Aerobically stable and substitutionally labile α-diimine rhenium dicarbonyl complexes

Abstract

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 thecis-cis-trans-[Re(CO)2(tBu2bpy)Br2]−anion (2, wheretBu2bpy 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). Compound2is stable in the solid state, but in solution it is oxidized by molecular oxygen back to1. Replacement of a single bromide of2by σ-donor monodentate ligands (Ls) yields stable neutral 18-electroncis-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 stablecis-cis-trans-[Re(CO)2(tBu2bpy)(L)2]+complexes in good yield. Ligand substitution of Re(i) complexes proceedsviapentacoordinate intermediates capable of Berry pseudorotation. In addition to thecis-cis-trans-complexes,cis-cis-cis- (allcis) isomers are also formed. In particular,cis-cis-trans-[Re(CO)2(tBu2bpy)(L)2]+complexes establish an equilibrium with allcisisomers 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 similarfac-[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(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 rheniumcis-[Re(CO)2]+complexes, offering a convenient entry in the chemistry of the core.