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1.  1,4-Dihydro­benzo[g]quinoxaline-2,3-dione 
The title compound, C12H8N2O2, was prepared by the reaction of the diethyl ester of naphthalene­bis­(oxamate) with tert-BuNH2. The mol­ecule is nearly planar, with an r.m.s. deviation of 0.017 Å from the plane through all 16 non-H atoms. In the crystal, a three-dimensional network is formed, composed of layers of mol­ecules along the b- and c-axis directions, due to the formation of inter­molecular N—H⋯O hydrogen bonds, as well as of chains along the a-axis direction due to parallel displaced sandwich-type π–π inter­actions with average distances between the inter­acting mol­ecules in the range 3.35–3.40 Å.
doi:10.1107/S1600536812047526
PMCID: PMC3589036  PMID: 23476272
2.  Acridin-10-ium 6-carb­oxy­pyridine-2-carboxyl­ate 
The title compound, C13H10N+·C7H4NO4 −, consists of a protonated acridinium cation and a 6-carb­oxy­pyridine-2-carboxyl­ate monoanion. The carboxyl­ate group of the anion appears to be delocalized on the basis of the nearly equivalent C—O bond lengths. In the crystal, the anions are connected by strong O—H⋯O hydrogen bonds, forming chains along the b axis. The acridinium cations are linked to the anionic chains by strong N—H⋯O hydrogen bonds between the carboxyl­ate group of the anion and the N—H group of the cation. Along the b axis, successive chains stack in opposite directions. Weak inter­molecular C—H⋯O hydrogen bonds further stabilize the crystal structure.
doi:10.1107/S1600536811053578
PMCID: PMC3254532  PMID: 22259478
3.  catena-Poly[[aqua­lithium(I)]-μ-3-carb­oxy-5,6-di­methyl­pyrazine-2-carboxyl­ato-κ4 O 2,N 1:O 3,N 4] 
The asymmetric unit of the title compound, [Li(C8H6N2O4)(H2O)]n, comprises three Li cations, two of which are located on a twofold rotation axis, two carboxylate anions and three water mol­ecules, of which two are situated on the twofold rotation axis being aqua ligands. Both carboxylate anions are in μ2-bridging mode. All Li ions show a trigonal–bipyramidal coordination mode; the two located in special positions are bridged through N,O-bonding sites generating a polymeric ribbon along the c-axis direction. The Li cation in a general position creates an independent polymeric ribbon through N,O-bonding sites of the two symmetry-related ligands; the trigonal–bipyramidal coordination is completed by an aqua ligand. In both carboxylate anions, a carboxyl­ate and a carb­oxy­lic acid group form an intra­molecular hydrogen bond. The polymeric ribbons running along [001] are inter­connected by hydrogen bonds in which the water mol­ecules act as donors and carboxyl­ate O atoms act as acceptors, giving rise to a three-dimensional architecture.
doi:10.1107/S1600536813030493
PMCID: PMC3884992  PMID: 24454167
4.  Adenin-1-ium hydrogen isophthalate di­methyl­formamide monosolvate 
In the title proton-transfer organic salt, C5H6.3N5 +·C8H4.7O4 −·C3H7NO, the adeninium moiety is protonated at the N atom in the 1-position of the 6-amino-7H-purin-1-ium (adeninium) cation. In the solid state, the second acidic proton of isophthalic acid is partially transferred to the imidazole N atom of the adeninium cation [refined O—H versus N—H ratio = 0.70 (11):0.30 (11)]. Through the partially transferred proton, the adeninium cation is strongly hydrogen bonded (N—H⋯O/O—H⋯N) to the isophthalate anion. This strong inter­action is assisted by another N—H⋯O hydrogen bond originating from the adeninium NH2 group towards the isophthalate keto O atom, with an R 2 2(8) graph-set motif. This arrangement is linked via N—H⋯O hydrogen bonds to the O atoms of the carboxyl­ate group of an isophthalate anion. Together, these hydrogen bonds lead to the formation criss-cross zigzag isophthalate⋯adeninium chains lying parallel to (501) and (50-1). The adeninium cations and the isophthalate anions are arranged in infinite π stacks that extend along the c-axis direction [inter­planar distance = 3.305 (3) Å]. Mol­ecules are inclined with respect to this direction and within the stacks they are offset by ca. half a mol­ecule each. Combination of the N—H⋯O and O—H⋯N hydrogen bonds with the π–π inter­actions forms infinitely stacked isophthalate⋯adeninium chains, thus leading to a two-dimensional supra­molecular structure with parallel inter­digitating layers formed by the π stacked isophthalate⋯adeninium chains. The DMF mol­ecules of crystallization are bonded to the adeninium cations through strong N—H⋯O hydrogen bonds and project into the lattice space in between the anions and cations. There are also C—H⋯O hydrogen bonds present which, combined with the other inter­actions, form a three-dimensional network. The crystal under investigation was found to be split and was handled as if non-merohedrally twinned.
