Jaheon Kim

Reticular synthesis and the design of new materials

The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal–oxygen–carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized.

A route to high surface area, porosity and inclusion of large molecules in crystals

One of the outstanding challenges in the field of porous materials is the design and synthesis of chemical structures with exceptionally high surface areas. Such materials are of critical importance to many applications involving catalysis, separation and gas storage. The claim for the highest surface area of a disordered structure is for carbon, at 2,030u2009m2u2009g-1 (ref. 2). Until recently, the largest surface area of an ordered structure was that of zeolite Y, recorded at 904u2009m2u2009g-1 (ref. 3). But with the introduction of metal-organic framework materials, this has been exceeded, with values up to 3,000u2009m2u2009g-1 (refs 4–7). Despite this, no method of determining the upper limit in surface area for a material has yet been found. Here we present a general strategy that has allowed us to realize a structure having by far the highest surface area reported to date. We report the design, synthesis and properties of crystalline Zn4O(1,3,5-benzenetribenzoate)2, a new metal-organic framework with a surface area estimated at 4,500u2009m2u2009g-1. This framework, which we name MOF-177, combines this exceptional level of surface area with an ordered structure that has extra-large pores capable of binding polycyclic organic guest molecules—attributes not previously combined in one material.

Advances in the chemistry of metal–organic frameworks

An extensive body of research results currently exists from the synthesis of metal–organic frameworks (MOFs), an area that has attracted widespread attention due to the facility with which well-defined molecular building blocks can be assembled into periodic frameworks and the promise that such a process holds for the logical design of materials. The synthesis of MOFs generally involves the copolymerization of organic links and metal ions in a polar solvent under mild temperatures (up to 200 °C) and autogenous pressures (up to 100 atm). Since most products can be considered kinetically driven and lie on local thermodynamic minima, factors such as solubility of the organic link and metal salt, solvent polarity, ionic strength of the medium, temperature and pressure play critical roles in determining the character of products. Indeed, slight perturbations in synthetic parameters have been the basis for the preparation of what seems to be na flood of new MOF compounds. n In the spirit of this discussion we advance the following ideas and developments that we believe contribute to the maturity of the field: (I) a conceptual framework that unifies the processes involved in the designed synthesis of MOFs, and which can be extended to other materials with extended structures; (II) a thesis concerning the possible structures that may form from building blocks with various shapes; (III) important considerations for achieving the design and synthesis of frameworks in which it is possible to change chemical functionality and metrics without changing the underlying framework topology; (IV) the inevitability of porosity for designed structures and some factors affecting framework stability; (V) insights on catenation: interpenetration versus interweaving, forbidden catenation, and duals. These points will be presented to an extent that will stimulate discussion—it nis not an attempt to be comprehensive or to give a thorough treatment of this rich field.

1,4-Benzenedicarboxylate derivatives as links in the design of paddle-wheel units and metal–organic frameworks

The square grid structure of MOF-2, constructed from npaddle-wheel units of Zn(II) and 1,4-benzenedicarboxylate (BDC) nlinks, persists for 2-amino-1,4-benzenedicarboxylate (ABDC) links but not nfor the sterically demanding 2,3,5,6-tetramethyl-1,4-benzenedicarboxylate n(TBDC); the dihedral angle between planes of the benzene and carboxylate ngroups play a determining role in the formation of the paddle-wheel nmotif.

Metal–organic frameworks constructed from pentagonal antiprismatic and cuboctahedral secondary building units

Two crystalline metal–organic frameworks formulated as nZn6(NDC)5(OH)2(DMF)2·4D nMF, MOF-48, and nZn7(m-BDC)6(OH)4(H2O) n2·6DMF·4H2O, MOF-49, (NDC = n1,4-naphthalenedicarboxylate; m-BDC=1,3-benzenedicarboxylate) have nbeen synthesized and fully characterized by single crystal X-ray ndiffraction studies, which reveal that the frameworks are constructed from npentagonal antiprismatic (MOF-48) and cuboctahedral (MOF-49) secondary nbuilding units respectively, however, both are reticulated into 3-D nstructures having the B network of CaB6.