Synthesis and structure of binuclear thioazobenzene pallada-cycles and their reaction with m-chloroperbenzoic acid: formation and structure of dinuclear azophenolates

Abstract

The cyclopalladation of 1,3-bis(2-thioazobenzene)propane (L¹H₂) and its methyl substituted derivative L²H₂ affords complexes of the type LPd₂Cl² (where L represents L¹ or L² which have been characterised by spectroscopy and X-ray crystallography. The crystal structure of L¹Pd²C² has revealed that each azobenzene fragment along with its thioether sulphur acts in the tridentate (C,N,S) fashion and the fourth coordination position is occupied by a chloride ion. The complexes are thus of type [Pd(C,N,S)Cl]₂, with a Pd Pd contact of 5.420(1) Å. The metallated carbon atom exerts a strong trans influence on the Pd---S bond. The reaction of LPd₂Cl₂ with am-chloroperbenzoic acid leads to smooth oxygen insertion into both the Pd---C bonds to give excellent yields of dinuclear azophenolato complexes o-LPd₂Cl₂ having the coordination sphere [Pd(O,N,S)Cl]₂. The crystal structure of o-L¹Pd₂Cl₂ is similar to that of L¹Pd₂Cl₂ but with a somewhat longer Pd...Pd distance of 5.890(1) Å. The insertion reaction has a negative entropy of activation, in keeping with an associative transition state for the electrophilic incorporation of oxygen. Reduction of the azophenolato complexes with hydrazine hydrate affords the free azophenols (o-LH₂) in high yields. Thus organometallic route to o-LH₂ from LH₂ is provided.