Infinite secondary building units and forbidden catenation in metal-organic frameworks.

Abstract

Catenation, in the form of interpenetrating and interweaving,[1] has been a major concern in the design of low-density (porous) structures due to the following widely held beliefs: a) the use of long links for the design of frameworks with large pores results in catenated structures and thus small pores, b) highly catenated frameworks typically have low porosity ( 20%), and c) catenation contributes negatively to the structural stability and porosity of open frameworks.[1, 2] We recently found in the chemistry of metal-organic frameworks (MOFs) that discrete secondary building units (SBUs) are important for designing structures with attributes that disprove the universality of b) and c); specifically, maximally interpenetrating MOFs have been shown to have highly porous ( 65%) structures, and interweaving in open frameworks has been recognized and used for the design of structures with reinforced walls and permanent porosity.[1c±d, 3] Herein, we introduce the use of infinite SBUs toward addressing the point presented in a). Specifically, we show that MOF-69A, [Zn3(OH)2(bpdc)2] ¥ 4DEF ¥ 2H2O (bpdc 4,4 -biphenyldicarboxylate; DEF N,N -diethylformamide), and its 2,6-naphthalenedicarboxylate (ndc) analogue MOF-69B have three-dimensional (3D) structures constructed from infinite Zn-O-C SBUs and long bpdc or ndc links that expand the Al net in SrAl2 and provide a framework where catenation is forbidden. Infinite Zn-O-C SBUs were produced by subjecting reaction mixtures that typically giveMOF-5 to increasing amounts of H2O2. These solutions were monitored for the appearance of crystalline solids. A single-crystal X-ray diffraction study[5a] performed on a rodlike colorless crystal of MOF-69A delocalized electrons on the linkers between the chains, exhibit very low TN values (4 ± 10 K). The presence of these electrons drastically enhances the magnetic characteristics of these solids above the strategic borderline of liquid nitrogen temperature. This renders porous solids magnetic with sufficiently high and available ordering temperatures which could now find applications, for example, in magnetic sorting.