Synthesis, Characterization, and Reactivity toward MeI of Carbonyl Rh(I) Complexes Containing a PNO Hydrazonic Ligand

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 HCl 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 MeI 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
MeI formed the cationic PNO coordinated acetyl complex [Rh(HL)(MeCO)I](CF3SO3) (10).
All the oxidative addition reactions were monitored by liquid IR spectroscopy.