Tuning the Electronic Properties of Main-Group Species by N-Heterocyclic Vinyl (NHV) Scaffolds

Molecules and materials with easily tunable electronic structures and properties are at the forefront of contemporary research. pi-Conjugation is fundamental in organic chemistry and plays a key role in the design of molecular materials. In this Account, we showcase the applicability of N-heterocyclic vinyl (NI-IV) substituents based on classical N-heterocyclic carbenes (NHCs) for tuning the structure, properties, and stability of main-group species (E) via pi-conjugation and/or pi-donation. NHVs such as [(NHC)=CR] (R = H or aryl) are monoanionic ligands formally derived by the deprotonation of N-heterocyclic olefins (NHOs), (NHC)=CHR. Further deprotonation of [(NHC)=CR] (R = H) is viable, giving rise to N-heterocyclic vinylidene (NHVD) species such as (NHC)=C. NHVs and NHVDs feature a highly polarizable exocyclic C-NHC =C bond because of the presence of adjacent pi-donor nitrogen atoms. The nature of the NHC, in particular the pi-acceptor property, has a direct consequence on the polarity of the C-NHC =C bond and hence on the magnitude of pi-conjugation in the derived molecules. Thus, the electronic structure, especially the energy and shape of frontier molecular orbitals, HOMO and LUMO, of derived species can be fine-tuned by a judicious choice of the carbene unit. For instance, the HOMO of classical diphosphenes (RP=PR) (R = alkyl or aryl) is invariably the phosphorus lone-pair orbital, while the P=P pi-bond is HOMO - 1 or HOMO - 2. In strong contrast, the HOMO of divinyldiphosphenes (R = NHV) is mainly the P=P pi-bond. This is owing to the pi-conjugation, resulting in the lowering of the LUMO and raising of the HOMO energy. They have a remarkably small HOMO-LUMO energy gap (4.15-4.50 eV) and readily undergo le-oxidations, giving rise to stable radical cations and dications. By employing a similar approach, one can access divinyldiarsenes and the corresponding radical cations and dications as crystalline solids. The use of divinyldiphosphenes and divinyldiarsenes as promising ligands in the stabilization of metalloradicals has been shown. By a logical selection of singlet carbenes, stable 2-phosha-1,3-butadiene and 2-arsa-1,3-butadiene compounds, as well as related radical cations and dications, can be prepared as crystalline solids. The relevance of NHV ligands as potent pi-donors has been demonstrated for the stabilization of elusive electrophilic phosphinidene and arsinidene complexes {(NHV)E}Fe(CO)(4) (E = P or As). Moreover, stable singlet diradicaloid [(NHC)CP](2) and p-quinodimethane derivatives [(NHC)CP2](2) based on an NHVD framework are accessible as stable solids. In this Account, a special emphasis is given to the contributions from this laboratory. The author hopes that this Account will serve as a useful reference guide for researchers interested in studying and applying NHV and NHVD scaffolds in modern molecular chemistry and materials sciences.

Automated summary

This paper discusses the tuning of structure and properties of main-group species (E) by controlling pi-conjugation and/or pi-donation using classical N-heterocyclic vinyl (NI-IV) ligands. By selecting appropriate carbene units, the electronic structure of derived species can be precisely controlled, resulting in stable solid materials.