Linear co-ordinative bonding at oxygen: a spectroscopic and structural study of phosphine oxide-Group 13 Lewis acid adducts

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Abstract
A number of adducts composed of phosphine oxides and Group 13 Lewis acids R3PO·EX3(R = Ph, NMe2, or PhO; E = B, Al, or Ga; X = F, Cl, or Br) have been spectroscopically characterised by multinuclear n.m.r. spectroscopy. Three isostructural derivatives have been structurally characterised by X-ray crystallography. Crystal data (all hexagonal, space group 3, Z= 6): Ph3PO·AlCl3, a= 13.663(2), c= 18.258(2)Å, R= 0.062; Ph3PO·AlBr3, a= 14.021 (6), c= 18.387(3)Å, R= 0.041; Ph3PO·GaCl3, a= 13.753(6), c= 18.345(6)Å, R= 0.079. The structures show a uniquely linear or almost linear P–O–E backbone, which lies on the three-fold axis, in contrast to the bent structures observed for the corresponding BF3 adducts and other related systems. Short Al–O bonds [X = Cl, 1.733(4); Br, 1.736(7)Å] are observed in both aluminium derivatives (E = Al). These compounds have narrow lines in the solution 27Al n.m.r. spectra, indicative of a highly symmetric environment for the aluminium centre, and consistent with a linear geometry in solution. The results provide experimental evidence for axially symmetric dative bonding by oxygen, support the triple-bond model for the phosphine oxide unit, and imply the possibility of a delocalised π interaction over the P–O–E framework.

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