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.

Keywords:

• cobalt
• trifluorophosphine
• density functional theory
• metal-metal bonding
• metal clusters

Acknowledgements

We are indebted to the 111 Project (B07012) and the National Natural Science Foundation (20873045) of China as well as the US National Science Foundation (Grants CHE-0749868 and CHE-0716718) for support of this research.