Diphosphines with strongly polarized P-P bonds: hybrids between covalent molecules and donor-acceptor adducts with flexible molecular structures.

A series of P-phospholyl-substituted N-heterocyclic phosphines was prepared and characterized by single-crystal X-ray diffraction and solution and solid-state (31)P NMR spectroscopy. The molecular structures are distinguished by the presence of P-P bonds of exceptionally variable lengths (2.35-2.70 A) that are all well beyond the standard distance of 2.21 A. The unique flexibility is best illustrated by a specimen 4f where minor conformational changes of remote substituents induce a deviation in P-P bond lengths of some 5 pm between crystallographically independent molecules in the same unit cell. Computational studies suggest to rationalize the bond elasticity as the consequence of a very flat potential energy basin that allows even weak forces to have large impact on bond lengths. Solid-state (31)P NMR studies show that the bond distance variation coincides with substantial changes in the magnitude and sign of (1)J(PP), which is explained in the context of a dominant Fermi contact contribution. A relation between increasing internuclear distance and decreasing magnitude of (1)J(PP) was experimentally proven by determination of effective dipolar coupling constants by the double-quantum dephasing experiment (DoDe) for the crystallographically independent conformers of 4f and further supported by comparison with calculated coupling tensors with inclusion of the anisotropic J-coupling. NMR studies revealed large discrepancies in the values of (1)J(PP) measured in solution and the solid state and a substantial temperature dependence of the former. Interpretation of this behavior was feasible by taking into account that the value of (1)J(PP) in solution is affected by both temperature-dependent equilibria between trans and gauche conformers and additional bond length relaxation that accompanies the dissolution process. Consideration of experimental observations and population analysis of computed electron densities suggested to classify the P-P bonds in the molecules under study as "dative" rather than "normal" covalent bonds and to address the compounds 4 as hybrids between covalent diphosphines and phosphenium-phospholide contact ion pairs.