Formation of Mixed Metal Cu3-xZnx(btc)2 Frameworks with Different Zinc Contents: Incorporation of Zn2+ into the Metal-Organic Framework Structure as Studied by Solid-State NMR

Incorporation of Zn2+ into the Cu-3(btc)(2) metal organic framework (HKUST-1) was successfully done with Zn2+ replacing Cu2+ in the paddle wheel unit up to 21% zinc. Detailed spectroscopic characterization of the resulting binuclear Cu-Zn paddle wheel units was carried out by H-1 and C-13 solid-state nuclear magnetic resonance (SSNMR). By intensity analysis of the NMR signals a quantitative determination of the Zn2+ content incorporated into the Cu-3(btc)(2) framework is possible. X-ray and EPR data verify the analysis. Surface areas were determined for the different samples that indicate a decreasing porosity with increasing zinc content. Water molecules coordinated to the zinc atoms remain in the structure even after evacuation. Their assignment was also confirmed by calculation of the pseudocontact or dipolar shift contribution from all unpaired electrons of surrounding paddle wheels.