Towards Spontaneous Heterolysis of the Homonuclear P-P Bond in Diphosphines: The Case of Diazaphospholeniumtriphospholides

Computational studies on a series of polyphospholyl-substituted N-heterocyclic phosphines (CH),(NR), P-P-n(CH)(5-n) (R-Me, n=1-5) disclosed that increasing formal replacement of CH units in the phosphole ring by phosphorus atoms is associated with an increase in P-P distances and charge separation, and a decrease in covalent bond orders. Altogether, these trends imply that the CH versus P substitution enhances ionic P-P bond polarization in these compounds. Experimental verification of this hypothesis was obtained for the triphospholyl diazaphospholenes (CR)(2)(NR')(2)P-P-3-(CtBu)(2) (8a: R=H, R'=tBu: 8b: R=Me, R'=Mesityl [Mes]), which were prepared through metathesis reactions from suitable precursors and identified by solution and solid-state NMR data and a single-crystal X-ray diffraction study of 8a. Analysis of J(PP) coupling patterns suggested that both species are characterized by the absence of a strong covalent P-P bond connecting both rings. This interpretation was confirmed by the finding of a unique P-P distance of 2.79 angstrom For crystalline 8a, and further supported by computational studies, which led to the conclusion that both species are better described as diazophospholenium-triphospolide contact ion pairs rather than covalent molecules. Variable-temperature (VT) NMR spectra of 8b showed a collapse of J(PP) couplings between atoms in different rings, which indicates scrambling of the diazaphospholenium and triphospholide units between different molecules in solution, and further substantiates the proposed view on the molecular structure.