Crystal engineering, the design of molecular solids, is the synthesis of functional solid-state structures from neutral or ionic building blocks, using intermolecular interactions in the design strategy. Hydrogen bonds, coordination bonds, and other less directed interactions define substructural patterns, referred to in the literature as supramolecular synthons and secondary building units. Crystal engineering has considerable overlap with supramolecular chemistry, X-ray crystallography, materials science, and solid-state chemistry and yet it is a distinct discipline in itself. The subject goes beyond the traditional divisions of organic, inorganic, and physical chemistry, and this makes for a very eclectic blend of ideas and techniques. The purpose of this Review is to highlight some current challenges in this rapidly evolving subject. Among the topics discussed are the nature of intermolecular interactions and their role in crystal design, the sometimes diverging perceptions of the geometrical and chemical models for a molecular crystal, the relationship of these models to polymorphism, knowledge-based computational prediction of crystal structures, and efforts at mapping the pathway of the crystallization reaction.

Crystal Engineering: A Holistic View

How do molecules aggregate in solution, and how do these aggregates consolidate themselves in crystals? What is the relationship between the structure of a molecule and the structure of the crystal it forms? Why do some molecules adopt more than one crystal structure? Why do some crystal structures contain solvent? How does one design a crystal structure with a specified topology of molecules, or a specified coordination of molecules and/or ions, or with a specified property? What are the relationships between crystal structures and properties for molecular crystals? These are some of the questions that are being addressed today by the crystal engineering community, a group that draws from the larger communities of organic, inorganic, and physical chemists, crystallographers, and solid state scientists. This Perspective provides a brief historical introduction to crystal engineering itself and an assessment of the importance and utility of the supramolecular synthon, which is one of the most important concepts in the practical use and implementation of crystal design. It also provides a look to the future from the viewpoint of the author, and indicates some directions in which this field might be moving.

Crystal engineering: from molecule to crystal.

Crystallization is vital to many processes occurring in nature and in the chemical, pharmaceutical, and food industries. Notably, crystallization is an attractive isolation step for manufacturing because this single process combines both particle formation and purification. Almost all of the products based on fine chemicals, such as dyes, explosives, and photographic materials, require crystallization in their manufacture, and more than 90% of all pharmaceutical products contain bioactive drug substances and excipients in the crystalline solid state. Hence control over the crystallization process allows manufacturers to obtain products with desired and reproducible properties. We judge the quality of a crystalline product based on four main properties: size, purity, morphology, and crystal structure. The pharmaceutical industry in particular requires production of the desired crystal form (polymorph) to assure the bioavailability and stability of the drug substance. In solution crystallization, nucleation...

Nucleation of crystals from solution: classical and two-step models.

Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid–liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted.

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Pre-nucleation clusters as solute precursors in crystallisation

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Little is known concerning the precise molecular pathway that links fluid-phase molecules to those in nascent crystal nuclei. In this paper the process of molecular self-assembly has been studied in concentrated solutions using FTIR spectroscopy. Three carboxylic acids, benzoic, tetrolic, and mandelic acids, have been chosen on the basis of their differing crystal chemistries, as reflected in observed hydrogen-bonding motifs. Using the solid-state spectra as a means of unambiguous assignment of carboxyl and hydroxyl vibrations associated with hydrogen bonding, spectroscopic data are reported for solutions as a function of both composition and solvent. In the cases of benzoic acid and tetrolic acid, a link between the growth synthon and the structural synthon is apparent. Mandelic acid, on the other hand, provides a more complex case in which strong solvation effects are evident, leading to the conclusion that significant molecular rearrangement must occur within the developing crystal nuclei.

Concerning the Relationship between Structural and Growth Synthons in Crystal Nucleation: Solution and Crystal Chemistry of Carboxylic Acids As Revealed through IR Spectroscopy

Linking solution chemistry to crystal nucleation: the case of tetrolic acid

There has been much recent interest in the role of solution chemistry and in particular the importance of molecular self-assembly in the nucleation of crystalline phases. Techniques such as FTIR and NMR have highlighted the existence of solution-phase dimers which in many cases mirror the structural synthons found in the resulting macroscopic crystals. However, there are no reported examples in which this new insight into the solution phase has been linked directly to the kinetics of crystal nucleation. Here for the first time, using a combination of solution FTIR, computational chemistry, and measured crystal nucleation rate data, such a link is demonstrated for p-aminobenzoic (PABA) and benzoic acids nucleating from polar and nonpolar solvents. Solute dimerization and desolvation are found to be rate-determining processes in the overall nucleation pathway.

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Revealing the roles of desolvation and molecular self-assembly in crystal nucleation from solution : benzoic and p -aminobenzoic acids

The design and preparation of two component crystalline solids, cocrystals, for potential pharmaceutical applications has been a significant activity in the field of crystal engineering over the past five years. In this contribution, we explore the solution chemistry of one particular system, diphenylamine−benzophenone. In particular, we elucidate the relationships between crystal chemistry, solution chemistry, and phase behavior in both binary and ternary systems. The combined use of FTIR spectroscopy and proton NMR reveals the molecular basis of negative and positive deviations from ideality and provides important insights into the crystal nucleation processes.

Cocrystallization: A Solution Chemistry Perspective and the Case of Benzophenone and Diphenylamine

Using combined X-ray diffraction and FTIR spectroscopy, a remarkable similarity is revealed between the local environments of trimesic acid in solution and in its most metastable trisolvate with DMSO. The implications of this for the nature of the nucleation process are discussed.

G. Dent

Papers

Concerning the Relationship between Structural and Growth Synthons in Crystal Nucleation: Solution and Crystal Chemistry of Carboxylic Acids As Revealed through IR Spectroscopy

Linking solution chemistry to crystal nucleation: the case of tetrolic acid

Revealing the roles of desolvation and molecular self-assembly in crystal nucleation from solution : benzoic and p -aminobenzoic acids

Cocrystallization: A Solution Chemistry Perspective and the Case of Benzophenone and Diphenylamine

Crystallising trimesic acid from DMSO solutions – can crystallography teach us anything about the process of crystal nucleation?