Limits to the heterogenization of a copper-bispidine aziridination catalyst on NU-1000

In this study, we investigated whether a tetradentate copper-bispidine complex can be heterogenized in the Zr-based metal-organic framework NU-1000. The ligand used is a rigid N-donor system based on the 3,7-diazabicyclo[3.3.1]nonane scaffold, and its copper complex is known for efficiently catalyzing the aziridination of styrene. Because different residual anions are commonly present in metal-organic frameworks, we first probed the influence of the counterion in homogeneous catalysis by synthesizing a series of copper-bispidine complexes bearing different counterions. In homogeneous
solution under an inert atmosphere, the counterion had a pronounced effect on the aziridination yields. Under air, however, oxygen oxidized the catalytically active Cu(I) species, while water/hydroxido ligation became dominant and acted as coligands, thereby diminishing the impact of the counterion. Immobilization was targeted via solvent-assisted ligand incorporation (SALI). To this end, the methyl esters on the ligand backbone were converted to carboxylates to enable binding to the Zr₆O₆ nodes of NU-1000. Immobilization of the copper complex in NU-1000 follows Langmuir behavior; however,
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powder X-ray diffraction and attenuated total reflection - Fourier transform infrared spectroscopy measurements show that, under the conditions employed, the immobilization is predominantly superficial. Significant leaching prevented reliable catalytic testing of the immobilisates. Overall, direct SALI-based loading of the copper-bispidine system onto NU-1000 proved unstable; the results identify the key limitations (surface deposition and leaching) and suggest that alternative support materials or anchoring strategies may enable robust heterogenization of this catalyst class.