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

Abstract

Synthesis and characterisation of the ruthenium complexes, [Ru^II(Q)(tppz)(Cl)]ClO₄ (1) and [{Ru^II(Q)Cl}₂- (µ-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 reported structures of analogous molecules establish that their valence configurations comprise the fully oxidized Q° and tppz^o along with the Ru(II) center as well as non-planarity of the coordinated tppz. The reversible Ru^II/Ru^III oxidation of (1) and two successive Ru^II/Ru^III couples for (2) appear at 0.95 V and 0.96, 1.11 V vs SCE in CH₃CN, respectively. The separation in potential of 0.15 V between the two successive oxidation processes in (2) leads to the comproportionation constant, K_c value of 3.5× 10², which implies a rather weakly coupled (electrochemical) valence localized class II mixed valent Ru^IIRu^III state in (2)⁺. However, the DFT calculated Mulliken spin densities of (2)⁺ (Ru1, Ru2, Q, tppz and Cl of 0.333, 0.412, 0.070, -0.006 and 0.221, respectively) suggest an almost valence averaged situation. The compositions of molecular orbitals of (1) and (2) suggest appreciable (d)Ru^II-> π (tppz)/π (Q) back-bonding. Both the complexes exhibit multiple close-by reductions within the potential range of 0 to -2.0 V vs SCE in CH₃CN which are assigned to be the ligand (Q/tppz) based reductions. The molecular orbital compositions predict Q based first reduction followed by tppz-based successive reductions in (1), whereas in (2) first reduction primarily takes place at the bridging tppz center followed by the reduction of Q. (1) and (2) exhibit multiple metal-to-ligand charge transfer transitions in the visible region due to the presence of two and three acceptor ligands, respectively. The key transitions in the visible region are assigned based on the TD-DFT calculations on optimized structures of (1) and (2).