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

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 H-1 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 P-31 and the C-13 chemical shifts of the electron-deficient carbene centres. It was found out that the is pi-donor and sigma-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 P=C bonds along sole ylide-like mesomeric contributors are existing. A phosphorus carbene stabilization mechanism, not yet specified, in addition or different to pi-donation, is badly needed. (c) 2022 Elsevier Ltd. All rights reserved.