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Dario Braga

Publications and source records attributed to Dario Braga.

At least 19 recordsLinked to original sources
PubMed2007

Polymorphism in crystalline cinchomeronic acid.

The structural relationship between the two crystal forms of cinchomeronic acid (CA 3,4-dicarboxypyridine) has been investigated by single crystal X-ray diffraction, IR and Raman spectroscopy and solid state NMR spectroscopy, showing that the two polymorphs form a monotropic system, with the orthorhombic form I being the thermodynamically stable form, while the monoclinic form II is unstable. In both forms CA crystallizes as a zwitterion and decomposes before melting. The crystal structure and spectroscopic analysis indicate that the difference in stability can be ascribed to the strength of the hydrogen-bonding patterns established by the protonated N-atom and the carboxylic/carboxylate O-atoms.

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PubMed2006

Mechanochemical preparation of molecular and supramolecular organometallic materials and coordination networks.

This Dalton Perspective deals with solvent-free reactions taking place within solids or between solids or involving a solid and a vapour. The focus is on reactions involving organometallic and coordination compounds and occurring via reassembling of non-covalent bonding, e.g. hydrogen bonds, and/or formation of ligand-metal coordination bonds. It is argued that reactions activated by mechanical mixing of solid reactants as well as those obtained by exposing a crystalline solid to a vapour can be exploited to "make crystals", which is the quintessence of crystal engineering. It is demonstrated through a number of examples that solvent-free methods, such as co-grinding, kneading, milling of molecular solids, or reactions of solid with vapours represent viable alternative, when not unique, routes for the preparation of novel molecular and supramolecular solids as well as for the preparation of polymorphic or solvate modifications of a same species. The structural characterization of the products requires the preparation of single crystals suitable for X-ray diffraction, a goal often achieved by seeding.

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PubMed2005

Hydrogen bonding and dynamic behaviour in crystals and polymorphs of dicarboxylic-diamine adducts: a comparison between NMR parameters and X-ray diffraction studies.

Fumaric, malonic, maleic, and hydromuconic (HOOCCH2(CH)2CH2COOH) acids were used to prepare a series of hydrogen-bonded adducts or salts, depending on whether acid-base proton transfer takes place, with the dibase [N(muCH2CH2)3N] in various stoichiometric ratios. The resulting compounds have been investigated by using the 1H MAS, 15N, and 13C cross polarisation magic-angle spinning (CPMAS) methods and discussed in relation to X-ray diffraction studies to ascertain the nature of the O-HO, NH-O, and N+-HO- hydrogen bonds between the various species. In addition, two polymorphic forms of the malonic compound and a hydrate in the maleic case were examined. We also present the correlations between the chemical shifts of the hydrogen-bonded protons and those from the proton transfer reaction (acid-to-base) with the heavy atom distances. The dynamic behaviour in the solid-state of the [N(muCH2CH2)3N] adducts with fumaric 2:1, maleic 1:1 hydrate, and hydromuconic acids, and a malonate 2:1 polymorph adduct have been investigated by using variable-temperature 1H spin-lattice relaxation times. A substantial agreement between the activation energies obtained from fitting the T1 data and the results of potential energy barrier calculations demonstrates that the facile reorientation of the [N(muCH2CH2)3N] molecule occurs in several of the adducts.

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PubMed2005

Novel organometallic building blocks for molecular crystal engineering. Part 4. Synthesis and characterization of mono- and bis-amido derivatives of [Co(III)(eta5-C5H4COOH)2]+ and their utilization as ligands.

The synthesis and structural characterization of the hexafluorophosphate salts of the substituted bis-amido molecular complexes [Co(III)(eta5-C5H4CONHC4H3N2)2]+ (1), [Co(III)(eta5-C5H4CONHCH2C5H4N)2]+ (2), [Co(III)(eta5-C5H4CON(C5H4N)2)2]+ (3), and of the amido-carboxyl complexes [Co(III)(eta5-C5H4CON(C5H4N)2)(eta5-C5H4COOH)]+ (4), and [Co(III)(eta5-C5H4CONHC2N3(C5H4N)2)(eta5-C5H4COOH)]+ (5) are reported. The pyridyl and pyrazine substituted amido ligands on the sandwich cores have been chosen because they allow both coordination to metal centres and participation in hydrogen bonding. The hydrogen bonding interactions established by the family of complexes in the solid state has been investigated. The utilization of complex 5 for the preparation of the complex of complexes[Cd(NO3)2{Co(III)(eta5-C5H4CONHC2N3(C5H4N)(C5H4NH))(eta5-C5H4COOH)}2]6+ (6) is reported as a first example of the potential of the substituted mono-and bis-amides as ligands. The isolation and structural characterization of the carbonyl chloride cation [Co(III)(eta5-C5H4COCl)2]+ (7) as its tetrachloro cobaltate anion salt is also described.

