Aerobically stable and substitutionally labile α-diimine rhenium dicarbonyl complexes.

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes are described. The molecules are prepared in high yield from the cis - cis - trans -[Re(CO) 2 ( t Bu 2 bpy)Br 2 ] - anion (2, where t Bu 2 bpy 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 σ-donor monodentate ligands (Ls) yields stable neutral 18-electron cis - cis - trans -[Re(CO) 2 ( t Bu 2 bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO) 2 ( t Bu 2 bpy)Br(L)] species can be further reacted with Ls to prepare stable cis - cis - trans -[Re(CO) 2 ( t Bu 2 bpy)(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 ) isomers are also formed. In particular, cis - cis - trans -[Re(CO) 2 ( t Bu 2 bpy)(L) 2 ] + complexes establish an equilibrium with all cis isomers 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 α-diimine unit. [Re(CO) 2 ( t Bu 2 bpy)Br 2 ] - and [Re(CO) 2 ( t Bu 2 bpy)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.