Solid-state 31P CP/MAS NMR vs. solution study of bis(tertiary phosphines)

Solid-state 31C CP/MAS results of several diphosphines are compared with solution phase data. Non-equivalence of the P atoms are observed in the solid state for the less flexible derivatives (including all chiral diphosphines studied), whereas the more flexible compounds with a linear alkyl chain such as the 1,2-ethane and the 1,4-butane derivatives give only rise to a single resonance. For the more strained bis(diphenylphosphino)methane contradictory data have been reported. Our experiments confirm the existence of an AB spin system, i.e., two different P atoms in solid state. Substantial chemical shift difference between P atoms have been observed in the solid state but not in solution of the chiral compounds. A possible explanation is suggested in terms of the concerted anisotropic effect of the phenyl rings attached to the phosphorus atom.