Impact of Halide (Cl vs I) Identity on the Preferred Positioning of Substituents between Al and M (M = Co, Rh, Ir) in PAlP Pincer Complexes

Protolysis of AlMe₃ or AlEt₃ with 2-diisopropylphosphinopyrrole (1) resulted in alane/bis(phosphine) pincer ligands containing two flanking phosphines and a central Al–Me (2-Me), Al–Et (2-Et) unit. Reactions of 2-Me with [(COD)MI]₂ (COD = 1,5-cyclooctadiene; M = Rh or Ir) in the presence of pyridine produced pincer complexes (3–Rh–I and 3–Ir–I) with M supported by the PAlP tridentate ligand, and pyridine, methyl, and iodide as monodentate ligands for Al or M. The analogous reaction of 2-Et with [(COD)MI]₂ and pyridine resulted in the formation of the analogous compounds 4–Rh–I and 4–Ir–I with hydride in place of methyl. DFT calculations were used to analyze the thermodynamic preferences for the positioning of pyridine, methyl or hydride, and the halide (chloride or iodide) on M vs Al. Cobalt was included with Rh and Ir among M for the purposes of DFT calculations. Theoretical studies suggested that different isomers are preferred for the iodide complexes (M–I and Al–Py) than for the chloride ones (M–py and Al–Cl, previously reported for Rh and Ir). X-ray structural study of 3–Rh–I and analysis of the ¹H NMR data of the Rh and Ir compounds in benzene corroborated these predictions.