The effect of an ancillary ligand proton on the photophysical properties of some RuIIN6 cores: a proton valve

Reaction of benzil mono-(2-pyridyl)hydrazone (LH; H is the dissociable proton) with cis-[Ru(N–N)2Cl2] 2H2O (N–N 2,20-bipyridine and 1,10-phenanthroline) in methanol yields complexes of the type [Ru(N–N)2(LH)](ClO4)2 (1a and 2a). Their pKa values in CH3CN are determined from solution conductivity as 4.35 and 3.83. Deprotonation of the two complexes by the addition of triethylamine in methanol gives complexes of the type [Ru(N–N)2L]ClO4 (1b and 2b). The X-ray crystal structures of 1a and 2b have been determined to confirm the presence/absence of the LH proton. In the electronic spectra of the deprotonated complexes 1b and 2b in CH3CN, a prominent metal centered transition is observed at 340 nm, which is generally not seen clearly in complexes containing the RuIIN6 core. A fast electron transfer process Ru(III) + e = Ru(II) is observed in cyclic voltammetry with an E1/2 of 0.8 V vs. NHE in 1b and 2b with a heterogeneous rate constant (ks,h) of 1.39 102 and 1.29 102 cm s1, respectively. Weak emissions at 615 nm with a quantum yield of 7 104–1 104 are observed in the fluorescence spectra of 1a and 2a when excited at 430 nm in deaerated CH3CN.