doi:10.1107/S1600536813034971
PMCID: PMC3998323  PMID: 24764884
5.  Bis(dimethyl­ammonium) 3,3′-dicarb­oxy-5,5′-(5,7,12,14-tetra­oxo-6,13-diaza­tetra­cyclo­[6.6.2.04,16.011,15]hexa­deca-1,3,8,10,15-penta­ene-6,13-di­yl)dibenzoate dihydrate 
The title compound, 2C2H8N+·C30H12N2O12 2−·2H2O, comprises dimethyl­ammonium cations, 3,3′-dicarb­oxy-5,5′-(5,7,12,14-tetra­oxo-6,13-diaza­tetra­cyclo­[6.6.2.04,16.011,15]hexa­deca-1,3,8,10,15-penta­ene-6,13-di­yl)dibenzoate dianions and water mol­ecules. The dianion is situated on a crystallographic inversion centre. Two very strong symmetry-restricted O⋯H⋯O hydrogen bonds are present which are situated about the crystallographic inversion centres. In one of these hydrogen bonds, the H atom is situated at its centre, while in the other one the H atom is disordered about its centre. Both H atoms are involved in the chain-like C 2 2(16) motif, and not in a more common motif R 2 2(8) that is composed of a pair of hydrogen carboxyl­ates with the H atoms situated about the centre between the pair of O atoms. In the crystal, inter­action of these hydrogen bonds results in formation of anionic layers of dianions parallel to (-111). The water mol­ecules donate their H atoms to one of two of the carboxyl­ate O atoms, forming strong hydrogen bonds. The dimethyl­ammonium donates a bifurcated hydrogen bond to an oxo group of the dianion, forming weak hydrogen bonds. All the hydrogen bonds form a three-dimensional hydrogen-bonded network.
doi:10.1107/S1600536812025470
PMCID: PMC3393979  PMID: 22798844
6.  Short strong hydrogen bonds in proteins: a case study of rhamnogalacturonan acetylesterase 
The short hydrogen bonds in rhamnogalacturonan acetylesterase have been investigated by structure determination of an active-site mutant, 1H NMR spectra and computational methods. Comparisons are made to database statistics. A very short carboxylic acid carboxylate hydrogen bond, buried in the protein, could explain the low-field (18 p.p.m.) 1H NMR signal.
An extremely low-field signal (at approximately 18 p.p.m.) in the 1H NMR spectrum of rhamnogalacturonan acetylesterase (RGAE) shows the presence of a short strong hydrogen bond in the structure. This signal was also present in the mutant RGAE D192N, in which Asp192, which is part of the catalytic triad, has been replaced with Asn. A careful analysis of wild-type RGAE and RGAE D192N was conducted with the purpose of identifying possible candidates for the short hydrogen bond with the 18 p.p.m. deshielded proton. Theor­etical calculations of chemical shift values were used in the interpretation of the experimental 1H NMR spectra. The crystal structure of RGAE D192N was determined to 1.33 Å resolution and refined to an R value of 11.6% for all data. The structure is virtually identical to the high-resolution (1.12 Å) structure of the wild-type enzyme except for the interactions involving the mutation and a disordered loop. Searches of the Cambridge Structural Database were conducted to obtain information on the donor–acceptor distances of different types of hydrogen bonds. The short hydrogen-bond inter­actions found in RGAE have equivalents in small-molecule structures. An examination of the short hydrogen bonds in RGAE, the calculated pK a values and solvent-accessibilities identified a buried carboxylic acid carboxylate hydrogen bond between Asp75 and Asp87 as the likely origin of the 18 p.p.m. signal. Similar hydrogen-bond interactions between two Asp or Glu carboxy groups were found in 16% of a homology-reduced set of high-quality structures extracted from the PDB. The shortest hydrogen bonds in RGAE are all located close to the active site and short interactions between Ser and Thr side-chain OH groups and backbone carbonyl O atoms seem to play an important role in the stability of the protein structure. These results illustrate the significance of short strong hydrogen bonds in proteins.
