How to elucidate and control the redox sequence in vinylbenzoate and vinylpyridine bridged diruthenium complexes

Abstract

Vinylbenzoate-bridged diruthenium complexes (RHC[double bond, length as m-dash]CH)(CO)(PiPr3)2Ru(μ-4-OOCC6H4–CH[double bond, length as m-dash]CH)RuCl(CO)(PiPr3)2 (R = Ph, 3a or CF3, 3b) and vinylpyridine-bridged (η6-p-cymene)Cl2Ru(μ-NC5H4-4-CH[double bond, length as m-dash]CH)RuCl(CO)(PiPr3)2 (3c) have been prepared from their monoruthenium precursors and investigated with respect to the sequence of the individual redox steps and electron delocalization in their partially and fully oxidized states. Identification of the primary redox sites rests on the trends in redox potentials and the EPR, IR and Vis/NIR signatures of the oxidized radical cations and is correctly reproduced by quantum chemical investigations. Our results indicate that the trifluoropropenyl complex 3b has an inverse FMO level ordering (Ru1-bridge-Ru2 > terminal vinyl-Ru1 site) when compared to its styryl substituted counterpart 3a such that the primary oxidation site in these systems can be tuned by the choice of the terminal alkenyl ligand. It is further shown that the vinylbenzoate bridge is inferior to the vinylpyridine one with regard to charge and spin delocalization at the radical cation level. According to quantum chemical calculations, the doubly oxidized forms of these complexes have triplet diradical ground states and feature two interconnected oxidized vinyl ruthenium subunits.