Barbiturate Anion Research Articles

The Sensitivity of Structure to Ionic Radius and Reaction Stoichiometry: A Crystallographic Study of Metal Coordination and Hydrogen Bonding in Barbiturate Complexes of All Five Alkali Metals Li-Cs.

A systematic study has been conducted on barbiturate complexes of all five alkali metals, Li-Cs, prepared from metal carbonates or hydroxides in an aqueous solution without other potential ligands present, varying the stoichiometric ratio of metal ion to barbituric acid (BAH). Eight polymeric coordination compounds (two each for Na, K, and Rb and one each for Li and Cs) have been characterised by single-crystal X-ray diffraction. All contain some combination of barbiturate anion BA- (necessarily in a 1:1 ratio with the metal cation M+), barbituric acid, and water. All organic species and water molecules are coordinated to the metal centres via oxygen atoms as either terminal or bridging ligands. Coordination numbers range from 4 (for the Li complex) to 8 (for the Cs complex). Extensive hydrogen bonding plays a significant role in all the crystal structures, almost all of which include pairs of N-H···O hydrogen bonds linking BA- and/or BAH components into ribbons extending in one dimension. Factors influencing the structure adopted by each compound include cation size and reaction stoichiometry as well as hydrogen bonding.

Iron(III) Complexation with Barbituric and 2-Thiobarbituric Acids in Aqueous Solution

Complexation in systems containing iron(III) chloride and barbituric (H2BA) or 2-thiobarbituric (H2TBA) acid has been studied by spectrophotometry and pH-metry in the pH range of 1.3–3.3 (I = 0.1 (NaCl), t = 20°C). The presence of 1 : 1 complexes with mono- and deprotonated forms of the ligand has been established, and their stability constants (in log units) have been determined: 1[FeHBA]2+ (3.49 ± 0.15), [FeHTBA]2+ (2.69 ± 0.07), [FeBA]+ (12.22 ± 0.13), and [FeTBA]+ (11.05 ± 0.08). It has been shown that the higher thermodynamic stability of barbiturate complexes compared to 2-thiobarbiturate ones is due to the greater basicity of the barbiturate anion. Based on the stability constants obtained, it has been proposed to use orthophosphate, fluoride, and ethylenediaminetetraacetate ions to eliminate the interfering effect of iron(III) in the determination of malondialdehyde by the thiobarbiturate method. Orthophosphoric acid is the most convenient for practical applications, as it makes it possible to mask iron(III) and to create a strongly acidic medium necessary for the formation of a colored malondialdehyde–H2TBA adduct.

Crystal structures of [Cu2(2,2′-bipyridine-N,N′)2(H2O)2(μ2-OH)2](barbiturate)2·2H2O and [Cu(2,2′-bipyridine-N,N′)(H2O)(barbiturate-O)Cl]·2H2O

Two mixed-ligand Cu(II) complexes [Cu2(Bipy)2(H2O)2(OH)2](Hba)2·2H2O (1) and [Cu(Bipy)(H2O)(Hba)Cl]·2H2O (2) (Bipy = 2,2′-bipyridine and Hba− = barbiturate anion) were synthesized and structurally characterized by single crystal X-ray diffraction. In 1, structural data revealed distorted square-pyramidal geometry for each of two crystallographic independent copper(II) atom with the basal plane formed by two nitrogen atoms of the 2,2′-bipyridine ligand and two oxygen atoms of bridging hydroxo groups. The apical positions are filled by the oxygen atoms from a water molecule. In 2, the Cu atom is also in distorted square pyramidal geometry, where the equatorial sites are occupied by two N atoms of one Bipy ligand, one O atom of the barbiturate ion and one chloride ion and the apical position by one aqua ligand. Numerous intermolecular hydrogen bonds OH⋯O, NH⋯O, CH⋯O in the structures 1–2 and in addition OH⋯Cl, CH⋯Cl in the structure 2 form the 3D networks. Topological analysis showed that these H-bonded networks are new. An additional factor that plays a role in the crystal packing of the discussed complexes is the π-π stacking interaction between the aromatic rings of Hba− and Bipy forming infinite chains –Hba–Bipy–Bipy–Hba–. The spectroscopic and thermal properties of compounds were analyzed.

