Mixed-Valence First-Row Metal Clusters for Catalytic Hydrodesulfurization and Hydrodeoxygenation

Abstract

Salt metathesis between CoCl2 and [KNPEt3]n provides a new route to the halide-substituted cobalt phosphoranimide cluster [CoCl(NPEt3)]4. This cluster is a useful precursor for the preparation of alkyl-capped cobalt phosphoranimide complexes. The methylated heterocubane [CoMe(NPEt3)]4 was synthesized in high yield by transmetallation of [CoCl(NPEt3)]4 with Me2Mg•dioxane in dioxane. The structural, magnetic, and electrochemical properties of the cluster have been rigorously evaluated. The type (III) mixed-valence cluster [CoMe(NPEt3)]4PF6 was prepared from [CoMe(NPEt3)]4 by outer-sphere electron transfer with Cp2FePF6. The cationic cluster has been fully characterized. Both [CoMe(NPEt3)]4 and [CoMe(NPEt3)]4PF6 are active precatalysts for reductive C–S bond cleavage. The catalytic activity of [CoMe(NPEt3)]4PF6 has been investigated in more detail. Under basic conditions, the mixed-valence cluster catalyzes the complete hydrodesulfurization of a number of organosulfur compounds under mild conditions (110–170°C, 1 atm H2): dibenzothiophene, 4,6-diethyldibenzothiophene, benzothiophene, 3-hexylthiophene, diphenyl sulfide, benzyl phenyl sulfide, and phenoxathiin. In all instances no products indicative of aromatic ring hydrogenation were noted. The desulfurization of dibenzothiophene was investigated over a range of temperatures. A preliminary mechanistic ‗road map‘ for the reaction was proposed. The mixed valence cluster [CoMe(NPEt3)]4PF6 also catalyzes C–O bond hydrogenolysis of unstrained ethers. Aromatic, benzyl, and aliphatic C–O bonds are all cleaved under mild conditions. The substrate scope includes diphenyl ether, 4-methoxybiphenyl, 2,3- dihydrobenzofuran, 1,3-bis(4-methoxyphenoxy)benzene, dibenzofuran, and benzofuran. In no case were products indicative of aromatic ring hydrogenation noted. Catalytic hydrogenolysis in the presence of the Lewis acid AlMe3 results in higher turnover frequencies than those not including AlMe3. The presence of the Lewis acid also promoted further C–O bond reductions to give complete hydrodeoxygenation of resistant substrates.