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Co-crystals of dextro-(R, R), levo-(S,S), meso-(R,S) and racemic (R,R-S,S)-tartaric acid with pyrazine were obtained by manual kneading and slurry experiments; subsequent reactions in the solid state between these co-crystals and the various forms of tartaric acid in the solid state and via slurry show that co-former exchange takes place according to the sequence of stability [(R, S)-ta](2) .py > (S,S/R,R)-ta . py > (R,R)-ta . py or (S,S)-ta . py.

A new silver(I) complex with 1-phenyl-3-(quinolin-5-yl)urea has been prepared, acting as a supramolecular supergelator in the presence of polar solvents. In open vials temperature dependent, reversible gel-to-xerogel transition is observed, while in sealed vials direct crystallization of four polymorphs of the same gelling compound is achieved, depending on the gelling solvent.

Reaction in solution and in the solid state of 18-crown[6] with Cs2CO3 and 5-substituted isophthalic acids yields the new organic–inorganic complexes 18-crown[6]·Cs[OH-isoH] (2), 18-crown[6]·Cs[CH3-isoH] (3), 18-crown[6]·Cs[Br-isoH] (4), 18-crown[6]·Cs[OCH3-isoH] (5), 18-crown[6]·Cs[NO2-isoH] (6) and 18-crown[6]·Cs[tert-Butyl-isoH] (8), which have all been characterized by X-ray diffraction. All compounds except the latter are isomorphous, in spite of the size, shape and chemical nature of the substituent and its ability to form hydrogen bonds.

Solid-state reactions of copper(I) iodide with 1,4-diazabicyclo[2.2.2]octane (DABCO) and piperazine in different conditions (kneading, vapour digestion, thermal treatment) resulted in the formation of four new luminescent systems, which have been characterized via X-ray diffraction and photo-luminescence spectroscopy. At room temperature all compounds exhibit a bright emission in the solid state that markedly shifts bathochromically on cooling to 77 K. The luminescence of the almost isostructural 2D-coordination polymers [Cu(2)I(2)(L)(2)](infinity) (L = DABCO, piperazine) exhibits an unusually long lifetime (125 and 53 us, respectively), which becomes even longer (609 and 420 us, respectively) at 77 K.

The synthesis of [Ir2Rh2(CO)12] (1) by the literature method gives a mixture 1/[IrRh3(CO)12] which cannot be separated using chromatography. The reaction of [Ir(CO)4]− with 1 mol-equiv. of [Rh(CO)2(THF)2]+ in THF gives pure 1 in 61% yield. Crystals of 1 are highly disordered, unlike those of its derivative [Ir2Rh2(CO)5(μ2-CO)3(norbornadiene)2] which were analysed using X-ray diffraction. The ground-state geometry of 1 in solution has three edge-bridging CO's on the basal IrRh2 face of the metal tetrahedron. Time averaging of CO's takes place above 230 K. The CO site exchange of lowest activation energy is due to one synchronous change of basal face, as shown by 2D- and VT-13C-NMR. Substitution of CO by X− in 1 takes place at a Rh-atom giving [Ir2Rh2(CO)8(μ2-CO)3X]− (X = Br, I). Substitution by bidentate ligands gives [Ir2Rh2(CO)7(μ2-CO)3(η4-L)] (L = norbornadiene, cycloocta-1,5-diene) where the ligand L is chelating a Rh-atom of the basal IrRh2 face. Carbonyl substitution by tridentate ligands gives [Ir2Rh2(CO)6(μ2-CO)3(μ3-L)] (L = 1,3,5-trithiane, tripod) with L capping the triangular basal face of the metal tetrahedron. Carbonyl scrambling is also observed in these substituted derivatives of 1 and is mainly due to the rotation of three terminal CO's about a local C3 axis on the apical Ir-atom.

The crystal structures of some high nuclearity carbonyl cluster anions of rhodium and cobalt have been investigated by means of empirical packing potential energy calculations, van der Waals volume analysis and computer graphics. Data was retrieved from the Cambridge Structural Database. The relationship between shape, size and charge of the component ions and the formation of preferential aggregates in the crystalline solid has been analyzed.

