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Computational Investigation on the Role of Disilene Substituents Toward N2O Activation
journal contribution
posted on 2016-12-07, 00:00 authored by Bholanath Maity, Debasis KoleyThe effect of substituents
in disilene mediated N2O activation was studied at the
M06-2X/QZVP//ωB97xD/TZVP level of theory. The relationship between
structural diversity and the corresponding reactivity of six disilenes
(IA–Ft) in the presence of four different substituents (−NMe2, −Cl, −Me, −SiMe3) is addressed
in this investigation. We primarily propose two plausible mechanistic
routes: Pathway I featuring disilene → silylene decomposition
followed by N2O coordination and Pathway II constituting
the N2O attack without Si–Si bond cleavage. Depending
on the fashion of N2O approach the latter route was further
differentiated into Pathway IIa and Pathway IIb detailing the “end-on”
and “side-on” attack to the disilene scaffold. Interestingly,
the lone pair containing substituents (−NMe2, −Cl,)
facilitates disilene → silylene dissociation; on the contrary
it reduces the electrophilicity at Si center in silylene, a feature
manifested with higher activation barrier during N2O attack.
In the absence of any lone-pair influence from substituents (−Me,
−SiMe3), the decomposition of disilenes is considerably
endothermic. Therefore, Pathway I appears to be the less preferred
route for both types of substituents. In Pathway IIa, the N2O moiety uniformly approaches via O-end to both the silicon centers
in disilenes. However, the calculations reveal that Pathway IIa, although
not operational for all disilenes, is unlikely to be a viable route
due to the predominantly higher transition barrier (ca. 36 kcal/mol). The most feasible route in this current study accompanying
moderately low activation barriers (∼19–26 kcal/mol)
is Pathway IIb, which involves successive addition of two N2O units proceeding via terminal N, O toward the Si centers and is
applicable for all disilenes. The reactivity of substituted disilenes
can be estimated in terms of the first activation barrier of N2O attack. Surprisingly, in Pathway IIb, the initial activation
barrier and hence the reactivity shows negligible correlation with
Si–Si bond strength, indicating toward the versatility of the
reaction route.
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