Ruthenium complexes of two different non-innocent ligands. Investigation of electronic structural aspects by experimental and DFT analysis

摘要

Synthesisand characterisation of the ruthenium complexes, [RuII(Q)(tppz)(Cl)]ClO4(1) and [{RuII(Q)Cl}2-(m-tppz)](ClO4)2 (2),incorporating redox noninnocent ligands, (Q = o-benzoquinonediimine,tppz = 2,3,5,6-tetrakis-(2-pyridyl)pyrazine) are reported. The crystal structure of (1) and DFT optimized structure of (2) in comparison with the reportedstructures of analogous molecules establish that their valence configurationscomprise the fully oxidized Qo and tppzo along with theRu(II) center as well as non-planarity of the coordinated tppz. The reversibleRuII/RuIII oxidation of (1) and two successive RuII/RuIII couples for(2) appear at 0.95 V and0.96, 1.11 V vs SCE in CH3CN, respectively. The separation inpotential of 0.15 V between the two successive oxidation processes in (2) leads to the comproportionationconstant, Kc value of 3.5X102, which implies a rather weaklycoupled (electrochemical) valence localized class II mixed valent RuIIRuIIIstate in (2)+.However, the DFT calculated Mulliken spin densities of (2)+ (Ru1, Ru2, Q, tppz andCl of 0.333, 0.412, 0.070, -0.006 and 0.221, respectively) suggest an almostvalence averaged situation. The compositions of molecular orbitals of (1) and (2) suggest appreciable (d)RuII->*(tppz)/*(Q) back-bonding. Both the complexes exhibit multipleclose-by reductions within the potential range of 0 to -2.0V vs SCE in CH3CN which are assigned to be the ligand (Q/tppz) basedreductions. The molecular orbital compositions predict Q based first reductionfollowed by tppz-based successive reductions in (1), whereas in (2) first reduction primarily takesplace at the bridging tppz center followed by the reduction of Q. (1) and (2) exhibit multiple metal-to-ligand charge transfer transitions inthe visible region due to the presence of two and three acceptor ligands,respectively. The key transitions in the visible region are assigned based onthe TD-DFT calculations on optimized structures of (1) and (2).