Synthesis, characterization, and reactivity toward Mel of carbonyl Rh(I) complexes containing a PNO hydrazonic ligand

Article abstract of DOI:10.1021/om000562r

The reaction of the potentially tridentate hydrazonic ligand 2-(diphenylphosphino)benzaldehyde benzoylhydrazone (HL, 1) with Rh₂(CO)₄Cl₂ in diethyl ether led to the isolation of Rh(HL)(CO)Cl (2), where the neutral ligand was PN bidentate. Addition of Et₃N or NaOMe caused the deprotonation of the ligand, with the consequent formation of the three-coordinated carbonyl Rh(I) complex Rh(L)(CO) (3), which was X-ray characterized. The 2 → 3 conversion was reversible, as it was found that 2 could be regenerated by bubbling a stoichiometric amount of HC1 into an ethereal solution of 3. On treating a solution of 2 with silver triflate, the cationic carbonyl complex [Rh(HL)(CO)](CF₃SO₃) (4) was obtained, where the neutral ligand showed a tridentate PNO behavior. All the complexes were allowed to react with Mel at room temperature. Complex 2 led to two different acetyl stereoisomers Rh(HL)(MeCO)ClI (5), where the neutral ligand coordinated in a PNO fashion. Complex 3 led to different products depending on the reaction solvent; thus the pentacoordinated acetyl complex Rh(L)(MeCO)I (8) and the solvated octahedral acetyl complex [Rh(L)(MeCO)(THF)I](THF) (9) were isolated in CH₂Cl₂ and in THF, respectively; the X-ray structure of 9 is reported. On carrying out the reaction in Et²O, the immediate precipitation of the hexacoordinated methylcarbonyl complex Rh(L)(CO)(Me)I (7) was observed, which rapidly rearranged into 8 after dissolution in CH₂Cl₂ or CHCl₃. Finally, the reaction between 4 and Mel formed the cationic PNO coordinated acetyl complex [Rh(HL)(MeCO)I](CF₃SO₃) (10). All the oxidative addition reactions were monitored by liquid IR spectroscopy.