Spectroscopic, luminescence and electrochemical studies on a pair of isomeric complexes [(bipy)2Ru(AB)PtCl2][PF6]2 and [Cl2Pt(AB)Ru(bipy)2][PF6]2, where AB is the bis-bipyridyl bridging ligand 2,2′:3′,2″:6″,2‴-quaterpyridine

Abstract

The positional isomers [{(bipy)₂Ru}AB(PtCl₂)][PF₆]₂ (Ru-AB-Pt) and [(Cl₂Pt)AB{Ru(bipy)₂}][PF₆]₂ (Pt-AB-Ru) (bipy=2,2′-bipyridine) were prepared by a stepwise ‘complexes-as-ligands’ method. Each contains {Ru(bipy)₂}²⁺ and {PtCl₂} fragments coordinated to the bridging ligand 2,2′:3′,2″:6″,2″-quaterpyridine (AB), which contains two inequivalent bipyridyl chelating sites. Electrochemical studies show a Ru^II/Ru^III couple and four ligand-centred reductions in each case, the first two reductions being centred on the two bipyridyl fragments (A and B) of the bridging ligand, and the second two being centred on the terminal bipy ligands. The absorption and luminescence properties are consistent with the inequivalent coordination properties of sites A and B of AB. In particular, from 77 K luminescence spectra, it is seen that the Ru-based energy level is 350 cm^-1 higher for Ru-AB-Pt than for Pt-AB-Ru. At room temperature, the Ru-based luminescence is stronger and longer-lived (φ=2.3×10^-2, τ=280 ns) for Ru-AB-Pt than for its isomeric counterpart (φ=7.5×10^-4, τ=13 ns).