Ligand substitution processes in tetranuclear carbonyl clusters. 9. Reactions of Co4(CO)9(tripod), tripod = HC(PPh2)3, and its derivatives

Article abstract of DOI:10.1021/om00120a016

Reactions of Co₄(CO)₉(tripod) with phosphorus donor ligands has resulted in a facile production of Co₄(CO)₈(L)(tripod) derivatives, where L = P(OMe)₃, PMe₃, and P(n-Bu)₃. The reaction proceeds too rapidly to occur via a CO dissociative (D) mechanism and hence is ascribed to a ligand-dependent pathway (I_a) for CO substitution. Phosphorus ligand substitution occurs stereoselectively at the apical cobalt sites. Further substitution by nucleophilic PMe₃ ligands leads first to additional apical incorporation of PMe₃, followed by a subsequent slower process that results in ligand rearrangement to relieve steric crowding where both apical and radial sites are occupied by PMe₃. A similar bis(phosphine) species is formed upon reaction of bis(diphenylphosphino)methane with Co₄(CO)₉(tripod). This derivative undergoes stereoselective carbonyl ligand exchange with ¹³CO at a rate which is not greatly different from the analogous process involving the parent compound Co₄(CO)₉(tripod). For example, the rate of dissociative CO loss in Co₄(CO)_7- (dppm) (tripod) is 5 times slower than the corresponding process in Co₄(CO)₉(tripod), indicative of only a slight perturbation in the reactivity of carbon monoxide at a cobalt center upon phosphine ligand substitution. A discussion of ¹³C and ³¹P NMR spectra of all the derivatives reported upon herein is included.