Probing Metal Ion Complexation of Ligands with Multiple Metal Binding Sites: The Case of Spiropyrans.

Among known molecular switches, spiropyrans attract considerable interest because of their reversible responsiveness to external stimuli and the deep conformational and electronic changes that characterize the switching process between the two isomeric forms [spiropyran (SP) and merocyanine (MC)]. Metal coordination is one of the most interesting aspects of spiropyrans for its potential in sensing, catalysis, and medicinal chemistry, but little is known about the details surrounding spiropyran-metal ion binding. We investigated the interplay between an N-modified 8-methoxy-6-nitrospiropyran (SP-E), designed to provide appropriate molecular flexibility and a range of competing/collaborative metal binding sites, with Mg2+ , Cu2+ and Zn2+ , which were chosen for their similar coordination geometry preferences while differing in their hard/soft character. The formed molecular complexes were studied by means of UV/Vis, fluorescence, and NMR spectroscopies and mass spectrometry, and the crystal structure of the SP-E-Cu complex was also obtained. The results indicate that the Mg2+ , Zn2+ and Cu2+ complexes have identical coordination stoichiometry. Furthermore, the Mg2+ and Zn2+ complexes display fluorescence properties in solution and visible-light responsiveness. These results provide important spectroscopic and structural information that can serve as a foundation for rational design of spiropyran-based smart materials for metal sensing and scavenging applications.

Crystal engineering of energetic materials: co-crystals of Ethylenedinitramine (EDNA) with modified performance and improved chemical stability.

In the area of energetic materials, co-crystallization is emerging as a new technology for modifying or enhancing the properties of existing energetic substances. Ethylenedinitramine (EDNA) is a known energetic material which requires attention partly due to its chemical instability originating with its two highly acidic protons. In order to stabilize EDNA, a co-crystallization approach targeting the acidic protons using a series of co-crystallizing agents with suitable hydrogen-bond acceptors was employed. Fifteen attempted co-crystallizations resulted in eight successful outcomes and six of these were crystallographically characterized and all showed evidence of hydrogen bonds to the intended protons. Calculated detonation properties and experimental thermal and impact data for the co-crystals were obtained and compared with those of pure EDNA. The co-crystal of EDNA and 1,2-bis(4-pyridyl)ethylene was recognized as a more thermally stable alternative to EDNA while the co-crystal of EDNA and pyrazine N,N'-dioxide showed comparable detonation strengths (and much improved chemical stability) compared with that of EDNA. The co-crystals EDNA:4,4'-bipyridine and EDNA:pyrazine N,N'-dioxide were found to be about 50 % less impact sensitive than EDNA, all of which illustrate how co-crystallizations can be utilized for successfully modifying specific aspects of energetic materials.

Chemical synthesis of tetracyclic terpenes and evaluation of antagonistic activity on endothelin-A receptors and voltage-gated calcium channels.

A class of tetracyclic terpenes was synthesized and evaluated for antagonistic activity of endothelin-1 (ET-1) induced vasoconstriction and inhibitory activity of voltage-activated Ca(2+) channels. Three repeated Robinson annulation reactions were utilized to construct the tetracyclic molecules. A stereoselective reductive Robinson annulation was discovered for the formation of optically pure tricyclic terpenes. Stereoselective addition of cyanide to the hindered α-face of tetracyclic enone (-)-18 was found and subsequent transformation into the aldehyde function was affected by the formation of bicyclic hemiiminal (-)-4. Six selected synthetic tetracyclic terpenes show inhibitory activities in ET-1 induced vasoconstriction in the gerbil spiral modiolar artery with putative affinity constants ranging between 93 and 319 nM. Moreover, one compound, (-)-3, was evaluated further and found to inhibit voltage-activated Ca(2+) currents but not to affect Na(+) or K(+) currents in dorsal root ganglion cells under similar concentrations. These observations imply a dual mechanism of action. In conclusion, tetracyclic terpenes represent a new class of hit molecules for the discovery of new drugs for the treatment of pulmonary hypertension and vascular related diseases.

Ranking relative hydrogen-bond strengths in hydroxybenzoic acids for crystal-engineering purposes.

Systematic co-crystallizations resulting in a total of six new crystal structures involving either 3-hydroxy- or 4-hydroxybenzoic acid, complemented by calculated molecular electrostatic potential surfaces and existing structural data, have shown that in a competitive molecular recognition situation, the -OH moiety is a more effective hydrogen-bond donor than the -COOH moiety which, in turn, highlights that electrostatic charge can offer more useful guidance than acidity for predicting competitive hydrogen-bond preferences.

