Cooperative Effect of Monopodal Silica-Supported Niobium Complex Pairs Enhancing Catalytic Cyclic Carbonate Production
Recent discoveries highlighted the activity, and the intrig-uing, mechanistic features of NIDCl5 as a molecular catalyst for the cycloaddffion, of CO2 and epoxides under ambient cooditions. This hag inSpired the preparation, of novel silica-supported Nb species by reacting a molecular niobium precursor, [NbCf5 center dot OEt2], with Silica dehydroxylated at 700 degrees C (SiO2-700) or at 200 degrees C (SiO2-200) to generate diversetrface complexes. The product of the reaction between SiO2-700 and [NbCl5 center dot OEt2] was identified as a monopodal supported surface species, [equivalent to SiONbCl4 center dot OBt(2)] (1a). The reactions, of SiO2-200 with the niobium precursor, according to two different protocols, generated surface complexes 2a. and 3a, presenting significant, but different,, populations of the monopodal surface complex along with bipodal [(equivalent to SiO)(2)NbCl3 center dot OEt2]. Nb-93 solid-state NMR spectra of 1a-3a and P-31 solid-state NMR On their PMe3 derivatives 1b-3b led to the unambiguous assignment of 1a as a single-site monopodal Nb species, while 2a and 3a were found to present two distinct surface-supported components, with 2a being mostly monopodal [equivalent to SiONbCl4 center dot OEt2] and 3a being mostly bipodal [(equivalent to SiO)(2)NbCl3 center dot OEt2]. A double-quantum/single-quantum 3113 NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface- for this species. 1a-3a were active heterogeneous catalysts for the synthesis of propylene carbonate from CO, and propylene Oxide under mild catalytic conditions,: the performance of 2a was found to significantly surpass that of la and 3a. With the support of a syStetnatic DFT study Carried' out model silica surfaces, the observed differences in catalytic efficiency were correlated with an unprecedented cooperative effect between two neighboring Nb centers on the surface of 2a. This is in an excellent agreement with our previbils discoveries regarding the mechanism Of NbCl5-catalyzed cycloaddition in the homogeneous phase.