Intramolecular Electron Transfer in Unsymmetric Azohetero-aromatic Radical Bridged Diruthenium(III) Set Up.

Abstract

The article demonstrated the stabilization of MLCT excited state in structurally characterized unsymmetric N,N/N,O- donating azoheteroaromatic radical bridged (acac)2RuIII(μ-L3•-)RuIII(acac)2 (2, S=1/2) [L3= deprotonated 5-(diethylamino)-2-((6-methybenzo[d] thiazol-2-yl)diazenylphenol, acac= electron rich acetylacetonate, N-N(azo) distance: 1.379(4) Å] via intramolecular electron transfer (IET) at the metal-azo interface. On the contrary, similar IET driven stabilization of MLCT excited state was failed to take place in N,N donating HL3 derived mononuclear counterpart (acac)2RuII(HL3) (1, S=0) with unperturbed azo function (N=N distance: 1.3361(18) Å) in spite of the fact of facile RuII/RuIII oxidation (0.28 V versus SCE) and azo reduction (-1.05 V versus SCE), presumably due to the influence of hydrogen bonding interaction at the back face of coordinated HL3. On the other hand, the use of electron poor bpy (2,2'-bipyridine) as a co-ligand facilitated exclusive generation of the mononuclear [(bpy)2RuIIL3]ClO4 ([3]ClO4, S=0) encompassing N,O--donating L3 with neutral azo group (N=N distance: 1.317(6) Å), unlike N,N donating HL3 in 1 due to the impact of σ-donating acac versus π-accepting bpy, which also devoided of IET step. DFT calculations further suggested the origin of multiple redox steps and absorption profiles of 1, 2, 3+.