jp6b12232_si_003.mpg (3.4 MB)
A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB12– Clusters (M = Co, Rh, Ir)
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posted on 2017-03-31, 00:00 authored by Ranajit Saha, Susmita Kar, Sudip Pan, Gerardo Martínez-Guajardo, Gabriel Merino, Pratim K. ChattarajStrong
binding of carbon monoxide (CO) and dinitrogen (N2) by
MB12– (M = Co, Rh, Ir) clusters
results in a spinning umbrella-like structure. For OCMB12– and NNMB12– complexes,
the bond dissociation energy values range within 50.3–67.7
kcal/mol and 25.9–35.7 kcal/mol, respectively, with the maximum
value obtained in Ir followed by that in Co and Rh analogues. COMB12– complex is significantly less stable
than the corresponding C-side bonded isomer. The associated dissociation
processes for OCMB12– and NNMB12– into CO or N2 and MB12– are highly endergonic in nature at 298 K, implying
their high thermochemical stability with respect to dissociation.
In OCMB12– and NNMB12– complexes, the C–O and N–N bonds are
found to be elongated by 0.022–0.035 Å along with a large
red-shift in the corresponding stretching frequencies, highlighting
the occurrence of bond activation therein toward further reactivity
due to complexation. The obtained red-shift is explained by the dominance
of L←M π-back-donation (L = CO, OC, NN) over L→M
σ-donation. The binding of L enhances the energy barrier for
the rotation of the inner B3 unit within the outer B9 ring by 0.4–1.8 kcal/mol, which can be explained by
a reduction in the distance of the longest bond between inner B3 and outer B9 rings upon complexation. A good correlation
is found between the change in rotational barrier relative to that
in MB12– and the energy associated with
the L→M σ-donation. Born–Oppenheimer molecular
dynamics simulations further support that the M-L bonds in the studied
systems are kinetically stable enough to retain the original forms
during the internal rotation of inner B3 unit.