CHM 2511 (Inorganic Chemistry)

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.