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PubMed2005

Making crystals from crystals: a green route to crystal engineering and polymorphism.

Supramolecular reactions between crystalline materials as well as reactions between a crystalline material and a vapour can be used to generate new crystalline substances. These solvent-free processes can be exploited to prepare both hydrogen-bonded co-crystals and coordination networks. Solid-vapour reactions do not differ from solid-vapour uptake/release processes, and can also be used to prepare polymorphs and solvates. It is argued that solvent-less reactions involving molecular crystals represent a green route to supramolecular solid-state chemistry and crystal engineering.

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PubMed2004

Reactions between or within molecular crystals.

Reactions that occur within or between molecular crystals, in particular those reactions that are activated by mechanical methods, are reviewed. The focus is on processes (whether intrasolid or intersolid) that are controlled primarily by supramolecular bonding, such as template cycloadditions, formation of inclusion compounds, reactions between molecular crystals by the reassembling of noncovalent bonds, and the formation of complexes and coordination compounds. It is proposed that solvent-free mechanochemical methods, for example, cogrinding, milling, and kneading, represent viable "green" routes for the preparation of novel molecular and supramolecular solids.

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PubMed2004

Crystal synthesis of hybrid organometallic-inorganic hydrogen bonded salts of acid oxoanions.

Partially deprotonated inorganic oxoanions derived from sulfuric and phosphoric acids have been used to assemble organometallic cations in inorganic-organometallic hybrid systems. The organometallic sandwich cations [(eta(5)-C(5)H(5))(2)Co]+, [(eta(5)-C(5)Me(5))(2)Co]+ and [(eta(5)-C(5)Me(5))(2)Fe]+ have been used because they do not interfere with hydrogen bonding formation forcing self-assembling of the inorganic acids anions HSO(4)(-) and H(2)PO(4)(-) into hydrogen bonded mono- and bi-dimensional networks.

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PubMed2004

Supramolecular complexation of alkali cations through mechanochemical reactions between crystalline solids.

The organometallic zwitterion [Co(III)(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] reacts quantitatively as a solid polycrystalline phase with a number of crystalline alkali salts MX (M = K(+), Rb(+), Cs(+), NH(4) (+); X = Cl(-), Br(-), I(-), PF(6)(-), although not in all cation/anion permutations) to afford supramolecular complexes of the formula [Co(III)(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)](2).M(+)X(-). In some cases, the mechanochemical complexation requires kneading of the two solids with a catalytic amount of water. The characterization of the solid-state products has been achieved by a combination of X-ray single-crystal and powder-diffraction experiments. The hydrogen-bonding interactions have been investigated by solid-state NMR spectroscopy. The mechanochemical reactions imply a profound solid-state rearrangement accompanied by breaking and forming of O-H...O hydrogen-bonding interactions between the organometallic molecules. All compounds could also be obtained by solution crystallization of the inorganic salts in the presence of the organometallic unit. The solid-state complexation of alkali cations by the organometallic zwitterion has been described as a special kind of solvation process taking place in the solid state.

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PubMed2004

1,4-hydroxybiradical behavior revealed through crystal structure-solid-state reactivity correlations.

Structure-reactivity correlations for triplet 1,4-hydroxybiradicals in solution are made difficult by the presence of multiple reactive conformers and the possibility of conformation-dependent intersystem crossing. These problems can be overcome by working in the crystalline state, where the conformations of the 1,4-hydroxybiradicals are fixed and determinable by X-ray crystallography of the parent ketones, assuming that hydrogen atom abstraction occurs with little or no change in conformation. This approach is applied to 15 bi- and tricyclic ketones designed to have slightly different biradical conformations, so that the effect of small and incremental changes in geometry on biradical behavior can be tested. The results indicate that, while geometry does have a strong influence on 1,4-hydroxybiradical partitioning between cyclization, cleavage, and reverse hydrogen transfer, a full understanding of the results requires that the strain involved in forming the cyclization products be taken into account.

