Polyelectrochromism and electronic coupling in vinylruthenium-modified carbazoles

Abstract

Six carbazole-derived mono-, di- and trinuclear ruthenium complexes featuring one to three square pyramidal, five-coordinated {Ru(CO)Cl(PiPr3)2(CH[dbnd]CH-)} entities were prepared by the regio- and stereospecific insertion of the hydride precursor HRu(CO)Cl(PiPr3)2 into the C[tbnd]C bond of the corresponding ethynylated N-substituted carbazoles. They were characterized by IR, UV/Vis, and NMR spectroscopy, electrochemistry, and, in up to four different oxidation states, by UV/Vis/NIR and IR spectroelectrochemistry. Their radical cations were additionally investigated by EPR spectroscopy. Experimental studies are complemented by (TD-)DFT calculations. Electrochemical studies on these complexes showed two to four reversible, consecutive one-electron oxidation processes. Half-wave potential separations ΔE½ between individual redox-waves and the derived comproportionation constants Kc indicate that intermediate oxidation states are sufficiently stable with respect to disproportionation to be generated and investigated in solution. Spectroscopic data indicate strong carbazole ligand contributions to every oxidation and notable involvement of the vinylruthenium moieties to the lower oxidation steps. UV/Vis/NIR spectroelectrochemistry reveals a polyelectrochomic behavior of all complexes with, however, less pronounced absorptivities when compared to the simple triarylamine counterparts. The pattern of two separate, moderately spaced Ru(CO) bands for the MV radical cations IIa+, IIb+ and III+ indicates that the unipositive charge is unevenly distributed over the two chemically equivalent vinylruthenium subunits.