Reticular Chemistry-Construction, Properties, and Precision Reactions of Frameworks

摘要

Reticular chemistry, the chemistry of linking molecular building blocks by strong bonds to make crystalline open frameworks, has significantly expanded the scope of chemical compounds and useful materials. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) exemplify the manner in which this chemistry is practiced and epitomize the molecular-level control being exercised over matter. Typically, MOFs are constructed from metal-oxide units joined by organic linkers through strong metal-carboxyl bond linkages, where it is possible to widely vary the combination of metal ions and organic linkers to achieve the desired framework composition and structure. COFs are constructed from organic building units to make two- and three-dimensional crystalline open organic frameworks with backbone structures that are entirely composed of light elements (e.g., B, C, N, O) and joined throughout by strong covalent bonds. The early work leading to MOFs and COFs required that the linking reactions be designed to overcome the "crystallization problem" and thus give crystalline products whose structure can be definitively characterized using X-ray and electron diffraction techniques. MOFs and COFs would have been considered as only beautiful "sculptures" with nothing else to offer, in terms of properties and reactivity, had it not been for another early advance, namely, that pertaining to their architectural stability and proof of their permanent porosity (i.e., demonstration of gases moving in and out of the pores without deformation or destruction of the framework). The gold standard to show this property is measurement of the N_(2(g)) or Ar_((g)) adsorption isotherm at 77 K, as originally reported for MOF-2 and MOF-5. The crystallinity of these MOF/COF compounds and their permanent porosity were the properties needed to develop this chemistry beyond mere beauty. Today, members of the MOF and COF compound families have been designed and made to have ultrahigh porosity (500-10 000 m~2/g), high thermal stability (300-500 ℃), and exceptional chemical stability in organic and aqueous media, acids, and bases. In the long line of progress in using covalent chemistry to build organic molecules and inorganic chemistry to build metal complexes, simple and complex alike, the chemistry of MOFs and COFs extends this precision molecular chemistry to frameworks. This framework chemistry would not be possible if not for the fact that metal complexation and covalent organic reactions can be carried out on MOF and COF structures, respectively, with preservation of framework crystallinity and porosity-giving rise to the "chemistry of the framework". Thus, the stage is set for study of the physical properties and chemical reactivity of MOFs and COFs, and indeed their development in a myriad of applications.