Immobilization of (NHC)NN-Pincer Complexes on Mesoporous MCM-41 Support

Unsymmetrical pincer-type-ligated {pincer = [C6H3N(CH2L1)(CH2L2)-2,6], L-1 = prolinamide, L-2 = NHC} gold and rhodium complexes have proven to be highly effective catalysts for the hydrogenation of alkenes. Immobilization on ordered mesoporous silica (MCM-41) using a grafting process offers significant potential advantages in the application of such catalysts particularly with respect to catalyst separation and recycling. We describe one approach toward such immobilization: covalent bonding to silica via a pendant alkoxysilane group. This approach yields catalysts that are robust, recyclable, and comparable to or even more active than the corresponding species in solution. Spectroscopic evidence (IR spectroscopy, solid-state CP/MAS NMR, SEM), elemental analysis, and studies of catalytic activity support the hypothesis that binding occurs at the prolinamide substituent with no complex degradation. Control experiments showed the true heterogeneous nature of the catalyst in this reaction. Analyses of the hybrid materials revealed that the mesoporous structure of these materials was retained during the immobilization process as well as during catalysis.