Synthesis and Reactivity of a Tricobalt Carbide Cluster

Abstract

Coordinatively-unsaturated transition metal clusters featuring contiguous open co- ordination sites serve as soluble, molecular models to inform on the complex surface chemistry evolved during heterogeneous catalysis. To this end, our group developed a chelating ligand FtbsLH6 which templates trinuclear clusters. The ligand scaffold enforces close metal-metal contacts, imposes coordinative unsaturation at each metal site, and orients adjacent coordination sites in a parallel arrangement to allow cooper- ative adsorption and activation of substrates. Chapter 1 discusses this cluster-surface analogy with regard to the catalytic hydrogenation of carbon monoxide during the Fischer-Tropsch process and polynuclear transition metal carbide clusters. The tricobalt cluster (FtbsL)Co3 presents a trimetallic face upon which the co- ordination chemistry of small molecules (CO, CNR, RCCR′) was examined. The tricobalt core effects substantial activation of the bound π-acids which enabled their functionalization to µ3-carbyne ligands (Chap. 2). While cleavage of the C≡X (X = O, NR, CCR2) bond could not be realized, the anionic tricobalt carbide clus- ter [(FtbsL)Co3(µ3–C)]1−was accessed through a multi-step synthesis wherein the carbidic ligand formally derives from the methyl substituent of a phosphonium salt (Chap. 3). Unlike previously reported polynuclear carbide clusters which generally stabilized inert or inaccessible carbide ligands, the apical carbide in [(FtbsL)Co3(µ3– C)]1−is exposed and readily performs alkylation chemistry and small molecule cap- ture (CO, CNR, CO2, C2H4) (Chap. 4). Hydrogenation of the tricobalt carbide traverses all four [Co3(µ3–CHx)]1−(x = 0, 1, 2, 3) species and ultimately arrests at the hydride cluster [(FtbsL)Co3(µ3–H)]1−. The former carbide ligand is released as methane, verified by isotope tracer experiments. Each of the [Co3(µ3–CHx)]1− species were independently synthesized and correspond to intermediates invoked in the Fischer-Tropsch process (Chap. 5). The nitride (FtbsL)Co3(µ3–N) and imido [(FtbsL)Co3(µ3–NR)]+ clusters were synthesized, and the isoelectronic series of clus- ters bearing C4−, CH3−, N3−, and NR2−ligands were examined by X-ray emission spectroscopy to establish electronic structures of the Co3 cores (Chap. 6).