doi:10.1107/S0907444908017083
PMCID: PMC2483496  PMID: 18645234
short hydrogen bonds; low-field NMR signals; rhamnogalacturonan acetylesterase
7.  catena-Poly[[diaqua­rubidium(I)](μ2-3-carboxy­pyrazine-2-carboxyl­ato)(μ2-pyrazine-2,3-dicarboxylic acid)] 
The structural unit of the title compound, [Rb(C6H3N2O4)(C6H4N2O4)(H2O)2]n, consists of one rubidium cation, one hydrogen pyrazine-2,3-dicarboxyl­ate anion, one pyrazine-2,3-dicarboxylic acid mol­ecule and two water mol­ecules. This formulation is repeated twice in the asymmetric unit as the rubidium cation lies on an inversion centre. Each anion or acid mol­ecule is linked to two rubidium cations, while the rubidium cation has close contacts to four symmetry-equivalent organic ligands, with two different coordination modes towards this cation. In addition, each rubidium cation is coordinated by two water O atoms, raising the coordination number to eight. One of the carboxyl groups of the acid holds its H atom, which forms a hydrogen bond to a coordinated water mol­ecule. The other carboxyl group is deprotonated in half of the ligands and protonated in the other half, taking part in a strong O—H⋯O hydrogen bond disordered over an inversion centre. The stabil­ization of the crystal structure is further assisted by O—H⋯O and O—H⋯N hydrogen-bonding inter­actions involving the water mol­ecules and carboxyl­ate O atoms.
doi:10.1107/S1600536809002001
PMCID: PMC2968173  PMID: 21581808
8.  catena-Poly[[di­aqua­bis­(4-formyl­benzoato-κO 1)copper(II)]-μ-pyrazine-κ2 N:N′] 
In the title polymeric compound, [Cu(C8H5O3)2(C4H4N2)(H2O)2]n, the CuII atom is located on a twofold rotation axis and has a slightly distorted octa­hedral coordination sphere. In the equatorial plane, it is coordinated by two carboxyl­ate O atoms of two symmetry-related monodentate formyl­benzoate anions and by two N atoms of the bridging pyrazine ligand, which is bis­ected by the twofold rotation axis. The axial positions are occupied by two O atoms of the coordinating water mol­ecules. In the formyl­benzoate anion, the carboxyl­ate group is twisted away from the attached benzene ring by 6.2 (2)°, while the benzene and pyrazine rings are oriented at a dihedral angle of 68.91 (8)°. The pyrazine ligands bridge the CuII cations, forming polymeric chains running along the b-axis direction. Strong intra­molecular O—H⋯O hydrogen bonds link the water mol­ecules to the carboxyl­ate O atoms. In the crystal, O—Hwater⋯Owater hydrogen bonds link adjacent chains into layers parallel to the bc plane. The layers are linked via C—Hpyrazine⋯Oform­yl hydrogen bonds, forming a three-dimensional network. There are also weak C—H⋯π inter­actions present.
doi:10.1107/S1600536813032297
PMCID: PMC3914040  PMID: 24526956
9.  Conformational heterogeneity within the Michaelis complex of lactate dehydrogenase† 
The journal of physical chemistry. B  2011;115(23):7670-7678.
A series of isotope edited IR measurements, both static as well as temperature jump relaxation spectroscopy, are performed on lactate dehydrogenase (LDH) to determine the ensemble of structures available to its Michaelis complex. There clearly has been a substantial reduction in the number of states available to the pyruvate substrate (as modeled by the substrate mimic, oxamate) and NADH when bound to protein compared to dissolved in solution, as determined by the bandwidths and positions of the critical C2=O band of bound substrate mimic and the C4-H stretch of NADH reduced nicotinamide group. Moreover, it is found that a strong ionic bond (characterized by a signature IR band discovered in this study) is formed between the carboxyl group of bound pyruvate with (presumably) Arg171, forming a strong ‘anchor’ within the protein matrix. However, conformational heterogeneity within the Michaelis complex is found that has an impact on both catalytic efficiency and thermodynamics of the enzyme.
doi:10.1021/jp2015929
PMCID: PMC3111758  PMID: 21568287
10.  (R)-Doxylaminium (R,R)-tartrate 
In the title compound (systematic name: (R)-dimeth­yl{2-[1-phenyl-1-(pyridin-2-yl)eth­oxy]eth­yl}aza­nium (R,R)-3-carb­oxy-2,3-dihy­droxy­propano­ate), C17H23N2O+·C4H5O6 −, the doxylaminium cation is protonated at the N atom. The tartrate monoanions are linked by short, almost linear O—H⋯O hydrogen bonds into chains extended along [100]. These chains are inter­linked by anion–pyridine O—H⋯N hydrogen bonds into a two-dimensional grid structure. WeakC—H⋯O inter­actions also play a role in the crystal packing. An intra­molecular hy­droxy–carboxyl­ate O—H⋯O hydrogen bond influences the conformation of the anion: the hydrogen-bonded fragment is almost planar, the maximum deviation from the mean plane being 0.059 (14) Å. In the cation, the aromatic rings are almost perpendicular [dihedral angle = 84.94 (8)°] and the conformation of the O—C—C—N chain is gauche(−), the dihedral angle is −76.6 (2)°. The absolute configuration was assigned on the basis of known chirality of the parent compound.
