Directing the Structure of Metal–Organic Frameworks by Oriented Surface Growth on an Organic Monolayer

Abstract

The concept of biomineralization implies control of crystallization in terms of phase and orientation through interactions with organic macromolecules. This is of particular interest for the synthesis of biomimetic materials. If one strives to mimic the enormous structure-directing power of biomineralization in materials science, an artificial organic interface is needed. For example, functionalized self-assembled monolayers (SAMs) have been shown to effect oriented growth and phase direction of dense-phase calcium carbonate. The oriented growth of other dense materials such as lead sulfide and zinc oxide, and even the oriented growth of porous materials such as zeolites, on SAMs has been reported. Studies on the growth of MOF-5 and HKUST-1 phases on organic monolayers were recently reported. However, to our knowledge, so far it has not been possible to control the crystal structure of a porous material through interactions with molecular layers. This is expected to be particularly difficult due to the large, complex unit cells of these systems. Herein we present a dramatic change in the crystallization of a porous metal–organic framework on moving from homogeneous nucleation to heterogeneous nucleation on an ordered SAM. Due to their many potential applications such as gas sorption, molecular separation, storage, and catalysis, metal– organic frameworks (MOFs) have been intensively studied. We have recently reported the tunable, oriented growth of the porous MOF HKUST-1 on different functionalized SAMs. Herein we investigate the crystal growth of MOFs on mercaptohexadecanoic acid (MHDA) SAMs in the system Fe/bdc (bdc= 1,4-benzenedicarboxylic acid or terephthalic acid). In this system several open-framework structures are known, includingMIL-53 andMIL-88. The structures of MIL53 and MIL-88 are very flexible, and the cell constants of these materials are strongly dependent on pore content. The framework flexibility of these materials enables their use for adsorption of different organic molecules and makes them interesting candidates for sensors. In the monoclinic structure of Fe(OH)(bdc)(py)0.85, the Fe analogue of MIL-53, chains of FeO6 octahedra are connected by benzenedicarboxylate anions. Thus, rhombshaped one-dimensional (1D) channels are formed that run along the a axis of the structure. The hexagonal 3D structure of MIL-88B is built up from trimers of FeO6 octahedra linked to benzenedicarboxylate anions. Thus, the 3D pore system of MIL-88B consists of tunnels along the c axis connected by bipyramidal cages (Scheme 1).