dichloromethane and tricyclohexylphosphine to form C₅₁H₇₆Cl₂P₂Ru. ¹H NMR spectroscopy of C₅₁H₇₆Cl₂P₂Ru yields the same series of peaks found around δ 7.3-7.8 that C₅₁H₄₀P₂Cl₂Ru exhibits, along with a faint series of peaks at δ 1.8-2.1. ³¹P NMR spectroscopy of both products shows a single peak around δ 29.5. This suggests what was believed to be C₅₁H₇₆Cl₂P₂Ru was actually mostly C₅₁H₄₀P₂Cl₂Ru. Catalytic measures of the two synthesized products were inconclusive due to their similar natures, however, it is expected that C₅₁H₇₆Cl₂P₂Ru is the better catalyst as it has bulkier, more readily dissociating substituents.

Introduction

The reaction of RuCl₂(PPh₃)₂ with THF and diphenylpropargyl alcohol under reflux yields C₅₁H₄₀P₂Cl₂Ru.¹ The reaction specifically takes place in the following manner:

Scheme 1

C₅₁H₄₀P₂Cl₂Ru can then react with dichloromethane and tricyclohexylphosphine to form C₅₁H₇₆Cl₂P₂Ru. The reaction occurs in the following manner:

Scheme 2

These products be distinguished via ¹H and ³¹ NMR spectroscopy. The ¹H NMR spectrum of the C₅₁H₇₆Cl₂P₂Ru will yield peaks representative of the newly added cyclo groups, which are missing in C₅₁H₄₀P₂Cl₂Ru. The ³¹P NMR spectra of each product should theoretically each show 1 peak, with the peak of C₅₁H₇₆Cl₂P₂Ru being downfield from C₅₁H₄₀P₂Cl₂Ru because of the lower electron density around the phosphorus.

The products from these two reaction are of interest because they are ruthenium alkidene complexes, which are alternatives to Grubbs’ catalysts and are much less difficult to prepare in the laboratory.¹ These two ruthenium indenylidene complexes can be used as catalysts in ring closing metathesis. Show below are the balanced reaction and mechanism in which diethyl diallylmalonate undergoes this process with the aid of a ruthenium catalyst:

Scheme 3

Scheme 4

The relative catalytic rates of the two ruthenium indenylidene complexes can be monitored via GC/MS. Determination of the starting material and product from this technique can show the relative percentages of each material within a solution. By comparing the ratio of reagent to product for each of the ruthenium complexes, it can be determined which is a better catalyst, as the more efficient catalyst will sport the lower ratio of reagent to product.

Experimental

All syntheses were carried out in nitrogen and the reagents and solvents were purchased from commercial sources and used as received unless otherwise noted. The synthesis of C₅₁H₄₀P₂Cl₂Ru (1A), C₅₁H₇₆Cl₂P₂Ru (1B), and C₁₁H₁₆O₄ (2) were based on reports published previously.¹

C₅₁H₄₀P₂Cl₂Ru (1A). A hot, dry 100 mL 3 neck round bottom flask was obtained from an oven and connected to it were a cold water condenser, septum, and sidearm stopcock. A gas inlet was connected to the condenser and a bubbler was connected to the gas inlet. All joints were greased. A stir bar was placed in the round bottom flask and the apparatus was connected to a nitrogen source. The condenser was connected to a cold water source. The round bottom flask was degassed with N₂ until cool, at which time RuCl₂(PPh₃)₂ (0.179 g, 1.87 x 10^-4 mol), THF (10 mL), and diphenylpropargyl alcohol (0.080 g, 3.84 x 10^-4 mol) were subsequently added to the reaction vessel. A sand bath was constructed and was used to heat the solution. The sand bath was set to 80% power and the mixture began to reflux a while later, but THF began to evaporate over time so the sand bath was turned down to around 40% power and an additional 30 mL of THF had to be added to the solution during the 2.5 h reflux period. The stir bar was spun at a moderate speed during this time.

After the reflux period had been completed, the reaction mixture was allowed to cool to room temperature. The solution was then transferred to a single neck 50 mL round bottom flask at which time the solution was taken off the N₂ supply and was exposed to air for the remainder of the synthesis. The solvent was removed via rotary evaporation leaving a thick, dark, brownish, reddish liquor. 1.5 mL dichloromethane was added to the liquor along with 9 mL hexane, which was slowly pipeted in. A dark red solid was precipitated and filtered using a small fritted funnel and was washed 3 times with about 2 mL hexane during each rinsing. The solid was vacuum dried and placed into a pre-weighed vial (9.698 g). The vial was stored in a dessicator for 1 week. The final weight of the vial was 9.767 g. The product was determined to be 1A (0.069g, 7.78 x 10^-5 mol, 41.6% yield based on the amount of RuCl₂(PPh₃)₂ used). ¹H NMR (CDCl₃): δ 7.3-7.8 (6 H, overlapping signal, Phand indenylidene). ³¹P NMR (CDCl₃): δ 29.5 (s, Ru-P). FTIR (ATR) ν 1928 cm^-1 (m).

C₅₁H₇₆Cl₂P₂Ru (1B). A hot, dry 100 mL 3 neck round bottom flask was obtained from an oven and connected to it were a cold water condenser, septum, and sidearm stopcock. A gas inlet was connected to the condenser and a bubbler was connected to the gas inlet. A stir bar was placed in the round bottom flask and the apparatus was connected to a nitrogen source. The condenser was connected to a cold water source. The round bottom flask was degassed with N₂ until cool, at which time 1A (0.050 g, 5.64 x 10^-5 mol), dichloromethane (7 mL), and tricyclohexylphosphine (0.055 g, 1.96 x 10^-4 mol) were subsequently added to the reaction vessel. The mixture was stirred at a moderate speed at room temperature for 1.5 h. 2 mL of additional dichloromethane was added to the solution during this time as some had evaporated off.

The solution was then transferred to a 50 mL single neck round bottom flask at which time the solution was taken off the N₂ supply and was exposed to air for the remainder of the synthesis. The solvent was removed via rotary evaporation. The remaining solid was suspended with 5 mL of hexane. This new solution was stirred at a moderate speed at ambient temperature for 0.5 h. The resulting solid was filtered using a small fritted funnel and was washed 3 times with about 2 mL hexane during each rinsing. The solid was vacuum dried and placed into a pre-weighed vial (9.737 g). This vial was stored in a dessicator for 1 week. The final weight of the vial was 9.824 g. The product was determined to be 1B (0.087g, 9.42 x 10^-5 mol, 167% yield based on the amount of 1A used). ¹H NMR (CDCl₃): δ 1.8-2.1 (5 H, overlapping signal, PCy₃), δ 7.1-7.9 (6 H, overlapping signal, Ph and indenylidene). ³¹P NMR (CDCl₃): δ 29.8 (s, Ru-P). FTIR (ATR) ν 1921 cm^-1 (m).

C₁₁H₁₆O₄ (2). A hot, dry 100 mL 3 neck round bottom flask was obtained from an oven and connected to it were a cold water condenser, septum, and sidearm stopcock. A gas inlet was connected to the condenser and a bubbler was connected to the gas inlet. A stir bar was placed in the round bottom flask and the apparatus was connected to a nitrogen source. The condenser was connected to a cold water source. The round bottom flask was degassed with N₂ until cool, at which time 1B (0.010 g, 1.08 x 10^-5 mol), anhydrous dichloromethane (6 mL), and diethyl diallylmalonate (0.100 g, 4.16 x 10^-4 mol) were subsequently added to the reaction vessel. The mixture was stirred at a moderate speed at room temperature for just over 1 h. The solution was then transferred to a 25 mL single neck round bottom flask at which time the solution was taken off the N₂ supply and was exposed to air for the remainder of the synthesis. The solvent was removed via rotary evaporation. ¹H NMR (CDCl₃): δ 1.2 (t, -CH₃), δ 2.6 (d, -CH₂), δ 4.1 (q, O-CH₂), δ 5.1 (m, =CH₂), δ 5.6 (tt, C-H), δ 6.8-7.7 (6 H, overlapping signal, Ph and indenylidene). GC-MS (CH₂Cl₂): 212 (2.5%, (2)), 241 (82.2%, (3)).

