From [RuX6] to [Ru(RCN)6]: synthesis of mixed halide–nitrile complexes of ruthenium, and their spectroelectrochemical characterization in multiple oxidation states

Abstract

A complete series of benzonitrile- and acetonitrile-substituted ruthenium halide complexes [RuX_6-n(RCN)_n]^z(n= 0–6), ranging stepwise from [RuX₆]^2– to [Ru(RCN)₆]²⁺, has been prepared and characterized. Three series were established, having RCN/X = PhCN/Cl, PhCN/Br, and MeCN/Cl. The strategy of reductive substitution has been developed to prepare [RuX₅(RCN)]^2–, trans-[RuX₄(RCN)₂]^–, mer-[RuX₃(RCN)₃] and trans-[RuX₂(RCN)₄] in turn from [RuX₆]^2– by systematic electrosynthetic routes, through detailed management of potential, temperature and RCN concentration. The monosubstituted pentahalogeno complexes are unstable and their preparation is only practicable via electrode-induced (Ru^IV→ Ru^III) halide displacement from [RuX₆]^2– at low temperature. At the divalent level, exhaustive substitution to form [RuX(RCN)₅]⁺ and [Ru(RCN)₆]²⁺ from [RuX₂(RCN)₄] required more forcing chemical conditions (Ag⁺ and CF₃SO₃H respectively). Voltammetric studies between –65 and 20 °C confirm that the family of mixed halide–nitrile monomeric complexes is rich in redox chemistry, spanning oxidation states V to II. Under reversible conditions, the various metal-based electrode potentials for [RuX_6-n(RCN)_n]^z are a linear function of the stoichiometry parameter, n, increasing by 0.45 (Ru^v–Ru^IV) or 0.6 V (Ru^IV–Ru^III and Ru^III–Ru^II) per halide replaced by nitrile. By use of spectroelectrogeneration techniques, the optical charge-transfer spectra for every available member of each family have been recorded in multiple oxidation levels, defining the states Ru^IV for n= 0–2, Ru^III for n= 0–5 and Ru^II for n= 2–6. For the present complexes, there are unmistakable complementary progressions in the halide-to-metal (X→Ru^III) and metal-to-ligand (Ru^II→RCN) bands, in accord with the underlying trend in E_½(Ru^III–Ru^II). These measurements present an unusual opprotunity to document and analyse the characteristic trends in appearance and location of both ligand-to-metal (Ru^III) and metal-to-ligand (Ru^II) charge-transfer manifolds as a function of stoichiometry.