Probing the Association of Frustrated Phosphine-Borane Lewis Pairs in Solution by NMR Spectroscopy

F-19,H-1 HOESY, diffusion, and temperature-dependent F-19 and H-1 NMR studies allowed us to unequivocally probe the association between the frustrated PR3/B(C6F5)(3) (1, R = CMe3; 2, R = 2,4,6-Me3C6H2) Lewis pairs in aromatic solvents. No preferential orientation is favored, as deduced by combining F-19,H-1 HOESY and DFT results, suggesting association via weak dispersion rather than residual acid/base interactions. The association process is slightly endoergonic [K = 0.5 M-1, Delta G(0) (298 K) = +0.4 kcal/mol for 2], as derived from diffusion NMR measurements.