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

Ruthenium-salicylate complexes incorporating azoimine based azopyridine ligands, L, of the type [RuII(L)2-(salicylate)] 1-5 [L = NC5H4N=NC6H4(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)C6H4C6H 4(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)C6H3=C6H 3(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.