Two novel mixed-ligand Ni(II) and Co(II) complexes with 1,10-phenanthroline: Synthesis, structural characterization, and thermal stability

Two nickel(II) and cobalt(II) complexes with phenanthroline, [Ni(Phen)(H2O)3Br]Br (1) and [Co(Phen)2(H2O)2](Hba)2·2H2O (2), Phen = 1,10-phenanthroline and Hba− = barbiturate anion, were synthesized and characterized by powder XRD, TGA and FT-IR. Their structures were determined by single crystal X-ray diffraction techniques. The Ni2+ ion is coordinated by two N atoms of Phen molecule, Br− ion and three H2O molecules forming an octahedron. Uncoordinated and coordinated Br− ions are connected with water molecules by OH⋯Br intermolecular hydrogen bonds with the formation of a 2D plane network which is extended into a 3D network by π−π stacking interactions. The [Co(Phen)2(H2O)2]2+ cation contains a six-coordinated cobalt atom chelated by two Phen ligands and two aqua ligands in the cis arrangement. NH⋯O, OH⋯O and CH⋯O intermolecular hydrogen bonds form a 3D net. NH⋯O hydrogen bonds form the infinite chains of Hba–. In addition, coordinated Phen molecules and lattice water molecules are linked via CH⋯OW hydrogen bonds to form infinite zigzag chains. These different chains are connected by OWH⋯O hydrogen bonds. π−π interaction plays an important role in the stabilization of structures 1–2. FT-IR, TGA, the diffuse reflectance, and UV–Vis spectra were also used to characterize these compounds.

Kinetics of Electrophilic Alkylations of Barbiturate and Thiobarbiturate Anions.

Second-order rate constants (k2) of the reactions of various barbiturate anions such as the parent barbiturate, 1,3-dimethylbarbiturate, 2-thiobarbiturate, and 1,3-diethyl-2-thiobarbiturate with diarylcarbenium ions and Michael acceptors have been determined in dimethyl sulfoxide solution at 20 °C. The reactivity parameters N and sN of the barbiturate anions were derived from the linear plots of log k2 versus the electrophilicity parameters E of these reference electrophiles, according to the linear-free-energy relationship log k2 (20 °C) = sN (E + N). Several reactions of these nucleophiles with benzylidenemalononitriles and quinone methides proceeded with reversible formation of the new C-C-bond followed by rate-determining proton shift. No evidence for initial attack of the electrophiles at the enolate oxygens of these nucleophiles was found by the kinetic measurements, in line with quantum chemical DFT calculations, which showed that in all cases C-attack is kinetically and thermodynamically preferred over O-attack. The nucleophilic reactivities of barbiturate anions were compared with those of structurally related carbanions, e.g., Meldrum's acid and dimedone anions.

Vibrational and structural properties of barbiturate anions in supramolecular compounds

In this work we have been successful in the synthesis, spectroscopic and structural characterization of five new supramolecular compounds of formulation [Ni(4dbpy2)2(H2O)2]Bb2·4H2O (1), [Mn(4dbpy2)2(H2O)2]Bb2·4H2O (2), [Mn(5dbpy2)2ClH2O][(C4O3N2H2)2-OH]·H2O (3), [Co(5dbpy2) (H2O)4]Bb2·2H2O (4) and [6dbpy2H] [(C4O3N2H2)2-OH]·H2O (5) which have been obtained from barbituric acid (HB) and 2,2′-bipyridine (4,4′, 5,5′ and 6,6′-dimethyl) building block ligands and transition metal ions, where Bb is the barbiturate monoanion and dbpy is the neutral substituted bipyridyl in the formulation. Compounds 1 and 2 are isomorphous, presenting the cationic block [M(4dbpy2)2(H2O)2]2+ neutralized by two uncoordinated barbiturate anions, whilst for compound 4 the different cationic units [Co(5dbpy2) (H2O)4]2+ are neutralized by the same anion. For the compounds 3 and 5 the formation of a new building block is recognized and characterized as the hydroxyhydurilate anion which has been generated in situ from the oxidation reaction between alloxan and the barbiturate anion. The Raman spectral analysis clearly shows the similarity of compounds 1 and 2 through similar wavenumber positions of all the vibrational bands, especially the band at 679cm−1 assigned to the ring breathing mode of the barbiturate anion which is present in compounds 1, 2 and 4. For compounds 3 and 5 the absence of this Raman band suggests that a noteworthy chemical modification has taken place in this building block and the appearance of a new band at 1745cm−1, assigned to the ν(CO) mode of the new hydroxyhydurilate anion generated in situ, supports this conclusion.

