Pyramidal Inversion at Phosphorus Facilitated by the Presence of Proximate Lewis Acids. Coordination Chemistry of Group 13 Elements with the Macrocyclic Bis(amidophosphine) Ligand [P2N2] ([P2N2] = [PhP(CH2SiMe2NSiMe2CH2)2PPh])

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Abstract
Investigations on the preparation of four- and five-coordinate aluminum and gallium bis(amidophosphine) derivatives are reported. The reaction of the macrocyclic ligand precursor anti-Li2(THF)2[P2N2] ([P2N2] = [PhP(CH2SiMe2NSiMe2CH2)2PPh]) with AlCl3 or GaCl3 in toluene at 25 °C leads to the formation of the four-coordinate species anti-MCl[P2N2] (M = Al (1), Ga (2)). An X-ray diffraction study of anti-GaCl[P2N2] shows it to be monomeric with a distorted tetrahedral geometry at Ga; only one of the phosphine donors of the [P2N2] ligand binds to the gallium, resulting in the retention of the anti-configuration. The solution NMR spectra are consistent with Cs symmetry. The addition of AlCl3 or GaCl3 to the macrocyclic ligand precursor syn-Li2(dioxane)[P2N2] in toluene at 25 °C yields the five-coordinate complexes syn-MCl[P2N2] (M = Al (3), Ga (4)). The X-ray crystal structure of syn-GaCl[P2N2] reveals a trigonal bipyramidal geometry about the metal atom, necessitating the coordination of both phosphorus atoms. The solution NMR spectra are consistent with a C2v symmetric complex. Heating the anti complexes results in the clean conversion to the syn complexes, with pyramidal inversion observed at phosphorus. The kinetics of this inversion were studied by 1H NMR spectroscopy and found to be first-order. Barriers to pyramidal inversion (ΔG⧧) were calculated to be 29.1 and 30.1 kcal mol-1 for the aluminum and gallium complexes, respectively; these barriers are approximately 2−3 kcal mol-1 lower than that determined for the metal-free, protonated compounds anti- and syn-H2[P2N2]. It is suggested that the role that the metals play in this inversion, based on the values of ΔG⧧, involves the large negative entropies of activation and thus help organize the transition state.