Diethyl(di-tert-butylphosphino)borane and related compounds: The first stable monomeric tetraalkyl phosphinoborane

Article abstract of DOI:10.1021/om00032a011

Four new phosphinoborane compounds were prepared and characterized by ¹H, ¹³C, ³¹P, and ¹¹B 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 ¹H NMR spectroscopy showed that ΔH = -93 kJ/mol and ΔS = -285 J/(mol·K) for the dimerization of Me₂BP(t-Bu)₂ and ΔH = -55 ± 12 kJ/mol and ΔS = -166 ± 42 J/(mol·K) for the dimerization of Me₂BP(SiMe₃)₂. Inverse 2D ¹H/¹³C correlation studies were applied to monomer-dimer solutions and were vital to assignment of ¹³C resonances. Solid-state ³¹P 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 P1, a = 8.366(3) A?, b = 8.0808(2) A?, c = 9.311(2) A?, α = 74.98(2)°, β = 78.87(2)°, γ = 64.32(2)°, Z = 1, and R = 0.0628.