1 Applications of Crystal Engineering Strategies in Solvent-free Reactions: Toward a Supramolecular Green Chemistry DARIO BRAGA, DANIELA D’ADDARIO, LUCIA MAINI, MARCO POLITO, STEFANO GIAFFREDA, KATIA RUBINI and FABRIZIA GREPIONI Dipartimento di Chimica G. Ciamician, University of Bologna, Via Selmi 2, 40126 Bologna, Italy. 1. INTRODUCTION Making crystals by design is the paradigm of crystal engineering [1]. The goal of this field of research is that of assembling functionalized molecular and ionic components into a target network of supramolecular interactions [2]. This “bottom-up” process generates collective supramolecular properties from the convolution of the physical and chemical properties of the individual building blocks with the periodicity and symmetry operators of the crystal (Figure 1) [3]. One can envisage two main subareas of crystal engineering, namely, those of coordi- nation networks [4] and of molecular materials [5], even though all possible intermediate situations are possible. The preparation of coordination networks or polymers can be appropriately described as periodical coordination chemistry and exploits the possibility of divergent ligand-metal coordination, as opposed to the more traditional convergent coordination chemistry operated by chelating polydentate ligands [6] (Figure 2). The possibility of exploiting engineered coordination networks for practical applica- tions (such as absorption of molecules, reactions in cavities, etc.) very much depends on whether the networks contain large empty spaces (channels, cavities, etc.) [7] or whether the network is close packed because of interpenetration and self-entanglement [8]. The Frontiers in Crystal Engineering. Edited by Edward R.T. Tiekink and Jagadese J. Vittal 2006 John Wiley & Sons, Ltd