The intercalation of zinc(II)-1,10-phenanthroline complexes in montmorillonite: An experimental and theoretical approach

Zinc(II)-phenanthroline complexes are widely used as building blocks to prepare solid-phase chiral catalysts. Adsorbing these complexes on montmorillonite - a green solid support with large external and interlayer surfaces - with the possibility of modulating the structure and the immobilization geometry would allow controlling the reactivity towards the substrate. Here, a series of [ZnPhenx]2+ complexes prepared by varying the phenanthroline/Zn2+ molar ratio in solution were adsorbed onto montmorillonite with the aim to grasp relationships between solution composition and the speciation and structure of the adsorbed complexes. The solid samples were characterized by elemental and chemical analysis, X-rays diffraction, thermogravimetric analysis, and NMR measurements. Atomic scale calculations based on Density Functional Theory (DFT) were also performed to define the structuring of the montmorillonite interlayer in the presence of different intercalated [ZnPhenx]2+ complexes. It was found that [ZnPhenx]2+ complexes were intercalated in a fashion which depends on the composition of the starting zinc(II)-phenanthroline solutions, without being its mirror. Properly tuning the phenanthroline/Zn2+ molar ratio in solution, however, the [ZnPhen]2+, [ZnPhen2]2+ and [ZnPhen3]2+ species and their mixtures were immobilized on montmorillonite in a predictable and reproducible way. This has been achieved through a careful control of the immobilization conditions of the complexes and characterization of the resulting materials. The interlayer structure was also characterized with a modelling approach. This work outlines the procedure to obtain the desired catalytic ZnII-montmorillonite hybrid materials useful as nanosized reaction environments from a starting solution containing several different species in equilibrium. This is a valuable tool for obtaining tailored "green" catalysts, as ZnII complexes have been proven to be effective for a wide variety of organic and polymerization reactions.