Notable differences between oxidized diruthenium complexes bridged by four isomeric diethynyl benzodithiophene ligands

Abstract

Four new diruthenium complexes [{(η⁵-C₅Me₅)Ru(dppe)}₂(μ-CC–L–CC)] featuring different bridging isomeric diethynyl benzodithiophenes viz. L = benzo[1,2-b;4,5-b′]dithiophene (complex 1), benzo[2,1-b;4,5-b′]dithiophene (complex 2), benzo[1,2-b;3,4-b′]dithiophene (complex 3) and benzo[1,2-b;4,3-b′]dithiophene (complex 4), were synthesized and characterized by molecular spectroscopic and crystallographic methods. The subtle changes in the molecular structure introduced by the diethynyl benzodithiophene isomers have a notable impact on the stability of the oxidized complexes and their absorption characteristics in the visible-NIR and IR spectral domains. Electronic properties of stable oxidized complexes [1]ⁿ⁺ and [4]ⁿ⁺ (n = 1, 2) were investigated by cyclic voltammetry, UV-vis-NIR and IR spectroelectrochemistry as well as DFT and TDDFT calculations. The results document the largely bridge-localized character of the oxidation of parents 1 and 4. Cations [2]⁺ and [3]⁺ are too unstable at ambient temperature to afford their unambiguous characterization. UV-vis-NIR absorption spectral data combined with TDDFT calculations (BLYP35) reveal that the broad electronic absorption of [1]⁺ and [4]⁺ in the NIR region has a mixed intraligand π–π* and MLCT character, with similar contribution from their spin-delocalized trans and cis conformers. A spin-localized (mixed-valence) rotamer was only observed for [1]⁺ at ambient temperature as a minor component on the time scale of IR spectroscopy.