The crystal structures of 28 hexanuclear carbonyl cluster anions have been investigated by means of empirical packing potential energy calculations, van der Waals volume analysis, and computer graphics. Data were retrieved from the Cambridge Structural Database. The relationship between shape, size, and charge of the component ions and the formation of preferential aggregates (piles, «snakes», layers) in the crystal lattice has been analyzed. It has been shown that, with large organic cations, the packing pattern is eventially that of molecular mixed crystals, while small cations drive toward monodimensional aggregation of the anions. It has been shown that there is a (fairly precise) ratio between the relative volumes of the component ions above which one-dimensional or two-dimensional networks are preferentially established

The new mixed-metal clusters [CO 2 Ir 2 (CO) 10 (cod)] ( 4 ) and [CoIr 3 (CO) 10 (cod)] ( 5 ) (cod = 1,5-cyclooctadiene) have been obtained from [CO 2 (CO) 8 ]( 2 ) and [IrCl(Cod) 2 ] ( 3 ). Alternatively, [CO 2 Ir 2 (CO) 12 ] ( 1 ) reacts with cod to yield 4 , which in the presence of CO reforms 1 . Under the same conditions, however, 5 is inert. The solid state structures of 4 and 5 have been established by X-ray analyses, and the disorder shown by the two species is discussed. Compound 4 is monoclinic, space group P 2 1 / c , a 8.891(2), b 10.746(3), c 22.648(8) A, β 95.16(3)°, Z = 4, R = 0.048, R′= 0.050, for 2393 [ I ⪢ 2σ( I )] of 2596 absorption corrected reflections; compound 5 is also monoclinic, space group P 2 1 / m , a 8.5994(9), b 12.047(3), c 10.799(3) A, β 98.30(1)°, Z = 2, R = 0.037, R′= 0.040, for 2071 [ I ⪢ 2σ( I )] of 2711 absorption corrected reflections.

The dienyl cluster compounds [M3H(CO)9(µ3-σ:η2:η2-C6H7)](M = Ru or Os) have been synthesised from [Os3H2(CO)10] or [M3(CO)10(MeCN)2](M = Ru or Os) with cyclohexa-1,3-diene. The molecular and crystal structure of [Ru3H(CO)9(C6H7)] has been established by single-crystal X-ray diffraction analysis: monoclinic, space group P21, a= 8.487(6), b= 12.031(3), c= 9.073(2)A, β= 92.43(4)° and Z= 2. The cyclohexadienyl ligand is involved in one σ and two π interactions with the metal triangle, while the H(hydride) ligand bridges the long Ru–Ru bond [3.052(1)A]. The other two Ru–Ru bond lengths are the same [2.837(1)A] and comparable to those observed in [Ru3(CO)9(µ3-η2:η2:η2-C6H6)]. The H(hyride) position afforded by the diffraction experiment has been compared with the result of potential-energy minimization procedures. The molecular organization within the lattice has been explored by means of atom–atom packing potential-energy calculations showing the presence of a network of C–H ⋯ O intermolecular hydrogen-bonding interactions.

Anhydrous rhodizonic acid H2C6O6 is obtained by (fully reversible) thermal dehydration of solid 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, H6C6O8, commonly known as rhodizonic acid dihydrate. Treatment of H6C6O8 with RbOH yields crystals of Rb2C6O6; the oxocarbon dianion C6O62− is shown to possess a flat, benzene-type structure, with C–C bonds shorter than expected for a (non-aromatic) ketonic-type structure.

Engineering crystal properties by a choice of adequate molecular building blocks has to cope with the fact that a given molecule can be isolated in many, at times very many, solid forms (single entity crystals, solvates, salts, co-crystals and their polymorphs) each with its own characteristics and properties. Preparation of new crystal forms of a same molecule, and investigation of the relationship among new crystal forms represent not only a further challenge to the solid state chemists, but also an opportunity of modifying properties of crystals with applicative implications (pharmaceuticals, agrochemicals, pigments, organic semiconductors, etc.). This feature article elaborates on this concept via a number of examples of multiple crystal forms and test cases.

The molecular organization in crystals of [Co2(CO)8] and [Fe2(CO)9] has been investigated by means of packing potential-energy calculations and computer graphic analysis. It is shown that the two molecules pack in nearly identical ways in their crystals. However, while crystalline [Fe2(CO)9] is compact, solid [Co2(CO)8] presents large empty channels (van der Waals width between 2.4 and 4.5 A, at room temperature) corresponding to the approximate location of possible residual electron density (whether due to Co–Co bonding electrons or to lone pairs on the Co atoms). Alternative crystal packings for [Co2(CO)8] have been generated, using improved atomic potential parameters adapted to carbonyl compounds. On purely steric grounds, it is shown that the [Co2(CO)8] molecule is compatible with more close-packed crystal arrangements than the one actually observed. In two alternative triclinic and monoclinic crystal structures the ‘empty’ bridging sites are shown to be available for molecular interlocking.

