Syntheses, spectroscopy, structures, and reactivity of neutral cubic group 13 molecular phosphonates

Abstract

The syntheses of cubic group 13 molecular phosphonate cages of the formulas [RPO₃MR']₄ (R = Me, Et, t-Bu, Ph; MR' = BEt, BBu-s, GaMe, InMe; all combinations, 1-16), [t-BuPO₃AlBu-i]₄ (17), and [MePO₃AlBu-t]₄ (18) have been achieved by the facile reactions of alkyl- or arylphosphonic acids with the corresponding alkyl compounds under appropriately chosen reaction conditions. Compounds 1-18 have been characterized by analytical and spectroscopic techniques. In all cases, the central M₄O₁₂P₄ inorganic cubic framework is shielded by alkyl/aryl groups. The phosphonates bearing tert-butyl groups on the phosphorus are highly soluble in common organic solvents such as n-hexane, Et₂O, and THF. The molecular structure of representative examples [t-BuPO₃BEt]₄ (3) and [t-BuPO₃BBu-s]₄ (7) as determined by single-crystal X-ray diffraction studies reveal the presence of a central cubic M₄O₁₂P₄ framework that is analogous to those found in alumino- and gallophosphate materials. Most of these cubic phosphonates are air and moisture stable. In the presence of excess phenol, it is possible to react one of the M-C bonds at the corners of the cube 3 and prepare its monophenoxy derivative [t-BuPO₃BEt]₃[t-BuPO₃B(OPh)] (19) in very low yields. However, in spite the presence of reactive M-C bonds, these cubic phosphonates are unreactive toward alcohols, amines, carboxylic acids, H₂O, and air under ambient conditions.