Mononuclear and dinuclear ruthenium(II)/(III) salicylates incorporating azoimine functionalities as ancillary ligands. Synthesis, spectroscopic and electron-transfer properties†

Abstract

Ruthenium–salicylate complexes incorporating azoimine based azopyridine ligands, L, of the type [RuII(L)2(salicylate)] 1–5 [L = NC5H4NNC6H4(R), R = H, m-Me/Cl or p-Me/Cl] have been synthesized and their spectroelectrochemical aspects investigated. The complexes systematically exhibit two 1e− and two 2e− oxidation processes and four successive one-electron reductions. The stepwise electrochemical oxidations were followed by electronic and EPR spectral studies on each oxidation step which indicate that the initial one-electron oxidation process corresponds to stereoretentive oxidation of the ruthenium(II) centre to ruthenium(III), [RuIII(L)2(salicylate)]+1++–5++. The second oxidation step corresponds to oxidation of the coordinated salicylate moiety in 1++–5++ to the ruthenium(III)–salicylate semiquinone cationic radical, [RuIII(L)2(salicylate)]2+E. The electrogenerated ruthenium(III) congeners (1++–5++) exhibit rhombic EPR spectra corresponding to distorted octahedral complexes. The electrogenerated semiquinone salicylate radical [RuIII(L)2(salicylate)]2+E undergoes a radical recombination process which leads to formation of antiferromagnetically coupled dimeric species, [(L)2RuIII(X)RuIII(L)2]4+F [(X = −O2C(O)C6H4C6H4(O)CO2−]. The next two 2e− oxidation processes are associated with oxidation of the bridging moiety of the dimeric species [(L)2RuIII(X′)RuIII(L)2]4+G [(X′ = −O2C(O)C6H3C6H3(O)CO2−] followed by oxidation of the metal centres, [(L)2RuIV(X′)RuIV(L)2]6+H. The chemical oxidation of the complexes 1–5 by HNO3 leads to formation of dimeric complexes, G, straightaway. The complexes display intense charge-transfer bands in the UV-visible region which have been found to be reasonably blue shifted while moving from 1–5 to 1++–5++ to F to G.