Marco Polito

Design of organometallic molecular and ionic materials

Abstract Organometallic crystal engineering is the modeling, synthesis, characterization and evaluation of crystalline materials constituted by organometallic molecules and ions. The properties of solids containing transition metal complexes are distinct and diverse from those of purely organic systems as well as from those of inorganic materials. In particular, while the periphery of (most) organometallic molecules are ‘organic’ in nature, since the outer atoms are usually those of the ligands, the ‘cores’ are formed by transition metal atoms in their (often variable) spin and charge states. These characteristics can be exploited to make crystalline materials with predefined physical properties as well as to organize organometallic molecules in complex supramolecular structures for absorption and desorption of solvent molecules. The possibility of utilizing the same building blocks in different ionic conditions (including neutral, e.g. in molecular crystals) permits tuning of the intermolecular bonding capacity via acid-base reactions. Organometallic polymorphism is discussed as a possibility for preparing and interconverting crystalline isomers. Pseudo-polymorphism is shown to be advantageous for the preparation of elusive crystal forms.

Design of hydrogen bonded networks based on organometallic sandwich compounds

Abstract The design, construction and evaluation of hybrid organic–organometallic and inorganic–organometallic crystalline materials held together by charge assisted hydrogen bonding interactions are described. It is shown that the convolution of the properties typical of coordination complexes (topology, oxidation and charge states etc.) with the extramolecular bonding capacity of ligands carrying hydrogen bonding groups allows one to prepare molecular and materials with desired architectures. The ionic or neutral nature of the building blocks can be utilized not only to control the strength of intermolecular bonding, but also to attain structure–function relationships and desired properties. In the case of organometallic species it is possible to vary in a controlled way both the redox properties of the metal centers and/or the acid/base behavior of the ligands. The possibility of utilizing the same building blocks in different neutral and ionic conditions can be exploited to construct complex structures for a variety of supramolecular applications.

Solvent effect in a “solvent free” reaction

Vapour digestion of a mixture of solid [Fe(η5-C5H4–C5H4N)2] and solid pimelic acid HOOC(CH2)5COOH in the presence of solvent vapours generates co-crystals of different stoichiometry depending on the protic or aprotic nature of the solvent. The nature of the products has been ascertained by a combined use of SSNMR and X-ray diffraction.

Crystal Polymorphism and Multiple Crystal Forms

This chapter discusses the phenomenon of polymorphism in organic and organometallic compounds. Polymorphism is first introduced and then, to give the work some context, background information is given concerning properties and techniques for characterizing the solid phases. In particular, desolvation and inter- converstion are examined, and the gas-solid reactions are presented as a successful route to obtaining new crystalline phases. Co-crystal definition is then described and the problem in distinguishing co-crystals and salts is evaluated.

Mechanochemical and solution preparation of the coordination polymers Ag[N(CH2CH2)3N]2[CH3COO]·5H2O and Zn[N(CH2CH2)3N]Cl2

The coordination polymer Ag[N(CH2CH2)3N]2[CH3COO]·5H2O has been obtained by co-grinding in the solid state and in the air of a stoichiometric amount of silver acetate and of [N(CH2CH2)3N]. Single crystals suitable for X-ray diffraction have been obtained from a water–methanol solution and used to compare calculated and experimental X-ray powder diffractograms. When ZnCl2 is used instead of AgCH3COO in the equimolar reaction with [N(CH2CH2)3N], the solution and solid-state treatments afford different products. Single crystals of the Zn-based coordination polymer Zn[N(CH2CH2)3N]Cl2 have been obtained by layering a solution of ZnCl2 in methanol over a solution of [N(CH2CH2)3N] in CH2Cl2. The product of the co-grinding, identified only on the basis of X-ray powder diffraction, converts, on prolonged grinding, into the crystalline material obtained from the reaction in solution, namely anhydrous Zn[N(CH2CH2)3N]Cl2.

