Mechanistic DFT Studies on the Protonation of Transition Metal-Nitride Complexes and the Decomposition of Metal-Alkoxides

Abstract

Knowledge of reaction mechanisms is of fundamental interest in chemistry. Understanding mechanisms can have immediate practical implications in the optimization of known reactions and the discovery of new ones. The advents in computer hardware and software technologies makes it possible at present to investigate reaction mechanisms of chemically relevant systems using modern quantum chemical methods such as density functional theory (DFT). In this thesis we report results on two mechanistic DFT investigations. The first investigation pertains to a peculiar observed reaction between four coordinate pincer-ligated metal-nitride complexes and carboxylic acid derivatives. The reaction converts the metal-nitride bond into a metal-ammine bond by an intramolecular metalligand cooperative (MLC) proton coupled electron transfer mode. The resulting ammine complex has an octahedral geometry with the benzoate anion acting as a chelating ligand. We calculate a series of elementary steps leading to the given transformation and we study how the activation and reaction energies change as a function of the carboxylic acid derivative and the metal center. Surprisingly, we find nearly no effects of the acid on the rates of the first step of metal-nitride protonation. Analysis of the results using thermodynamic cycles show this behavior follows because nitride protonation in this system in fact proceeds by a concerted bifunctional addition step in which the proton from the carboxylic acid goes to the nitride while the carbonyl group of the acid simultaneously coordinates to the metal. The stronger acids that favor nitride protonation appear to simply disfavor coordination of the carbonyl to the metal. The two effects thus cancel out. The calculations reveal more pronounced dependence of the reaction rates on the nature of the metal. In all cases, the calculations reveal that the carboxylic acid addition step is not the rate limiting step. In light of this observation we explored the possibility of coupling the carboxylic acid addition step with an external hydrogen atom donor. The calculations predict a mixture of carboxylic acid and TEMPOH is likely to transform the metal-nitride bond to metal-ammine one at rates comparable to the intramolecular ones involving the ligand. Our second investigation pertains to the mechanism and products of decomposition of the octahedral complex trans-(H)(OMe)Ir(Ph)(PMe3)3. In the presence of methanol as a catalyst, this complex is known to decompose into the trans-dihydride complex trans-(H)2Ir(Ph)(PMe3)3 along with organic products that were reported as formaldehyde oligomers. Our detailed calculations on this system reproduce the experimental observation that methanol can catalyze decomposition by an unconventional outer-sphere beta-hydride elimination from the metal-alkoxide to give the observed trans-dihydride and formaldehyde. However, our calculations indicate strongly that this step is thermodynamically uphill. As an alternative to formaldehyde oligomerization, we show a formaldehyde molecule produced from one metal-alkoxide can react with another metal-alkoxide to give another transdihydride complex and methyl formate. The latter reaction is computed to be highly exergonic and can be viewed as an H/OMe metathesis taking place by an outer-sphere mechanism. The calculations also show that a methyl formate product can in turn undergo another low barrier H/OMe metathesis with another metal alkoxide to give dimethyl carbonate.