Synthesis, characterization, electrochemical and theoretical study of substituted phenyl-terpyridine and pyridine-quinoline based mixed chelate ruthenium complexes

In the present work, we report two methoxy-substituted phenyl-terpyridine ruthenium complexes with pyridine carboxyquinoline and NCS as ancillary ligands, [Ru(OMePhtpy)(pcqH)(NCS)](PF6) (1) and [Ru(triOMePhtpy)(pcqH)(NCS)](PF6) (2) (where OMePhtpy = (4'-(4-methoxy) phenyl-2,2': 6', 2 ''-terpyridine, triOMePhtpy = (4'-(3,4,5-trimethoxy) phenyl- 2,2': 6', 2 ''-terpyridine and pcqH = pyridine-carboxyquinoline). Both complexes have been characterized by spectroscopic techniques e.g., mass, H-1-NMR and FTIR. UV-vis spectrophotometric and electrochemical studies for both complexes have been performed. The substitution pattern of the-OMe groups have been successfully utilized to tune the redox potential of the metal complexes. On the anodic side of cyclic voltammogram, 1 and 2 show an irreversible wave corresponding to Ru-II/III couple at 0.95 and 0.85 V, respectively. The lower Ru-II/III oxidation potential for 2 may be attributed to increased electron density on ruthenium due to three (+R) methoxy-groups appended to the phenyl moiety of triOMePhtpy. DFT optimization of structure and energy calculation reveals that in both complexes, HOMO is metal-and thiocyanate-based, whereas the LUMO is based on pcqH. Correlation of TDDFT results with experimental electronic spectrum indicates that bands at 502 nm (1) and 528 nm (2) are of MLLCT character from ruthenium-thiocyanate to pcqH.