Ligand Exchange and Redox Reactions of Chromium(V): Isolation of the Chromium(V) Oxalato Complexes From the Ligand Exchange Reaction of Bis[2-ethyl-2-hydroxybutanoato(2-)]oxochromate(V) With Oxalate in 50% Aqueous Acetic Acid

Abstract

The moderately stable chromium(v) complex, trans-bis[2-ethyl-2-hydroxybutanoato(2-)] oxo -chromate(v), [Cr( ehba )₂O]^- (1), has been assessed for its suitability as a synthetic intermediate for the preparation of other chromium(v) complexes. This complex undergoes a ligand exchange reaction with excess oxalic acid to form an equilibrium mixture of chromium(v) complexes which have been assigned the structures [Cr( ehba )₂O]^-, [Cr( ehba )(ox)O]^- (2), [Cr(ox)₂O]^- (3) and cis -[Cr(ox)₂(OH₂)O]^- (4). The products of the ligand exchange reaction were characterized by a combination of u.v ./visible spectroscopy, e.p.r . spectroscopy and electrochemistry. The equilibrium mixture of chromium(v) complexes decays to form chromium(III) products by a redox reaction which is much slower than the initial ligand exchange reaction. The difference in the rates of ligand exchange and subsequent reduction allows the extraction of the chromium(v) ligand exchange products into dichloromethane. n.m.r. spectroscopy and electrochemical techniques were used to identify these products. Under appropriate conditions (high oxalic acid concentrations) the major species extracted are the bis [ oxalato (2-)] oxochromate (v) complexes which have been isolated as oily solids in relatively pure form (≥90% pure) for the first time. n.m.r. spectroscopic evidence indicates that the isolated solid was a mixture of (3) and (4) which explains the difficulty encountered in crystallizing the solid. These ligand exchange reactions may be of general utility in the synthesis of chromium(v) complexes.