Pentacoordinated iridium(I) complexes incorporating azopyridine chelation. Synthesis, structure and bonding

Abstract

The reaction of [Ir(COD)Cl]₂ with 2-(arylazo)pyridine (L) in dichloromethane solution has afforded the nonelectrolytic pentacoordinated species of type Ir(L)(COD)(Cl) from which the corresponding bromides and iodides have been synthesised by metathesis (COD=1,5-cyclooctadiene). L ligands used are: 2-(phenylazo)pyridine (L¹); 2-(m-tolylazo)pyridine (L²) and 2-(p-chlorophenylazo)pyridine (L³). The X-ray structures of Ir(L²)(COD)(Cl)·0.5 CH₂Cl₂ and Ir(L³)(COD)(I) have been determined revealing square-pyramidal geometry. The relatively short Ir---N(azo) lengths (~2.00 Å) and the relatively long N---N bond distance (~1.30 Å) are consistent with significant dπ---π (azo) back-bonding. The HOMO (50% Ir and 15% azo character) and LUMO (50% azo and 30% Ir character) are primarily localised in the Ir---L fragment and the absorption bands near 600 nm is assigned tentatively to the HOMO→LUMO transition. The stability of the pentacoordinated structures and the inertness to oxidative addition of the present complexes in contrast to the behaviour of corresponding 2,2'-bipyridine species (tetracoordinated, reactive) is rationalised in terms of p-acidity order L»bpy.