The process described above was repeated by a laboratory partner using 1A in lieu of 1B. ¹H NMR (CDCl₃): δ 0.9 (t), δ 1.25 (s), δ 1.55 (s), δ 1.84 (t), δ 3.74 (t), δ 6.8-7.7 (6 H, overlapping signal, Ph and indenylidene). GC-MS (CH₂Cl₂): 212 (0.12%, (2)), 241 (59.1%, (3)).

C₁₃H₂₀O₄ (3). The ¹H NMR spectrum of (3) was obtained from Sigma Aldrich.^{2 1}H NMR (CHCl₃): δ 1.25 (t, -CH₃), δ 2.6 (d, -CH₂), δ 4.2 (q, O-CH₂), δ 5.1 (m, =CH₂), δ 5.7 (tt, C-H).

Results

The reaction of RuCl₂(PPh₃)₂ with diphenylpropargyl alcohol yielded 0.069g of product, which was determined to be 1A. This translated to 7.78 x 10^-5 mol and thus a 41.6% yield based on the amount of RuCl₂(PPh₃)₂ used, which was the limiting reagent in the reaction. Proton NMR spectroscopy of 1A yielded one series of peaks of interest. From δ 7.3-7.8 there was a sequence of peaks representing the 6 different aromatic hydrogens from the phenyl and indenylidene groups. ³¹P NMR spectroscopy elicited one peak at δ 29.5 which can be attributed to phosphorus coordinated with the metal, Ru. The IR spectrum of the substance gave one notable peak at 1928 cm^-1, but the identity of this peak was unable to be determined.

The reaction of 1A with dichloromethane and tricyclohexylphosphine yielded 0.087g of product, which was determined to be 1B. This translated to 9.42 x 10^-5 mol and thus a 167% yield based on the amount of 1A used, which was the limiting reagent in the reaction. Proton NMR spectroscopy of 1B yielded two series of peaks of interest. From δ 1.8-2.1 were noted a faint sequence of overlapping signals, which were thought to be due to the 5 different hydrogens from the PCy₃ groups. From δ 7.1-7.9 there was a string of peaks representing the 6 different aromatic hydrogens from the phenyl and indenylidene groups. ³¹P NMR spectroscopy elicited one peak at δ 29.8 which can be attributed to phosphorus coordinated with the metal, Ru. The IR spectrum of the substance gave one notable peak at 1921 cm^-1, but again the identity of this peak was unable to be determined.

The reaction using 1B as a catalyst to perform ring closing metathesis on diethyl diallylmalonate produced a product with a ¹H NMR spectrum containing several peaks of interest. The triplet δ 1.2 was thought to be due to the methyl group, the doublet at δ 2.6 was thought to be due to –CH₂ groups, the quartet at δ 4.1 was thought to be due to the O-CH₂ groups, the multiple peaks at δ 5.1 were thought to be from =CH₂, the triplet of triplets at δ 5.6 was thought to be from C-H, and lastly the extremely weak overlapping signals at δ 6.8-7.7 were thought to be from phenyl and indenylidene groups. These assumptions are made taking into consideration that the ¹H NMR spectrum of diethyl diallylmalonate was identical, save for the almost negligible peaks from δ 6.8-7.7.² The GC/MS of 1B gave what were thought to be signals of interest at times 6.648 min and 6.945 min. The reading at 6.648 min accounted for 2.5% of the scan and was thought to be C₁₁H₁₆O₄ because its m/z of 212 appeared as a peak. The reading at 6.945 min accounted for 82.2% of the scan and was thought to be diethyl diallylmalonate because its m/z of 241 appeared as a peak, albeit very small. This gave a proposed ratio of 33:1, reactant to product.

When using 1A as the catalyst in lieu of 1B in this reaction, proton NMR spectroscopy of the product elicited several peaks, most of which were not able to be identified. The sequence of overlapping peaks from δ 6.8-7.7 was attributed to the 6 different hydrogens from phenyl and indenylidene groups, but the triplet at δ 0.9, the singlet at δ 1.25, the singlet at δ 1.55, the triplet at δ 1.84, and the triplet at δ 3.74 could not be determined. A standard ¹H NMR spectrum of the desired product C₁₁H₁₆O₄ was unobtainable for comparison. The GC/MS of 1A gave what were thought to be signals of interest at times 6.648 min and 6.974 min. The reading at 6.648 min accounted for 0.12% of the scan and was thought to be C₁₁H₁₆O₄ because its m/z of 212 appeared as a peak. The reading at 6.974 min accounted for 59% of the scan and was thought to be diethyl diallylmalonate because its m/z of 241 appeared as a peak, again albeit very small. This gave a proposed ratio of 491:1, reactant to product.

Discussion

The results of this experiment are inconclusive. The first reaction seemed to give a decent percent yield of 1A and it was identifiable through ¹H and ³¹P NMR spectroscopy, however there were a few erroneous peaks noted on the ¹H NMR spectrum and the peaks of interest were somewhat weak. The ³¹P NMR spectrum of 1A was inconclusive at first, so a new scan was done at a later time with a different sample. These facts seem to suggest that the original 1A obtained was not very pure. During the procedure, the sand bath was not adequately controlled, and this is most likely what caused the impure product. Because the reaction was overheated, side products may have formed or the original reagents did not react to completion, and in turn, the percent yield was in reality not as high as it appeared. This also attributes to the extra peaks that showed up on the ¹H NMR spectrum. The oxidation state of 1A is +4 and its electron count is 16.

The second reaction resulted in a percent yield of 167% of what was thought to be 1B, which again suggests some error. The ³¹P of this product gave a peak in nearly the exact same position as 1A, so this seems to confirm that the product obtained from the second reaction was not 1B, but mostly 1A. The peak should have shifted downfield to about δ 41, which is what colleagues have reported. The proton NMR spectrum does show faint peaks from δ 1.8-2.1 which is where one would expect hydrogens attached to non-aromatic cyclo groups to be found. This means that some of the -PPh₃ groups did convert to -PCy₃ groups, but a significant amount on the whole. The IR spectra of the products after reactions one and two are also quite similar, again hinting that nothing really transpired during reaction two. The procedure during reaction two went as detailed by the laboratory manual, so this means the starting material was probably impure and thus could not react to completion.¹ The oxidation state of 1B would also be +4 with an electron count of 16.

Because reaction one and reaction two seemed to yield the same product, the ring closing reactions cannot accurately be compared for catalytic activity. Theoretically 1B is the better catalyst, as it has PCy₃ ligands opposed to the PPh₃ ligands characteristic of 1A. PPh₃ ligands have a cone angle of 145^o while PCy₃ ligands have a cone angle of 170^o.³ The larger the cone angle, the bulkier the ligand and the faster it dissociates, allowing for expedited ring closing metathesis.³ The mechanism in which this takes place can be seen in Scheme 4. The reaction of diethyl diallylmalonate with 1B did not seem to elicit the ring closing mechanism. The ¹H NMR spectra of the product looks identical to that of the starting material, diethyl diallylmalonate, save for one area around δ 6.8-7.7 where traces of what looks like aromatic structures, namely phenyl and indenylidene groups can be found. It looks like there was such a minute amount of catalyst available that it never interacted with diethyl diallylmalonate to close the ring.

GC/MS of 1B shows two signals which may account for diethyl diallylmalonate and the closed ring. At time 6.648, a peak accounting for 2.5% all that was picked up by the scan contains a signal of 212 can be seen which corresponds with the m/z of C₁₁H₁₆O₄. At time 6.945 min, a peak accounting for 82.2% of the scan contains a signal of 241 can be seen which corresponds with the m/z of diethyl diallylmalonate. This gives a ratio of 33:1, reactant to product, which means the yield was rather poor. It does seem to suggest that some product may have been formed, however product was not visible on the ¹H NMR spectrum, so this interpretation may be inaccurate.