doi:10.1107/S160053681200935X
PMCID: PMC3344015  PMID: 22589924
11.  Crystal structure of anhydrous poly[bis­(μ2-sarcosinato-κ3 O,N:O′)copper(II)] 
The copper(II) ion of the anhydrous form of bis­(sarcosinato)copper(II) exhibits a [4 + 2] coordination sphere with four shorter equatorial bonds to the N and carboxyl­ate O atoms of two sarcosinate anions, and two longer axial bonds to O atoms of neighboring complexes, leading to a sheet structure parallel to (001).
The title compound, [Cu(C3H6NO2)2]n, is a bis-complex of the anion of sarcosine (N-methyl­glycine). The asymmetric unit consists of a copper(II) ion, located on a center of inversion, and one mol­ecule of the uninegative sarcosinate anion. The copper(II) ion exhibits a typical Jahn–Teller distorted [4 + 2] coordination geometry. The four shorter equatorial bonds are to the nitro­gen and carboxyl­ate O atoms of two sarcosinate anions, and the longer axial bonds are to carboxyl­ate O atoms of neighboring complexes. The overall structure is made up from two chains formed by these longer axial Cu—O bonds, one extending parallel to [011] and the other parallel to [0-11]. Each one-dimensional array is connected by the equatorial bridging moieties to the chains on either side, creating an extended two-dimensional framework parallel to (100). There is a single inter­molecular hydrogen-bonding inter­action within the sheets between the amino NH group and an O atom of an adjacent mol­ecule.
doi:10.1107/S1600536814020418
PMCID: PMC4257172  PMID: 25484653
crystal structure; anhydrous bis­(sarcosinato)copper(II); non-proteinogenic amino acid
12.  2-Amino-5-methyl­pyridinium 2-carb­oxy­benzoate 
In the title salt, C6H9N2 +·C8H5O4 −, the hydrogen phthalate anion is essentially planar, with a maximum deviation of 0.011 (2) Å. In the crystal structure, the protonated N atom of the pyridine ring and the 2-amino group of the cation are hydrogen bonded to the carboxyl­ate O atoms of the anion via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. In the hydrogen phthalate anion, there is a very strong, almost symmetric, intra­molecular O—H⋯O hydrogen bond, generating an S(7) motif [O⋯O = 2.382 (3) Å]. Furthermore, these two molecular motif rings are connected by a bifurcated N—H⋯(O,O) hydrogen-bonded motif R 1 2(4), forming a supra­molecular ribbon along the b axis. The crystal structure is further stabilized by π–π inter­actions between the cations and anions [centroid–centroid distance = 3.6999 (10) Å].
doi:10.1107/S160053681003000X
PMCID: PMC3008103  PMID: 21588569
13.  Poly[[diaqua­bis­[μ4-5-nitro­isophthalato-κ4 O 1:O 1:O 3:O 3′]bis­[μ3-pyridine-4-carboxyl­ato-κ3 O:O′:N]tricobalt(II)] tetra­hydrate] 
The title compound, {[Co3(C6H4NO2)2(C8H3NO6)2(H2O)2]·4H2O}n, exhibits a two-dimensional layer-like structure in which the CoII ions exhibit two kinds of coordination geometries. One nearly octa­hedral CoII ion with crystallographic inversion symmetry is coordinated to six carboxyl­ate O atoms from four bridging 5-nitro­isophthalate (NIPH) ligands and two isonicotinate (IN) anions, while the other type of CoII ion binds with one N atom and one carboxyl­ate O atom from two IN anions, two carboxyl­ate O atoms from two different NIPH anions and one ligated water mol­ecule, displaying a distorted square-pyramidal coordination geometry. Three adjacent CoII ions are bridged by six carboxyl­ate groups from four NIPH ligands and two IN anions to form a linear trinuclear secondary building unit (SBU). Every trinuclear SBU is linked to its nearest neighbours in the ab plane, resulting in a two-dimensional layer-like structure perpendicular to the c axis. Along the a-axis direction neighbouring mol­ecules are connected through carboxyl­ate and pyridyl units of the IN anions, along the b axis through carboxyl­ate groups of the NIPH ligands. The H atoms of one free water mol­ecule are disordered in the crystal in a 1:1 ratio. Typical O—H⋯O hydrogen bonds are observed in the lattice, which include the following contacts: (a) between coordinated water mol­ecules and carboxyl­ate O atoms of the NIPH anions, (b) between lattice water mol­ecules and carboxyl­ate O atoms of the NIPH anions, and (c) between coordinated and lattice water mol­ecules. These inter­molecular hydrogen bonds connect the two-dimensional layers to form a three-dimensional supra­molecular structure.
