Synthesis, spectroscopic and a ZINDO study of cis- and trans-(X2)bis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) complexes (X=Cl−, H2O, NCS−)

Abstract

Ruthenium complexes of the type trans-[Ru(dcbpyH2)2(Cl)2], trans-[Ru(dcbpyH2)2(H2O)2] and trans-[Ru(dcbpyH2)2(NCS)2] (dcbpyH2=4,4′-dicarboxylic acid-2,2′-bipyridine) were synthesized and characterized by NMR, UV–vis absorption and emission spectroscopy. The proton NMR spectra of the trans isomers, in the aromatic region, show only three peaks corresponding to the two dcbpy ligands in which all the pyridine rings are trans to each other and magnetically equivalent. The 13C-NMR spectrum of the trans isomer is characterized by a relatively simple pattern of resonances from the four equivalent pyridine rings. The lowest energy metal-to-ligand charge-transfer transition maximum of the trans-[Ru(dcbpyH2)2(Cl)2] complex is at 14500 cm−1 in DMF solution and shows onset of weak and broad emission signals above 11100 cm−1. This is a significantly larger red shift compared with any of the trans ruthenium(II) polypyridyl complexes reported to date. The absorption and emission maxima of the trans complexes are red shifted compared with the cis analogues, which is due to the CO2H contribution in the trans isomer that stabilizes the LUMO relative to the LUMO of the cis isomer. The enhanced red response of the trans complexes renders them as suitable dyes for dye-sensitized titanium dioxide mesoporous electrodes. The electronic spectra of cis- and trans-[Ru(dcbpyH2)2(Cl)2] complexes were calculated by ZINDO/S and compared with the experimental data.