Supramolecular hierarchy among halogen-bond donors.

Through a combination of structural chemistry, vibrational spectroscopy, and theory, we have systematically examined the relative structure-directing importance of a series of ditopic halogen-bond (XB) donors. The molecular electrostatic potential surfaces of six XB donors were evaluated, which allowed for a charge-based ranking. Each molecule was then co-crystallized with 21 XB acceptors and the results have made it possible to map out the supramolecular landscape describing the competition between I/Br-ethynyl donors, perfluorinated I/Br donors, and I/Br-phenyl based donors. The results offer practical guidelines for synthetic crystal engineering driven by robust and directional halogen bonds.

Synthesis and antiviral activity of substituted quercetins.

Influenza viruses are important pathogens that cause respiratory infections in humans and animals. In addition to vaccination, antiviral drugs against influenza virus play a significant role in controlling viral infections by reducing disease progression and virus transmission. Plant derived polyphenols are associated with antioxidant activity, anti-carcinogenic, and cardio- and neuro-protective actions. Some polyphenols, such as resveratrol and epigallocatechin gallate (EGCG), showed significant anti-influenza activity in vitro and/or in vivo. Recently we showed that quercetin and isoquercetin (quercetin-3-ß-d-glucoside), a glucoside form of quercetin, significantly reduced the replication of influenza viruses in vitro and in vivo (isoquercetin). The antiviral effects of isoquercetin were greater than that of quercetin with lower IC(50) values and higher in vitro therapeutic index. Thus, we investigated the synthesis and antiviral activities of various quercetin derivatives with substitution of C3, C3', and C5 hydroxyl functions with various phenolic ester, alkoxy, and aminoalkoxy moieties. Among newly synthesized compounds, quercetin-3-gallate which is structurally related to EGCG showed comparable antiviral activity against influenza virus (porcine H1N1 strain) to that of EGCG with improved in vitro therapeutic index.

Diaqua-[(1R,2S,4R,8R,9S,11R)-2,9-di-methyl-1,4,8,11-tetra-aza-cyclo-tetra-decane]nickel(II) dichloride dihydrate.

The crystal structure of the title complex, [Ni(C(12)H(28)N(4))(H(2)O)(2)]Cl(2)·2H(2)O, displays O-H⋯Cl and O-H⋯O hydrogen bonding. The tetra-aza-cyclo-tetra-decane ligand inter-acts with the Ni(II) atom in the cis V configuration and the final two ligand binding sites are occupied by water.

Bis-N-heterocyclic carbene palladium(IV) tetrachloride complexes: synthesis, reactivity, and mechanisms of direct chlorinations and oxidations of organic substrates.

This Article describes the preparation and isolation of novel octahedral CH(2)-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPd(IV)Cl(4) [L = (NHC)CH(2)(NHC)] from LPd(II)Cl(2) and Cl(2). In intermolecular, nonchelation-controlled transformations LPd(IV)Cl(4) reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C-H bonds were converted into C-Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE Pd(IV) complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a Pd(IV)-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl(+)-transfer from the pentacoordinated Pd(IV)-intermediate LPd(IV)Cl(3)(+) to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl(2) from a reductive elimination process. Instead, a ligand-mediated direct Cl(+)-transfer from LPd(IV)Cl(3)(+) to the π-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed LPd(IV)Cl(3)(+). The presence of a large excess of added Cl(-) slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both Pd(IV)-Cl ionization and Cl(+)-transfer from LPd(IV)Cl(3)(+). (1)H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl(-)···H-C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center.

Synthesis of 2,24-Diene-12,13,15,16,34,35,37,38-octaphenyl[4.4]-triphenylparacyclophane.

A new octaphenyl[4.4]triphenylparacyclophanediene was readily synthesized in six steps from p-xylene via the installment of bromine atoms, replacement with a vinyl group, carbonylative coupling, intermolecular followed by intramolecular double Grubbs olefin metathesis, Knoevenagel condensation, and Diels-Alder cycloaddition. The belt-shaped structure and trans-stereochemistry of the alkene moieties of the octaphenyl[4.4]triphenylparacyclophane and a synthetic intermediate, 2,21-dioxo-11,30-diene[3.4.3.4]paracyclophane, were determined by X-ray crystallography. The synthetic methodology leading to octaphenyl[4.4]triphenylparacyclophane is applicable for the synthesis of substituted triphenylparacyclophanes and possibly their corresponding bis-hexabenzocoronenylparacyclophanes via a Scholl-Mullen oxidative aryl-aryl coupling reaction.

Synthesis of cyclododeciptycene quinones.

