Phosphorus stabilization of the carbene function in P-analogues of non-cyclic carbenes, N-heterocyclic carbenes and cyclic(alkyl)-(amino)carbenes − an assessment on basis of geometry, 13C, 31P chemical shifts and the anisotropy effects of the carbene electron deficient centres

Abstract

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of the P-analogues of non-cyclic, N-heterocyclic carbenes (NHCs) and cyclic (alkyl) (amino)carbenes (CAACs) have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and the results visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values (actually the anisotropy effects experimentally measurable in 1H NMR spectroscopy) are employed to qualify and quantify the position of the present mesomeric equilibria (P-carbenes ↔ P-ylides ↔ P-acetylenes/P-allenes). Results are confirmed by geometry (bond angles and bond lengths), the 31P and the 13C chemical shifts of the electron-deficient carbene centres. It was found out that the π-donor and σ-acceptor power of phosphorus in planar and not pyramidalized PR2 substituents to stabilize P-carbenes (compared with NR, OR, SR and Hal substituents) is only minor, but complete PC bonds along sole ylide-like mesomeric contributors are existing. A phosphorus carbene stabilization mechanism, not yet specified, in addition or different to π-donation, is badly needed.