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Stability and Electrophilicity of Phosphorus Analogues of Arduengo Carbenes—An Experimental and Computational Study
Author(s) -
Gudat Dietrich,
Haghverdi Asadollah,
Hupfer Heike,
Nieger Martin
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20000915)6:18<3414::aid-chem3414>3.0.co;2-p
Subject(s) - chemistry , lewis acids and bases , hyperconjugation , crystallography , computational chemistry , ionic bonding , covalent bond , substituent , frustrated lewis pair , electrophile , stereochemistry , molecule , ion , organic chemistry , catalysis
A variety of differently substituted 1,3,2‐diazaphospholenium salts and P ‐halogeno‐1,3,2‐diazaphospholenes (X=F, Cl, Br) were synthesized, and their molecular structures, bonding situation, and Lewis acid properties were characterized by experimental (single‐crystal X‐ray diffraction, NMR and IR/Raman spectroscopy, MS, conductometry, titrations with Lewis bases) and computational methods. Both experimental and computational investigations confirmed that the structure and bonding in the diazaphospholenium cations of OTf and BF 4 salts resembles that of neutral Arduengo carbenes and that the cations should not be described as genuinely aromatic. P ‐Halogenodiazaphospholenes are, in contrast to earlier assumptions, molecular species with covalent P−X bonds whose bonding situation can be expressed in terms of hyperconjugation between the six π electrons in the C 2 N 2 unit and the σ*(P−X) orbital. This interaction induces a weakening of the P−X bonds, whose extent depends subtly on substituent influences and contributes fundamentally to the amazing structural similarity of ionic and covalent diazaphospholene compounds. A further consequence of this effect is the unique polarizability of the P−Cl bonds in P ‐chlorodiazaphospholenes, which is documented in a considerable spread of P−X distances and bond orders. Measurement of the stability constants for complexes of diazaphospholene compounds with Lewis bases confirmed the lower Lewis acidities and higher stabilities of diazaphospholenium ions as compared with non‐conjugated phosphenium ions; this had been inferred from computed energies of isodesmotic halide‐transfer reactions, and permitted also to determine equilibrium constants for P−Cl bond dissociation reactions. The results suggest, in accord with conductance measurements, that P ‐chlorodiazaphospholenes dissociate in solution only to a small extent. On the basis of these findings, the unique solvatochromatic behavior of NMR chemical shifts of these compounds was attributed to solvent‐dependent P−Cl bond polarization rather than to shifts in dissociation equilibria.