The reaction of diethyl diallylmalonte with 1A yielded a different ¹H NMR than the reaction with 1B did. It also shows overlapping peaks at δ 6.8-7.7 indicative of phenyl and indenylidene groups, but these peaks are much more noteworthy, meaning there was an abundance of catalyst available, where in the other reaction there was almost no catalyst available. Hence, upfield peaks are seen and are believed to be product, but these peaks are unable to be confirmed. A standard ¹H NMR spectrum of C₁₁H₁₆O₄ is unobtainable for comparison. The peaks reminiscent of the starting material seen in the ¹H NMR spectrum for the reaction with 1B are not visible, which means there was some sort of change in the starting material.

GC/MS analysis however does not seem to confirm the presence of a closed ring product. At time 6.648, a peak accounting for 0.12% all that was picked up by the scan contains a signal of 212 can be seen which corresponds with the m/z of C₁₁H₁₆O₄. At time 6.974 min, a peak accounting for 59% of the scan contains a signal of 241 can be seen which corresponds with the m/z of diethyl diallylmalonate. This gives a ratio of 491:1, reactant to product, which means the almost no product formed at all despite the presence of what seems to be a copious amount of catalyst. The reaction with 1B has a ratio of 33:1 and had almost no visible catalyst in its ¹H spectrum, so it could be possible that the GC/MS was analyzed improperly.

The sources of error are difficult to pinpoint, but one issue may have been the flow of nitrogen through the system. If the flow was too great, solvent would have been lost and it would have hindered the reactions. If the hot 100 mL three neck round bottom flask was not allowed to cool completely, that may have also caused a side reaction to occur due to the unwarranted heat. Also, as noted earlier during the synthesis of 1A, the reaction was overheated, which could have caused side products to form and thus inhibit the results of the following syntheses.

Conclusion

The main purposes of the experiments were to synthesize 1A and 1B, confirm their structures via ¹H, ³¹P, and IR spectroscopy, and to determine their relative catalytic rates during ring closing metathesis of diethyl diallylmalonate. 1A was identifiable by a series of overlapping peaks at δ 7.3-7.8 representative six different hydrogens attached to phenyl and indenylidene groups. This material was collected in a 46% yield, but in reality the yield was likely lower due to contaminants. 1B was synthesized with 167% yield, which suggests error. It was vaguely identifiable through its ¹H NMR spectrum by a series of peaks found at δ 1.8-2.1 representative of protons attached to cyclo groups, namely the PCy₃ substituents. This spectrum also contained the same series of overlapping peaks found around δ 7.3-7.8 for 1A. The ³¹P NMR and IR spectra for 1A and 1B are nearly identical, suggesting that almost no change in structure took place during the synthesis of 1B from 1A.

Because 1B did not properly synthesize, or did in an extremely low proportion, it was not feasible to measure 1A and 1B in comparison of their catalytic properties. It would be expected that 1B would be a better catalyst, as it contains bulkier groups which in theory dissociate faster.³ The 1A and 1B synthesized were both used as catalysts for ring closing metathesis of diethyl diallylmalonate. The product from the synthesis with 1B gave a ¹H spectrum nearly identical to that of the starting material, diethyl diallylmalonate, which says that there was too low of a concentration of catalyst for the reaction to occur in the time allotted. The product from the synthesis with 1A gave a different ¹H with peaks that are thought to be the desired product, but no standard ¹H NMR spectrum of the product is obtainable. The results from the GC/MS of both products runs contrary to the belief that any significant amount of C₁₁H₁₆O₄ was synthesized at all, and thus the results from this laboratory experiment are inconclusive.

References

(1) Pappenfus et al. Synthesis and Catalytic Performance of Ruthenium Carbene Complexes for Olefin Metathesis: A Microscale Organometallic Experiment. Journal of Chemical Education. 2007, 84, 1998-2000.

(2) http://www.sigmaaldrich.com/spectra/fnmr/FNMR005436.PDF

(3) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry: Third Edition. Pearson Prentice Hall: Upper Saddle River, 2004; pp 523-546.

Synthesis and Determination of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃

Abstract

The reaction of mesitylene with Mo(CO)₆ under reflux yields [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ in low percent yield (around 1%). ¹H NMR of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ shows singlets at δ 2.25 and 5.23 with absorption ratios of 9:3, respectively. ¹H NMR of mesitylene shows singlets at δ 2.25 and 6.78, also with absorption ratios of 9:3, respectively. This suggests addition of the metal complex to mesitylene causes downfield shifting of the signal for protons attached directly to the ring as they are unshielded from the backbonding of carbonyl groups. The IR spectrum of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ shows a strong antisymmetric C-O stretch at 1852 cm^-1 and a medium symmetric C-O stretch at 1942 cm^-1 with peak areas of 64.462 cm^-1 and 9.111 cm^-1 respectively. The calculated OC-Mo-CO bond angle is 108.32°.

Introduction

The reaction of mesitylene with Mo(CO)₆ under reflux yields [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃. The reaction specifically takes place in the following manner:

Scheme 1

This is compound of interest because it a metal-arene complex and can be considered to be an octahedral rather than tetrahedral complex. This is because the OC-Mo-CO bond angles are close to 90° instead of the expect 109.5° for tetrahedrals.¹ In order to determine the structure of said substance from its ¹H NMR, the peaks must be compared to the same spectrum of mesitylene for indication of identical methyl group peaks and downfield shifting a peak indicative of protons attached directly to the ring. The IR spectrum of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ can be analyzed for peaks indicative of symmetrical and antisymmetrical carbonyl stretching, whose areas can be used to calculate the bond angle between the carbonyl groups attached the to metal.

Experimental

All syntheses were carried out in nitrogen and the reagents and solvents were purchased from commercial sources and used as received unless otherwise noted. The synthesis of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ (1) was based on reports published previously.¹

[1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ (1). Mo(CO)₆ (2.083 g, 7.92 mmol) and mesitylene (10 mL, 72 mmol) were added subsequently to a 100 mL 3 neck round-bottom flask along with a small magnetic stir bar. A sand bath was constructed and set to 50% power. A greased sidearm, stopcock, and 30 cm cold water condenser were attached to the round-bottom flask. A greased gas inlet was then attached to the condenser and connected to a bubbler. The condenser was not connected to a cold water source; it was used only to allow air to circulate. The sidearm was connected to a nitrogen source, and the system was allowed to degas for 5 minutes. The system was then put on the sand bath and the stir bar was spun at a moderate speed via a magnetic stirring instrument.

After 5 minutes, the solution in the round-bottom flask was not boiling as outlined, so the sand bath was turned up to 70% power. The sand bath was turned up to 85% another 5 minutes later. A rigorous boil was achieved when the sand bath was set to 95% 5 minutes after that. It was then set to 85% power in efforts to obtain a less extreme boil. After a total of 0.33 h of reflux, the solution was taken off the sand bath and allowed to cool to room temperature. When the apparatus was removed from the sand bath, it was dropped and roughly more than 60% of the solution was lost. The remaining solution cooled to a blackish yellow color.

The following and final procedures took place in the presence of air. Once cool, the solution was washed with 15 mL of hexane via suction filtration in a 15 mL frit. The solution was then washed with another 5 mL of hexane. About 10 mL CH₂Cl₂ was added to the blackish yellow powder precipitate remaining in the frit. The powder was washed with 25 to 30 mL of hexane and vacuum dried. This powder was discarded and the collected yellowish washings were rotovapped for about 0.33 h to obtain the desired product. The product was vacuum filtered, as it would not completely dry under the rotovap. The resulting yellowish powder was determined to be 1 (0.028 g, 1.18% yield based on the amount of Mo(CO)₆ used). ¹H NMR (CH₂Cl₂): δ 2.25 (s, -CH₃), 5.23 (s, C-H). FTIR (ATR) ν_(C-O) 1852 cm^-1 (s, C-O linkage), ν_(C-O) 1942 cm^-1 (m, C-O linkage).

1,3,5-C₆H₃(CH₃)₃ (2). The ¹H NMR spectrum of 2 was extrapolated from the literature.^{1 1}H NMR (CHCl₃): δ 2.25 (s, -CH₃), 6.78 (s, C-H).

