The Diversity of Difluoroacetylene Coordination Modes Obtained by Coupling Fluorocarbyne Ligands on Binuclear Manganese Carbonyl Sites

Abstract

Recent work has shown that the fluorocarbyne ligand CF, isoelectronic with the NO ligand, can be generated by the defluorination of CF(3) metal complexes, as illustrated by the 2006 synthesis by Hughes et al. of [C(5)H(5)Mo(CF)(CO)(2)] in good yield by the defluorination of [C(5)H(5)Mo(CF(3))(CO)(3)]. The fluorocarbyne ligand has now been investigated as a ligand in the manganese carbonyl complexes [Mn(CF)(CO)(n)] (n = 3, 4) and [Mn(2)(CF)(2)(CO)(n)] (n = 4-7) by using density functional theory. In mononuclear complexes, such as [Mn(CF)(CO)(4)], the CF ligand behaves very much like the NO ligand in terms of pi-acceptor strength. However, in the binuclear complexes the two CF ligands couple in many of the low-energy structures to form a bridging C(2)F(2) ligand derived, at least formally, from difluoroacetylene, FC[triple bond]CF. The geometries of such [Mn(2)(C(2)F(2))(CO)(n)] complexes suggest several different bonding modes of the bridging C(2)F(2) unit. These include bonding through the orthogonal pi bonds of FC[triple bond]CF, similar to the well-known [R(2)C(2)Co(2)(CO)(6)] complexes, or bonding of the C(2)F(2) unit as a symmetrical or unsymmetrical biscarbene. This research suggests that fluorocarbyne metal chemistry can serve as a means for obtaining a variety of difluoroacetylene metal complexes, thereby avoiding the need for synthesizing and handling the very unstable difluoroacetylene.