Empowering the Base Metals: Rational Design of Homogeneous Catalysts for Hydrotreatment, and Reduction of Organic Unsaturates

Abstract

Hydrocarbon-soluble, low-coordinate, phosphoranimide-supported complexes of the base metals (iron, cobalt and nickel) were developed and evaluated for a range of stoichiometric and catalytic bond-forming and bond-breaking transformations. An unprecedented family of tetrametallic clusters of Co(I) and Ni(I), with formula [M(NPtBu3)]4, was synthesized in high yields and characterized by single crystal X-ray analysis and by solution-phase magnetic susceptibility measurements. These ancillary-ligand-free clusters exhibit high room-temperature magnetic susceptibility and are structurally unique; each metal center in the coplanar, tetranuclear core is linear, two-coordinate. A rare homoleptic, mononuclear, three-coordinate Co(III) d6 complex [Co(NPtBu3)3] was also synthesized in high yield and characterized using NMR spectroscopy and single crystal X-ray analysis. The ancillary ligand-free “surface-mimetic” clusters constitute a new class of high-activity homogeneous catalysts for room-temperature hydrogenation and hydrosilylation of polar and nonpolar unsaturates. Both clusters hydrogenate unactivated and sterically hindered alkenes and alkynes to the corresponding alkanes (1 atm H2). These clusters also hydrosilylate a range of functionalized aldehydes and ketones, utilizing a diverse scope of organosilyl hydride reagents. Radical clock hydrosilylation studies revealed differences in the mechanistic behavior of the isostructural Co(I) and Ni(I) clusters: the cobalt catalyst functions through a non-radical mechanism while nickel catalyst shows contributions from radical-type pathway(s). More importantly, both Co(I) and Ni(I) clusters are high-activity catalysts for the hydrogenolysis of carbon-heteroatom bonds under remarkably mild conditions (90-150 °C, 1 atm H2). Both clusters constitute the very first set of homogeneous, first-row transition metal catalysts for the mild-condition hydrodesulfurization of dibenzothiophene derivatives. The tetranuclear clusters also activate C-O bonds in symmetrical and unsymmetrical aryl ethers and dibenzofuran-type substrates, producing aromatic hydrocarbons and phenols. In some preliminary experiments, the Ni(I) cluster and the mononuclear Co(III) complex were shown active for the hydrogenolysis of C-N bond in neutral and basic aromatic nitrogenous substrates. Results of hydrogenolysis studies show catalyst selectivity for the direct hydrogenolysis of Caryl–X bonds (X = S, O, N) with complete suppression of aromatic ring hydrogenation pathways. Lastly, the both Co(I) and Ni(I) clusters effect the deoxygenation of a range of functionalized carbonyl substrates, producing unique mixtures of hydrocarbons under mild hydrosilylation conditions.