Results

The reaction of Mo(CO)₆ with mesitylene yielded 0.028 g of the product, [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃. This translated to 0.09328 mmol, and thus a 1.18% yield based on the amount of Mo(CO)₆ used, which was the limiting reagent in the reaction. Mo(CO)₆ reacted to form the product in a 1:1 ratio, and 7.92 mmol of Mo(CO)₆ was used to start, so that proportion was taken into account when calculating the percent yield. Proton NMR spectroscopy yielded a two peaks of interest. A peak found at δ 2.25 was indicative of methylhydrogens and a peak noted at δ 5.23 was suggestive of hydrogens attached directly to the aromatic ring.¹ These peaks were noted with relative intensities of 9 to 3, respectively. Peaks seen at δ 7.25 and 1.54 were attributed to solvent and hexane, respectively. The ¹H NMR spectrum of mesitylene in CHCl₃ showed absorptions at δ 2.25 and 6.78 with relative intensities of 9 to 3, respectively.¹ The peak at δ 2.25 hinted of methyl protons and the peak at δ 6.78 was suggestive of protons bonded directly the ring. The mass spectrum of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ showed a peak at m/z = 302.0, which is nearly equal to the molar mass of said substance.¹ The IR spectrum showed a strong peak around 1852 cm^-1 of area 64.462 cm^-1 indicative of antisymmetrical C-O stretching and a medium peak around 2942 cm^-1 of area 9.111 cm^-1 indicative of symmetrical C-O stretching. These areas were used to calculate a OC-Mo-CO bond angle of 108.32°.

Discussion

The percent yield of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ is very poor. Much of this quantitative shortcoming can be attributed to clumsiness, as much of the solution containing future product was lost when the reaction vessel was dropped. This not only resulted in a direct loss of solution, but also exposed the solution to air. The solution had been kept under nitrogen as to prevent decomposition of the products. This exposure to air undoubtedly had a contribution to the poor percent yield. The solution was also not heated as desired because it was difficult to control the sand bath. The solution was to be brought to a moderate boil, but could not be controlled to do so. It would not boil, then boiled rigorously a moment later. Attempts were made to subdue the boiling, but were unsuccessful. This overheating was probably not favorable for the reaction. More adept control of the sand bath would have resulted in a better yield of product.

Washing the product with excess amounts of hexane and CH₂Cl₂ also may have added to the loss of product. Excess washing would make it difficult to extract the product from the solution, as there would have been a relatively small amount of product compared to the amount of solution it was dissolved in. The solution could not be completely dried with the rotovap, which means there was an excess of hexane and/or mesitylene in the solution. Vacuum filtration then had to be used to collect the product, which was not ideal. Best case scenario, the rotovap would have completely dried the product and it would have been scraped out of the flask. Vacuum filtration gives a better chance for loss of product.

The product seemed pure as it produced clear ¹H NMR and IR spectra readings. The ¹H NMR spectrum shows a methyl peak at δ 2.25 and a C-H peak at δ 5.23, and the reagent in the reaction, mesitylene, also gives a peak at δ 2.25. This seems to confirm the structure and addition of the metal complex, as only the C-H peak was shifted downfield. The methyl protons are too shielded to be affected by the metal. The downfield shift is caused by backbonding of the carbonyls. IR spectroscopy revealed 3 C-O stretches, 2 of which were accounted for by a strong peak at 1852 cm^-1 and the 3^rd of which was accounted for by a medium peak at 1942 cm^-1. Two peaks were seen because of symmetrical and antisymmetrical stretching of the carbonyls.¹ The two antisymmetical modes have exactly identical absorption frequencies, and the symmetrical mode has a different absorption frequency than them, which means that a total of two peaks should be seen.¹ The areas of these peaks, 64.462 cm^-1 for antisymmetical stretch and 9.111 cm^-1 for symmetrical stretch, allowed for discovery of the bond angle between the CO ligands. The calculated angle was 108.32°. This seems to make sense as [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ is predicted to be a tetrahedral complex and the expected bond angle for tetrahedral complexes is 109.5°, but in reality [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ acts more like an octahedral complex and the bond angle should be slightly less than 90°.¹ This error could be due to excess solvent or impure an sample, which resulted in skewed IR spectrum readings and thus incorrect peak areas. The oxidation state and electron count of Mo in [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ are 0 and 6 electrons, so it is an 18 electron complex.

Conclusion

The main purpose of the experiment was to interpret the ¹H NMR and IR spectra of [1,3,5-C₆H₃(CH₃)₃]Mo(CO)₃ to confirm the product and to decipher the bond angle between the carbonyls. The ¹H NMR spectrum of the product showed peaks at δ 2.25 and 5.23, while the ¹H NMR spectrum of mesitylene gave peaks at δ 2.25 and 6.78, both in ratios of 9:3, respectively. This ratio seems to confirm methyl groups and single protons attached to the ring. The downfield shifting of the second peak is attributed to the coordination of mesitylene to the metal complex. The protons attached directly to the ring are affected by backbonding of the carbonyl groups. The methyl hydrogens are shielded, and thus are not affected by the metal complex. The IR spectrum yielded two peaks near 2000 cm^-1. These two peaks account for 3 C-O stretches, 2 of which are accounted for by a strong peak at 1852 cm^-1 and the 3^rd of which are accounted for by a medium peak at 1942 cm^-1. The strong peak accounts for antisymmetrical stretching of the carbonyls and the medium peak accounts for symmetrical stretching of the carbonyls. The areas of these peaks, 62.462 cm^-1 and 9.111 cm^-1 respectively, provide for a theoretical angle between the carbonyls of 108.32°. In reality, this angle should be only nearly 90°. The percent yield for the reaction was poor and could have improved with a more steady hand, more precise heating of the reagents, less exposure of the solution to air, and less solvent used for washing.

References

(1) Angelici, R. J.; Girolami, G. S.; Rachufuss T. B. Synthesis and Technique in Inorganic Chemistry: A Laboratory Manual; University Science Books: Sausilito, CA, 1999; pp 161-170.

Synthesis and Determination of Polypyrazolylborates:

K[HB(3,5-C₅H₇N₂)₃] and HB(3,5-C₅H₇N₂)₃Cu(CO)

Abstract

The reaction of KBH₄ heated with 3,5-dimethylpyrazole produces potassium tris(3,5-dimethylpyrazolyl)hydroborate at an unknown percent yield. ¹H NMR spectroscopy of K[HB(3,5-C₅H₇N₂)₃] shows singlets at δ 1.79 and 2.06 indicative of –CH₃ whereas the same spectroscopy of 3,5-dimethylpyrazole shows only one singlet in the same area. IR spectroscopy of K[HB(3,5-C₅H₇N₂)₃] shows a B-H stretch at 2420 cm^-1, which is absent from the IR spectrum of 3,5-dimethylpyrazole. The reaction of K[HB(3,5-C₅H₇N₂)₃] with CuI and CO gives rise to HB(3,5-C₅H₇N₂)₃Cu(CO) at 265% yield. ¹³C NMR spectroscopy of HB(3,5-C₅H₇N₂)₃Cu(CO) produces a singlet at δ 172.4 indicative of C-O bonding, which does not appear on the ¹³C NMR spectrum of K[HB(3,5-C₅H₇N₂)₃]. IR spectroscopy of HB(3,5-C₅H₇N₂)₃Cu(CO) produces a C-O stretch at 2053 cm^-1, which is absent from the IR spectrum of K[HB(3,5-C₅H₇N₂)₃]. The analysis of these spectra seems to validate the supposed products from the reactions.

Introduction

The reaction of KBH₄ with 3,5-dimethylpyrazole yields tris(3,5-dimethylpyrazolyl)hydroborate. The reaction specifically takes place in the following manner:

This is compound of interest because it a polypyrazolylborate, or scorpionate, formed from a binary born hydride, which are difficult to handle.^1,2 In order to determine the structure of said substance from a ¹H NMR and IR spectrum, the peaks and stretches must be compared to the same spectra for 3,5-dimethylpyrazole to look for indications of boron in the structure and differentiation in methyl groups. The spectra for the two compounds should be similar save for those two main differentiations.

