The quest for trifluorophosphine as a bridging ligand in homoleptic binuclear and tetranuclear cobalt complexes

Abstract

Transition metal complexes with bridging PF3 groups are unknown analogous to metal carbonyls with bridging carbonyl groups. The possibility of bridging PF3 groups in binuclear Co2(PF3) n complexes (n = 9, 8, 7, 6, 5) and the tetranuclear Co4(PF3)12 has now been investigated by density functional theory (DFT). The lowest energy Co2(PF3)8, Co2(PF3)7, and Co4(PF3)12 structures are predicted to be unbridged structures with cobalt-cobalt bond distances of ∼2.8 Å, ∼2.5 Å, and ∼2.6 Å, respectively. The lowest energy Co2(PF3)6 structure is an unsymmetrical unbridged structure with a Co–Co distance of ∼2.5 Å. For the more highly unsaturated Co2(PF3)5 both triplet and singlet structures are found, consisting of two Co(PF3)2 units linked by two bridges, namely a PF2 group and a F atom. Such Co2(PF3)5 structures have square planar cobalt coordination and a Co…Co distance too long for direct bonding. In addition, a singlet Co2(PF3)5 structure is found, with two semi-bridging PF3 groups and a very short Co ··· Co distance of ∼2.1 Å indicative of the formal quadruple bond required to give both cobalt atoms the favoured 18-electron configuration. Bridging PF3 groups with no cobalt-cobalt bond are found in a high energy structure of Co2(PF3)7 and in a Co2(PF3)9 structure unstable with respect to PF3 loss to give Co2(PF3)8. The dissociation of Co2(PF3)8 into two Co(PF3)4 radicals is predicted to be only slightly endothermic at −14.3 kcal/mol (B3LYP) or 7.1 kcal/mol (BP86). However, in contrast to its carbonyl analogue Co4(CO)12, the dissociation of Co4(PF3)12 into 2 Co2(PF3)6 is highly exothermic. Optimization of the structure of the experimentally known but not structurally characterized bis(difluorophosphido) derivative Co2(PF3)6(µ-PF2)2 predicts the expected structure with a long Co ··· Co distance of ∼3.5 Å indicating the lack of a direct cobalt-cobalt bond. DFT predicts tetrahedral structures for Co(PF3)4 and HCo(PF3)3 as well as the experimentally known trigonal bipyramidal structure for HCo(PF3)4.