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

Abstract

Reaction of benzil mono-(2-pyridyl)hydrazone (LH; H is the dissociable proton) with cis-[Ru(N–N)₂Cl₂]·2H₂O (N–N ≡ 2,2′-bipyridine and 1,10-phenanthroline) in methanol yields complexes of the type [Ru(N–N)₂(LH)](ClO₄)₂ (1a and 2a). Their pK_a 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 Ru^IIN₆ core. A fast electron transfer process Ru(III) + e⁻ = Ru(II) is observed in cyclic voltammetry with an E_1/2 of 0.8 V vs. NHE in 1b and 2b with a heterogeneous rate constant (k_s,h) of 1.39 × 10⁻² and 1.29 × 10⁻² cm s⁻¹, respectively. Weak emissions at 615 nm with a quantum yield of 7 × 10⁻⁴–1 × 10⁻⁴ are observed in the fluorescence spectra of 1a and 2a when excited at 430 nm in deaerated CH₃CN. But 1b and 2b do not fluoresce. Such different photophysical behaviour is explained in terms of the fact that, while a ³MLCT is the lowest excited state in 1a and 2a (where the LH proton is present), the reactive ³MC is the lowest excited state in 1b and 2b (where the LH loses the proton). It is pointed out that the proton acts as a valve in the conduit for the radiative pathways. Various experimental results obtained here are supported by detailed density functional theory (DFT) calculations at the BP86/LanL2DZ level.