Highly Reactive Ru(II) Complexes Stabilized by Agostic Interaction/N2 Binding : small molecule binding and utility in hydrogenation catalysis

Includes references.

PhD;2024;Inorganic and Physical Chemistry.

Binding and activation of chemically inert and strong sigma H−X bonds (X = H, B, Si, C) in small molecules such as H2, H3B.NR3, HSiR3, and CH4 have been the subject of intense research interest. The binding of a small molecule to the metal center involves donation of the bonding electron pair of the H−X bond to the vacant d-orbital of the metal, which is reinforced via backdonation of electron density from filled d-orbital of the metal to the σ* orbital of the H−X bond. This intermediate state of is a non-classical 3-center, 2-electron interaction between the sigma bond of a small molecule and the metal center, known as a sigma complex, along the reaction coordinate for the homolysis or heterolysis of the H–X bond. With an objective to achieve the binding and activation of small molecules, PNP pincer ligated, coordinatively unsaturated, ruthenium complexes, [RuH(CO)(PPhNiPrPPh)][BArF4] and [RuH(N2/sol)(CO)(PPhNNpPPh)][BArF4] have been synthesized and characterized. These reactive species were stabilized by agostic interaction or nitrogen(N2) or solvent molecule at the sixth coordination site. Reactivity studies of these species with small molecules such as H2, H3B.NMe3, and HSiRR’2 resulted in the formation of the corresponding sigma complexes. The σ-borane complex was isolable and found to be stable under inert atmosphere at 298 K, whereas the sigma-H2 and sigma-silane complexes were observed at low temperature (<243 K). Further, the catalytic activity of these weakly stabilized ruthenium species was explored for the hydrogenation of olefinic substrates under mild conditions. In addition, PNP pincer-bridged binuclear ruthenium complex, [{RuCl2(3-PPhNNpPPh)}2(μ-2-PPhNNpPPh)] has been synthesized and structurally characterized. Highly reactive electrophilic binuclear [{RuCl(3-PPhNNpPPh)}2(μ-2-PPhNNpPPh)][BArF4]2 and [{Ru(3-PPhNNpPPh)}2(μ-2-PPhNNpPPh)][BArF4]4 complexes were generated via abstraction of the chloride from the [{RuCl2(3-PPhNNpPPh)}2(μ-2-PPhNNpPPh)][BArF4]2 complex. Reactivity study of these complexes towards H2(g) revealed the heterolytic cleavage of the H−H bond at 298 K. The heterolytic cleavage of the H–H bond was established using variable temperature NMR spectroscopy.