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.