doi:10.1107/S1600536812011269
PMCID: PMC3343849  PMID: 22589823
14.  Diaqua­bis(2,2′-biimidazole)cobalt(II) 4,4′-dicarboxy­biphenyl-3,3′-di­car­boxylate 
In the title compound, [Co(C6H6N4)2(H2O)2](C16H8O8), the CoII cation and the organic anion occupy different crystallographic inversion centres and, as a consequence, the asymmetric unit comprises two half-mol­ecules. The benzene groups are coplanar. The four coordinating N atoms of the two bidentate biimidazole ligands define the equatorial plane of a slightly distorted octa­hedral CoO2N4 geometry, and the water O atoms lie in the axial coordination sites. Translational (a,) and inversion-related symmetry operations link the Co complex mol­ecules and the negatively charged carboxyl­ate anions via inter­molecular N—H⋯O and O—H⋯O hydrogen bonds into sheets parallel to (01). The coordinated water mol­ecules connect the sheets through O—H⋯O hydrogen bonds, forming a three-dimensional framework. In addition, two intra­molecular O—H⋯O hydrogen bonds are observed between the carboxyl and carboxyl­ate groups.
doi:10.1107/S1600536809007764
PMCID: PMC2968794  PMID: 21582332
15.  9-Amino­acridin-10-ium 4-amino­benzo­ate dihydrate 
The asymmetric unit of the title hydrated salt, C13H11N2 +·C7H6NO2 −·2H2O, consists of two independent 9-amino­acridinium cations, two 4-amino­benzoate anions and four water mol­ecules. Both 9-amino­acridinium cations are essentially planar, with maximum deviations of 0.034 (1) and 0.025 (2) Å, and are protonated at the pyridine N atoms. The 4-amino­benzoate anions are approximately planar, with dihedral angles of 9.16 (19) and 5.4 (2)° between the benzene ring and the carboxyl­ate group. In the crystal, the two independent anions are connected by N—H⋯O hydrogen bonds, forming a layer parallel to (100). The layers are connected through the cations by N—H⋯N and N—H⋯O hydrogen bonds. The water mol­ecules, which form O—H⋯O hydrogen-bonded chains along the b-axis direction, connect the anions and the cations by O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds. The crystal structure also features π–π inter­actions [centroid–centroid distances = 3.6343 (9)–3.8366 (10) Å] and a C—H⋯π inter­action.
doi:10.1107/S160053681401023X
PMCID: PMC4051093  PMID: 24940241
16.  Isopropyl­aminium 2-carb­oxy-4,5-di­chloro­benzoate 
In the structure of the 1:1 proton-transfer compound of isopropyl­amine with 4,5-dichloro­phthalic acid, C3H10N+·C8H3Cl2O4 −, the three cation H-atom donors associate with three separate carboxyl O-atom anion acceptors, giving conjoint cyclic R 4 4(12), R 4 4(16) hydrogen-bonding cation–anion inter­actions in a one-dimensional ribbon structure. In the anions, the carboxyl groups lie slightly out of the plane of the benzene ring [maximum deviations = 0.439 (1) for a carboxylic acid O atom and 0.433 (1) Å for a carboxyl­ate O atom]. However, the syn-related proton of the carboxylic acid group forms the common short intra­molecular O—H⋯Ocarbox­yl hydrogen bond.