Cycloiptycenes are elusive and synthetically challenging molecules. We report the first synthesis of two substituted cyclododeciptycene tetraquinones via a sequence of intermolecular and intramolecular Diels-Alder reactions from cis,cis-heptiptycene tetraquinone 2 and substituted 7,16-dihydro-7,16-(o-benzeno)heptacenes 3. Heptiptycene tetraquinone 2 was made from triptycene bisquinone 4 and 1,4-dimethoxyanthracene in three steps, and 6,8,15,17-tetramethoxy-7,16-dihydro-7,16-(o-benzeno)heptacene (3a) was synthesized from triptycene bisquinone 4 and 1,4-dihydro-2,3-benzoxathiin-3-oxide in four steps. The structure of a cyclododeciptycene, 1a, was determined by a single-crystal X-ray analysis. The synthetic sequence is general and should allow the incorporation of various alkoxy and acetoxy substituents appended to the cycloiptycene framework.

Using cocrystals to systematically modulate aqueous solubility and melting behavior of an anticancer drug.

Five cocrystals of an anticancer compound have been assembled using a well-defined hydrogen-bond-based supramolecular approach that produced the necessary structural consistency in the resulting solids. These cocrystals contain aliphatic even-numbered dicarboxylic acids of increasing chain length, and as a result, the physical properties of the cocrystals can be related to the molecular structure of the acid. The melting points of the five cocrystals show an excellent correlation with the melting points of the individual acids, and it has also been shown that aqueous solubility can be increased by a factor of 2.5 relative to that of the individual drug. Consequently, cocrystals can offer a range of solid forms from which can be chosen an active ingredient where a particular physical property can be dialed in, provided that the cocrystals show considerable structural consistency and that systematic changes are made to the participating cocrystallizing agents.

Establishing amide...amide reliability and synthon transferability in the supramolecular assembly of metal-containing one-dimensional architectures.

On the basis of a combination of new structural data (eleven single-crystal structure determinations are presented) and information from the Cambridge Structural Database, it has been shown that self-complementary hydrogen-bond based amide...amide dimers can be relied upon as effective supramolecular synthons for the assembly and organization of acac- and paddle-wheel complexes of a variety of metal(II) ions. The targeted molecular recognition event and intended extended one-dimensional motif appear with a supramolecular yield of 78% (a total of 28 structures were examined). Despite the fact that the hydrogen bonds that give rise to the R2(2)(8) motif can be disrupted by both carboxylate- and acac-ligands, as well as by solvent molecules, they remain remarkably resilient and therefore represent useful synthetic tools in inorganic crystal engineering.

Cyanoximes as effective and selective co-crystallizing agents.

The ability of cyanoxime-based synthons to act as versatile synthetic tools for the construction of co-crystals is demonstrated through the preparation and structural characterization of seven co-crystals. Cyanoximes can bind effectively to both five-membered and six-membered N-heterocyclic hydrogen-bond acceptors, and they also display selectivity towards the best acceptor (as determined using semi-empirical AM1 calculations) in ditopic compounds.

Total syntheses of (+/-)-ovalicin, C4(S *)-isomer, and its C5-analogs and anti-trypanosomal activities.

Total syntheses of (+/-)-ovalicin, its C4(S( *))-isomer 44, and C5-side chain intermediate 46 were accomplished via an intramolecular Heck reaction of (Z)-3-(tert-butyldimethylsilyloxy)-1-iodo-1,6-heptadiene and a catalytic amount of palladium acetate. Subsequent epoxidation, dihydroxylation, methylation, and oxidation led to (3S( *),5R( *),6R( *))-5-methoxy-6-(tert-butyldimethylsilyloxy)-1-oxaspiro[2.5]octan-4-one (2), a reported intermediate. The addition of a side chain with cis-1-lithio-1,5-dimethyl-1,4-hexadiene (27) followed by oxidation afforded (+/-)-ovalicin. The functional group manipulation afforded a number of regio- and stereoisomers, which allow the synthesis of analogs for bioevaluation. The structure of 44 was firmly established via a single-crystal X-ray analysis. The stereochemistry at C4 generated from the addition reactions of alkenyllithium with ketones 2, 40, and 45 is dictated by C6-alkoxy functionality. Anti-trypanosomal activities of various ovalicin analogs and synthetic intermediates were evaluated, and C5-side chain analog, 46, shows the strongest activity. Compound 44 shows antiproliferative effect against HL-60 tumor cells in vitro. Compounds 46 and a precursor, (3S( *),4R( *),5R( *),6R( *))-5-methoxy-4-[(E)-(1',5'-dimethylhexa-1',4'-dienyl)]-6-(tert-butyldimethylsilyloxy)-1-oxaspiro[2.5]octan-4-ol (28), may be explored for the development of anti-parasitic drugs.

