Syntheses and electronic and optical properties of complexes of the bis(2, 2′-bipyrazyl)ruthenium unit. (16th August 2015)

• Record Type: Journal Article
• Authors: Coe, Benjamin J.; Peers, Martyn K.; Scrutton, Nigel S.
• Is Part Of: Polyhedron. Volume 96(2015)
• Page Start: 57
• Page End: 65
• Publication Date: 2015-08-16
• Subjects: Ruthenium complexes -- 2, 2′-Bipyrazyl -- UV–Vis spectroscopy -- Electrochemistry -- Density functional theory; Chemistry, Inorganic -- Periodicals; Chimie inorganique -- Périodiques; Organometaalverbindingen; Anorganische chemie; 546.05
• DOI: 10.1016/j.poly.2015.04.028
• Languages: English
• ISSNs: 0277-5387

Abstract

Graphical abstract: New complexes cis -[Ru II (bpz)2 (L–L)] n + (bpz = 2, 2′-bipyrazyl, n = 2 or 4, L–L = a 4, 4′-disubstituted 2, 2′-bipyridyl derivative) have been prepared and studied by using techniques including UV–Vis spectroscopy, cyclic voltammetry and TD-DFT calculations. Their photoredox properties are highly tunable, with the bpz ligands acting as stronger electron acceptors than any of the L–L ligands (even the –CF3 derivative). Abstract: Seven new complexes of the form cis -[Ru II (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 1 H 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 Ru III/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 Ru II → bpz MLCT, while further such transitions occur along with MLCT to L–L at higher energies.