Stability and electrophilicity of phosphorus analogues of Arduengo carbenes - An experimental and computational study

A variety of differently substituted 1,3,2-diazaphospholenium salts and P-h alogeno-1,3,2-diazaphospholenes (X = E Cl, Br) were synthesized, and their molecular structures, bonding situation, and Lewis acid properties were cha racterized by experimental (single-crystal X-ray diffraction, NMR and IR/Ra man spectroscopy, MS, conductometry, titrations with Lewis bases) and compu tational methods. Both experimental and computational investigations confir med that the structure and bonding in the diazaphospholenium cations of OTf and BF4 salts resembles that of neutral Arduengo carbenes and that the cat ions should not be described as genuinely aromatic. P-Halogenodiaza-phospho lenes are, in contrast to earlier assumptions, molecular species with coval ent P-X bonds whose bonding situation can be expressed in terms of hypercon jugation between the six pi electrons in the C2N2 unit and the sigma*(P-X) orbital. This interaction induces a weakening of the P-X bonds, whose exten t depends subtly on substituent influences and contributes fundamentally to the amazing structural similarity of ionic and covalent diazaphospholene c ompounds. A further consequence of this effect is the unique polarizability of the P-CI bonds in P-chlorodiazaphospholenes, which is documented in a c onsiderable spread of P-X distances and bond orders. Measurement of the sta bility constants for complexes of diazaphospholene compounds with Lewis bas es confirmed the lower Lewis acidities and higher stabilities of diazaphosp holenium ions as compared with nonconjugated phosphenium ions; this had bee n inferred from computed energies of isodesmotic halide-transfer reactions, and permitted also to determine equilibrium constants for P-CI bond dissoc iation reactions. The results suggest, in accord with conductance measureme nts, that P-chlorodiazaphospholenes dissociate in solution only to a small extent. On the basis of these findings, the unique solvalochromatic behavio r of NMR chemical shifts of these compounds was attributed to solvent-depen dent P-Cl bond polarization rather than to shifts in dissociation equilibri a.