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

Abstract

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.