The crystal structures of chloro and methylortho-benzoic acids and their co-crystal: rationalizing similarities and differences

A range of crystallisation experiments seeking to investigate the chloro/methyl structural similarity of o-toluic acid (o-methylbenzoic acid) and o-chlorobenzoic acid, including heterogeneous seeding experiments, has produced single crystals of both acids, and a 1 : 1 co-crystal (1) in which the chlorine atom and methyl group are disordered. All structures are characterized by the same hydrogen bonded ribbons, but they are not isostructural because of significant differences in the packing of the ribbons. Additional polymorphs of o-toluic acid were detected: microcrystalline form II, and form III with a large unit cell. The observed structures were compared with computationally generated isomorphs and ordered models, and alternative low energy structures generated in computational searches. These calculations show that the structures generated by interchanging the chloro and methyl groups are slightly less stable than the experimentally observed forms. The small energy differences associated with interchanging the two functional groups and between alternative packing arrangements of the hydrogen-bonded ribbons is consistent with the polymorphism of o-toluic acid and the observed disorder in the co-crystal.

Polymorphic gabapentin: thermal behaviour, reactivity and interconversion of forms in solution and solid-state

The various crystal forms of the neuroleptic drug gabapentin have been investigated, and in some cases re-investigated, by a combination of differential scanning calorimetry, hot stage microscopy and variable temperature powder diffraction methods in order to establish the relative stability of both its anhydrous and hydrated forms. A series of steps involving slurrying, heating, de-hydration and reaction with vapours of HCl have been performed. In this latter case, it has been possible to show that the reaction with HCl vapour leads to the same product as that obtained in solution. In slurry experiments in the absence of water, the most stable form, Form II, is invariably obtained, whereas in water, the slurry leads to the conversion of all crystal forms to the hemihydrated Form I. The conditions for the solid-state formation of the gabapentin-lactam de-hydration product have been analysed. Co-crystal formation has also been attempted. In the course of one such experiment, 1 : 1 co-crystals of gabapentin-lactam and benzoic acid were obtained.

Mechanochemical and solution reactions between AgCH3COO and [H2NC6H10NH2] yield three isomers of the coordination network {Ag[H2NC6H10NH2]+}∞

Solid-state co-grinding of silver acetate and solid trans-1,4-diaminocyclohexane, [H2NC6H10NH2] yields two isomeric coordination networks depending on the crystallization conditions; a third isomeric form is obtained when the same reaction is carried out in solution.

Unexpected solid–solid reaction upon preparation of KBr pellets and its exploitation in supramolecular cation complexation

Pressing solid [CoIII(eta 5-C5H4COOH)(eta 5-C5H4COO)] with KBr to prepare samples for IR spectroscopy leads to a profound solid state rearrangement with formation of the supramolecular complex [CoIII(eta 5-C5H4COOH)(eta 5-C5H4COO)]2.K+Br-, which can also be obtained from solution crystallization. Similar solid-solid supramolecular complexation has been observed with K[PF6] and [NH4][PF6].

Tunable Supramolecular Synthons and Versatile, Water‐Soluble Building Blocks for Crystal Engineering: [(η5‐C5H4COOH)2CoIII]+ and its Zwitterionic Form [(η5‐C5H4COOH)(η5‐C5H4COO)CoIII]

It is shown that the water-soluble dicarboxylic cationic acid [(eta5-C5H4COOH)2Co(III)]+ (1) is an extremely versatile building block for the construction of organometallic crystalline edifices. Removal of one proton from 1 leads to formation of the neutral zwitterion [(eta5-C5H4COOH)(eta5-C5H4COO)Co(III)] (2), while further deprotonation leads to formation of the dicarboxylate monoanion [(eta5-C5H4COO)2Co(III)]- (3). Compounds 1. 2 and 3 possess different hydrogen-bonding capacity and participate in a variety of hydrogen-bonding networks. The cationic form 1 has been characterised as its [PF6]- and Cl- salts 1-[PF6] and 1-Cl.H2O, as well as in its co-crystal with urea, 1-Cl.3(NH2)2CO, and with the zwitterionic form 2, [(eta5-CH4COOH)(eta5-C5H4COO)Co(III)][(eta5-C5H4COOH)2Co(III)]+[PF6]-, 2.1-[PF6]. The neutral zwitterion 2 behaves as a supramolecular crown ether: it encapsulates the alkali cations K+, Rb+ and Cs+ as well as the ammonium cation NH4+ in cages sustained by O-H...O and C-H...O hydrogen bonds to form co-crystalline salts of the type 2(2)-M[PF6] (M = K, Rb, Cs) and 2(2)-[NH4][PF6]. The deprotonated acid 3 has been characterised as its Cs+ salt, Cs+-3.3H2O.