doi:10.1107/S1600536809052672
PMCID: PMC2980220  PMID: 21580024
17.  catena-Poly[[di­aqua­bis­(4-formyl­benzo­ato-κO 1)nickel(II)]-μ-pyrazine-κ2 N:N′] 
In the title polymeric compound, [Ni(C8H5O3)2(C4H4N2)(H2O)2]n, the NiII atom is located on a twofold rotation axis and has a slightly distorted octa­hedral coordination sphere. In the equatorial plane, it is coordinated by two carboxyl­ate O atoms of two symmetry-related monodentate formyl­benzoate anions and by two N atoms of the bridging pyrazine ligand, which is bis­ected by the twofold rotation axis. The axial positions are occupied by two O atoms of the coordinating water mol­ecules. In the formyl­benzoate anion, the carboxyl­ate group is twisted away from the attached benzene ring by 7.0 (6)°, while the benzene and pyrazine rings are oriented at a dihedral angle of 66.2 (3)°. The pyrazine ligands bridge the NiII cations, forming polymeric chains running along the b-axis direction. Intra­molecular O—H⋯O hydrogen bonds link the water ligands to the carboxyl­ate O atoms. In the crystal, water–water O—H⋯O hydrogen bonds link adjacent chains into layers parallel to the bc plane. Pyrazine–formyl C—H⋯O hydrogen bonds link the layers, forming a three-dimensional network. There are also weak C—H⋯π inter­actions present. The title compound is isotypic with the copper(II) complex [Çelik et al. (2014a). Acta Cryst. E70, m4–m5].
doi:10.1107/S160053681400155X
PMCID: PMC3998267  PMID: 24764828
18.  catena-Poly[[lithium-μ2-(di­hydrogen pyrazine-2,3,5,6-tetra­carboxyl­ato)-κ6 O 2,N 1,O 6;O 3,N 4,O 5-lithium-di-μ-aqua-κ4 O:O] 2.5-hydrate] 
The title coordination polymer, {[Li2(C8H2N2O8)(H2O)2]·2.5H2O}n, is built up from mol­ecular ribbons propagating in the c-axis direction of the ortho­rhom­bic unit cell; the ligand bridges two Li+ ions using both its N,O,O′-bonding sites and adjacent Li+ ions are bridged by pairs of water mol­ecules. The coordination geometry of the metal ion is distorted trigonal bipyramidal, with the ligand O atoms in the axial sites. Two of the carboxyl­ate groups of the ligand remain protonated and form short symmetric O—H⋯O hydrogen bonds. In the crystal, the ribbons inter­act via a network of O—H⋯O hydrogen bonds in which coordinating water mol­ecules act as donors and carboxyl­ate O atoms within adjacent ribbons act as acceptors, giving rise to a three-dimensional framework. O—H⋯N inter­actions are also observed. The asymmetric unit contains quarter of the ligand and the complete ligand has 2/m symmetry; the Li+ ion lies on a special position with m.. site symmetry. Both bridging water mol­ecules have m2m site symmetry and both lattice water mol­ecules have m.. site symmetry; one of the latter was modelled with a site occupancy of 0.25.
doi:10.1107/S160053681401174X
PMCID: PMC4050999  PMID: 24940214
19.  Crystal structure of (E)-4-{2-[4-(all­yloxy)phen­yl]diazen­yl}benzoic acid 
The title compound has an E conformation about the azo­benzene linkage and the benzene rings are almost coplanar to one another [dihedral angle = 1.36 (7)°]. In the crystal, a combination of O—H⋯O and C—H⋯O hydrogen bonds and C—H⋯π inter­actions leads to the formation of slabs parallel to (001).
The title compound, C16H14N2O3, has an E conformation about the azo­benzene [—N=N– = 1.2481 (16) Å] linkage. The benzene rings are almost coplanar [dihedral angle = 1.36 (7)°]. The O atoms of the carb­oxy­lic acid group are disordered over two sets of sites and were refined with an occupancy ratio of 0.5:0.5. The two disordered components of the carb­oxy­lic acid group make dihedral angles of 1.5 (14) and 3.8 (12)° with the benzene ring to which they are attached. In the crystal, mol­ecules are linked via pairs of O—H⋯O hydrogen bonds, forming inversion dimers. The dimers are connected via C—H⋯O hydrogen bonds, forming ribbons lying parallel to [120]. These ribbons are linked via C—H⋯π inter­actions, forming slabs parallel to (001).
doi:10.1107/S1600536814023745
PMCID: PMC4257410  PMID: 25552976
crystal structure; azo­benzene; benzoic acid; liquid crystal; nematic phase
20.  Poly[hexa­aqua­(μ9-cyclo­hexane-1,2,3,4,5,6-hexa­carboxyl­ato)trimanganese(II)] 
The asymmetric unit of the title compound, [Mn3(C12H6O12)(H2O)6]n, comprises one MnII ion, one third of a cyclo­hexane-1,2,3,4,5,6-hexa­carboxyl­ate anion and two aqua ligands. The anion is completed by application of a -3 axis. The MnII ion is six-coordinated by six O atoms from two aqua ligands and three different cyclo­hexa­carboxyl­ate anions in an octa­hedral geometry. The six carboxyl­ate groups adopt a bridging bidentate mode to ligate the MnII ions. Thus, each cyclo­hexane-1,2,3,4,5,6-hexa­carboxyl­ate anion adopts a μ9-connected mode, ligating nine different MnII ions and forming a three-dimensional framework. In the framework, there are strong O—H⋯O hydrogen-bonding inter­actions, which further stabilize the crystal structure.
