Direct Observation of Molecular-Level Template Action Leading to Self-Assembly of a Porous Framework

The molecular steps involved in the self-assembly of Cu-3(BTC)(2) (BTC = 1,3,5-benzenetricarboxylic acid) metal organic frameworks that enclose Keggin-type H3PW12O40 heteropolyacid molecules were unraveled by using solution O-17, P-31, and W-183 NMR spectroscopy, small-angle X-ray scattering, near-IR spectroscopy, and dynamic light scattering. In aqueous solution, complexation of Cu2+ ions with Keggin-type heteropolyacids was observed. Cu2+ ions are arranged around the Keggin structure so that linking through benzenetricarboxylate groups results in the formation of the Cu-3(BTC)(2) MOF structure HKUST-1. This is a unique instance in which a templating mechanism that relies on specific molecular-level matching and leads to explicit nanoscale building units can be observed in situ during formation of the synthetic nanoporous material.