Migration of Hydride, Methyl, and Chloride Ligands between Al and M in (PAlP)M Pincer Complexes (M = Rh or Ir)

Protolysis of AlMe₃ or AlBuⁱ₃ with 2-diisopropylphosphinopyrrole (1) yields molecules containing two flanking phosphines and a central Al–Me (2-Me), Al-ⁱBu (2-ⁱBu), or Al–H (2-H) unit. The reactions of 2-Me with [L₂MCl]₂ (L = cyclooctene or 1/2 1,5-cyclooctadiene and M = Rh or Ir) in the presence of pyridine produces PAl^ClP pincer complexes (3-Rh and 3-Ir) with Al–Cl and M–Me bonds. The analogous reaction of a mixture of 2-ⁱBu and 2-H with [L₂MCl]₂ and pyridine resulted in the formation of analogous Rh–H (4-Rh) and Ir–H (4-Ir) complexes. Treatment of 3-Rh with NaBEt₃H produced compound 5-Rh with an Al–Me and a Rh–H bond; the analogous reaction of 3-Ir did not result in a clean product. 4-Ir accepted an equivalent of H₂ to produce 6-Ir with two terminal Ir–H bonds and one bridging Al–H–Ir moiety, whereas 4-Rh did not react with H₂. The density functional theoretical treatment is in accord with this finding, highlights the likely mechanism for the H₂ addition, and supports the bonding picture in 6-Ir arising from NMR and X-ray diffraction (XRD) observations. Spectroscopic data and XRD studies are consistent with distorted square-pyramidal structures (about Rh or Ir) for compounds 3–5, with an alane occupying the apical position. Complexes 3 and 4 possess some of the shortest known Rh–Al or Ir–Al distances.