Synthesis, characterization, thermal behaviour and single crystal X-ray analysis of two new insensitive high energy density materials [8-hydroxyquinolinium 5-(2,4,6-trinitrophenyl)barbiturate (I) and 8-hydroxyquinolinium 5-(5-chloro-2,4-dinitrophenyl)-1,3-dimethyl barbiturate (II)

Barbiturates I and II have been synthesized as maroon red and red orange coloured solids by mixing the ethanolic solutions of 2-chloro-1,3,5-trinitrobenzene (TNCB), pyrimidine-2,4,6(1H,3H,5H)-trione [barbituric acid (BA)] and 8-hydroxyquinoline and 1,3-dichloro-4,6-dinitrobenzene (DCDNB), 1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione(1,3-dimethylbarbituric acid) and 8-hydroxyquinoline respectively. The structures of these two barbiturates have been predicted from the spectral studies (UV-VIS, IR, 1H NMR, 13C NMR, mass) and elemental analysis. Qualitative tests have been carried out to infer the presence of nitrogen and nitro groups and also chlorine atom in barbiturate II. Slow evaporation of ethanol-dimethylsulphoxide/ethanol solutions of barbiturate I/barbiturate II at 293 K yielded good for X-Ray diffraction crystals. Single crystal X-ray diffraction studies of the crystals further confirm the putative structures of the barbiturates. The asymmetric unit of the barbiturate I comprises of 8-hydroxyquinolinium cation, 5-(2,4,6-trinitrophenyl) barbiturate anion and a molecule of dimethylsulphoxide (DMSO), which is used as a recrystallizing solvent. It crystallizes in the triclinic system with space group \(P\bar 1\) (centrosymmetric). Barbiturate II crystallizes in the orthorhombic system with space group P212121 (non-centrosymmetric). Barbiturates I and II are stable towards an impact sensitivity test, when a weight of 2 kg mass hammer is dropped from a height of 160 cm of the instrument. TGA/DTA analyses at four different heating rates (5, 10, 20, and 40 K/min) imply that they undergo exothermic decomposition (∼85%) in three different stages between 273 and 873 K. Activation energies for these decomposition processes have been calculated by employing Kissinger and Ozawa plots. Impact sensitivity test and activation energies have revealed that the titled barbiturates are insensitive high energy density materials (IHEDMS).

Crystal structure of 4-amino-pyridinium 5-(5-chloro-2,4-dinitrophenyl)-1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate hemihydrate.

The title mol-ecular salt, C5H7N2 (+)·C12H8ClN4O7 (-)·0.5H2O, crystallizes as a hemihydrate. The two rings in the barbiturate anion are not coplanar but make a dihedral angle of 43.17 (16)°. The two nitro groups deviate from the ring to which they are attached; the nitro group ortho with respect to the ring junction is more deviated [39.3 (4)°] than that in the para position [4.2 (5)°], probably to overcome steric hindrance. As a result of this, the latter nitro group is more involved in delocalizing the negative charge of the anion than the former nitro group. In the crystal, the cations and anions are linked via N-H⋯O hydrogen bonds forming zigzag chains along [10-1]. The chains are linked by O-H⋯O and C-H⋯O hydrogen bonds, forming slabs lying parallel to (10-1). Further C-H⋯O hydrogen bonds link the slabs, forming a three-dimensional structure.