Metal complex HB(3,5-C₅H₇N₂)₃Cu(CO) can be synthesized from the following reaction:

The identity of this product can be confirmed by comparing its ¹³C NMR and IR spectra to the same spectra of the reagent K[HB(3,5-C₅H₇N₂)₃]. In this case, the spectra are compared to look for the presence of C-O bonding. The spectra should appear similar aside from peaks and stretches indicative that bond. The systematic addition of functional groups to these two compounds is what makes them comparable and identifiable to and from one another in their ¹H NMR, ¹³C NMR, and IR spectra.

Experimental

All syntheses were carried out in air and the reagents and solvents were purchased from commercial sources and used as received unless otherwise noted. The synthesis of K[HB(3,5-C₅H₇N₂)₃] (1) and HB(3,5-C₅H₇N₂)₃Cu(CO) (2) were based on reports published previously.¹

K[HB(3,5-C₅H₇N₂)₃] (1). KBH₄ (1.028 g, 19.1 mmol) and 3,5-dimethylpyrazole (7.002 g, 72.8 mmol) were added subsequently to a 100 mL round-bottom flask along with a small magnetic stir bar. A cold water condenser with greased joint was inserted into the round-bottom flask containing the solution. This connection was further secured with a keck clip. A silicone oil bath was constructed with a glass dish containing a paper clip as a stirring instrument. The bath was placed on a hot plate and the round bottom flask was placed in the oil bath. The cold water condenser was not connected to a cold water source; it was used to allow air to circulate.

Once secure, the hot plate was turned on to 230 °C and the stirring instruments were spun at a moderate speed. A thermometer was inserted into oil bath to monitor the temperature, which fluctuated between 230 °C and 250 °C during the experiment. The solution was allowed to heat for 1 h. After this time, the round-bottom flask was taken off the oil bath and the condenser was removed. A white solid precipitate submerged in liquid remained and was allowed to cool to 90 °C, again using the thermometer to measure temperature. 50 mL of toluene was added to the flask and the solution was vacuum filtered with a 60 mL frit. A total of about 100 mL more toluene was added to wash the resulting white solid precipitate. The precipitate was washed a final time with 50 mL of diethyl ether. The precipitate was then vacuum dried for 0.33 h. The precipitate was a powdery white substance 1. ¹H NMR (D₂O): δ 1.79 (s, -CH₃), 2.06 (s, -CH₃), 5.82 (s, C-H). ¹³C NMR (D₂O): δ 11.05 (s, -CH₃), 12.44 (s, -CH₃), 104.9 (s, C-H), 146.1 (s, C-CH₃), 148.9 (s, C-CH₃). FTIR (ATR) ν_(C=N) 1560 cm^-1 (s, pyrazolyl), ν_(B-H) 2420 cm^-1 (s, B-H linkage), ν_(C-H) 2950 cm^-1 (br, C-H linkage).

HB(3,5-C₅H₇N₂)₃Cu(CO) (2). CuI powder (0.394 g, 2.07 mmol) and acetone (35 mL) were subsequently added to a 100 mL round-bottom flask along with a small magnetic stir bar. A septum was attached to the flask. Previously synthesized 1 (0.191 g, 0.567 mmol) was dissolved in a minimal amount of acetone (3 to 5 mL). CO gas was bubbled into the round-bottom flask for about 5 min. At this time, the solution of 1 and acetone was injected into the round-bottom flask, and CO gas was allowed bubbled in for another few minutes. A yellowish liquid resulted and the flask was put on ice for 1 h to allow for recrystallization. The solution was then roto-vaporized for 5 to 10 minutes to make up for inadequate recrystallization.

A grayish, greenish powder remained in the round-bottom flask, which was scraped out using a spatula and determined to be 2 (0.584 g, 265% yield based on the amount of 1 used). ¹H NMR (CDCl₃): δ 2.30 (s, -CH₃), 2.50 (s, B-H), 5.68 (s, C-H). ¹³C NMR (CDCl₃): δ 12.50 (s, -CH₃), 13.92 (s, -CH₃), 104.37 (s, C=C), 143.60 (s, C=N), 147.42 (s, C-N), 172.4 (s, C-O). FTIR (ATR) ν_(C=N) 1543 cm^-1 (s, pyrazolyl), ν_(C-O) 2053 cm^-1 (s, carbonyl), ν_(B-H) 2499 cm^-1 (s, B-H linkage), ν_(C-H) 2921 cm^-1 (br, C-H linkage).

C₅H₈N₂ (3). The ¹H NMR and IR spectra of (3) were obtained from Sigma Aldrich.^{3,4 1}H NMR (CDCl₃): δ 2.25 (s, -CH₃), 5.8 (s, C-H). FTIR (ATR) ν_(C-N) 1030 cm^-1 (s, pyrazolyl), ν_(C-H) 2860 cm^-1 (br, C-H linkage), ν_(C-H) 2930 cm^-1 (br, C-H linkage).

Results

The reaction of KBH₄ and 3,5-dimethylpyrazole was not measured for yield of the product, K[HB(3,5-C₅H₇N₂)₃], but theoretical yield would be 19.1 mmol. Theoretical yield of H₂ gas, though not measured, was 57.3 mmol, based on the amount of KBH₄ used, which was the limiting reagent. KBH₄ reacts to form H₂ in a 1:3 ratio, and 19.1 mmol of KBH₄ was used to start, so that proportion was taken into account when calculating the theoretical yield. ¹H and ¹³C NMR spectroscopy of the product yielded several peaks. The ¹H NMR spectrum presented a two singlets found at δ 1.79 and 2.06, representative of methyl groups. A singlet found at δ 5.82 was indicative of the hydrogen attached directly to pyrazolyl ring. The ¹³C NMR spectrum yielded a pair of singlets found at δ 11.05 and 12.44, which was suggestive of methyls attached to the pyrazolyl ring. A singlet found at δ 104.9 was from the C-H bond on the ring, and two final singlets found at δ 146.1 and 148.9 were from the carbons on the ring attached to the methyl groups. The IR spectrum showed a sharp peak around 1560 cm^-1 indicative of a C=N bond forming the pyrazolyl ring, a sharp peak around 2420 cm^-1 indicative of B-H linkage, and finally a broad peak near 2950 cm^-1 suggestive of C-H bonding.

The reaction of CuI, K[HB(3,5-C₅H₇N₂)₃], CO, and acetone yielded 0.584 g of product, HB(3,5-C₅H₇N₂)₃Cu(CO). This translated to 1.502 mmol, and thus was a 265% yield. ¹H and ¹³C NMR spectroscopy of the product yielded several peaks. The ¹H NMR spectrum contained a singlet found at δ 2.30, representative of methyl groups. A singlet found at δ 2.50 was indicative of the hydrogen bonded to boron. A third singlet found at δ 5.68 was from protons bonded to the pyrazolyl ring. The ¹³C NMR spectrum produced a two singlets found at δ 12.50 and 13.92, which suggested methyls carbons. A singlet found at δ 104.37 was from double bonded carbons, a singlet found at δ 143.60 was from carbon double bonded to nitrogen, another singlet found at δ 147.42 was from carbon singly bonded to nitrogen, and one final singlet at δ 172.4 was representative of carbon bonded to oxygen. The IR spectrum yielded a sharp peak around 1543 cm^-1 indicative of a C=N bond forming the pyrazolyl ring, a sharp peak around 2053 cm^-1 indicative of C-O bonding, a sharp peak around 2499 cm^-1 indicative of B-H linkage, and finally a broad peak near 2921 cm^-1 suggestive of C-H bonding.

Discussion

The percent yield of K[HB(3,5-C₅H₇N₂)₃] was not able to be determined. The weight of this product was either never obtained or the figure was lost during the experiment. Percent error, though not measured, could possibly have been affected from heating the KBH₄ and 3,5-dimethylpyrazole solution at too high a temperature, as it went above the 230 °C limit specified by the experimental guidelines.¹ The solution was heated for only 1 h, when the suggested time was 1 to 1.5 h, which means the reagents may not have completely reacted. When the solution was taken off the oil bath and allowed to cool, it cooled more quickly than expected, and dropped to 90 °C or lower before adding the 50 mL of toluene when 100 °C was specified the addition temperature.¹ There was some confusion as far as the protocol at this point, so the solution with the toluene added was allowed to cool for a short while, when the guidelines asked for the residue to be filtered and washed hot. The solution was still warm when filtered and washed, but not nearly as hot as it could have been. These types of errors would have resulted in loss of potential product and negatively affected the percent yield, had it been measured.

