Carbonyl rhodium(I) complexes containing a hydrazonic tridentate HNN′O ligand. Synthesis, X-ray structure and reactivity toward methyl iodide

The ligand (2-pyridinecarboxaldehyde) benzoylhydrazone (HNN′O) has been reacted with [Rh2(μ-Cl)2(CO)4] in diethyl ether, isolating the carbonyl complex [Rh(κ2-HNN′)(CO)Cl] (1), where the neutral ligand coordinates the metal through the nitrogen atoms, the amidic oxygen been excluded by the coordination sphere. Repeated attempts aimed to force the ligand to an anionic tridentate coordination, both by prior deprotonation of the free ligand or deprotonation of 1, have resulted in extensive deposition of metallic rhodium. The reaction between HNN′O and [Rh2(μ-Cl)2(CO)4] in basic media and in the presence of PPh3, has led to the isolation of the complex [Rh(κ2-N′O)(PPh3)(CO)]·1/2CH2Cl2 (2), where the anionic ligand is N′O bidentate. Compound 1 has been reacted with an excess of MeI in CH2Cl2 or THF, isolating [Rh(κ2-HNN′)(CH3CO)ClI] (4) and [Rh(κ2-HNN′)(CH3CO)(THF)ClI] (5), respectively. Although the oxidative addition step is practically instantaneous in both cases, the migratory insertion step results faster in THF, as established by liquid film IR spectroscopy. Compound 2 has been reacted with MeI in THF isolating the complex [Rh(κ2-N′O)(PPh3)(CO)(Me)I] (6), which, however, does not transform into the corresponding acyl complex. The crystal structure of complex 2·1/4CH2Cl2 has been solved.