&Metal–Organic Frameworks A Boiling-Water-Stable, Tunable White-Emitting Metal–Organic Framework from Soft-Imprint Synthesis

A new avenue for making porous frameworks has been developed by borrowing an idea from molecularly imprinted polymers (MIPs). In lieu of the small molecules commonly used as templates in MIPs, soft metal components, such as CuI, are used to orient the molecular linker and to leverage the formation of the network. Specifically, a linear dicarboxylate linker with thioether side groups reacted simultaneously with Ln ions and CuI, leading to a bimetallic net featuring strong, chemically hard Eu–carboxylate links, as well as soft, thioetherbound Cu2I2 clusters. The CuI block imparts water stability to the host; with the tunable luminescence from the lanthanide ions, this creates the first white-emitting MOF that is stable in boiling water. The Cu2I2 block also readily reacts with H2S, and enables sensitive colorimetric detection while the host net remains intact. Templating is a tried-and-true strategy of wide use in materials synthesis, with examples including molecularly imprinted polymers (MIP) and the organic template-directed syntheses of zeolitic materials. In the former, the small-molecule template is intended as a mold to orient the functional groups on the growing polymer, in order to impart enzyme-like recognition properties to the (subsequently evacuated) polymer host; the supple nature of the polymer backbone could, however, compromise the structural memory. By comparison, in the very rigid zeolitic grid, the pore size and geometry templated by the organic molecule are often well retained, even when the organic templates have been burned away. The common, long-standing practice of templating, is, however, conspicuously under-explored in the assembly of metal– organic frameworks, in which the use of rigid, open-shaped molecular linkers, in combination with metal cluster nodes, serves to impose the porous characters of the resultant solid grid. In other words, the use of templates to purposely direct structure and create cavities appears to be unnecessary, and is therefore often not factored into the synthetic design of MOF materials. Among the scarce examples of deliberately using templates to influence the framework formation, the template acts either as an auxiliary ligand bound to the metal center or simply as a space holder without well-defined contacts with the framework atoms. Instead of closely molding the shape and function of the cavities, as in MIPs, templates generally do not leave distinct imprints on the MOF grid. We therefore suspect that there is room for further deploying the templating strategy in the synthesis of metal–organic frameworks. In this work, we envision the template in the form of secondary, soft metal ions that stand apart from the primary (often hard) metal ions incorporated within the host net. This thought traces back to reported works on hard and soft linkers for MOF construction, as exemplified by the carboxylate–sulfur combination. Previously, we reported the simultaneous reaction of a bifunctional molecule with two distinct metal species: one chemically hard metal (e.g. , Eu) for interconnecting the carboxyl ends; the other, chemically soft (e.g. , AgCl), intended for the thioether units. The soft metal, being bound by the weaker thioether donors, plays a lesser role in determining the structural integrity of the Eu–carboxylate host net. For example, the soft metal can be dislodged by treatment with H2S to form Ag2S particles, without collapsing the primary metal–carboxylate scaffold. In general, such treatment represents a unique method for imbedding semiconducting metal chalcogenide nanostructures within well-defined MOF hosts, a method that also chimes with the popular practice of postsynthetic modification (PSM). 9] We describe herein an exercise in MOF synthesis that further highlights the advantages of this strategy of soft imprinting. Unlike our previous work, in which the soft metal did not affect the connectivity of the metal–carboxylate host net, the soft part (i.e. , CuI) in the present case acts as a structural modulator that opens up the framework—a role that more closely connects to the templating strategy in molecularly imprinted polymers. The experiment involved the reaction of the thioether–carboxylate ligand H2L (see the Supporting Information for details of the synthesis) with lanthanide ions (e.g. , Eu) while [a] Prof. J. He, J. Huang, Y. He, P. Cao School of Chemical Engineering and Light Industry Guangdong University of Technology Guangzhou 510006, Guangdong (P.R. China) E-mail : [email protected] [b] Dr. M. Zeller, Dr. A. D. Hunter Department of Chemistry, Youngstown State University One University Plaza, Youngstown, OH 44555 (USA) [c] Prof. Z. Xu Department of Biology and Chemistry, City University of Hong Kong 83 Tat Chee Avenue, Kowloon, Hong Kong (P.R. China) E-mail : [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201504941. CCDC 1421986 and 1422000 contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre. Chem. Eur. J. 2016, 22, 1597 – 1601 Ó 2016 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1597 Communication DOI: 10.1002/chem.201504941