Non-innocent behaviour of ancillary and bridging ligands in homovalent and mixed-valent ruthenium complexes [A2Ru(µ-L)RuA2]n, A = 2,4-pentanedionato or 2-phenylazopyridine, L2− = 2,5-bis(2-oxidophenyl)pyrazine

Abstract

Structurally characterised 2,5-bis(2-hydroxyphenyl)pyrazine (H₂L) can be partially or fully deprotonated to form the complexes [(acac)₂Ru(µ-L)Ru(acac)₂], [1], acac⁻ = acetylacetonato = 2,4-pentanedionato, [(pap)₂Ru(µ-L)Ru(pap)₂](ClO₄)₂, [2](ClO₄)₂, pap = 2-phenylazopyridine, or [(pap)₂Ru(HL)](ClO₄), [3](ClO₄). Several reversible oxidation and reduction processes were observed in each case and were analysed with respect to oxidation state alternatives through EPR and UV-VIS-NIR spectroelectrochemistry. In relation to previously reported compounds with 2,2′-bipyridine as ancillary ligands the complex redox system [1]ⁿ is distinguished by a preference for metal-based electron transfer whereas the systems [2]ⁿ and [3]ⁿ favour an invariant Ru^II state. Accordingly, the paramagnetic forms of [1]ⁿ, n = −, 0, +, exhibit metal-centred spin whereas the odd-electron intermediates [2]⁺, [2]³⁺ and [3] show radical-type EPR spectra. A comparison with analogous complexes involving the 3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine reveals the diminished π acceptor capability of the pyrazine-containing bridge.