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 Bu2bpy)Br2]- anion (2, where t Bu2bpy 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 Bu2bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO)2( t Bu2bpy)Br(L)] species can be further reacted with Ls to prepare stable cis-cis-trans-[Re(CO)2( t Bu2bpy)(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 Bu2bpy)(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 Bu2bpy)Br2]- and [Re(CO)2( t Bu2bpy)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.
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