Reactivity of Rh(i) and Rh(iii) complexes containing a PNO hydrazonic ligand toward HCl

Abstract

The tridentate hydrazonic ligand 2-(diphenylphosphino)benzaldehyde benzoylhydrazone (HPNO) reacts with Rh₂(CO)₄Cl₂ in toluene or THF leading to the formation of the chloro Rh(III) complex [RhCl₃(HPNO)] (5) and of the carbonyl Rh(I) complex [Rh(PNO)(CO)] (3); the reaction occurs with evolution of CO and H₂. The same products are obtained on refluxing a THF or a toluene solution of the chloro-carbonyl Rh(I) complex [RhCl(HPN)(CO)] (2). Furthermore, complex 5 also forms on bubbling gaseous HCl into a solution of 3, but in this case its formation is preceded by the precipitation of another species that is tentatively identified as the hydride-carbonyl Rh(III) complex [RhCl₂H(HPN)(CO)] (4). Pure 5 is synthesized by reacting equimolar amounts of HPNO and hydrated RhCl₃ in THF at room temperature. Once dissolved in dmso, complex 5 transforms into complex [RhCl₂(PNO)(dmso)]·dmso (6) by elimination of a HCl molecule; complex 6 has been structurally characterized. On bubbling an excess of HCl into a solution of the acetyl Rh(III) complex [RhI(MeCO)(PNO)] (7), complex 5 and acetaldehyde are produced. Conversely, on using a stoichiometric amount of HCl, the reaction brings to the acetyl complex [RhCl_1.6I_0.4(MeCO)(HPNO)]·CH₂Cl₂ (8), which has been characterized by X-ray crystallography.