First-Row Metal Catalysts for Hydrogenolysis of CarbonâSulfur and CarbonâOxygen Bonds.

Abstract

A series of phosphoranimide-supported first-row transition metal clusters were prepared, structurally characterized, and investigated for catalytic hydrogenolysis of carbon-heteroatom bonds. Homoleptic heterocubane clusters, as well as two-dimensional bi- and tetranuclear compounds of manganese, nickel and copper were isolated and characterized. Noteworthy among these is the unique low-valent cluster [Cu4(NPtBu3)4Li2], which revealed the ion-specific cation-induced redox modulation of tetranuclear [M(NPtBu3)]4 clusters (M = Ni, Cu). Binuclear and tetranuclear nickel clusters, [Ni(NPtBu3)2]2 and [Ni(NPEt3)2]4, are effective precatalysts for the hydrodesulfurization of dibenzothiophene, mediated by stoichiometric potassium tert-butoxide, under relatively mild conditions (150-200 °C and 1-34 atm hydrogen). Under the reaction conditions, the nickel clusters are transformed to nanoparticles, which function as active catalysts for hydrodesulfurization. Supported heterogeneous catalysts were rationally prepared by protolytic grafting of nickel phosphoran¬imide clusters onto alumina and silica. These alumina- and silica-supported -catalysts display improved reactivity and selectivity for hydrodesulfurization, compared to the unsupported analogues. Simple Ni(II) alkoxides, derived in situ, and soluble Ni(0) complexes are also effective precatalysts for hydrodesulfurization under these conditions, establishing that the phosphoranimide ligand serves only to control the grafting and initiate hydrogenolysis. For all desulfurization reactions, the promotional effect of potassium ions was determined, attributable to the formation of cation-Ï interactions with the aromatic rings of dibenzothiophene and potassium-promoted hydrogenolysis of nickel sulfide. Finally, a simple copper/molecular sieves system for catalytic hydrodeoxygenation of aryl ketones under very mild conditions (120 °C, 1 atm H2) is reported. The copper catalysts are conveniently generated in situ from the decomposition of CuOtBu under hydrogen. While a homogeneous catalytic specie is sufficient to hydrogenate the ketone to the corresponding alcohol, full deoxygenation requires the formation of a heterogeneous copper catalyst, in the presence of molecular sieves.