Stepwise Construction of Silica-supported Tantalum/Iridium Heteropolymetallic Catalysts Using Surface Organometallic Chemistry

Abstract A stepwise surface organometallic chemistry (SOMC) methodology consisting in using a silica-supported tantalum species, [(≡SiO)Ta(CHtBu)(CH2tBu)2], as a reactive center to coordinate an iridium site, was developed to construct tantalum/iridium heterobimetallic edifices. The resulting material, MAT-2, exhibits enhanced catalytic performances - both in H/D isotopic exchange and alkane metathesis reactions - in comparison to MAT-1, which was prepared from the direct grafting on silica of a well-defined heterobimetallic Ta/Ir complex. We projected that the difference in catalytic activity was due to the presence of distinct active sites and we used a combination of advanced spectroscopic methods (IR, solid-state NMR and XAS spectroscopies) as well as modeling (computational studies and molecular models) to identify the structure of these surface species. These investigations point towards the presence of three types of active sites at the surface of MAT-2: the heterobimetallic surface species, [≡SiOTa(CH2tBu)2{IrH2(Cp*)}] 2-s, which is also found in MAT-1, as well as an unanticipated heterotrimetallic species, [≡SiOTa(CH2tBu)2{IrH2(Cp*)}], 3-s along with some unreacted monometallic Ta sites [(≡SiO)Ta(CHtBu)(CH2tBu)2], 1-s. The well-defined trimetallic surface species 3-s was independently prepared and characterized to support this hypothesis. This study highlights the importance of the synthetic methodology used for the preparation of heterobimetallic species through SOMC, and the difficulty to obtain true single-sites surface species.