Figure 2 Illustrates the relationship between C2-S/Se5 WBI and ∆E2 amongst different NHC systems
Figure 2 illustrates how the variation in the WBI values of C2-S/Se5 bond affects the corresponding ∆E2 values. For systems (IH, ICN, IOH, IOMe, BH, BMe, TH and TMe) where the WBI values lies in the range of 0.8-1, the ∆E2 values are markedly high in comparison with the other systems, where WBI < 0.2 (IMe, INH2, ISH and ISMe). It was also interesting to note that NHC catalyzed pathways with electron donating N,N-disubstitution proceeded through INT’ and TS2 with much reduced C2-S/Se5 interaction and have lowered energy barriers as compared to parent system (Table 2, Supplementary information). However with electron withdrawing N,N-disubstitution case, the pathway proceeds through INT and TS2 with C2-S/Se5 interaction, characterized by increased ∆E values (Table 2, Supplementary information). Since the presence of HCCH does not affect the C2-S/Se5 interactions in intermediates, a detailed study on the potential energy surface of protonated carbene-thiolate (or selenolate) complex alone will be sufficient to shed light on the reason for this interaction changes.
Different possible orientations of thiolate/selenolate around protonated NHCs were characterized and the outcome suggested the possibility of three conformations (Figure 3) out of which one lacked C2-S/Se5 interaction (conformer III). Conformer study was done on complexes of thiolate/selenolate with IH, IMe, INH2, ICN, IMes, IOH, IOMe, ISH, ISMe, BH, BMe, TH and PH. Conformers I and II were computed for all the studied systems while conformer III was possible for complexes of IMe, INH2, IMes, ISH, ISMe, BMe. In the case of hydro- thiolation and selenation conformers I and II have energies differing by less than 1 kcal/mol. In comparison with parent NHC system (IH), N,N-disubstitution with electron-withdrawing substituents (ICN) tend to enhance the C2-S/Se5 interaction (evident from WBI values and pyramidalization of carbene carbon; see supplementary information for details), while electron-donating substituents (IMe, INH2) weaken it. This result substantiates the observed geometrical differences. Similar to ICN, other systems like IOH, IOMe, ISH, ISMe, BH, BMe, TH and PH systems show almost close or enhanced C2-S/Se5 in comparison to IH (Table, Supplementary information). Thus it can be concluded that presence of hetero atom, ring expansion and conjugation also has the effect of enhancing the C2-S/Se5 interaction in addition to electron withdrawing N,N-disubstitution.