Due to quantum confinement, the electronic states in metal clusters can be grouped into electronic shells. This shell structure controls the reactivity of clusters with reagents like O2. For example, silver cluster anions [Agn]- have recently been found to exhibit variable reactivity with [Ag13]- exhibiting unique stability against reactivity with oxygen. Similar behavior has been observed in the reactivity of Aln- clusters with oxygen where an Al13- cluster is found to be relatively inert. I will demonstrate that these reactivity patterns are related to the spin accommodation. The ground state of O2 is a spin triplet and the reactivity entails a spin multiplicity 3?1 excitation of the cluster. The reactivity is then correlated with the spin excitation energy and clusters with filled shells and the large separation between filled and unoccupied shells require large energy for spin excitation and exhibit reduced reactivity. While the electronic shells control the reactivity with O2, reactivity with molecules having polar covalent bonds exhibits different patterns. Experiments on the reactivity of aluminum cluster anions with water, methanol and formaldehyde show that while some clusters with open electronic shells such as Al20- do not show significant reactivity, selected clusters with closed electronic shells exhibit strong reactivity. I will show that these behaviors are rooted in the non-uniform distribution of charge densities and have a geometric origin. Clusters with non-uniform charge distributions are marked by Lewis-acid and Lewis-base complementary sites that stimulate the breaking of polar bonds.  Such pairs can break hydroxyl and even stronger C=O carbonyl bond of formaldehyde. I will show that such pairs can be created by attaching ligands offering a novel route to designer catalysts.

Spin accommodation and geometric effects in catalysis: Breaking polar covalent bonds via complementary active sites

247th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.