Dinuclear ruthenium(II) and/or osmium(II) complexes of a non-symmetric bis-chelating quaterpyridine ligand. Synthesis, electrochemical behaviour, absorption spectra, luminescence properties and intercomponent energy transfer

Abstract

We have synthesized the hexafluorophosphate salts of the mono- and di-nuclear compounds Ru–LL′, Os–LL′, Ru–LL′–Ru, Os–LL′–Os, Ru–LL′–Os and Os–LL′–Ru, where Ru and Os are {Ru(bipy)2}2+ and {Os(bipy)2}2+ fragments (bipy = 2,2′-bipyridine), and LL′ is the bis-chelating 2,2′ : 3′,2″:6″,2‴- quaterpyridine bridging ligand with inequivalent bipy-type binding sites (L and L′). The compound [(bipy)2Os(LL′)Ru(bipy)2][PF6]4·3MeCN has been crystallographically characterized, the co-ordination environment about the OsII centre is essentially identical to that of [Os(bipy)3]2+, but the co-ordination environment about the RuII centre is somewhat distorted with one particularly long Ru–N bond, due to the inherent sterically hindered nature of the L′ site of the bridging ligand. Electrochemical studies show that a given metal ion is slightly easier to oxidize when in the L co-ordination site, but the difference in the properties of the two moieties of the LL′ bridging ligand is much smaller than difference in the properties of RuII and OsII, so that in the mixed-metal complexes Ru–LL′–Os and the metal easier to oxidize is always Os and the luminescent moiety is always the Os-based one. The lowest energy (luminescent) level in the homodinuclear compounds is located on the L co-ordination site. In Ru–LL′–Os and Os–LL′–Ru electronic energy transfer from the higher energy (Ru-based) to the lower energy (Os-based) moiety is very fast and 100% efficient.