Activation of Molecular Hydrogen and Oxygen by PSiP Complexes of Cobalt

Abstract

The syntheses of CoII halide complexes supported by κ3‐(2‐Cy2PC6H4)2SiMe (Cy‐PSiP) ligation are detailed. Reduction of (Cy‐PSiP)Co(PMe3)I could be achieved under mild conditions using magnesium metal to generate the CoI complex (Cy‐PSiP)Co(PMe3)N2 in high yield. When this reaction was carried out under an atmosphere of CO, (Cy‐PSiP)Co(CO)2 was obtained. Unlike the facile reduction of CoII to CoI, attempts to access CoIII species supported by Cy‐PSiP ligation proved challenging. Attempted oxidative addition reactions involving (Cy‐PSiP)Co(PMe3)N2 were generally unsuccessful, with the sole exception of H2, which reacted to afford the dihydride complex (Cy‐PSiP)Co(PMe3)(H)2. The dihydride complex undergoes Co‐H site exchange in solution and readily eliminates H2. The CoI precursor (Cy‐PSiP)Co(PMe3)N2 is a competent precatalyst for the hydrogenation of terminal alkenes. Exposure of (Cy‐PSiP)CoI to O2 gas under anhydrous conditions led to rapid ligand oxidation at Si and P, with no evidence observed at low temperature for a CoIII superoxo or peroxo intermediate. Exclusive oxidation at Si to afford a CoII‐siloxy complex was observed upon treatment of (Cy‐PSiP)CoI with one equiv. Me3NO. While this siloxy complex did not react further with O2, treatment with a second equiv. of Me3NO led to subsequent oxidation involving one phosphino donor. This observation supports the notion that in the ligand oxidation reactivity observed with O2, the O atoms incorporated at both Si and P are likely derived from the same O2 molecule.