Mesoporous silica–supported metalloSalen and metalloSalphen: synthesis and application in heterogeneous catalysis

Salen and Salphen ligands have attracted important attention due to their easy synthesis and ability to form stable complexes with transition metals for main application in catalysis. MetalloSalen and metalloSalphen complexes exhibited good catalytic activity for many chemical transformations. But they suffer from some limitations including difficult separation from the reaction mixture and non-reusability. Immobilizing these complexes into solid supports could solve such issues and afford efficient and recyclable heterogeneous catalysts. Mesoporous silica materials (MSMs), such as SBA-15, MCM-41, and MCM-48, are ideal support materials due to their important features, such as large surface areas, highly ordered nanostructure, narrow pore size distribution, tunable porosity, easy to functionalize, and high chemical and hydrothermal stability. These properties allowed high dispersion and better catalytic activity of the reported heterogeneous catalysts. This review article provides an overview about the synthesis of metalloSalen/Salphen complexes, their immobilization into MSMs, and their utilization as heterogeneous catalysts for different types of reactions such as the oxidation and hydrogenation of hydrocarbons. Different techniques to incorporate these complexes into MSMs have been discussed, including grafting, co-condensation, periodic mesoporous organosilicas (PMOs), and coordination-assisted grafting. The developed heterogeneous catalysts combine the benefits of metalloSalen/Salphen complexes as efficient homogeneous catalysts and the advantageous features of MSMs as support. Compared to their homogeneous analogues, the MSM-supported complexes are recyclable and often exhibit higher catalytic activity due to the high active site’s distribution.