Arylazo-oximates of ortho-palladated azobenzenes: synthesis, isomerism, and redox behaviour

Abstract

The synthesis and characterisation of new planar mixed species of the type [PdLA](A =ortho-metallated azobenzene or its substituted derivative) is achieved by splitting the halogeno-bridge in [Pd₂Cl₂A₂] by arylazo-oximes (HL). Triphenylphosphine cleaves the Pd-N(azo) bond in the PdL fragment, giving [PdLA(PPh₃)]. Many of the [PdLA] species occur as isomeric mixtures whose compositions have been established using ¹H n.m.r. data. The isomerism which originates from different modes of ortho-metallation of A is carried over to [PdLA] from the precursor complex [Pd₂Cl₂A₂]. A simple and accurate 1H n.m.r. method of isomer analysis in [Pd₂Cl₂A₂] is described. Substituent effects on isomer population are consistent with the electrophilic nature of the palladation reaction but steric effects also play dominant roles. The electrochemistry of [PdLA] has been thoroughly examined using cyclic voltammetry. Three couples are observed near –0.8, –1.2, and –1.5 V vs. standard calomel electrode, in acetonitrile. The first two couples correspond to successive one-electron reductions of the azo-functions of L and A respectively. The third couple is due to two-electron metal reduction.