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.