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PubMed2004

Solid-state versus solution preparation of two crystal forms of [HN(CH2CH2)3NH][OOC(CH2)COOH]2. Polymorphs or hydrogen bond isomers?

The reaction of [N(CH2CH2)3N] with malonic acid [HOOC(CH2)COOH] in the molar 1:2 ratio yields two different crystal forms of the salt [HN(CH2CH2)3NH][OOC(CH2)COOH]2 depending on the preparation technique and crystallization speed: form I, containing malonate anions with intramolecular hydrogen bonds, is obtained by solid-state co-grinding or by rapid crystallization, whereas form II containing intermolecular hydrogen bonds is obtained by slow crystallization. Form I and II do not interconvert, and form I undergoes an order-disorder phase transition on cooling.

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PubMed2003

Mechanochemical preparation of hydrogen-bonded adducts between the diamine 1,4-diazabicyclo[2.2.2]octane and dicarboxylic acids of variable chain length: an X-ray diffraction and solid-state NMR study.

Mechanical mixing of solid dicarboxylic acids of variable chain length HOOC(CH(2))(n)COOH (n = 1-7) with solid 1,4-diazabicyclo[2.2.2]octane generates the corresponding salts or co-crystals of the formula [N(CH(2)CH(2))(3)N]-H-[OOC(CH(2))(n)COOH] (n=1-7). Preparation of the same systems from solution has been instrumental for a full characterization of the mechanochemical products by means of single-crystal and powder-diffraction X-ray analyses, as well as by solid-state NMR. The acid-base adducts, whether involving proton transfer from the COOH group to the N-acceptor, that is having ((-))O...H-N((+)) interactions, or the formation of neutral O-H...N hydrogen bonds, show a melting point alternation phenomenon analogous to that shown by the neutral carboxylic acids. The carbon chemical shift tensors of the COOH group obtained from the sideband intensity of low speed spinning NMR spectra provide a reliable criterion for assigning the protonation state of the adducts.

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PubMed2003

Assembly of hybrid organic-organometallic materials through mechanochemical acid-base reactions.

Manual grinding of the organometallic complex [Fe(eta(5)-C(5)H(4)COOH)(2)] with a number of solid bases, namely 1,4-diazabicyclo[2.2.2]octane, C(6)H(12)N(2), 1,4-phenylenediamine, p-(NH(2))(2)C(6)H(4), piperazine, HN(C(2)H(4))(2)NH, trans-1,4-cyclohexanediamine, p-(NH(2))(2)C(6)H(10), and guanidinium carbonate [(NH(2))(3)C](2)[CO(3)], generates quantitatively the corresponding adducts, [HC(6)H(12)N(2)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (1), [HC(6)H(8)N(2)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (2), [H(2)C(4)H(10)N(2)][Fe(eta(5)-C(5)H(4)COO)(2)] (3), [H(2)C(6)H(14)N(2)][Fe(eta(5)-C(5)H(4)COO)(2)].2 H(2)O, (4.2 H(2)O), and [C(NH(2))(3)](2)[Fe(eta(5)-C(5)H(4)COO)(2)].2 H(2)O, (5.2 H(2)O), respectively. Crystallization from methanol in the presence of seeds of the ground sample allows the growth of single crystals of these adducts; therefore we were able to determine the structures of the adducts by single-crystal X-ray diffraction. This information was used in turn to identify and characterize the polycrystalline materials obtained by the grinding process. In the case of [HC(6)N(2)H(12)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (1), the base can be removed by mild treatment regenerating the starting dicarboxylic acid, while in all other cases decomposition is observed. The solid-solid processes described herein imply molecular diffusion through the lattice, breaking and reassembling of hydrogen-bonded networks, and proton transfer from acid to base.

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PubMed2002

Mechanochemical assembly of hydrogen bonded organic-organometallic solid compounds.

Solvent-free reactions with molecular systems have been exploited to prepare hybrid organic-organometallic solids: grinding of the complex [Fe(eta 5-C5H4COOH)2] with solid bases B generates quantitatively the corresponding hydrogen bonded salts [Fe(eta 5-C5H4COOH)(eta 5-C5H4COO)][HB] (B = 1,4-diazabicyclo[2.2.2]octane, 1,4-phenylenediamine); gas-solid reactions are also possible with volatile bases.

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