On the Nature of the Bonding of Lone Pair Ligands to Small Metal Clusters

Density functional study of molecular nitrogen adsorption on gold-copper and gold-silver binary clusters

Density functional theory calculations were performed to investigate the adsorption behaviors of nitrogen molecule on small bimetallic AunCum and AunAgm clusters, with n + m ≤ 5. In all cases the N2 forms a linear or quasi-linear M-N-N structure (M = Au, Cu or Ag). The adsorption energies of N2 on pure metal clusters follow the order CunN2 > AunN2 > AgnN2, which is due to the weaker orbital interaction between silver and N2. N2 prefers to bind to a copper atom in AunCumN2 complexes and prefers to bind to a silver atom in AunAgmN2 complexes. The combination of Cu atoms into Aun clusters makes the cluster more reactive toward N2 while the combination of Ag atoms into Aun clusters makes the cluster less reactive toward N2. The electrostatic interaction is strengthened while the back-donation from metal to N2 is reduced in bimetallic cluster nitrides, as compared to the mono cluster nitrides. The N-N stretching frequencies are all red-shifted upon adsorption and the M-N stretching frequencies are highly correlated to the atoms to which the N is attached.

The nature of the supramolecular association of 1,2,5-chalcogenadiazoles

Organochalcogen-nitrogen heterocycles such as the 1,2,5-chalcogenadiazoles have a distinct tendency to establish intermolecular links in the solid state through secondary bonding interactions E...N (E = S, Se, Te). The association of these molecules was examined in detail using relativistic density functional theory. Although there is an important electrostatic component, the interaction between these molecules is dominated by contributions arising from orbital mixing, which can be interpreted as the donation of a nitrogen lone pair into the chalcogen-centered antibonding orbitals. Because of its more polar character and lower-lying antibonding orbitals, the tellurium derivatives possess the strongest association energies; these are so large that the binding strength is comparable to that of some hydrogen bonds. In the absence of steric constraints, telluradiazoles associate in a coplanar fashion forming ribbon polymers. However, bulky susbstituents could be used to direct the formation of either helical chains or discrete dimers. In addition to its strength, the coplanar dimer is characterized by being rigid, yet no activation barrier is expected for the association/dissociation process. These attributes strongly indicate that tellurium-nitrogen heterocycles have great potential as building blocks in supramolecular architecture.