Cyclometallation and N=N bond cleavage of 2-(arylazo)phenols by osmium. Synthesis, structure and redox properties
Journal
Journal of the Chemical Society, Dalton Transactions
Journal Issue
3
Pages
284-288
Date Issued
2001
Author(s)
Abstract
Reaction of 2-arylazo-4-methylphenol, (H2ap-R, where H2 indicates the two protons (the phenolic proton and one phenyl proton at the ortho position of the arylazo fragment) that undergo dissociation upon complexation) with [Os(PPh3)3Br2] in the presence of a base afford two types of organometallic complexes of osmium(III), viz. red [Os(PPh3)2(ap-R)Br] and brownish-green [Os(PPh3)(ap-R)(N-O)] [N-O is an iminosemiquinonate ligand generated from the 2-(arylazo)phenol via N=N bond cleavage]. The structures of [Os(PPh3)2(ap-Cl)Br] and [Os(PPh3)(ap-Cl)(N-O)] have been determined by X-ray crystallography. In the [Os(PPh3)2(ap-R)Br] complexes, the ap-R ligand coordinates to osmium as a tridentate C,N,O donor ligand forming two five-membered chelate rings and the two PPh3 ligands are trans. In the [Os(PPh3)(ap-R)(N-O)] complexes, osmium is bound to one PPh3, one ap-R ligand coordinated as a tridentate C,N,O donor ligand and one iminosemiquinonate ligand. The [Os(PPh3)2(ap-R)Br] complexes are paramagnetic (low-spin d5, S = 1/2) and show anisotropic EPR spectra at 77 K. The [Os(PPh3)-(ap-R)(N-O)] complexes are diamagnetic and show sharp 1H and 13C NMR signals. In acetonitrile solution all the complexes display intense charge-transfer transitions in the visible region. Cyclic voltammetry on these complexes in acetonitrile solution shows an osmium(m)-osmium(iv) oxidation positive to SCE and an osmium(II)-osmium(III) reduction negative to SCE. Two irreversible oxidations are also displayed by all the [Os(PPh3)(ap-R)(N-O)] complexes in the range 1.10-1.76 V vs. SCE. © The Royal Society of Chemistry 2001.
Type
journal article