Amorphous silica as a versatile supermolecular ligand for NiII amine complexes:: Toward interfacial molecular recognition

Selective adsorption of Ni-II amine complexes used as precursors for supported catalysts was studied on amorphous silica surfaces. The nature of the adsorption sites was probed by [Ni(en)(dien)(H2O)](2+), [Ni(en)(2)(H2O)(2)](2+) and [Ni(dien) (H2O)(2)](3+) (en=ethylene-diamine, dien=diethylenetriamine), which respectively contain one, two, and three labile aqua ligands. The silica surface acts as a mono- or polydenate ligand that can substitute the aqua ligands of the Ni-II complexes in an inner-sphere adsorption mechanism. Room-temperature adsorption isotherms indicate that each nickel complex selects a limited number of adsorption sites; different sites are recognised by the three complexes, even though they have the same charge and compatible sizes. Several spectroscopic techniques (UV/Vis/NIR, EXAFS, and Si-29 NMR) were used to confirm the selective character of the interaction of Ni-II amine complexes with silica surface. The specific sites include both silanol/silanolate groups in the same number as the original labile ligands and other surface groups that probably act as hydrogen-bond acceptors. These two types of groups cooperate to result in interfacial molecular-recognition phenomena with interactional complemenatary.