Probing the association of frustrated phosphine-borane Lewis pairs in solution by NMR spectroscopy.

(19)F,(1)H HOESY, diffusion, and temperature-dependent (19)F and (1)H 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 (19)F,(1)H 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), ΔG(0)(298 K) = +0.4 kcal/mol for 2], as derived from diffusion NMR measurements.