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

Markus Blum,Martin Nieger,Dietrich Gudat,Christoph M Feil

doi:10.1002/zaac.202000252

Markus Blum, Martin Nieger + Show 2 more

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https://doi.org/10.1002/zaac.202000252

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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.

Highlights

Phosphinyl radicals are divalent phosphorus compounds of general composition R2P that are distinguished by an open shell electron structure with a formal 7 VE electron configuration
The substantially higher dissociation enthalpy ΔHDiss of 135.2(26) kJ·mol–1 for {7}2 implies that PP bond homolysis is in this case energetically less favorable than in all reference compounds in Table 2, and it is only due to the compensation of the unfavorable energy term by an unusually large positive entropy contribution [ΔSDiss = 154(8) J·K–1·mol–1] that radical formation at ambient temperature is at all observable
DFT calculations on radical 7 and diphosphine {7}2 were carried out at the ωB97x-D/cc-pVDZ level of theory that had already been employed in earlier studies on radicals 5, 6.[11]. The subsequent discussion is based on molecular geometries of 7 and {7}2 that were obtained by energy optimization using the solid-state structures of 8 and {7}2 as starting points, and established as local minima on the energy hypersurface by frequency calculations

Summary

Introduction

Phosphinyl radicals are divalent phosphorus compounds of general composition R2P that are distinguished by an open shell electron structure with a formal 7 VE electron configuration. The formation of radicals through thermally induced PP bond homolysis was monitored by recording EPR spectra of a solution of {7}2 in anhydrous and degassed toluene.

Results

Conclusion