Simple Route to the Doubly ortho-Palladated Azobenzenes:  Building Blocks for Organometallic Polymers and Metallomesogens

A new class of doubly cyclopalladated complexes, {PdCl(dmf)}₂(μ-azb) (1) and {PdCl(dmf)}₂(μ-aazb) (2), has been prepared in dimethylformamide (dmf) by reaction of azobenzene (azb) and 4-aminoazobenzene (aazb), respectively, with an excess of PdCl₂(CH₃CN)₂ complex. Recrystallization of 1 and 2 in dimethyl sulfoxide (dmso) yields complexes {PdCl(dmso)}₂(μ-azb) (3) and {PdCl(dmso)}₂(μ-aazb) (4), respectively. The crystal structures of 1 and 4 have been determined by X-ray diffraction. All complexes are characterized by ¹H and ¹³C NMR spectra and elemental analysis. In both crystal structures, solvent molecules are bound to palladium through oxygen atoms and oriented trans to carbon. In view of greater preference of palladium to nitrogen and sulfur atoms, the experimental structures were rationalized by quantum-chemical calculations and confirmed as the most stable isomers.