Osmium mediated decarbonylation of aldehydes and ketones. Synthesis, structure and reactivities of some osmium carbony1 complexes

Abstract

Reaction of [Os(PPh₃)₃Br₂] with salicylaldehyde or 2-hydroxyacetophenone or 2-hyd roxynaphthaldehyde in the presence of a base results in the decarbonylation of these phenolic ligands affording an osmium carbonyl complex, <i>viz</i> . [Os(PPh₃)₂(CO)₂Br₂]. The same [Os(PPh₃)₂(CO)₂Br₂] complex has also been obtained from the reaction of [Os(PPh₃)₃Br₂]

with benzaldehyde, acetophenone and 1-naphthaldehyde in the absence of any base. Reacti on of [Os(PPh₃)₃Br₂] with formaldehyde in the presence of a base affords another carbonyl complex, <i>viz, </i>[Os(PPh₃)₂(CO)₂(HCOO)₂]. Structure of both the carbonyl complexes has been determined by X-ray crystallography. Reaction of the carbonyl complexes has been carried out with three bidentate ligands, <i>viz. </i>tetrachlorocatechol (H₂Cl₄Cat), quinolin -8-ol (Hq) and 2,2'-bipyridine (bpy),which results in the formation of [Os(PPh₃)₂(CO)₂(Cl₄Cat)], [Os(PPh₃)₂(CO)(q)(H)] and [Os(PPh₃)₂(CO)(bpy)(H)]⁺ complexes respectively. Structure of [Os(PPh₃)₂(CO)₂(Cl₄Cat)] has also been determined by X-ray crystallography. All the complexes are diamagnetic and show intense charge- transfer transitions in the visible and/or ultraviolet region. Cyclic voltammetry of all the complexes in dichloromethane solution shows an osmium(II)-osmium(III) oxidation within 0.56 - 1.15 V vs SCE.

The [Os(PPh₃)₂(CO)(q)(H)] complex also shows an osmium(III)-osmium(IV) oxidation at 1.1 5 V vs SCE and in the [Os(PPh₃)₂(CO)(bpy)(H)]⁺ complex, a reduction of the coordinated 2,2'-bipyridine ligand is observed at -1.21 V vs SCE.