Modulating the physical properties of solid forms of urea using co-crystallization technology.

The solid-form landscape of urea was explored using full interaction maps (FIMs) and data from the CSD to develop optimum protocols for synthesizing co-crystals of urea. As a result, 49 of the 60 attempted reactions produced new co-crystals, and the crystal structures of four of these are presented. Moreover, the goal of reducing the solubility and lowering the hygroscopicity of the parent compound was achieved, which in turn offers new opportunities for application as a slow-release fertilizer with limited hygroscopicity, thereby reducing many current problems of transport, handling, and storage of urea.

Constructing, deconstructing, and reconstructing ternary supermolecules.

A hypothesis-driven protocol comprising precise and predictable molecular recognition events based upon an electrostatic view of competing hydrogen-bond interactions is proposed and subsequently employed in the construction of ternary co-crystals.

Combining halogen bonds and hydrogen bonds in the modular assembly of heteromeric infinite 1-D chains.

Hydrogen bonds and halogen bonds operate in concert in the directed assembly of infinite 1-D chains in binary co-crystals of iodine iso-nicotinamide (2 : 2), 1 and tetrafluorodiiodobenzene iso-nicotinamide (1 : 2) 2.

A new tecton with parallel halogen-bond donors: a path to supramolecular rectangles.

A new tecton, 1,8-diiodoethynylanthracene, with two halogen-bond donor sites was synthesized and characterized. This tecton is capable of forming two parallel halogen bonds at once, which makes it a useful building block for the construction of a variety of supramolecular squares and rectangles.

Crystal structure of {(R)-N2-[(benzo[h]quinolin-2-yl)meth-yl]-N2'-[(benzo[h]quinolin-2-yl)methyl-idene]-1,1'-binaphthyl-2,2'-di-amine-κ4N,N',N'',N'''}(trifluoromethane-sulfonato-κO)zinc(II)} trifluoromethane-sulfonate di-chloro-methane 1.5-solvate.

The zinc(II) atom in the title compound, [Zn(C48H31N4)(CF3SO3)](CF3SO3)·1.5CH2Cl2, adopts a distorted five-coordinate square-pyramidal geometry. It is coordinated by one tri-fluoro-methane-sulfonate ligand and four N atoms of the N2-[(benzo[h]quinolin-2-yl)meth-yl]-N2'-[(benzo[h]quinolin-2-yl)methyl-idene]-1,1'-binaphthyl-2,2'-di-amine ligand. The complex is present as a single-stranded P-helimer monohelical structure incorporating π-π and/or σ-π inter-actions. One of the imine bonds present in the original ligand framework is reduced, leading to variations in bond lengths and torsion angles for each side of the ligand motif. The imine-bond reduction also affects the bond lengths involving the metal atom with the N-donor atoms located on the imine bond. There are two mol-ecules of the complex in the asymmetric unit. One of the mol-ecules exhibits positional disorder within the coordinating tri-fluoro-methane-sulfonate ion making the mol-ecules symmetric-ally non-equivalent.

Crystal structure of ({(1R,2R)-N,N'-bis-[(quino-lin-2-yl)methyl]cyclo-hexane-1,2-di-amine}-chlorido-iron(III))-µ-oxido-[tri-chlorido-ferrate(III)] chloro-form monosolvate.

The first FeIII atom in the solvated title compound, [Fe2Cl4O(C26H28N4)]·CHCl3, adopts a distorted six-coordinate octa-hedral geometry. It is coordinated by one chloride ligand, four N atoms from the (1R,2R)-N,N'-bis-[(quinolin-2-yl)methyl]cyclo-hexane-1,2-di-amine ligand, and a bridging oxido ligand attached to the second FeIII atom, which is also bonded to three chloride ions. A very weak intra-molecular N-H⋯Cl hydrogen bond occurs. In the crystal, the coordination complexes stack in columns, and a grouping of six such columns create channels, which are populated by disordered chloro-form solvent mol-ecules. Although the Fe-Cl bond lengths for the two metal atoms are comparable to the mean Fe-Cl bond lengths as derived from the Cambridge Structural Database, the Fe-O bond lengths are notably shorter. The solvent chloro-form mol-ecule exhibits 'flip' disorder of the C-H moiety in a 0.544 (3):0.456 (3) ratio. The only directional inter-action noted is a weak C-H⋯Cl hydrogen bond.