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) 2 (NR) 2 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 8 a. 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 A for crystalline 8a, and further supported by computational studies, which led to the conclusion that both species are better described as diazaphospholenium-triphospholide 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.