Transition metal–carbon bonds. Part XIV. Allylic complexes of rhodium

Abstract

A convenient synthesis of complexes of the type [Rh₂Cl₂(all)₄](all = allyl, 1-methyl-, 2-methyl-, or 2-chloro-allyl) is described from the oxidative hydrolysis of [Rh₂Cl₂(CO)₄] in the presence of an allylic chloride (allCl) in aqueous methanol. If the synthesis is carried out in the presence of potassium hydroxide the reactions are much faster and the yields higher; [Rh₂Cl₂(1-phenylallyl)₄] was prepared by this method. With 2-methylallyl chloride and [Rh₂Cl₂(CO)₄] reacting in aqueous methanol at ca. 0° the major product is [Rh₂Cl₂(2-methylallyl)₄] but at 60° only [Rh₂Cl₂(2,5-dimethylhexa-1,5-diene)₂] was isolated. Deuteriation and other studies show that reversible protonation of the 2-methylallyl groups plays an essential part in the formation of the 2,5-dimethylhexa-1,5-diene ligand. Complexes of the type [Rh₂Cl₂(all)₄] contain asymmetrically bonded allylic groups, the nature of which is discussed. Some complexes of the type [Rh₂Cl₂(all)₄] react with alcoholic potassium hydroxide to give an olefin (allH). They can also be used to prepare complexes of the types: [Rh₂X₂(all)₄](X = Br, l, or Ac); [RhCl(all)₂L] and [Rh(all)₂py₂]⁺(L = py, PR₃, or AsR₃); [RhCl(PPh₃)₃]; [Rh₂Cl₂(cyclo-octa-1,5-diene)]; [Rh(acac)(all)₂]; [Rh(σ-all)(π-all)(C₅H₅)]; [RhCl₂(all)]_x and [Rh(all)₃]. [Rh(all)₃] reacts with Na₂PdCl₄ to give [Rh₂Cl₂(all)₄] and [Pd₂Cl₂(all)₂], or with PPh₃ to give [Rh(all)(PPh₃)₂]. Complexes of the types [RhCl₂(all)L₂](L = py or PMe₂Ph) and [Rh₂Cl₄(all)₂(CO)₂], and the complex [RhCl₂(C₅H₅)]_x are described. Near-infrared, far-infrared, and nuclear magnetic resonace data are given and discussed.