Bis(N,N) chelated rhenium(III): the [ReL2Cl2]Cl family [L = 2-(arylazo)pyridine]

Abstract

The reaction of mer-[Re(MeCN)(PPh₃)₂Cl₃] with 2-(arylazo)pyridines (L) affords dark green, diamagnetic, 1 : 1 electrolytes [ReL₂Cl₂]Cl (aryl of L = Ph, m-MeC₆H₄ or p-ClC₆H₄). The ReCl₂ moiety is cis(two Re–Cl stretches) and the cation has an overall two-fold symmetry axis (¹H NMR spectra). The co-ordinated pyridine-nitrogen atoms and azo-nitrogen atoms of each ligand are assigned relative trans and cis geometries respectively. The NN stretching frequencies are low (1280–1290 cm^–1) implying significant Re(t₂)–azo(π*) back bonding. In MeCN solution three nearly reversible redox couples occur with E_½ values of ≈ 0.9 V [rhenium(IV)–rhenium(III)] and ≈–0.4, ≈–0.8 V (azo redox)vs. the saturated calomel electrode (SCE). A metal-to-ligand charge-transfer (m.l.c.t.) absorption is observed near 600 nm. The reduction potentials and m.l.c.t. frequencies are mutually consistent. The remarkable spectral and electrochemical similarities between [Re^IIIL₂Cl₂]Cl and [Os^IIL₂Cl₂] are noted and rationalised.