Synthesis and Determination of Polypyrazolylborates:
K[HB(3,5-C5H7N2)3] and HB(3,5-C5H7N2)3Cu(CO)
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.
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.
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 1H 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).
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.
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.
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.
(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.