Theoretical Study of Reductive Functionalization of Methyl Ligands of Group 9 Complexes Supported by Two Bipyridyl Ligands: A Key Step in Catalytic Hydrocarbon Functionalization

Abstract

A density functional theory (DFT) study was performed to understand the factors that control the reactivity of bipyridine (bpy)-ligated Rh(III) methyl complexes toward nucleophiles to produce functionalized methane and Rh(I) complexes. The effect of the structure of the complex, the nucleophile, the identity of the ancillary ligand, the electronic properties of the bipyridine ligand, and the identity of the metal were considered. Many similarities were found between the reaction of Rh(III) methyl complexes supported by bipyridyl ligands and classic organic S N 2 reactions, including a strong dependence of the reaction on the nucleophile identity and modifications to the complex that facilitate rhodium as a leaving group. Using these concepts, a comparison of reductive functionalization of Rh(III) alkyl complexes supported by porphyrin versus two bipyridyl ligands was made, and modifications that could lead to more active complexes were proposed.

DFT study of reductive functionalization in cis and trans cobalt–methyl–bipyridine complexes

“…Additional preliminary calculations were done with the B3LYP functional and the CEP-31G(d) basis set, but little difference in predicted results found. Complexes were optimized in continuum dimethylsulfoxide solvent (DMSO, SMD model [17]), which was deemed to be most experimentally relevant and most appropriate for stabilizing anions and cations [2] in nucleophilic attack reactions. Enthalpies and free energies were calculated at 298.15 K and 1 atm, and were recorded in kcal/mol.…”

“…On the basis of previous research by our group, the impact of the co-ligand X upon the barriers to nucleophilic attack, Scheme 1, by hydroxide on the Co(III)-methyl bond was investigated for a series of co-ligands [2]. To facilitate comparison among the modeled complexes, the ligands were fixed at 2,2 0 -bipyridine, a common experimentally relevant supporting ligand for this chemistry [2].…”

“…The direct selective partial oxidation of an alkane to an alcohol, especially methane to methanol, is a process of interest for alternative energy production [1,2]. An abundant source of energy, worldwide natural gas reserves were estimated in 2013 to be around 185.7 trillion cubic meters [3].…”

“…In addition, in comparison with methanol, methane is unreactive due to the strength of its C-H bond [bond dissociation enthalpy = 105 kcal/mol], low acidity (pK a = 48), high ionization potential (291 kcal/mol), and low proton affinity (130 kcal/mol) [1,6,7]. A cost-effective solution to methane's aforementioned disadvantages could be catalytic methane-to-methanol conversion at relatively low pressures and temperatures (e.g., 6500 psi and 6250°C) [2,8]. As a liquid, methanol would serve as a much more manageable source of fuel, especially as the U.S. already relies on pipelines to transport liquid materials over long distances [3,9].…”

“…In the proposed mechanism, a bipyridyl methyl cobalt complex with supporting ligands undergoes nucleophilic attack by a hydroxide nucleophile, forming methanol and a metal-based leaving group. Because previous organometallic studies provided evidence that gold [13] and rhodium [2,14] could be effective electrophilic catalysts in the methane to methanol process, the possibility of cobalt as a more inexpensive, yet effective, Earth-abundant transition metal catalyst in this reaction was pursued [2].…”