Ligand substitution and electronic structure studies of bis(Phosphine)cobalt cyclooctadiene precatalysts for alkene hydrogenation

Diene self-exchange reactions of the 17-electron, formally cobalt(0) cyclooctadienyl precatalyst, (R,R)-(^iPrDuPhos)Co(COD) (P₂CoCOD, (R,R)-^iPrDuPhos = 1,2-bis((2R,5R)-2,5-diisopropylphospholano)benzene, COD = 1,5-cyclooctadiene) were studied using natural abundance and deuterated 1,5-cyclooctadiene. Exchange of free and coordinated diene was observed at ambient temperature in benzene-d₆ solution and kinetic studies support a dissociative process. Both neutral P₂CoCOD and the 16-electron, cationic cobalt(I) complex, [(R,R)-(^iPrDuPhos)Co(COD)][BAr^F4] (BAr^F4 = B[(3,5-(CF₃)₂)C₆H₃]₄) underwent instantaneous displacement of the 1,5-cyclooctadiene ligand by carbon monoxide and generated the corresponding carbonyl derivatives. The solid-state parameters, DFT-computed Mulliken spin density and analysis of molecular orbitals suggest an alternative description of P₂ CoCOD as low-spin cobalt(II) with the 1,5-cyclooctadiene acting as a LX₂-type ligand. This view of the electronic structure provides insight into the nature of the ligand substitution process and the remarkable stability of the neutral cobalt complexes toward protic solvents observed during catalytic alkene hydrogenation.