The use of discrete metal cluster complexes as models of chemisorption systems in surface chemistry is an attractive hypothesis and has been emphasised by numerous workers in recent years. In this paper we shall examine possible boundary conditions to such a cluster-surface analogy, drawing extensively on the interfacial chemistry of benzene. The motivation for this work is provided by the molecular chemistry of a new class of arene clusters in which the arene molecule is bonded to an M3 triangulo-face within the cluster. The parent compounds in these studies, [M3(CO)9(μ3:η2:η2:η2-C6H6)] (M = Ru or Os), contain benzene in this important new face-capping bonding mode that accurately models benzene adsorption at a threefold site on the surface of a close-packed metal lattice. X-ray diffraction studies combined with NMR (1H and13C) and infrared spectroscopic studies lead to the conclusion that the benzene is coordinated to the metal triangle and is best viewed as a bond-localised cyclohexa-1,3,5-triene.

The molecular and crystal structure of [Fe3(CO)12] has been reinvestigated from single-crystal X-ray diffraction data collected at 100, 160, 250 and 320 K. The asymmetric bridging carbonyl ligands become progressively more symmetric as the temperature is decreased. At the lowest temperature the molecule possesses nearly exact C2v, symmetry. The crystal structure has been examined in the two limiting hypothetical P21 and P1 crystals composing the disordered P21/n structure.

The electronic structure of [Fe3(CO)12] has been investigated by means of extended-Huckel calculations and compared to that of [Ru3(CO)12], showing that the bridged structure, observed in the solid state, arises because of electronic reasons, namely a weaker metal–metal repulsion in the case of the lighter cluster. This is associated with the bridge-formation process and becomes determining for the energetic balance in compounds of heavier metals. The possible existence of crystal structures which are alternative to the observed disordered one known for [Fe3(CO)12] has been explored by means of the atom–atom pairwise potential-energy method. Alternative ordered molecular arrangements have been generated and compared with the experimental crystal structure in terms of packing efficiency and cohesive energy.

Structural variability and flexibility are distinctive properties of organometallic molecules which are reflected in the patterns of intermolecular interactions established by mononuclear and polynuclear coordination complexes in the solid state. The study of van der Waals interactions, hydrogen bonding, and anion–cation interactions allows us to understand how organometallic molecules self-recognize and self-organize and to gain insight into the relationship between molecular and supramolecular crystal structures. This establishes a basis on which a strategy of organometallic crystal engineering can be developed.

Patterns of hydrogen bonding in organometallic crystals and the relationship with the hydrogen bonding patterns in organic solids have been discussed. It has been shown, inter alia, that organometallic systems provide new types of hydrogen bonding donor and acceptor groups, such as the M-H system and the CO ligand in its various bonding modes, which have no counterpart in organic crystals. Emphasis is given to the recognition of packing motifs and to the importance of hydrogen bonds in the construction of crystalline materials based on co-crystallization of organic and organometallic molecules chosen on the basis of complementarity of shape and size.

Crystal engineering is a bridge between supramolecular and materials chemistry. The use of organometallic molecules or ions in the construction of crystals with predefined architectures is a means to introduce into the crystals the variable oxidation and spin states of metal atoms as well as the specific topological requirements of coordination bonds. The perspective of an emerging organometallic crystal engineering discipline has been discussed. It has been shown through a number of examples that the presence of metal atoms makes organometallic crystal engineering distinct from organic crystal engineering. The effect of ligand topology, the ability to tune the acid/base behavior of ligands by means of ligand–metal(s) coordination, the direct participation of metal atoms in extramolecular interactions in the solid state, as well as the consequence of the ionic charges on the strength of weak and strong intermolecular hydrogen bonds have been discussed. In particular, a design strategy to obtain organic–organometallic crystals by means of a selective use of strong hydrogen bonds between the organic component and of weak hydrogen bonds between organic and organometallic components is illustrated with emphasis on the role of charge-assisted C–Hδ+⋯Oδ− hydrogen bonds.

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.

Engineering crystal properties by a choice of adequate molecular building blocks has to cope with the fact that a given molecule can be isolated in many, at times very many, solid forms (single entity crystals, solvates, salts, co-crystals and their polymorphs) each with its own characteristics and properties. Preparation of new crystal forms of a same molecule, and investigation of the relationship among new crystal forms represent not only a further challenge to the solid state chemists, but also an opportunity of modifying properties of crystals with applicative implications (pharmaceuticals, agrochemicals, pigments, organic semiconductors, etc.). This feature article elaborates on this concept via a number of examples of multiple crystal forms and test cases.

