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

The reaction of [Ir(COD)Cl](2) with 2-(arylazo)pyridine (L) in dichlorometh ane solution has afforded the nonelectrolytic pentacoordinated species of t ype Ir(L)(COD)(CI) from which the corresponding bromides and iodides have b een synthesised by metathesis (COD = 1,5-cyclooctadiene). L ligands used ar e: 2-(phenylazo)pyridine (L-1); 2-(m-tolylazo)pyridine (L-2) and 2-(p-chlor ophenylazo)pyridine (L-3). The X-ray structures of Ir(L-2)(COD)(Cl).0.5 CH2 Cl2 and Ir(L-3)(COD)(I) have been determined revealing square-pyramidal geo metry. The relatively short Ir-N(azo) lengths (similar to2.00 Angstrom) and the relatively long N-N bond distance (similar to1.30 Angstrom) are consis tent with significant d pi-pi* (azo) back-bonding. The HOMO (50% Ir and 15% azo character) and LUMO (50% azo and 30% Ir character) are primarily local ised in the Ir-L fragment and the absorption bands near 600 nm is assigned tentatively to the HOMO --> LUMO transition. The stability of the pentacoor dinated structures and the inertness to oxidative addition of the present c omplexes in contrast to the behaviour of corresponding 2,2'-bipyridine spec ies (tetracoordinated, reactive) is rationalised in terms of pi -acidity or der L >> bpy. (C) 2001 Elsevier Science Ltd. All rights reserved.