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 Os^II centre is essentially identical to that of [Os(bipy)₃]²⁺, but the co-ordination environment about the Ru^II 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 Ru^II and Os^II, 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.