Pyrrolyl–Silicon Compounds as Precursors for
Donor–Acceptor Systems Stabilized by Noncovalent Interactions
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Abstract
Pyrrolyl–silicon compounds
were investigated by different
theoretical approaches. Model monomers consisted of a pyrrole ring
N-substituted with silylmethoxy and silylhydroxy end groups through
a propyl chain spacer, designated as PySi and PySiOH. Geometrical,
vibrational, and electronic properties, as well as chemical reactivity,
are discussed and compared with pyrrole (Py) and <i>N</i>-propylpyrrole (<i>N</i>-PrPy) that were studied in parallel
for reference purposes and methods validation. The electronic distribution
between PySi and PySiOH differs importantly, the former being an electron
donor, as Py and <i>N</i>-PrPy. Conversely, PySiOH presents
donor–acceptor character with the LUMO energy level localized
on the silanol end group. Global and local reactivity descriptors
predict PySiOH more reactive than PySi with two preferential reactive
sites: electron-rich Py ring and electron-deficient silanol group.
On the basis of experimental studies, oligomers of PySiOH linked α–α′
via Py rings (α–α′Py<sub><i>n</i></sub>SiOH, <i>n</i> = 2, 3) were considered as model molecules
of hydrolyzed PySi. The most stable structures were derived from randomly
generated α–α′Py<sub><i>n</i></sub>SiOH that were optimized at semiempirical AM1 and refined with M05-2X/6-31G(d,p).
Conformational analysis of dimer and trimer structures points to stability
enhanced by molecular packing. Nonetheless, NBO and RDG results indicate
that oligomer stability is dictated by the cooperative contribution
of hydrogen bonding between silanol end groups and dispersive vdW
interactions between silanol and the π system of the Py ring.
The latter interaction resulting from electron delocalization induced
by an electron-deficient silanol group seems to determine the smaller
gap energy of T-shaped OH−π arrangements. The theoretical
findings support the peculiar chemical behavior revealed by experiment