Formation, structural characterization, and reactions of a unique cyclotrimeric vicinal Lewis pair containing (C6F5)2P-Lewis base and (C6F5)BH-Lewis acid components

Abstract

The synthesis of the new vicinal frustrated Lewis pair 5 containing (C₆F₅)₂P-Lewis base and (C₆F₅)BH-Lewis acid functionality is described. It forms a unique cyclotrimer (5)₃ which was structurally characterized by X-ray crystallography and high-resolution solid-state NMR spectroscopy. The relevant NMR Hamiltonian parameters (¹¹B and ³¹P chemical shielding tensors, ¹¹B quadrupolar coupling tensors, and ³¹P–¹¹B spin–spin coupling constants) indicate significant intramolecular covalent B⋯P interactions, consistent with results from density functional theory (DFT) calculations. In addition, the ¹¹B/³¹P and ³¹P/³¹P three-spin geometries are accurately reproduced by suitable high-resolution hetero- and homonuclear dipolar NMR experiments. As predicted from the bonding character portrayed by the solid-state NMR results, the cyclotrimer (5)₃ possesses only moderate catalytic activity. However, it undergoes an addition reaction with pyridine and hydroboration reactions with benzaldehyde and tert-butylacetylene. The products of the hydroboration reactions form stable adducts with pyridine.