The pronounced melting point alternation of α,ω-alkanedicarboxylic acids (HOOC–(CH2)n−2–COOH, n = 4–10) is maintained in co-crystals of the same acids with 1,2-bis(4-pyridyl)ethane (BPA), which contains an even number of carbon atoms in the chain, and is reversed in co-crystals with 1,2-bis(4-pyridyl)propane (BPP), which contains an odd number of carbon atoms in the aliphatic chain.

Five new polymorphs and one hydrated form of 2-thiobarbituric acid have been isolated and characterised by solid-state methods. In both the crystalline form II and in the hydrate form, the 2-thiobarbituric molecules are present in the enol form, whereas only the keto isomer is present in crystalline forms I (reported in 1967 by Calas and Martinex), III, V and VI. In form IV, on the other hand, a 50:50 ordered mixture of enol/keto molecules is present. All new forms have been characterised by single-crystal X-ray diffraction, 1D and 2D ((1)H, (13)C, and (15)N) solid-state NMR spectroscopy, Raman spectroscopy and X-ray powder diffraction at variable temperature. It has been possible to induce keto-enol conversion between the forms by mechanical methods. The role of hydrogen-bond interactions in determining the relative stability of the polymorphs and as a driving force in the conversions has been ascertained. To the best of the authors' knowledge, the 2-thiobarbituric family of crystal forms represents the richest collection of examples of tautomeric polymorphism so far reported in the literature.

Solid-state adducts (co-crystals and molecular salts) of 1,4-diazabicyclo-[2.2.2]-octane (DABCO) with aromatic polycarboxylic acids (isophthalic acid, isoH2, dinicotinic acid, dinH2 and dipicolinic acid, dipH2) were prepared in the solid state by grinding and kneading techniques, and fully characterized via X-ray diffraction. The polycarboxylic acids differ for the presence/absence and position of a nitrogen atom in the aromatic ring; the extent of proton transfer, from the carboxylic groups on the acids to the nitrogen atoms on DABCO, reflects the trend of solution acidity of the three polycarboxylic acids.

Pressing solid barbituric acid with KBr to prepare samples for IR spectroscopy leads to the formation of an ionic co-crystal, in which the co-former is a classical ionic salt; co-crystal formation is also obtained with the other alkali bromides (LiBr, NaBr, RbBr and CsBr) and with caesium iodide. The simultaneous presence of alkali and halide ions affects the dissolution properties of barbituric acid in water.

Co-crystals of phenazine and acridine with vanillin have been obtained by solvent-free reaction or thermal treatment of the solid reactants: their structures, thermal behaviour and eutectic formation have been investigated via single crystal X-ray diffraction, differential scanning calorimetry (DSC), variable temperature X-ray powder diffraction and hot-stage microscopy (HSM). Polymorph screening of the reagents has also been carried out.

The use of discrete metal cluster complexes as models of chemisorption systems in surface chemistry is an attractive hypothesis and has been emphasised by numerous workers in recent years. In this paper we shall examine possible boundary conditions to such a cluster-surface analogy, drawing extensively on the interfacial chemistry of benzene. The motivation for this work is provided by the molecular chemistry of a new class of arene clusters in which the arene molecule is bonded to an M3 triangulo-face within the cluster. The parent compounds in these studies, [M3(CO)9(μ3:η2:η2:η2-C6H6)] (M = Ru or Os), contain benzene in this important new face-capping bonding mode that accurately models benzene adsorption at a threefold site on the surface of a close-packed metal lattice. X-ray diffraction studies combined with NMR (1H and13C) and infrared spectroscopic studies lead to the conclusion that the benzene is coordinated to the metal triangle and is best viewed as a bond-localised cyclohexa-1,3,5-triene.

