Stretching the P-C Bond. Variations on Carbenes and Phosphanes.

Zsolt Kelemen,László Nyulászi,Dániel Buzsáki

doi:10.1021/acs.jpca.0c00641

Abstract

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene–pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene–phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX—phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

Highlights

Carbenes (Carb this notation will be used throughout) are divalent carbon compounds, stabilized mainly by heteroelements like nitrogen or sulfur,[1−5] yielding N-heterocyclic carbenes (NHCs) or cyclic aminocarbenes (CAACs) just to mention the most widely applied ligands of transition metals.[6−9] as strong nucleophiles,[10] these ligands can stabilize otherwise highly reactive molecules formed from p-block elements.[11,12]
The products from the reactions between different NHCs and phosphanes are more diverse (Figure 1: A−E), depending on the reaction conditions and the nature of the starting materials.[13−19,22,25−32,34−40] The parent H-phosphanes did not react with imidazol-2-ylidene (1 R:Dipp);[22] imidazolidine-2-ylidene (2 R:Dipp)[22] and CAAC (3)[19] were activating the PH bond, yielding primary phosphanes A (X = alkyl or aryl, Y:H see Figure 1)
We investigate the relative stability of the possible isomers, in case of the different carbenes, and discuss comprehensively the effect of the formal increase of the phosphorus valency on the P−NHC bond of the G-type compounds

Summary

■ INTRODUCTION

The formation of any λ5-P containing F-type structure, where the NHC and the ylidic structure with opposing polarizing effects on the double bond (Figure 3), is clearly an interesting possibility; this type of adduct was not observed in the reactions between NHCs and phosphanes considered so far. This might be in accordance with the known destabilizing effect[72] of the amino substituents on phosphorus ylides; some C-amino-phosphorus ylides could be synthesized.[124,125].

■ RESULTS AND DISCUSSION

CONCLUSIONS

■ REFERENCES

The Detection of Unstable Molecules by Microwave Spectroscopy