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.     

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.     

Steric control over hydrogen bonding in crystalline organic solids: a structural study of N,N'-dialkylthioureas

Hydrogen bonding in crystalline N,N'-dialkylthioureas was examined with the help of single-crystal X-ray diffraction, DFT calculations, and Cambridge Structural Database (CSD) analysis. A CSD survey indicated that unlike the related urea derivatives, which persistently self-assemble into one-dimensional hydrogen-bonded chains, the analogous thioureas can form two different hydrogen-bonding motifs in the solid state: chains, structurally similar with those found in ureas, and dimers, that further associate into hydrogen-bonded layers. The formation of one motif or another can be manipulated by the bulkiness of the organic substituents on the thiourea group, which provides a clear example of steric control over the hydrogen bonding arrangement in crystalline organic solids.     

Orientational control of guest molecules in an organic intercalation system by host polymer tacticity

Four kinds of stereoregular poly(muconic acid)s, which are synthesized by topochemical polymerization and subsequent solid-state hydrolysis, are used as the organic host materials for intercalation. We describe the reaction behavior and layered structure of intercalation compounds using stereoregular poly(muconic acid)s and n-alkylamines as host and guest, respectively. The packing structure of the guest alkylamines was determined by X-ray diffractions as well as IR and Raman spectroscopies. We have found that the orientation of the guest molecules is controlled by the host polymer tacticity, depending on the structure of the two-dimensional hydrogen-bonding network formed in the polymer sheets of the crystals.     

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.     

Thermal Expansion Properties and Mechanochemical Synthesis of Stoichiometric Cocrystals Containing Tetrabromobenzene as a Hydrogen- and Halogen-Bond Donor

The solution and mechanochemical synthesis of two cocrystals that differ in the stoichiometric ratio of the components (stoichiometric cocrystals) is reported. The components in the stoichiometric cocrystals interact through hydrogen or hydrogen/halogen bonds and differ in π-stacking arrangements. The difference in structure and noncovalent interactions affords dramatically different thermal expansion behaviors in the two cocrystals. At certain molar ratios, the cocrystals are obtained concomitantly; however, by varying the ratios, a single stoichiometric cocrystal is achieved using mechanochemistry.     

A Robust Nanoporous Supramolecular Metal–Organic Framework Based on Ionic Hydrogen Bonds

Hydrogen‐bond assembly of tripod‐like organic cations [H₃‐MeTrip]³⁺ (1,2,3‐tri(4′‐pyridinium‐oxyl)‐2‐methylpropane) and the hexa‐anionic complex [Zr₂(oxalate)₇]^6? leads to a structurally, thermally, and chemically robust porous 3D supramolecular framework showing channels of 1 nm in width. Permanent porosity has been ascertained by analyzing the material at the single‐crystal level during a sorption cycle. The framework crystal structure was found to remain the same for the native compound, its activated phase, and after guest resorption. The channels exhibit affinities for polar organic molecules ranging from simple alcohols to aniline. Halogenated molecules and I₂ are also taken up from hexane solutions by this unique supramolecular framework.     

Enantioselective intramolecular crossed Rauhut-Currier reactions through cooperative nucleophilic activation and hydrogen-bonding catalysis: scope and mechanistic insight

A highly efficient and enantioselective intramolecular crossed Rauhut-Currier (RC) reaction of nitroolefins with tethered enonates has been developed through cooperative nucleophilic activation and a hydrogen-bonding catalytic strategy (≤98% ee and 98% yield). The reaction features simple experimental procedures and is completely chemoselective and atom-economic in character. The potential synthetic applications have been demonstrated by the conversion of the RC reaction products into biologically and pharmaceutically valuable compounds with highly diastereoselectivity. In addition, computational investigations were employed to support the proposed mechanism and to obtain a good understanding of the origin of the stereoselectivity in RC reactions.     

Guest-induced supramolecular isomerism in inclusion complexes of T-shaped host 4,4-bis(4'-hydroxyphenyl)cyclohexanone

The T-shaped host molecule 4,4-bis(4'-hydroxyphenyl)cyclohexanone (1) has an equatorial phenol group and a cyclohexanone group along the arms and an axial phenol ring as the stem. The equatorial phenyl ring adopts a "shut" or "open" conformation, like a windowpane, depending on the size of the guest (phenol or o/m-cresol), for the rectangular voids of the hydrogen-bonded ladder host framework. The adaptable cavity of host 1 expands to 11x15-18 A through the inclusion of water with the larger cresol and halophenol guests (o-cresol, m-cresol, o-chlorophenol, and m-bromophenol) compared with a size of 10x13 A for phenol and aniline inclusion. The ladder host framework of 1 is chiral (P2(1)) with phenol, whereas the inclusion of isosteric o- and m-fluorophenol results in a novel polar brick-wall assembly (7x11 A voids) as a result of auxiliary C-H...F interactions. The conformational flexibility of strong O-H...O hydrogen-bonding groups (host 1, phenol guest), the role of guest size (phenol versus cresol), and weak but specific intermolecular interactions (herringbone T-motif, C-H...F interactions) drive the crystallization of T-host 1 towards 1D ladder and 2D brick-wall structures, that is, supramolecular isomerism. Host 1 exhibits selectivity for the inclusion of aniline in preference to phenol as confirmed by X-ray diffraction, 1H NMR spectroscopy, and thermogravimetry-infrared (TG-IR) analysis. The T(onset) value (140 degrees C) of aniline in the TGA is higher than those of phenol and the higher-boiling cresol guests (T(onset)=90-110 degrees C) because the former structure has more O-H...N/N-H...O hydrogen bonds than the clathrate of 1 with phenol which has O-H...O hydrogen bonds. Guest-binding selectivity for same-sized phenol/aniline molecules as a result of differences in hydrogen-bonding motifs is a notable property of host 1. Host-guest clathrates of 1 provide an example of spontaneous chirality evolution during crystallization and a two-in-one host-guest crystal (phenol and aniline), and show how weak C-H...F interactions (o- and m-fluorophenol) can change the molecular arrangement in strongly hydrogen-bonded crystal structures.     

Hetero‐Seeding and Solid Mixture to Obtain New Crystalline Forms

Para‐methyl benzyl alcohol (p‐MeBA II) and para‐chloro benzyl alcohol (p‐ClBA) are quasi‐isostructural and share the same hydrogen‐bond patterns, but their crystals are not isomorphous. No new polymorphs could be obtained by conventional polymorph screening based on different solvents and different crystallization conditions. Formation of a new polymorph of p‐MeBA named p‐MeBA I, isomorphous with the crystal of p‐ClBA, was induced by hetero‐seeding with a small quantity of powdered p‐ClBA added to a supersaturated solution of p‐MeBA in hexane, while seeding of p‐ClBA with p‐MeBA II failed to give a new phase of p‐ClBA isomorphous with known crystalline p‐MeBA II. Mixed crystals of p‐MeBA and p‐ClBA were also prepared with different p‐MeBA/p‐ClBA ratios to understand the role of the different functional groups in the crystal structure. Crystal phases were characterized by combined use of single‐crystal and powder X‐ray diffraction, differential scanning calorimetry, and solid‐state NMR spectroscopy.