Synthesis and Thermally and Light Driven Cleavage of an N‐Heterocyclic Diphosphine with Inorganic Backbone

Abstract

A diphosphine with an unsupported PP bond connecting two carbon‐free “inorganic” 1,3,2,4,5‐diazaphosphadisilolidine rings was prepared by reductive coupling of a P‐chloro‐substituted monocyclic precursor molecule. VT‐EPR studies revealed that the diphosphine exists in solution, like other compounds of this kind, in dynamic equilibrium with the corresponding phosphinyl radicals. Determination of the radical concentration from the EPR spectra permitted to calculate thermochemical parameters for the homolytic PP bond fission. The results disclose that both the enthalpy and entropy of dissociation are higher than in topologically related bi(diazaphospholidines). The impact of the entropy term allows explaining that, regardless of the presence of an energetically rather stable PP bond, the onset of dissociation is observable even at ambient temperature. Irradiation experiments showed that radical formation cannot only be induced thermally, but also by photolysis.

Stability of Carbocyclic Phosphinyl Radicals: Effect of Ring Size, Delocalization, and Sterics

“…To expand the scope of this study, we involved phosphinyl radicals with exocyclic C=C double bonds as well ( Figure 5 ). Furthermore, the cyclic structure is of great importance to modify or constrain a given conformation, 44 which may also influence delocalization between the P-center and the π-system.…”

“…The steric stabilization can be achieved by careful selection of bulky substituents, and several species have been accessed employing this methodology. − The origin of this effect is that the flanking large ligands cause steric congestion via Pauli repulsion; thus, the crowded radical centers are less prone to dimerization. However, a large number of weak dispersion interactions may arise between the sterically demanding ligands, and this can counterproductively result in substantial stabilization of the dimeric form. , In contrast, the electronic stabilization can be enhanced by introducing functional groups next to the P-center enabling resonance stabilization via delocalization, that is electronic communication of the lone electron with the neighboring centers.…”

“…However, a large number of weak dispersion interactions may arise between the sterically demanding ligands, and this can counterproductively result in substantial stabilization of the dimeric form. 44 , 45 In contrast, the electronic stabilization can be enhanced by introducing functional groups next to the P-center enabling resonance stabilization via delocalization, that is electronic communication of the lone electron with the neighboring centers.…”