Diazaphospholenes: N-heterocyclic phosphines between molecules and Lewis pairs

The interest in geometrically distorted or electronically polarized molecules is often motivated by the realization that unusual structures can engender unprecedented physical or chemical properties. 1,3,2-Diazaphospholenes are N-heterocyclic phosphines (NHPs) that have ring structures similar to N-heterocyclic carbenes (NHCs). Although NHPs were initially mainly of interest as precursors for carbene-analogous phosphenium cations, it was noted that they exhibit quite peculiar structural features and remarkable chemical behavior on their own. In this Account, we discuss both structure and chemistry in connection with the special bonding situation that is characterized by significant n(N)-σ*(P-X) hyperconjugation and induces a strong ionic polarization of the P-X bonds. This induced polarization is surprisingly maintained even when P and X have similar or like electronegativities (for example, X = H, P) and offers the possibility to design compounds with polarized homonuclear bonds. An exemption from the general pattern was only noted for some P-amino-NHPs in which reverse hyperconjugation weakens endocyclic P-N bonds and was predicted to facilitate ring fragmentation and formation of phosphinidenes. An important corollary of the P-X bond polarization in NHPs is a unique bond lengthening, which not only can be fine-tuned by adjusting intramolecular steric and electronic interactions but also responds to intermolecular influences and solvent effects. Insight from crystallographic, spectroscopic, and computational studies allows the development of concepts for controlled manipulation of the bonding, up to a point where P-X bonds are dominated by electrostatic interactions and species behave as Lewis pairs rather than covalent molecules. An appealing aspect lies in the fact that this P-X bond polarization induces reactivities that have hardly any precedence in phosphine chemistry. Examples include (i) reactions of P-hydrogen-substituted NHPs as hydride transfer reagents, (ii) metatheses and additions to multiple bonds (diphosphination) of phosphino-NHPs, which can be used to catalyze P-C cross-coupling reactions and to synthesize 1,2-bisphosphine ligands, (iii) cyclopentadienyl (Cp) transfer reactions of P-Cp-NHPs, and (iv) metal insertion into the P-X bonds of P-halogeno-NHPs. In many aspects, these reactions have potentially useful mechanistic or synthetic implications, and their future exploitation might help to convert NHPs from academically interesting species into useful reagents.