Reshaping molecules through cocrystallization. Comparison of the structures of Hg{Co(CO)4}2 and Hg{Co(CO)4}2·C60·toluene

Cocrystallization of Hg{Co(CO)4}2 with C60 produces Hg{Co(CO)4}2·C60·toluene in which the geometry of the Hg{Co(CO)4}2 molecule is rearranged to fit between the remarkably well ordered fullerenes.

Crystal engineering: from molecule to crystal

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.

A cartography of the van der Waals territories

The distribution of distances from atoms of a particular element E to a probe atom X (oxygen in most cases), both bonded and intermolecular non-bonded contacts, has been analyzed. In general, the distribution is characterized by a maximum at short E···X distances corresponding to chemical bonds, followed by a range of unpopulated distances--the van der Waals gap--and a second maximum at longer distances--the van der Waals peak--superimposed on a random distribution function that roughly follows a d(3) dependence. The analysis of more than five million interatomic "non-bonded" distances has led to the proposal of a consistent set of van der Waals radii for most naturally occurring elements, and its applicability to other element pairs has been tested for a set of more than three million data, all of them compared to over one million bond distances.

Mononuclear manganese(II) complexes of phenyl 2-pyridyl ketoxime – models for studies of Q-band EPR properties

Structure and properties of a series of mononuclear manganese(II) complexes with phenyl 2-pyridyl ketoxime (ppkoH) as ligand are presented. [Mn(ClC6H4COO)2 (ppkoH)2] · 0.5 CH3OH and [Mn(ClC6H4COO)2(ppkoH)2](1 and 2) are isolated from the same reaction system and differ by disordered solvent content and conformation of the complex molecules. [MnCl2(ppkoH)2] · 0.02 C2H5OH(3) was obtained in order to modify the central Mn2+ ion coordination sphere with respect to 1/2. Strong intramolecular hydrogen bonds of the O–H···O and O–H···Cl type are present in 1/2 and 3, respectively. Moreover, weak C–H···O (1–3), C–H···Cl (3), π···π (1–2) and Cl···π (2) contacts can be found. The complexes display well-resolved Q-band EPR spectra and may serve as model compounds for further studies.

Structural isomerism in tris-tolyl halo-phosphonium and halo-arsonium tri-halides, [(CH3C6H4)3EX][X3], (E = P, As; X = Br, I)

The group 15 ligands (o-CH(3)C(6)H(4))(3)P, (m-CH(3)C(6)H(4))(3)P, (p-CH(3)C(6)H(4))(3)P, Ph(3)As, (o-CH(3)C(6)H(4))(3)As and (p-CH(3)C(6)H(4))(3)As have been reacted with two equivalents of di-iodine or di-bromine to yield complexes of formula R(3)EX(4) (E = P, As; X = I, Br). These halogenated group 15 compounds are ionic, [R(3)EX][X(3)] consisting of halo-phosphonium or halo-arsonium cations and trihalide anions. These adducts exhibit structural isomerism and may exist either as simple 1:1 ion pairs, [R(3)EX][X(3)], isomer (A), which display a weak XX interaction between cation and anion, or as a 2:1 complex, which consists of a [{R(3)EX}(2)X(3)](+) cationic species made up of two [R(3)EX](+) cations interacting with one [X(3)](-) anion. The overall charge is balanced by a second [X(3)](-) anion. These 2:1 species also exhibit structural isomerism due to subtle differences in the connectivity of the [{R(3)EX}(2)X(3)](+) fragment, as the {R(3)EX}(+) units may either interact at the same end of the [X(3)](-) ion, to give a Y-shaped motif, isomer (B), or at opposite ends, giving a Z-shaped motif, isomer (C). The type of structural isomer formed is related to the way in which [Ar(3)EX](+) cations pack together via aryl embraces. Isomer (A) and (C) structures form chains of side-to-side, anti-parallel embracing cations. In (A) and (C) structures a square-like stacking motif of cations is observed. In contrast, isomer (B) structures feature side-to-side, parallel embracing cations, and do not exhibit the square motif.

Crystalline molecular machines: function, phase order, dimensionality, and composition

The design of molecular machines is stimulated by the possibility of developing new materials with complex physicochemical and mechanical properties that are responsive to external stimuli. Condensed-phase matter with anisotropic molecular order and controlled dynamics, also defined as amphidynamic crystals, offers a promising platform for the design of bulk materials capable of performing such functions. Recent studies have shown that it is possible to engineer molecular crystals and extended solids with Brownian rotation about specific axes that can be interfaced with external fields, which may ultimately be used to design novel optoelectronic materials. Structure/function relationships of amphidynamic materials have been characterized, establishing the blueprints to further engineer sophisticated function. However, the synthesis of amphidynamic molecular machines composed of multiple "parts" is essential to realize increasingly complex behavior. Recent progress in amphidynamic multicomponent systems suggests that sophisticated functions similar to those of simple biomolecular machines may eventually be within reach.

Stable radicals during photodecarbonylations of trityl-alkyl ketones enable solid state reactions through primary and secondary radical centers

The solid state photoexcitation of several triphenylmethyl-alkyl ketones resulted in the loss of CO and the exclusive formation of radical-radical combination products. Differences in reactivity suggest a stepwise mechanism with the unprecedented formation of primary and secondary radicals in some of the radical pair intermediates in the solid state.

Structure:function relationships in molecular spin-crossover complexes

Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).

Bis{μ-2-[(2-carbamoylhydrazin-1-ylidene)methyl]phenolato}bis[chloridozinc(II)] methanol disolvate, with non-aromatic-aromatic π-π stacking and N-H...Cl-Zn hydrogen bonding

Centrosymmetric dimers of Zn(II) with singly deprotonated 2-[(2-carbamoylhydrazin-1-ylidene)methyl]phenolate, [Zn(2)(C(8)H(8)N(3)O(2))Cl(2)]·2CH(3)OH, form an infinite one-dimensional hydrogen-bonded chain which is further aggregated by non-aromatic-aromatic π-π stacking and nonclassical N-H...Cl hydrogen bonding.