The product did seem pure, as it was a clean white color, and its ¹H NMR, ¹³C NMR, and IR spectra yielded clear readings. The ¹H spectrum shows methyl peaks at δ 1.79 and 2.06 whereas the ¹H spectrum for the reagent in the reaction, 3,5-dimethylpyrazole, shows only one methyl peak at δ 2.25. This seems to validate that addition of the boron to the molecule, as it would cause make each methyl group slightly different from the other. Polypyrazolylborates produce sharp a B-H stretch in their IR spectra, and this is evident in the IR spectrum reading for K[HB(3,5-C₅H₇N₂)₃].² A sharp peak is noted at 2420 cm^-1, whereas the IR spectrum for 3,5-dimethylpyrazole does not contain said stretch, again supporting the claim for addition of boron to the molecule.

The percent yield for HB(3,5-C₅H₇N₂)₃Cu(CO) was not accurate. The product obtained from the roto-vaporization was not washed, so it is suspected that the other product of the reaction, KI, was mixed in with the desired product. It is believed that the greenish powder was HB(3,5-C₅H₇N₂)₃Cu(CO) while the greyish powder was KI. This is why the percent yield was above 100%. Aside from that inaccuracy, the only 0.567 mmol of K[HB(3,5-C₅H₇N₂)₃] was used, when the protocol called for 2 mmol to be used.¹ This would not affect the percent yield, as the amount of K[HB(3,5-C₅H₇N₂)₃] used would still be the limiting reagent, but could have affected the NMR and IR spectrums. However, it was intuitively noted that the greenish powder was the desired product, and an effort was made to extract only that powder from the product for the spectroscopy determinations.

The fact that recrystallization did not seem take place as detailed¹ and that a roto-vaporizer had to be used to dry the product most likely did not help the yield of product either. Product may have been lost during this process. If the powder had been washed with acetone, a more accurate percent yield would have been obtained because the KI would have been washed away, but this was not extremely necessary for the purposes of this experiment. The product obtained did give clear ¹H NMR, ¹³C NMR, and IR spectra, meaning it was fairly pure. The addition of the CO to the molecule from K[HB(3,5-C₅H₇N₂)₃] is evident in the ¹³C NMR and IR spectra. There is a distinct peak at δ 172.4 on the ¹³C NMR spectrum which is not noted on the same spectrum for K[HB(3,5-C₅H₇N₂)₃]. The IR spectrum of HB(3,5-C₅H₇N₂)₃Cu(CO) shows a tall sharp stretch at 2053 cm^-1 distinctive of C-O bonding; the IR spectrum of K[HB(3,5-C₅H₇N₂)₃] shows no such stretch. Peaks and stretches for the spectra were labeled with the help of colleagues. An acknowledgement is made that are more than likely downfield or upfield shifts of some of the peaks from one product to another because of changes in chemical structure, but these postulates were not explored. The oxidation state and electron count of Cu in HB(3,5-C₅H₇N₂)₃Cu(CO) are +1 and 10 electrons, so it is a 18 electron complex.

Conclusion

The main purpose of the experiment was to decipher the structural changes from 3,5-dimethylpyrazole to potassium tris(3,5-dimethylpyrazolyl)hydroborate to a copper complex of potassium tris(3,5-dimethylpyrazolyl)hydroborate through ¹H NMR, ¹³C NMR, and IR spectra. Addition of boron to 3,5-dimethylpyrazole was apparent in the ¹H NMR and IR spectra of the first product. The ¹H spectrum shows methyl peaks at δ 1.79 and 2.06 whereas the ¹H spectrum for the reagent in the reaction, 3,5-dimethylpyrazole, shows only one methyl peak at δ 2.25, seemingly confirming the addition of boron as this would make each methyl group differentiable. The IR spectrum of this product showed a sharp stretch around 2420 cm^-1, indicative of B-H bonding, which is absent in the IR spectrum for 3,5-dimethylpyrazole. All of these finding seem to validate K[HB(3,5-C₅H₇N₂)₃] as being the product of the reaction.

The ¹H NMR, ¹³C NMR, and IR spectra of the second product also seem to confirm its expected structure. The ¹³C NMR spectrum for the second product shows a peak at δ 172.4, which is an area suggestive of C-O bonding. The ¹³C NMR spectrum of K[HB(3,5-C₅H₇N₂)₃] contains no peak in this area. The IR spectrum of the second product shows a sharp stretch around 2053 cm^-1, which is also indicative of C-O bonding. The IR spectrum of K[HB(3,5-C₅H₇N₂)₃] contains no stretch in this area. These noted findings on the spectra all point towards to product being HB(3,5-C₅H₇N₂)₃Cu(CO).

The percent yield for the first reaction was not monitored, but would have been aversely affected by factors such as poor temperature control, short reaction time, and more prompt washing technique. The percent yield for the second reaction was poor, but could have been improved by washing the product with acetone and by allowing for a longer recrystallization period.

References

(1) Bochmann, M. Preparation and Complexation of Tris(3,5-dimethylpyrazoyl)hydroborate. pp 33-35.

(2) Trofimenko, S. Polypyrazolylborates: Scorpionates. Journal of Chemical Education. 2005, 82, 1715-1720.

(3) http://www.sigmaaldrich.com/spectra/fnmr/FNMR010068.PDF

(4) http://www.sigmaaldrich.com/spectra/ftir/FTIR007818.PDF

Tinkering with Tin: Synthesis of SnCl(CH₂C₆H₅)₃ and SnCl₄[OS(CH₃)₂]₂

Abstract

The reaction of tin with benzyl chloride under reflux yields SnCl(CH₂C₆H₅)₃ in low percent yield (around 15%) due to various factors. ¹H NMR of SnCl(CH₂C₆H₅)₃ shows a singlet at δ 3.15 with satellites containing coupling constant between Sn-H of 78.1 Hz. The reaction of SnCl₄ with OS(CH₃)₂ yields SnCl₄[OS(CH₃)₂]₂ with poor percent yield (around 140%) due to various factors. The IR spectrum of SnCl₄[OS(CH₃)₂]₂ shows a large S-O stretch at 899.7 cm^-1, which is a lower frequency than the S-O stretch of 1042.5 cm^-1 from the IR spectrum of OS(CH₃)₂, suggesting coordination on the metal complex at oxygen.

Introduction

The reaction of tin with benzyl chloride yields the metal complex SnCl(CH₂C₆H₅)₃. The reaction specifically takes place in the following manner:

[Figure missing.]

This is compound of interest because of the different isotopes of tin that naturally occur. In order to determine the structure of said substance from a ¹H NMR spectrum, the peaks must be analyzed for satellites occurring from coupling of hydrogen to isotopes of tin. The observed spectrum is a combination of SnCl(CH₂C₆H₅)₃ molecules containing different isotopes of Sn, namely ¹¹⁷Sn and ¹¹⁹Sn, among others. The value for J_Sn-H between satellites can confirm the presence of SnCl(CH₂C₆H₅)₃. Mass spectrum can also be used to confirm the presence of SnCl(CH₂C₆H₅)₃ in the product by looking for a peak at m/z equal to the molecular mass of the substance (427 g/mol).

SnCl₄[OS(CH₃)₂]₂ can be synthesized from the following reaction:

[Figure missing.]

It is a reaction of interest because it can be used to determine where OS(CH₃)₂ coordinates to Sn. Coordination of S to Sn would result in a higher frequency of S-O stretch on an IR spectrum than the frequency of the S-O stretch on the IR spectrum of OS(CH₃)₂ due to a strengthening of the S-O bond. Coordination of O to Sn would result in a lower frequency of S-O stretch because of a weakened S-O bond from the resonance form needed to secure that coordination.¹

Experimental

All syntheses were carried out in air and the reagents and solvents were purchased from commercial sources and used as received unless otherwise noted. The synthesis of SnCl(CH₂C₆H₅)₃ (1) and SnCl₄[OS(CH₃)₂]₂ (2) were based on reports published previously.¹

SnCl(CH₂C₆H₅)₃ (1). 325 mash-Aldrich Sn powder (1.939 g, 16.34 mmol), 99% Aldrich benzyl chloride (6.0 mL, 52 mmol), and deionized H₂O (4.0 mL) were added subsequently to a 50 mL round-bottom flask. A small stir bar was added then added to this solution. A sand bath was placed over a stir plate and a cold water condenser with greased joint was inserted into the round-bottom flask containing the solution. This connection was further secured with a keck clip. The round bottom flask was placed in the sand bath and the condenser was connected to the cold water source.

