Bond Dissociation Energies of Carbene–Carbene and Carbene–Main Group Adducts

Abstract

A range of carbene structures and their adducts with one another and with a selection of small-molecule electrophiles and nucleophiles were examined at the composite correlated molecular orbital theory G3MP2 level to explore ground-state "carbenic" structures, their stabilities, and reactivities. Differences between carbene general classification as a singlet electrophilic carbene or singlet nucleophilic carbene and their given reactivity are discussed. A key quantity is the carbon−carbon bond dissociation energy for carbene dimers or the carbene-adduct dissociation energy for other species. The carbene dimer bond dissociation energies span a wide range from 10 to 170 kcal/mol. The hydrogenation energies and singlet−triplet splitting were found to correlate best with the carbene's self-dimerization energy, whereas other descriptors do not. The proton and fluoride affinities of the carbenes alone prove inadequate for classifying reactivity among classes of carbenes. The self-dimerization bond dissociation energy, hydrogenation energy, and singlet−triplet splitting of various carbenes, despite sometimes large differences in proton affinity and other indicators of reactivity, provide usable metrics to correlate substantial amounts of thermodynamic and kinetic (reactivity) information regarding these structures.

Chalcogen-Substituted Carbenes: A Density Functional Study of Structure, Stability, and Donor Ability

“…There have been many experimental and theoretical studies aimed at quantifying salient properties and finding useful descriptors of NHCs, including donor ability by Tolman electronic parameters (TEPs) 18,19 and 77 Se NMR of selenourea derivatives, 20,21 steric encumbrance via percent buried volume, 22 and stability and reactivity by calculation of singlet-triplet gaps 23,24 and dimerization energy. 25 The carbene dimerization mechanism has also been modeled computationally.…”

“…27,38 The smaller the singlet-triplet gap, the stronger the C=C bond in the resulting carbene dimer, and hence the more thermodynamically favoured dimerization will be. 23,24 In the context of the chalcogensubstituted NEHC carbenes, given their small singlet-triplet gaps, the direct uncatalyzed dimerization process is possible. Arduengo and coworkers reported a stable thiazolylidene (featuring methylation on the backbone and a Dipp substituent on nitrogen) which formed an anti-configuration dimer when catalyzed by the protonated thiazolium salt.…”

Chalcogen-substituted carbenes: a density functional study of structure, stability, and donor ability

“…27,42 The smaller the singlet-triplet gap, the stronger the C]C bond in the resulting carbene dimer, and hence the more thermodynamically favoured dimerization will be. 23,24 In the context of the chalcogen-substituted NEHC carbenes, given their small singlet-triplet gaps, the direct uncatalyzed dimerization process is possible. Arduengo and coworkers reported a stable thiazolylidene (featuring methylation on the backbone and a Dipp substituent on nitrogen) which formed an anti-conguration dimer when catalyzed by the protonated thiazolium salt.…”

“…There have been many experimental and theoretical studies aimed at quantifying salient properties and nding useful descriptors of NHCs, including donor ability by Tolman electronic parameters (TEPs) 18,19 and 77 Se NMR of selenourea derivatives, 20,21 steric encumbrance via percent buried volume, 22 and stability and reactivity by calculation of singlet-triplet gaps 23,24 and dimerization energy. 25 The carbene dimerization mechanism has also been modeled computationally, [26][27][28] and energies of isodesmic reactions between carbenes and methane have been correlated to the carbene dimerization energies.…”

Halogen bonds with carbenes acting as Lewis base units: complexes of imidazol-2-ylidene: theoretical analysis and experimental evidence

“…12 The role of NHCs in numerous processes of organic chemistry was discussed in earlier 13 and more recent studies. 14 However attention is paid in this study to the carbene species that are linked with Lewis acid centres through halogen bonds. Several studies related to the latter topic may be mentioned here; the study on the stable carbene iodine adduct since the halogen bond in the 1,3-diethyl-2-iodo-4,4-dimethylimidazolium iodide crystal structure was analyzed, 15 the theoretical calculations were performed on simple complexes linked through halogen bonds, [16][17][18] the hydrogen bond and halogen bond interactions for singlet and triplet diphenylcarbenes were discussed on the basis of high level DLPO-CCSD(T) calculations, 19 and the halogen bond interaction in complexes of anionic N-heterocyclic carbenes was analyzed in a recent study.…”