Trimethylammonium 5-(2,4-dinitrophenyl)-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate 0.125-hydrate

The asymmetric unit of the title salt C3H10N+·C10H5N4O7 −·0.125H2O [trivial name: trimethyl­ammonium 5-(2,4-dinitro­phen­yl)barbiturate 0.125-hydrate], contains two independent cations, two independent anions and a 0.25-occupancy solvent water mol­ecule. In one of the cations, the C atoms are disordered over two sets of sites with refined occupancies of 0.538 (8) and 0.462 (8). In the anions, the dihedral angles between the pyrimidine and benzene rings are 42.77 (6) and 46.55 (7)°. In the crystal, N—H⋯O hydrogen bonds connect anions and cations into chains along [010]. Within these chains, R 2 2(8) ring motifs are formed by inversion-related barbiturate anions. The H atoms of the partial occupancy water mol­ecule were not located nor included in the refinement.

N,N-Diethylanilinium 5-(5-chloro-2,4-dinitrophenyl)-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate

In the anion of the title salt, C10H16N+·C10H4ClN4O7 − [trivial name = N,N-diethyl­anilinium 5-(3-chloro-4,6,-dinitro­phen­yl)barbiturate], the dihedral angle between the benzene and pyrimidine rings is 45.49 (6)°. The mean plane of the nitro group, which is ortho-substituted with respect to the pyrimidine ring, is twisted by 41.57 (13)° from the benzene ring, while the mean plane of the nitro group, which is para-substituted, is twisted by 14.41 (12)° from this ring. In the crystal, N—H⋯O hydrogen bonds link cations and anions into chains along [1-10]. Within the chains, inversion-related anionic barbiturate anions form R 2 2(8) ring motifs.

Acid–base equilibrium in aqueous solutions of 1,3-dimethylbarbituric acid as studied by 13C NMR spectroscopy

Abstract 13 C NMR spectra of 1,3-dimethylbarbituric acid in aqueous solutions of various acidities and for various solute concentrations have been recorded and interpreted. The spectra recorded at pH = 2 and below contain the signals of the neutral solute molecule exclusively, while the ones recorded at pH = 7 and above only the signals of the appropriate anion, which has been confirmed by theoretical GIAO-DFT calculations. The signals in the spectra recorded for solutions of pH via a simple transfer of the C–H proton to water molecules but also through a process with participation of the barbiturate anions. Moreover, the importance of tautomerism, or association, or both these phenomena for the kinetics of the acid–base transformations in the investigated system has been shown. Qualitatively similar changes of 13 C NMR spectra with the solution pH variation have been observed for the parent barbituric acid.

N,N-Diethyl-2-hydroxyethanaminium 2,6-dioxo-5-(2,4,6-trinitrophenyl)-1,2,3,6-tetrahydropyrimidin-4-olate dihydrate

In the title mol­ecular salt, C6H16NO+·C10H4N5O9 −·2H2O, which crystallizes as a dihydrate, O—H⋯O hydrogen bonds link the barbiturate anion, the ethanaminium cation and the water mol­ecules of crystallization. The dihedral angle between the rings in the anion is 43.71 (8)°. In the crystal, an R 2 2(8) ring motif hydrogen-bonding pattern is also found involving inversion-related barbiturate rings with N—H⋯O hydrogen bonds. As a result of the various hydrogen bonds an infinite two-dimensional network, propagating in (10), is formed.

Two New Supramolecular Compounds that Involve Trans-1,2-bis(4- pyridyl)ethylene (bpe), Barbituric Acid and Fe(II) ions

