Metal-free dihydrogen activation chemistry: structural and dynamic features of intramolecular P/B pairs

Hydroboration of allyl(dimesityl) phosphane with HB(C6F5)(2) gives the intramolecular five-membered P-B Lewis pair 7, that was characterized by X-ray diffraction. Similarly, HB(C6F5)(2) addition to the substrates (mesityl)(2)P-CR=CH2(R=CH3, Ph) yield the corresponding (mesityl)(2)P(mu-CHRCH2) B(C6F5)(2) products 9a (R=CH3) and 9b (R=Ph) that show a weak intramolecular P... B interaction. The activation energy of the (reversible) P... B cleavage of these substrates was determined by dynamic F-19 NMR spectroscopy (9a: Delta G(inv)(not equal) (280 K) = 11.7 +/- 0.4 kcal mol(-1)). Compounds 9b and 9c show similar values. Compound 9c was prepared by HB(C6F5)(2) addition to (mesityl)(2)P-CH=CHSiMe3. Compound 9a reacts rapidly with dihydrogen (2.5 bar) at room temperature in pentane to give the zwitterionic H-2-activation product (mesityl)(2)PH+(mu-CHMeCH2)BH-(C6F5)(2)(11).