Once secure, the sand bath was set to 50% power and the stir bar was spun a shade over moderate speed. The solution was allowed to reflux for 2.75 h. Once reflux was complete, the 50 mL round-bottom flask was removed from the condenser and allowed to cool in an ice bath until it was cold. The liquid was decanted from the white precipitate and discarded. The white precipitate was saved. Ethyl acetate (10 mL) was added to the precipitate and this solution was heated using a sand bath at 65% power and stirred at moderate speed until the white solid had completely dissolved. The round-bottom flask was then taken off the heat and submerged in an ice bath for approximately 0.5 h to allow for recrystallization.

The stir bar was removed and a high vacuum was used for about 10 minutes to extract the extraneous liquid and leave a white powder. Several mL of ether were added to the round-bottom flask and a glass stir rod was used to break up the chunks of powder and dissolve it in the ether. This solution was then suction filtered with a 30 mL glass frit and allowed the dry. The precipitate collected was 1 (0.471 g, 13.49% based on the amount of Sn powder used). ¹H NMR (CDCl₃): δ 7.25 (s, CDCl₃), 7.19 (s, Ph-H), 7.17 (s, Ph-H), 7.05 (s, Ph-H), 3.15 (s, J_Sn-H = 78.1 Hz, CH₂).

SnCl₄[OS(CH₃)₂]₂ (2). SnCl₄ (2.25 mL, 19 mmol), anhydrous diethyl ether (45 mL), DMSO (2.9 mL, 41 mmol), and ether (5 mL) were subsequently added to a 125 mL Erlenmeyer flask. The DMSO was added using a syringe for safety purposes while the other reagents were measured and added using graduated cylinders. This solution yielded a precipitate which was isolated by suction filtering the solution through a 50 mL glass frit. Several extra mL of ether were added to the Erlenmeyer flask to help aid in transfer of all the precipitate to the filter. Once dry, the white powder precipitate 2 was collected and weighed. 11.04 g (26.49 mmol) were recovered, giving a yield of 139.4%. FTIR (ATR) ν_(S-O) 899.7 cm^-1 (s S-O coordination to Sn).

OS(CH₃)₂ (3). The IR spectrum of (3) was taken by Dr. Graham. FTIR (ATR) ν_(S-O) 1042.5 cm^-1 (s, S-O linkage).

Results

The reaction of Sn, benzyl chloride, and H₂O yielded 0.471 g of the product, SnCl(CH₂C₆H₅)₃^. This translated to 1.102 mmol, and thus was a 13.49% yield based on the amount of Sn used, which was the limiting reagent in the reaction. Sn reacted to form the product in a 2:1 ratio, and 16.34 mmol of Sn was used to start, so that proportion was taken into account when calculating the percent yield. Proton NMR yielded several peaks. A set of peaks were found at δ 7.19, 7.17, and 7.05 were indicative of phenyl resonances derived from protons on the phenyl ring of the product.¹ A sharp peak located at δ 7.25 was due to the chloroform solvent ¹. Coupling between Sn and H produced a singlet at δ 3.15 with satellites ¹J_Sn-H equal to 78.1 Hz due to the presence of isotopes ¹¹⁷Sn and ¹¹⁹Sn.¹ The mass spectrum of SnCl(CH₂C₆H₅)₃ showed a peak at m/z = 427.0,¹ which is the molar mass of said substance.

The reaction of SnCl₄ and DMSO yielded 11.04 g of product, SnCl₄[OS(CH₃)₂]₂. This translated to 26.49 mmol, and thus was a 139.4% yield. IR spectrum of SnCl₄[OS(CH₃)₂]₂ showed its largest peak at 899.7 cm^-1 while the IR spectrum of OS(CH₃)₂ gave its largest peak at 1042.51 cm^-1.

Discussion

The percent yields for each product are less than stellar. During the synthesis of SnCl(CH₂C₆H₅)₃, letting the reagents reflux for a longer time period, closer to 3 hours, may have been beneficiary to resulting in more product. The reagents may not have all completely reacted. Some of the precipitate may have been accidentally removed during decanting after reflux, and much ethyl acetate may have been added to that precipitate. A minimal amount should have been used while trying to dissolve the white solid precipitate in the sand bath. The less ethyl acetate needed and used would have resulted in a better percent yield.

The fact that recrystallization did not seem take place as detailed¹ and that a vacuum had to be used to dry the product most likely did not help the yield of product either. Product may have been lost during the vacuuming process. The glass frit used for filtering was not of the utmost quality, and so product may have escaped during that process, also. The product was also not completely dry, and gave a misleading mass measurement, meaning the percent yield is even lower than recorded. The product obtained did seem to give a clear ¹H NMR spectra, meaning it was fairly pure. The coupling between Sn and H due to isotopes ¹¹⁷Sn and ¹¹⁹Sn ¹ in the product is distinctly visible around δ 3.15 with a J_Sn-H of 78.1 Hz. The NMR chemical shifts of the Sn-CH₂ protons and the C­₆H₅ protons of SnCl(CH₂C₆H₅)₃ are so different because of the 117 and 119 isotopes of Sn. They affect the coupling with H, producing satellites. The C₆H₅ protons are not affected by this coupling. The literature states that peaks for phenyl resonance appear around δ 7 and chloroform appears around δ 7.24, which seems to validate the experimental values obtained (δ 7.05 to 7.19 for phenyl resonances ad δ 7.25 for chloroform).¹

The percent yield of SnCl₄[OS(CH₃)₂]₂ is likely thrown off because the product was not dry when it was weighed. Several extra mL of ether were used for transport of the precipitate to the glass frit for filtering, which would most likely lead to loss of product. Not all of the precipitate was able to be transferred from the Erlenmeyer flask to the frit. Product was distinctly lost due to a seemingly defective frit, too. Attempts to refilter the filtrate were unsuccessful. The fact that the frit quality was poor meant that the product was not able to be dried well and thus carried extra weight. The product seemed to smoke away as it was allowed to dry further in exposure to air, and lost mass over time. This conundrum could not be adequately explained. IR spectrum of the product SnCl₄[OS(CH₃)₂]₂ shows its largest stretch at 899.7 cm^-1, hinting that the coordination to Sn occurs at oxygen, as this frequency is lower than that of the largest stretch from the OS(CH₃)₂ spectrum (1042.51 cm^-1). This is due to resonance and weakening of the S-O bond.¹ The two spectra are similar save for the shifting of that one peak. The stretch at 1042.51 cm^-1 is consistent with the value stated in the literature for S-O stretching (approximately 1100 cm^-1).¹

Conclusion

The main purposes of the experiments were to decipher the ¹H NMR for SnCl(CH₂C₆H₅)₃ in order to determine J_Sn-H and to interpret the IR spectra of OS(CH₃)₂ and SnCl₄[OS(CH₃)₂]₂ to tell whether O or S coordinates to Sn. The J_Sn-H was found to be 78.1 Hz. The observed spectrum is a combination of SnCl(CH₂C₆H₅)₃ molecules containing different isotopes of Sn, namely ¹¹⁷Sn and ¹¹⁹Sn, among others. This is what makes the ¹H NMR difficult to interpret, but the J_Sn-H value is validation of the presence of SnCl(CH₂C₆H₅)₃. The percent yield for that reaction was poor, due to varying factors. Longer reflux time, less ethyl acetate used during recrystallization, and better filtration techniques all would have contributed to an improved percent yield.

