Remote Substituent Effects on the Structures and Stabilities of P═E π-Stabilized Diphosphatetrylenes (R2P)2E (E = Ge, Sn).

Keith Izod,Paul G Waddell,Peter Evans,Michael R Probert

doi:10.1021/acs.inorgchem.6b01566

Abstract

A rare P-E π interaction between the lone pair of a planar P center and the vacant p orbital at the Ge or Sn center provides efficient stabilization for P-substituted tetrylenes (R2P)2E (E = Ge, Sn) and enables isolation of the first example of a compound with a crystallographically authenticated P═Sn bond. Subtle changes in the electronic properties of the bulky aryl substituents in these compounds change the preference for planar versus pyramidal P centers in the solid state; however, variable-temperature NMR spectroscopy indicates that in solution these species are subject to a dynamic equilibrium, which interconverts the planar and pyramidal P centers. Consistent with this, density functional theory studies suggest that there is only a small energy difference between the planar and pyramidal forms of these compounds and reveal a small singlet-triplet energy separation, suggesting potentially interesting reactivities.

Highlights

The stability of diaminocarbenes may be attributed to the presence of electronegative N atoms adjacent to the carbene center, which increase the s character of the carbene lone pair and the π donation of the electron density from the N lone pairs into the vacant p-orbital at the carbene C atom, which mitigates the electron deficiency of this center
While it is a commonly held belief that pπ−pπ overlap will be poorer for P than N because of the larger, more diffuse valence orbitals of the former, calculations suggest that the inherent πdonor capacity of a planar P center is equal to, or better than, that of N.5
As part of an extended program investigating the chemistry of heavier group 14 carbene analogues,7h,12,13 we recently reported the synthesis of the first two-coordinate diphosphatetrylene to exhibit significant pπ−pπ interactions between a planar P center and GeII, {(Dipp)2P}2Ge [Dipp = 2,6iPr2C6H3] (6Ge; Chart 2).[14]

Summary

■ INTRODUCTION

Despite the fact that singlet phosphacarbenes, in which a PIII center lies directly adjacent to a divalent C atom, were among the first stable carbenes to be isolated, their chemistry is significantly less well-established than that of their Nsubstituted analogues.[1−3] This is striking given the widespread use of diaminocarbenes (such as N-heterocyclic carbenes, NHCs) as supporting ligands in transition-metalbased catalysis and organocatalysis and the major advances made in the manipulation of the steric and electronic properties of these compounds.[1,4]. P-substituted heavier group 14 carbene analogues (phosphatetrylenes, (R2P)EX, and diphosphatetrylenes, (R2P)2E; E = Si, Ge, Sn, Pb) are poorly represented, in spite of the relative accessibility of the 2+ oxidation state for many of the heavier group 14 elements.[6−12] While the first crystallographically characterized two-coordinate diphosphatetrylenes [{(Tripp)(tBu)(F)Si}(iPr3Si)P]2E [E = Sn (4Sn), Pb (4Pb); Tripp = 2,4,6-iPr3C6H2] were reported by Driess and co-workers in 1995,8 further examples of such species were, until recently, limited to a small number of compounds whose monomeric nature has been inferred from spectroscopic and/or cryoscopic data.[9] In both 4Sn and 4Pb, the two P centers adopt a trigonal-pyramidal geometry; there is no evidence for the π-type electron donation usually associated with the stabilization of NHCs and related compounds. This study, detailed below, reveals that relatively small changes in the steric and electronic properties of the ligands may have a profound effect on the structures and properties of their group 14 compounds

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES