Zr vs. Ti molecular and silica-supported imido complexes in stoichiometric and catalytic oxo/imido heterometathesis

Abstract

Surface Organometallic Chemistry (SOMC) aims at selective preparation of unusual or highly reactive species on the surfaces of inorganic supports in order to reach a better understanding of classical heterogeneous catalysts and discover new reactivities and catalytic transformations. SOMC relies on the application of both surface science techniques and molecular organometallic chemistry; in particular it requires rational design of the molecular precursors to ensure their selective reactivity with the supports. In this report we apply the SOMC protocols to prepare well-defined silica-supported Zr imido complexes. We describe the synthesis of Zr imido pyrrolyl complexes Zr(NR)(Me₂Pyr)₂(py)₂ (R = tBu (1a), Ar (1b); Ar = 2,6-iPr₂C₆H₃, Me₂Pyr = 2,5-dimethylpyrrolyl, py = pyridine), investigate their grafting onto the surface of partially dehydroxylated silica and evaluate the obtained materials as oxo/imido heterometathesis catalysts in comparison with the previously reported highly active supported Ti systems. We also investigate the stoichiometric oxo/imido exchange reactivity of Zr imides using Ph₂CO as a probe molecule, compare them with isostructural Ti analogs Ti(NR)(Me₂Pyr)₂(py)₂ (R = tBu (2a), Ar (2b)), and discuss how these results correlate with the differences in their electronic structure and catalytic performance of silica-supported species.