doi:10.1107/S1600536813015626
PMCID: PMC3772420  PMID: 24046563
21.  Poly[[tetra­aqua­tetra­kis­[μ3-5-(pyridine-4-carboxamido)­isophthalato]cobalt(II)dierbium(III)] tetra­hydrate] 
In the centrosymmetric polymeric title compound, {[CoEr2(C14H8N2O5)4(H2O)4]·4H2O}n, the ErIII cation has a coordination number of eight and is surrounded by seven carboxyl­ate O atoms from four 5-(pyridine-4-carboxamido)­isophthalate (L) ligands and one water mol­ecule, forming a distorted square-anti­prismatic arrangement. The CoII cation is located on an inversion center and is coordinated by two pyridine N atoms, two carboxyl­ate O atoms and two water mol­ecules in a distorted octa­hedral geometry. The asymmetric unit contains two anionic L ligands. One bridges two ErIII cations and one CoII cation through two carboxyl­ate groups and one pyridine N atom, while the other bridges two ErIII cations and one CoII cation through two carboxyl­ate groups. Extensive O—H⋯O, O—H⋯N and N—H⋯O hydrogen-bonding inter­actions are present in the crystal, involving all uncoordinated water mol­ecules and the uncoordinated pyridine N atom of one of the ligands bonded to an adjacent coordinated water mol­ecule. The title compound is isotypic with the gadolinium analogue.
doi:10.1107/S1600536811039948
PMCID: PMC3246915  PMID: 22219735
22.  4-[Bis(4-fluoro­phen­yl)meth­yl]-1-[(2E)-3-phenyl­prop-2-en-1-yl]piperazin-1-ium 3-carb­oxy­propano­ate 
In the title salt, C26H27F2N2 +·C4H5O4 −, the piperazine N atom bearing the vinylic substituent is protonated. The piperazine ring adopts a chair conformation. In ther crystal, the succinate monoanions are connected via short O—H⋯O hydrogen bonds between the carb­oxy­lic acid and carboxyl­ate groups into undulating chains extending along [001] and the flunarizinium monocations are attached to these chains via N+—H⋯O− hydrogen bonds. C—H⋯O inter­actions connect these chains into a three-dimensional network. The shortest centroid–centroid distance of 3.7256 (10) Å was found between one of the fluorinated benzene rings and the non-fluorinated phenyl ring in the neighbouring mol­ecule related by a glide plane.
doi:10.1107/S1600536813000706
PMCID: PMC3569791  PMID: 23424537
23.  Bis(2-carb­oxy-N-{[1-(2-hy­droxy­eth­yl)-3,3-dimethyl­indolin-2-yl­idene]methyl­imino}­anilinium) sulfate monohydrate 
The asymmetric unit of the title compound, 2C20H22N3O3 +·SO4 2−·H2O, contains four cations, two sulfate anions and two lattice water mol­ecules. One of the four cations shows a different conformation of the hy­droxy­ethyl group; the remaining three are all essentially superimposable. Two cations exhibit two-site orientational disorder [ratios = 0.524 (5):0.476 (5) and 0.616 (6):0.384 (6)] of the last two atoms of their hy­droxy­ethyl groups, and one water mol­ecule is disordered over two positions in a 0.634 (13):0.366 (13) ratio. Each imine H atom is intra­molecularly in contact with the adjacent carboxyl O atom, forming an S(6) motif, while all the carb­oxy­lic acid H atoms are hydrogen bonded to O atoms of the sulfate anions. Other notable hydrogen-bond inter­actions involve (methyl­ene, phenyl and imine chain) C—H⋯O (sulfate and carbox­yl) and O—H⋯O(water) contacts, making up a comprehensive three-dimensional network involving D 2 2(n), with n = 4–6 and 15–16, and C 2 2(17) classical hydrogen-bond motifs. The crystal investigated was twinned by pseudomerohedry with a twin component ratio of 0.4745 (12):0.5255 (12).
doi:10.1107/S1600536813003188
PMCID: PMC3588437  PMID: 23476537
24.  Crystal structures and hydrogen bonding in the proton-transfer salts of nicotine with 3,5-di­nitro­salicylic acid and 5-sulfosalicylic acid 
The crystal structures of the 1:1 salts of nicotine with 3,5-di­nitro­salicylic acid and with 5-sulfosalicylic acid both show polymeric hydrogen-bonded and π–π-bonded structures but these differ in that in the first example, cations and anions form separate cation chains or anion columns which are unassociated through formal hydrogen bonds while in the second, hydrogen-bonded cation–anion chains are found.
