Assembling molecular species into 3D frameworks:: Computational design and structure solution of hybrid materials

We present here the computational prediction of hybrid organic-inorganic extended lattices. The production of candidate crystal structures is successfully performed by direct-space assembly of building-units using the AASBU (Automated Assembly of Secondary Building Units) method, mixing independent organic and inorganic units. Hybrid candidates that are compatible with the imposed metal:organic ratio are generated with their cell parameters, space group, atomic positions, along with their simulated diffraction pattern. Since no explicit limit regarding the nature, number, and size of the inorganic and organic units, or hybrid building-block is involved, the method offers boundless potential for exploring hybrid frameworks in terms of the topological diversity. The most appealing development arises from the computer-assisted design of hybrid frameworks. Indeed, in a significant number of systems, it is well-known that controlled synthesis conditions can promote the occurrence of specific building-units, which serve to "propagate" the infinite crystal structure. We believe that the computational approach presented herein is valuable to create virtual libraries of viable hybrid polymorphs. We further show how it has proven to be, for the first time in the realm of hybrids, a tangible route towards structure solution in direct space, exemplified here with the computational structure determination of two complex hybrid structures, MIL-100 and MIL-101. This challenging area is of special interest when high quality diffraction data are not available or when very large cell sizes are involved. The development of a structural model in direct space, starting with minimal knowledge such as the metal:organic ratio, is shown here to be possible. With such a method in hand, formerly intractable structural problems when using methods based on conventional reciprocal space become feasible in direct space. (C) 2005 Published by Elsevier Ltd.