A facile and preferential synthesis of the complexes, cis-trans-cis-Ru^II[NC₅H₄---N=N---C₆H₄(R)]₂ (R = H, o-Me/Cl, m-Me/Cl, m-Me/Cl) synthesis, spectroscopic characterization and electron-transfer properties

Abstract

A group of seven complexes of the type ctc-Ru^IIL₂Cl₂ (2a-2g) (L = NC₅H₄-N=N-C₆H₅(R), R = H, o-Me/Cl, m-Me/Cl, m-Me/Cl and ctc = cic-traps-cis with respect to chlorides, pyridine and azo nitrogens, respectively) have been synthesized and characterized. The complexes are diamagnetic (Ru^II, t_2g⁶, S = 0) and electrically neutral. The molecular geometry of the complexes (2) in solution has been established by ¹H NMR spectroscopy. They exhibit a strong metal to ligand charge-transfer band in the range 582-603 nm and intra ligand π-π^∗ transition near 320 nm. In acetonitrile solution the complexes display reversible ruthenium (II) ruthenium(III) oxidation couples in the range 1.02 → 1.36 V vs SCE. Two successive quasi-reversible ligand reductions are observed for each complex in the ranges -0.42 →-0.60 V and -0.68 → -0.85 V vs SCE, respectively. The complexes 2a and 2e have been oxidized to the corresponding trivalent species (3a and 3e) by using HNO₃ as oxidizing agent and isolated in solid state as perchlorate salts. Complexes are one-electron paramagnets (Ru^III, t_2g⁵, S = ½ ) and show 1 : 1 conductivity in acetonitrile solution. The presence of perchlorate counter ion in the complexes has been evidenced by the strong infrared bands near 1100 and 600 cm^-1. Complexes exhibit a strong ligand to metal charge-transfer band near 530 nm and intra ligand π-π^∗ transition near 370 nm. In glassy condition (77 K) complexes display rhombic EPR spectra corresponding to the distorted octahedral geometry.