Ruthenium and rhenium complexes with silyl-substituted bipyridyl ligands

The preparation of 5,5′-bis(trimethylsilyl)- (1a) and 5,5′-bis(pentamethyldisilanyl)-2,2′-bipyridines (1b) by dehalogenative coupling of the corresponding 2-bromo-5-silylpyridines is described. Silyl substitution causes broad and red shifted π→π* and σ→π* UV–vis absorption bands; electrochemical reduction is facilitated. With these ligands, a series of ruthenium complexes [Ru(bpy)2(L)](PF6)2 (3a, L=1a; 3b, L=1b) and [RuL3](PF6)2 (4a, L=1a; 4b, L=1b), as well as rhenium compounds Re (L)(CO)3Cl (5a, L=1a; 5b, L=1b) (bpy=2,2′-bipyridine) were synthesized. These complexes give rise to red-shifted metal-to-ligand charge-transfer absorptions in the region of 460–480 nm for the ruthenium complexes and around 400 nm for the rhenium complexes. While the oxidation potentials of ruthenium complexes 3a, 3b, 4a, and 4b are almost the same as that of [Ru(bpy)3](PF6)2, reduction of the ruthenium and rhenium complexes occurs at more positive potentials than that of [Ru(bpy)3](PF6)2 and Re(bpy)(CO)3Cl. Band maxima of the metal-to-ligand charge-transfer emission of the ruthenium and the rhenium complexes were observed at 620 and 610 nm, respectively. The results indicate that the LUMO levels of 2,2′-bipyridine and its metal complexes are lowered by electron-accepting effects of trimethylsilyl and pentamethyldisilanyl substituents, while the HOMO level of bpy is elevated by pentamethyldisilanyl substitution due to the effective σ–π conjugation between an SiSi bonding orbital and a bpy π orbital.