Syntheses and electronic and optical properties of complexes of the bis(2,2′-bipyrazyl)ruthenium unit

Seven new complexes of the form cis-[RuII(bpz)2(L-L)]n+ (bpz = 2,2′-bipyrazyl: n = 2; L-L = 4,4′-bis(tert-butyl)-2,2′-bipyridyl, 4,4′-diphenyl-2,2′-bipyridyl, 4,4′-dichloro-2,2′-bipyridyl, 4,4′-diamino-2,2′-bipyridyl, 4,4′-bis(trifluoromethyl)-2,2′-bipyridyl, 4,4′-bis(methoxycarbonyl)-2,2′-bipyridyl: n = 4; L-L = N″,N′″-dimethyl-4,4′:2′,2″:4″,4′″-quaterpyridinium) are prepared and isolated as their PF6− and Cl− salts. Improved methods for synthesising bpz and 4,4′-bis(trifluoromethyl)-2,2′-bipyridyl are described also. Characterisation involves various techniques including 1H NMR spectroscopy and mass spectrometry. The new compounds are studied alongside the known species where n = 2 and L-L = 2,2′-bipyridyl, 4,4′-dimethyl-2,2′-bipyridyl or 2,2′-bipyrimidine. Their UV-Vis spectra display intense intraligand π → π∗ absorptions, and also metal-to-ligand charge-transfer (MLCT) bands with two resolved maxima in the visible region. Red-shifts in the MLCT bands occur as the electron-donating strength of L-L increases. Cyclic voltammograms show reversible RuIII/II oxidation waves, and several ligand-based reductions that are also mostly reversible. The variations in the redox potentials correlate with changes in the MLCT energies. Time-dependent density functional theory calculations give relatively good correlations with the experimental UV-Vis spectra for selected complexes when using the M06 functional and basis sets Def2-QZVP (on Ru) and Def2-SVP (on all other atoms) in acetonitrile. The lowest energy visible absorption band is confirmed to be due to RuII → bpz MLCT, while further such transitions occur along with MLCT to L-L at higher energies.