Electronic Structure Trends in N‐Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHCTransition‐Metal Bond Properties

Abstract

Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the σ-lone pair at Ccarbene and the π-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the pπ population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metalNHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHCgold and the NHCtitanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.

Estimating π binding energy of N‐Heterocyclic carbenes: The role of polarization

“…In agreement with the literature, no descriptor could be found for the total Δ E π energy. Interestingly, the donation energy correlates ( R 2 = 0.913) with the π ‐HOMO eigenvalue of the bare NHC (see Fig.…”

“…Different strategies have been used in the literature for estimating separately these two contributions, as the deletion of selected orbitals, natural bond orbital (NBO) analysis, or the extended transition state (ETS) method for energy decomposition analysis (EDA) combined with the natural orbitals for chemical valence (NOCV) theory. Even if these procedures have significantly increased our understanding of the π donor–acceptor interactions, the interpretation of the results is hindered by the limitations of these approaches and the complexity of the systems under study, which prevent to associate accurately each energetic term to a single contribution (vide infra).…”

“…Most of the research published regarding the π properties of NHCs has been devoted to imidazolylidenes and pyrimidinylidenes. Tuning the π properties of NHCs by manipulating the position of the heteroatoms in the NHCs has yet to be considered in literature, even though, together with the exocyclic substituents, it has been observed to play a major role on these properties …”

Generating and Trapping Metalla‐N‐Heterocyclic Carbenes

“…Zuschriften complexes 5a, 5b and 6,r espectively,a nd on their corresponding metalla-azolium cations [MNHC-H] + .A ss hown in Table 1, these MNHCs are singlet in the ground state,but the singlet-triplet gap is much smaller than for classical NHCs (imidazol-2-ylidenes;T able 1e ntry 1) [13] or abnormal NHCs (entry 2) [13a,b] and is similar to the acyclicalkyl amino carbenes described by Bertrand and co-workers (26 kcal mol À1 ). [14] Curiously,t he average C carbene ÀNb ond lengths in 7a, 7b, (Table 1).…”

Activation of boryl-, borylene and metalloborylene complexes by isonitriles

“…DFT calculations on the [:C(NtBu)B(IMe)tBu] fragment describe it as a ground-state singlet, with a substantially smaller singlet-triplet gap (+49.1 kcal mol À1 ) than typical NHCs (e.g., IMe = 84.8 kcal mol À1 ), 39 but similar to the singlet-triplet gaps of cyclic (alkyl)(amino)carbenes (CAACs, +45 kcal mol À1 ). DFT calculations on the [:C(NtBu)B(IMe)tBu] fragment describe it as a ground-state singlet, with a substantially smaller singlet-triplet gap (+49.1 kcal mol À1 ) than typical NHCs (e.g., IMe = 84.8 kcal mol À1 ), 39 but similar to the singlet-triplet gaps of cyclic (alkyl)(amino)carbenes (CAACs, +45 kcal mol À1 ).…”