Synthesis of Iron Catalysts Confined in Mesoporous Silica

Abstract

Mesoporous materials as catalyst support are promising due to their exceptional properties (e.g. high surface area, large/tunable pore structures). High specific surface areas covered with free silanol groups allow introduction of high quantities of organic functionalities through grafting; these functionalities can be used to create metal active sites for catalytic purposes. The present study was dedicated to the investigation of heterogeneous mesoporous supported propyl-urea Fe(III) active sites for the Mizoroki-Heck reaction. Little reports have been made about iron catalyzed Mizoroki-Heck couplings; the activity of iron for this reaction has only been reported for a few cases. MCF demonstrated to be a suitable support material for these active sites; however, no significant catalytic activity was observed for the Mizoroki-Heck reaction using the synthesized catalyst. Moreover, base-catalyzed Mizoroki-Heck coupling was observed by the use of KOtBu.

The use of nanoporous materials such as cage-like modified mesocellular foams (m-MCFs) has shown to be interesting for the development of nanoconfined metal complexes due to their controllable large cavities and small entrance sizes. In this study, successful control of structural properties of m-MCFs was demonstrated by application of different temperatures during hydrothermal treatments. 3D cage-like structures were obtained with variations in cavity sizes (16 – 21 nm) and total pore volumes (0.65 – 0.96 cm3) together with a successful decrease in entrance sizes (down to 2.0 nm); further decrease of entrance sizes was allowed by n-propyl functionalization. These properties were exploited for the entrapment of the Fe(III)-salen complex through a ship in a bottle synthesis. The highest loading of Fe(III)-salen corresponding to 20.8 complexes/100 nm3 was observed in m-MCF with average entrance sizes of 0.7 nm. The nanoconfined Fe(III)-salen complex showed lower activity and stereoselectivity for asymmetric thioether oxidations than its homogeneous counterpart. The obtained heterogeneous m-MCF confined Fe(III)-salen complex could be successfully recovered via centrifugation and recycled without significant loss in stereoselectivity.