Aerobically Stable and Substitutionally Labile α-Diimine Rhenium Dicarbonyl Complexes

New synthetic routes to aerobically stable and substitutionally labile a-diimine rhenium(I) dicarbonyl complexes are described. The molecules are prepared in high yield from the cis-cis-trans-[Re(CO)₂(^tBu₂bpy)Br₂]^- anion (2, where ^tBu₂bpy is 4,4'-di-tert-butyl-2,2'-bipyridine), which can be isolated from the one electron reduction of the corresponding 17-electron complex (1). Compound 2 is stable in the solid state, but in solution it is oxidized by molecular oxygen back to 1. Replacement of a single bromide of 2 by s-donor monodentate ligands (Ls) yields stable neutral 18-electron cis-cis-trans-[Re(CO)₂(^tBu₂bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO)₂(^tBu₂bpy)Br(L)] species can be further reacted with Ls to prepare stable cis-cis-trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes in good yield. Ligand substitution of Re(I) complexes proceeds via pentacoordinate intermediates capable of Berry pseudorotation. In addition to the cis-cis-trans-complexes, cis-cis-cis- (all cis) enantiomers are also formed. In particular, cis-cis-trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes establish an equilibrium with all cis enantiomers in solution. The solid state crystal structure of nearly all molecules presented could be elucidated. The molecules adopt a slightly distorted octahedral geometry. In comparison to similar fac-[Re(CO)₃]⁺complexes, Re(I) diacarbonyl species are characterized by a bend (ca. 7°) of the axial ligands towards the a-diimine unit. [Re(CO)₂(^tBu₂bpy)Br₂]^- and [Re(CO)₂(^tBu₂bpy)Br(L)] complexes may be considered as synthons for the preparation of a variety of new stable diamagnetic dicarbonyl rhenium cis-[Re(CO)₂]⁺ complexes, offering a convenient entry in the chemistry of the core.