Changes in ligating abilities of the singlet and triplet states of normal, abnormal and remote N-heterocyclic carbenes depending on their aromaticities

SEVİNÇEK R., KARABIYIK H., KARABIYIK H.

JOURNAL OF MOLECULAR MODELING, cilt.19, sa.12, ss.5327-5341, 2013 (SCI-Expanded) identifier identifier identifier

  • Yayın Türü: Makale / Tam Makale
  • Cilt numarası: 19 Sayı: 12
  • Basım Tarihi: 2013
  • Doi Numarası: 10.1007/s00894-013-2027-1
  • Dergi Adı: JOURNAL OF MOLECULAR MODELING
  • Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Sayfa Sayıları: ss.5327-5341
  • Anahtar Kelimeler: Abnormal NHC, Aromaticity, CSD search, FLU, HOMA, N-Heterocyclic carbene (NHC), Remote NHC, Triplet state, CORRELATED MOLECULAR CALCULATIONS, CAMBRIDGE STRUCTURAL DATABASE, GAUSSIAN-BASIS SETS, ELECTRON DELOCALIZATION, STABLE CARBENES, ORGANIC-CHEMISTRY, MAXIMUM HARDNESS, NHC LIGANDS, COMPLEXES, INDEX
  • Dokuz Eylül Üniversitesi Adresli: Evet

Özet

Quantum chemical calculations at B3LYP/aug-cc-pVTZ level about singlet N-heterocyclic carbene (NHC) ligands, imidazol-2-ylidene, imidazol-4-ylidene, pyrazol-3-ylidene and pyrazol-4-ylidene, and their protonated analogues show that they are considerably aromatic except for pyrazol-3-ylidene. This result is experimentally verified by approximately five thousand NHC transition metal complexes retrieved from the Cambridge Structural Database (CSD). CSD search discloses that NHCs can participate in pi-stacking interactions, albeit scarce. Geometry-based HOMA and electronic aromaticity index FLU rather than NICS provide a satisfactory description of the bonding situations in NHC ligands. Singlet state of the normal NHC has electron-deficient aromaticity as compared to those of the abnormal and remote NHCs. Depending on the transition from the singlet to triplet state, NHCs become electron-deficient ligands except for remote NHC. Computational studies regarding electronic nature of free NHC ligands show that the pi-electronic population of the formally vacant p(pi) orbital on the carbene atoms in abnormal and remote NHC is occurred as a result of the aromaticity of NHCs, not as a result of the direct electron donation from LP-orbitals of N atoms to carbene atom according to putative push-pull effect used in understanding the electronic stabilization of normal NHC. Increase in the aromaticity raises sigma-donating ability of both imidazol-and pyrazol-based NHC ligands. Free abnormal and remote NHC ligands have higher sigma-donation ability than normal NHC ligands. The lack of sigma-donating ability of normal NHC is compensated by its relatively high pi-accepting ability, whereas pi-back donation abilities of abnormal and remote NHCs are prohibited by their almost fully occupied pi-orbitals. Aromaticities of the triplet NHC ligands are higher than that of the lowest-lying triplet state of benzene. Increase in the aromaticity of NHC ligands decreases van der Waals shortening in TM-NHC bonds mainly due to diminishing dative character of these bonds.