The structures of the 1:1 anhydrous salts of nicotine (NIC) with 3,5-di­nitro­salicylic acid (DNSA) and 5-sulfosalicylic acid (5-SSA), namely (1R,2S)-1-methyl-2-(pyridin-3-yl)-1H-pyrrolidin-1-ium 2-carb­oxy-4,6-di­nitro­phenolate, C10H15N2 +·C7H3N2O7 −, (I), and (1R,2S)-1-methyl-2-(pyridin-3-yl)-1H-pyrrolidin-1-ium 3-carb­oxy-4-hy­droxy­benzene­sulfonate, C10H15N2 +·C7H5O6S−, (II), are reported. The asymmetric units of both (I) and (II) comprise two independent nicotinium cations (C and D) and either two DNSA or two 5-SSA anions (A and B), respectively. One of the DNSA anions shows a 25% rotational disorder in the benzene ring system. In the crystal of (I), inter-unit pyrrolidinium N—H⋯Npyridine hydrogen bonds generate zigzag NIC cation chains which extend along a, while the DNSA anions are not involved in any formal inter-species hydrogen bonding but instead form π–π-associated stacks which are parallel to the NIC cation chains along a [ring-centroid separation = 3.857 (2) Å]. Weak C—H⋯O inter­actions between chain substructures give an overall three-dimensional structure. In the crystal of (II), A and B anions form independent zigzag chains with C and D cations, respectively, through carb­oxy­lic acid O—H⋯Npyridine hydrogen bonds. These chains, which extend along b, are pseudocentrosymmetrically related and give π–π inter­actions between the benzene rings of anions A and B and the pyridine rings of the NIC cations C and D, respectively [ring centroid separations = 3.6422 (19) and 3.7117 (19) Å]. Also present are weak C—H⋯O hydrogen-bonding inter­actions between the chains, giving an overall three-dimensional structure.
doi:10.1107/S1600536814023253
PMCID: PMC4257264  PMID: 25484766
crystal structure; nicotine; proton-transfer salts; 3,5-di­nitro­salicylic acid; 5-sulfosalicylic acid; hydrogen-bonding; π–π inter­actions
25.  Conformation and crystal structures of 1-amino­cyclo­hexa­neacetic acid (β3,3Ac6c) in N-protected derivatives1  
The gauche conformation of backbone torsion angles (ϕ, θ) for β3,-Ac6c-OH is observed in the N-protected derivatives of 1-amino­cyclo­hexa­neacetic acid.
N-Protected derivatives of 1-amino­cyclo­hexa­neacetic acid (β3,3-Ac6c), namely Valeroyl-β3,3-Ac6c-OH [2-(1-pentanamidocyclohexyl)acetic acid, C13H23NO3], (I), Fmoc-β3,3-Ac6c-OH [2-(1-{[(9H-fluoren-9-yloxy)carbonyl]amino}cyclohexyl)acetic acid, C23H25NO4], (II), and Pyr-β3,3-Ac6c-OH {2-[1-(pyrazine-2-amido)cyclohexyl]acetic acid, C13H17N3O3}, (III), were synthesized and their conformational properties were determined by X-ray diffraction analysis. The backbone torsion angles (ϕ, θ) for β3,3-Ac6c-OH are restricted to gauche conformations in all the derivatives, with a chair conformation of the cyclo­hexane ring. In the crystal structure of (I), the packing of mol­ecules shows both carb­oxy­lic acid R 2 2(8) O—H⋯O and centrosymmetric R 2 2(14) N—H⋯O hydrogen-bonding inter­actions, giving rise to chains along the c-axis direction. In (II), centrosymmetric carb­oxy­lic acid R 2 2(8) O—H⋯O dimers are extended through N—H⋯O hydrogen bonds and together with inter-ring π–π inter­actions between Fmoc groups [ring centroid distance = 3.786 (2) Å], generate a layered structure lying parallel to (010). In the case of compound (III), carb­oxy­lic acid O—H⋯Npyrazine hydrogen bonds give rise to zigzag ribbon structures extending along the c-axis direction.
doi:10.1107/S1600536814020777
PMCID: PMC4257263  PMID: 25484721
crystal structure; disubstituted-β-amino acids; π–π inter­action; hydrogen bonds; conformation

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