INTERMOLECULAR EXCHANGE IN A TRIARYLBORANE PHOSPHINE COMPLEX - A MULTINUCLEAR MAGNETIC-RESONANCE STUDY

It has been shown using H-1, F-19, and P-31 NMR spectroscopy that the tris(pentafluorophenyl)borane-phosphine adduct undergoes intermolecula r exchange in toluene solution involving transfer of the phosphine gro up. The rate coefficient (k) for the exchange process was measured usi ng the P-31 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 i ndependent of the excess phosphine concentration. The mechanism for th e exchange process is proposed to be two step, a slow initial dissocia tion of the adduct followed by a rapid recombination step. The rate co efficient 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 H-1 two-dimensional exchange spectrum (NOESY; is described in terms of the competing effects of nuc lear spin relaxation and chemical exchange.