Isomeric ruthenium terpyridine complexes [Ru(trpy)(L)Cl]n+ containing the unsymmetrically bidentate acceptor L = 3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine. Synthesis, structures, electrochemistry, spectroscopy and DFT calculations

Abstract

The isomeric title complexes were obtained in almost equimolar ratio from the reaction of Ru(trpy)Cl₃ and L. Crystal structure analyses of the perchlorate hemihydrates, electrochemical and spectroscopic (NMR, UV/VIS, EPR) studies, supported by DFT calculations, reveal distinct differences between the isomeric redox series [1]ⁿ⁺ (tetrazine-N_ttrans to Cl) and [2]ⁿ⁺ (pyrazolyl-N_ptrans to Cl; n = 0, 1, 2). The latter system with the π acceptors trpy and tetrazine in the equatorial plane and the pyrazolyl and chloride donors in the axial positions exhibits facilitated oxidation, lower energy MLCT transitions, more balanced chelate coordination, and a higher g anisotropy in the oxidised (Ru^III) state. According to partially resolved EPR spectra of one-electron reduced neutral compounds 1 and 2 they have the unpaired electron predominantly in the tetrazine ring of L.