The IR spectrum of OS(CH₃)₂ showed its largest stretch around 1050 cm^-1 while the IR spectrum of SnCl₄[OS(CH₃)₂]₂ gave its largest stretch around 900 cm^-1. This lower frequency of stretch suggests that O and not S coordinates to the Sn. This is due in part to the resonance form needed to form the complex, which weakens the S-O bonding, and thus lowers the stretching frequency. The yield for the reaction to form SnCl₄[OS(CH₃)₂]₂ was also poor, which can be mostly attributed to poor filtering and drying techniques, along with the overuse of ether.

References

(1) “Synthesis and Techniques in Inorganic Chemistry,” Third Edition, G. S. Girolami, T.R. Rauchfuss, and R. Angelici, University Science Books, 1999.

Otherwise known as: “A Reusable Polymeric Asymmetric Hydrogenation Catalyst Made by Ring-Opening Olfein Metathesis Polymerization”

This was a PowerPoint presentation I did for class.

It is widely known among inorganic chemists that multiply bonded metal-ligand species take part in a diverse set of atom and group transfer reactions. It is common to witness CR₂ groups transferred to unsaturated organic substrates, but viewing the insertion of CH₂ into C-H bonds to yield saturated product is quite unusual. In the case of [TolC(NSiMe₃)₂]₂Ta(CH₂)CH₃, its electrophilic nature allows for an improbable double group transfer to occur when exposed to pyridine N-oxides. This reactions yields [TolC(NSiMe₃)₂]₂Ta(O)CH₃ due to simultaneous deoxygenation and regioselective methylation of the pyridine N-oxide.

The benzamidinate tantalum ethylidene complex is also able to react with nitrones, which are similar in structure for pyridine N-oxides. It is not however able to react with weak oxidants such as styrene oxide and triphenylphosphine oxide. Only one equivalent of the pyridine N-oxide was needed for the aforementioned reaction to take place. 2-Methylpryidine is produced as well, as confirmed by comparison using NMR integration versus a trimethoxybeneze internal standard. The trimethoxybenze reacts further with 2-methylpyridin N-oxide to ultimately yield 2,6-dimethylpyridine and oxo complex.

It should be noted that methylation occurs regioselectively at the unsubstituted ortho position in each pyridine N-oxide. Also, the substituted pyridine N-oxide species react much slower than the unsubstituted variant, comparatively in minutes versus microseconds.

Proton and carbon 13 NMR, IR spectroscopy, and X-ray crystallography were all used to verify the tantalum oxo complex product. The IR spectrum shows a strong stretch at 922 cm^-1, which is a feasible number to indicative of terminal Ta-O multiple bonds (typically 850-1000 cm^-1). X-ray crystallography reveals a distorted-octahedral coordination geometry surrounding the tantalum and thus confirmed the presence of a terminal oxo character. The measured bond length of the tantalum atom to oxygen bond is reported to be 1.76 Å, which is in line with previously reported figures for Ta-O multiple bonds. Thus, all the statistics seem to confirm that a double group transfer does indeed take place.

The mechanism of this reaction is thought to take place via two possible schemes involving a total of three mechanisms, but it is not known which scheme or mechanism is correct. There is an absence of intermediates in the reaction as evidenced by UV, IR, and ¹H NMR spectroscopy, so deuterium labeling is used to distinguish these potential routes of formation. GC-MS shows parent ion at m/z 95 and 111 corresponding to the methyl and dimethylpyridine products, respectively. At 2.40 ppm there is a 1:1:1 triplet indicative of the CH₂D group. This group also appears in both the proton and carbon 13 NMR spectras, which in all suggests that the mechanism of reaction takes place via scheme one and a mechanism label B.

Finally, nitrones which is similar in structure to pyridine N-oxides are also reacted with the benzamidinate tantalum ethylidene complex to see if they have a comparable interaction. Only after heating the complex with N-tert-butyl-α-phenylnitrone at 45 °C for 40 hours did styrene and another new organometallic product come to fruition. The new product is suspected to be [TolC(NSiMe₃)₂]­₂TA(O)(N^tBuMe) through ¹H, ¹³C{¹H} NMR, IR, and mass spectroscopic techniques.

In conclusion, it is the enhanced electrophilicity of the benzamidinate tantalum ethylidene which allows for the reaction pathway to occur. The atom transfer reactions allow for Ta-O double bonds and organic product with new C-C bonds to be formed. Further investigation into these matters is ongoing. I believe that following steps that could be taken would to delve into other metals complexes that could allow for double group transfers. Logically, I would think that the next metals to investigate would be other group 5 metals, possibly replacing Ta with Nb or Db. These metals should have the most similar properties in relationship to Ta. Reactants other than N-oxides and nitrones could also be analyzed to see if it is possible to replicate the double group transfer.

In a related study performed by ….

Unlike the well know and oft studied chemistry of double bonds between carbons, the chemistry of boron-boron double bonds is for the most part unexplored. It is believed that boron should behave similarly to carbon due to its relativity to the element on the periodic table. Anions containing boron double bonds, specifically [R₂BBR­­₂]^2-, have in the past been predicted to be possible structures of interest to synthesize in the laboratory, however such efforts have failed for the most part.

It was then proposed to explore neutral diborenes, even though they in theory should be highly reactive compounds due to their triplet ground states and two one-electron π-bonds according to molecular orbital theory. The electron deficiency in this structure could however be stabilized by the addition of Lewis base ligands. The stabilizing ability of different ligand groups were assessed, including CO and NHC, which were chosen based on their strong electron donating capabilities. The ligand group that ultimately experimentally produced an actual neutral diborene was :C{N(2,6-PRⁱ₂C₆H₃)CH}₂. Previous work from using this ligand group for stabilizing carbenes suggested that this would be a potential stabilizing ligand for a diborene.

This compound, R(H)B=B(H)R, where R is the aforementioned ligand group, was synthesized beginning with RBBr₃ and KC₈ in diethyl ether. Two products were isolated from this reaction, including the desired diborene R(H)B=B(H)R. It was shown that a ratio of 1:5.4 of RBBr₃ to KC₈ yielded the highest percentage of R(H)B=B(H)R (12%). Any excess amount of RBBr₃ over this ratio resulted in a decrease of R(H)B=B(H)R and thus in increase of the other product, R(H)₂B-B(H)₂R.

A few methods were utilized in order to determine the chemical makeup of these products. NMR resonances of RBH₃, R(H)₂B-B(H)₂R, and R(H)B=B(H)R were respectively reported to be -35.38, -31.62, and +25.30 ppm. The ¹¹B signal of R(H)B=B(H)R produced a quartet, while the other two compounds elicited singlets. This alone could suggest double bond character between borons.

X-ray structural analysis shows a bond distance of 1.828 Å for R(H)₂B-B(H)₂R. This number seems to be on point with calculated B-B bond lengths for similar structures such as the CO-ligated analogue (1.819 Å) and an activated m-terphenyl based diborate (1.83 Å). Crystallization of R(H)B=B(H)R reveals B-C bond distances of 1.547 Å, which is marginally shorter than that of the other molecules. In addition to this, it is calculated that the angles between the C₃N₂ carbene rings and the core are strikingly different than that of the other compounds used and produced. Finally, the B=B bond distance in R(H)B=B(H)R was measured to be much shorter than the B-B distance reported in R(H)₂B-B(H)₂R, again implying a double bond.

DFT computations were also used to support the nature of R(H)B=B(H)R. The analysis was performed on the simplified model, where R=:C(NHCH)_2­. The experimental bond lengths for the non-simplified model seem to be in concordance with the computed B-B and B-C bond lengths, and well as the B-B-C bond angle calculated from the simplified model analyzed using DFT. The bond character of these bonds was also delved into via HOMO representations of the compounds among other computational techniques.

In conclusion, the authors of the paper were able to successfully prove that they had synthesized and characterized the first neutral diborene compound. They also ventured into the nature of the elusive boron-boron double bond. Though it was not necessarily expected that this phenomenon could feasibly be synthesized due to the expected reactivity of the boron-boron double bond, these chemists found a way to isolate the compound. In context to the larger field of chemistry, I suppose that the authors could determine other possible ligand groups that would produce a stable neutral diborene. They could also venture into increasing the percent yield, as 12% is on the low side. Finally, they could explore other group 13 elements, such as Al and Ga to see if they can replicate similar double bond properties.