A new series of Re- and Ru-complexes having a lowest σπ* excited state that varies from reactive to stable and long lived

Abstract

The complexes Re(L)(CO)3(α-diimine) and Ru(E)(E′)(CO)2(α-diimine), in which L and E/E′ represent an alkyl group or metal fragment bound to the metal by a high-lying σ orbital, have a lowest 3σπ* state from which they normally decompose into radicals. In the case of Re(benzyl)(CO)3(iPr-DAB) the rate of the radical formation was found to be strongly dependent on the coordinating ability of the solvent. Decay to the ground state is slow due to a weak distortion of the complex in its 3σπ* state. This weak distortion is evident from the time-resolved IR spectra, which show that the v(CO) vibrations hardly shift upon σ→π* excitation. By the introduction of two σ-bonded groups (E and E′) in Ru(E)(E′)(CO)2(α-diimine) the σ-bonding orbital becomes delocalised over Ru, E, E′ and the α-diimine ligand and distortion in the 3σπ* state is further decreased. As a result, the complex Ru(SnPh3)2(CO)2(iPr-DAB) has, in a glass at 77 K, an emission lifetime of 264 μs, which is very long compared to those of related complexes having a lowest 3MLCT state (e.g. τ= 300 ns for Ru(Cl)(Me)(CO)2(iPr-DAB)). Contrary to these 3MLCT states, the 3σπ* state may thus vary from very labile and short lived to stable and very long lived depending on the strength and delocalisation of the σ bond.