DIETHYL(DI-TERT-BUTYLPHOSPHINO)BORANE AND RELATED-COMPOUNDS - THE 1ST STABLE MONOMERIC TETRAALKYL PHOSPHINOBORANE

Four new phosphinoborane compounds were prepared and characterized by H-1, C-13, P-31, and B-11 NMR spectroscopy and elemental analysis. Monomeric Et2BP(t-Bu)2 Was isolated as a volatile yellow liquid and [Me2BP(t-Bu)2]2 as a colorless solid using salt elimination reactions. The colorless dimeric solids [Me2BP(SiMe3)2]2 and [Et2BP(SiMe3)2]2 were prepared via trimethylsilyl halide elimination reactions. The dimeric compounds dissociate in benzene or toluene at ambient temperature, and an equilibrium with the monomeric species is established. Variable-temperature H-1 NMR spectroscopy showed that DELTAH = -93 kJ/mol and DELTAS = -285 J/(mol.K) for the dimerization of Me2BP(t-Bu)2 and DELTAH = -55 +/- 12 kJ/mol and DELTAS = -166 +/- 42 J/(mol.K) for the dimerization of Me2BP(SiMe3)2. Inverse 2D H-1/C-13 correlation studies were applied to monomer-dimer solutions and were vital to assignment of C-13 resonances. Solid-state P-31 NMR confirmed that [Me2BP(t-BU)2]2 and [Me2BP(SiMe3)2]2 are dimeric solids. The structure of [Me2BP(t-BU)2]2 was determined by single-crystal X-ray diffraction. Crystal data: triclinic, space group P1BAR, a = 8.366(3) angstrom, b = 8.0808(2) angstrom, c = 9.311(2) angstrom, alpha = 74.98(2)-degrees, beta = 78.87(2)-degrees, gamma = 64.32(2)-degrees, Z = 1, and R = 0.0628.