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 [Ru^II(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), [Ru^III(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,[Ru^III(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 [Ru^III(L)₂(salicylate)]²⁺E undergoes a radical recombination process which leads to formation of antiferromagnetically coupled dimeric species, [(L)₂Ru^III(X)Ru^III(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)₂Ru^III(X')Ru^III(L) ₂]⁴⁺G [(X'=-O₂C(O)C₆H₃C ₆H₃(O)CO₂^-] followed by oxidation of the metal centres, [(L)₂Ru^IV(X')Ru^IV(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.