Deprotonation of [Ir4(CO)11(PPh2H)]1 in the pressence of [Au(PPh3)]PF6 yields the novel species [Ir4(CO)11(PPh2AuPPh3)]2 which possesses a tetrahedral framework bearing a terminally bonded PPh2AuPPh3 ligand. Changing the order of addition of the reagents results in the formation of [Ir4(CO)10(µ-PPh2)(µ-AuPPh3)]4 with bridging phosphido and Au(PPh3)+ units. Reaction of 4 with PPh3 yields [Ir4(CO)9(PPh3)(µ-PPh2)(µ-AuPPh3)]6, while with the more basic phosphine P(C6H4OMe-4)3 substitution of the PPh3 at the Au(PPh3)+ unit occurs producing [Ir4(CO)10(µ-PPh2){µ-AuP(C6H4PMe-4)3}]. Deprotonation of [Ir4(CO)10(PPh3)(PPh2H)] in the presence of [Au(PPh3)]PF6 yields [Ir4(CO)10(PPh3)(PPh2AuPPh3)], which undergoes rearrangement to 6. The kinetics of decarbonylation of compound 2 and give 4 has been investigated and a dissociative mechanism is suggested, confirmed by the activation parameters ΔH‡= 135.6 ± 3.8 kJ mol–1 and ΔS‡= 81.2 ± 10.9 J K–1 mol–1.

The dinuclear complexes [Fe2(η5-C5H5)2(µ-CO)(CO)2{µ-CH(CN)}]1 and [Fe2(η5-C5H5)2(µ-CO)(CO)2{µ-C(CN)2}]2 have been obtained from the sulfonium cation [Fe2(η5-C5H5)2(µ-CO)(CO)2{µ-C(SMe2)(CN)}]+via SMe2 displacement with H– and CN–, respectively. Complex 1 is present in solution and in the solid state as a mixture of isomers, the relative composition depending on the solvent. The structures of the cis isomers of 1(cis-1a) and 2(cis-2) have been determined by single-crystal X-ray diffraction: cis-1a, monoclinic, space group P21/m, a= 6.408(1), b= 13.58(1), c= 8.004(1)A, β= 93.17(2)°, Z= 2, 2225 measured, 1392 unique observed reflections [I > 2.0σ(I)], R= 0.035, R′= 0.036; cis-2, orthorhombic, space group Pnma, a= 10.346(3), b= 12.313(4), c= 12.590(3)A, Z= 4, 1645 measured, 1115 unique observed reflections [I > 2.0σ(I)], R= 0.027, R′= 0.029. The dynamic behaviour of the two complexes in the solid state has been investigated by variable-temperature 1H spin–lattice relaxation time measurements and 13C magic angle spinning NMR spectroscopy. The activation energies for the reorientational processes have been estimated. The separate intra- and intermolecular contributions to the total reorientational barriers have been evaluated by means of potential-energy barrier calculations within the pairwise atom–atom approach.

18-Crown[6] has been used to prepare new organic–inorganic complexes with Cs2CO3 and aromatic mono-, di- and tricarboxylic acids (benzoic, isophthalic, terephthalic and trimesic acid). The complexes 18-crown[6]·Cs[trimH2] (1), 18-crown[6]·Cs[isoH] (2), 18-crown[6]·Cs2[isoH]2·2H2O (3), (18-crown[6])3·Cs3[trim]·9H2O (4), 18-crown[6]·Cs[ben]·H2O (5) and (18-crown[6])6·Cs4[tereH]2[tere]·5H2O (6) have been characterized by single-crystal X-ray diffraction. The thermal behaviour of 1, 2 and 4 has been investigated by a combination of solid-state techniques including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and variable temperature X-ray powder diffraction (VT-XRPD); in the case of 2 hot stage microscopy was used to clarify its thermal behaviour.

The reaction of the trinuclear anion [HOs 3 (CO) 11 ] − with Ph 2 C 2 unexpectedly leads to the pentanuclear species [H 2 Os 5 (CO) 13 (Ph 2 C 2 )(PhC 2 (C)(C 6 H 4 )], which has been shown by an X-ray crystal structure determination to have a twisted bow-tie arrangement of metal atoms bearing a μ 3 -η 1 -PhCCPh and an orthometallated μ 2 -η 2 -Ph-CC(H)-C 6 H 4 ligands, this ligand being formed by an intramolecular H-transfer reaction from a phenyl group to the acetylene group.

The dynamic behavior in the solid state of (C 6 Me 6 )Cr(CO) 3 , (1,2,3-C 6 H 3 Me 3 )Cr(CO) 3 , and (1,2,4,5-C 6 H 2 Me 4 )Cr(CO) 3 has been investigated by means of variable-temperature 1 H spin−lattice relaxation time T 1 measurements and potential energy barrier calculations. Structural characterization at room temperature by single-crystal X-ray diffraction has been carried out. Crystal data: space group P2 1 /n