Intermolecular exchange in a triarylborane-phosphine complex: a multinuclear magnetic resonance study

It has been shown using 1 H, 19 F, and 31 P NMR spectroscopy that the tris -(pentafluorophenyl) borane-phosphine adduct undergoes intermolecular exchange in toluene solution involving transfer of the phosphine group. The rate coefficient (k) for the exchange process was measured using the 31 P NMR selective inversion experiment on a series of samples containing excess phosphine. At 254 K, k is 3.60 ± 0.15 s -1 and is independent of the excess phosphine concentration. The mechanism for the exchange process is proposed to be two step, a slow initial dissociation of the adduct followed by a rapid recombination step. The rate coefficient is strongly temperature dependent in the range 243-263 K and a value of 126 kJ mol -1 is obtained for the activation energy of the dissociation step. The appearance of the 1 H two-dimensional exchange spectrum (NOESY) is described in terms of the competing effects of nuclear spin relaxation and chemical exchange.