A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB12 – Clusters (M = Co, Rh, Ir)

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 121; no. 15; pp. 2971 - 2979
• Main Authors: Saha, Ranajit, Kar, Susmita, Pan, Sudip, Martínez-Guajardo, Gerardo, Merino, Gabriel, Chattaraj, Pratim K
• Format: Journal Article
• Language: English
• Published: American Chemical Society 20.04.2017
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.6b12232

Strong 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.