In this study, the synthesis, spectroscopic properties (infrared and Raman) and crystal structures of two new compounds named (H2bpe)B2 . 2H2O (1) and (Fe(bpe)(H2O)4)B2·4H2O (2) have been reported, where bpe is trans-1,2- bis(4-pyridil)-ethylene, H2bpe is protonated bpe and B - is the barbiturate anion. For both complexes, we observed the presence of supramolecular interactions, such as electrostatic, hydrogen bonding and � -stacking interactions. Compound 1 crystallised in a triclinic system with the space group P-1, which was generated by cationic and anionic units formed by protonated bpe, the barbiturate anion and water molecules. These molecules interacted through weak intermolecular inter- actions and were responsible for the crystalline stability of the synthesised compounds. For compound 2, an orthorhombic crystal system was observed with a Pbcn space group. In this crystal, the metallic site adopted a distorted octahedral ge- ometry that was coordinated by two pyridine nitrogen atoms and four oxygen atoms from coordinated water molecules. The three-dimensional arrangement of the structure consisted of one covalent one-dimensional linear cationic (Fe(bpe)(H2O)4) 2+ chain, which interacts by forming hydrogen-bonding, � -stacking and electrostatic interactions with bar- biturate anions and water molecules that were trapped in the crystal. The vibrational spectra of the two compounds were very similar despite the different crystalline arrangements that were adopted by the two structures. In the infrared spectra, two intense bands at 1684 and 1630 cm -1 were observed and assigned to the CO stretching ( CO) of the barbiturate anion and  C=C of the bpe ligand, respectively. In the Raman spectra of the compounds, the most important bands were attrib- uted to the bpe ligand at 1629 and 1006 cm -1 , which were assigned to the  C=C and  ring, respectively. The presence of the barbiturate anion was confirmed by the presence of a band of low intensity located at 680 cm -1 and was assigned as the

Rubidium 2,4,6-trioxo-1,3-diazinan-5-ide–1,3-diazinane-2,4,6-trione–water (1/1/1)

The asymmetric unit of the title compound, Rb+·C4H3N2O3 −·C4H4N2O3·H2O, consists of one rubidium cation, a barbituric acid mol­ecule, a barbiturate anion and one water mol­ecule. The rubidium ion has seven close-contact inter­actions with O atoms, with Rb⋯O distances ranging from 2.8594 (16) to 3.2641 (14) Å. These seven O atoms together with an eighth O atom at 3.492 (2) Å away from Rb form a distorted polyhedron with shape inter­mediate between an anti­prism and a dodeca­hedron. The Rb+ ions connect layers built of organic components and water mol­ecules linked via N—H⋯O and O—H⋯O hydrogen bonds.

Synthesis, structural studies and vibrational spectroscopy of Fe 2+ and Zn 2+ complexes containing 4,4′-bipyridine and barbiturate anion

Two new coordination polymers of general formula FeB 2bpy·10H 2O ( 1) and ZnB 2bpy·10H 2O ( 2) (where B = barbiturate anion; bpy = 4,4′-bipyridine) were synthesized and characterized by X-ray diffraction, vibrational spectroscopy (Raman and infrared) and thermal analysis. The two complexes are isostructural and crystallize in a hexagonal crystal system with space group P6 422. The compound contains the 4,4′-bipyridine ligand coordinated to the metal sites in a bis-monodentate mode, and also four water molecules, in a slightly distorted octahedral geometry, giving rise to the cationic unit with two barbiturate anions. Each structure shows one covalent linear [M(bpy)(H 2O) 4] 2+ one-dimensional chain, which interacts via hydrogen bond with barbiturate anions and crystallization water molecules, resulting in a three-dimensional arrangement. The vibrational spectra of both compounds are very similar; in all infrared spectra is observed an intense band at 1686 cm −1 assigned to the νCO stretching mode of the barbiturate anion, as well as a medium band around 1610 cm −1, attributed to the νCC/ νCN stretching modes of the 4,4′-bipyridine ligand. The Raman spectra show intense and conclusive bands for the nitrogen ligand at 1615, 1295 and 1020 cm −1, assigned to the νCC/ νCN, ν ring + δCH and ν ring modes, respectively. The barbiturate anion is observed through the presence of a marker band at 680 cm −1, assigned to the ring breathing mode.

Supramolecular architectures constructed with the skeletons of zinc(II) 2,2′-bipyridine and barbiturate anion: Synthesis and characterization

The barbiturate 1,5-dihydro-5-[5-pyrimidine-2,4(1H,3H)-dionyl]-2H-chromeno[2,3-d] pyrimidine-2,4(3H)-dione) (LH 4) 1 forms a complex with Zn(bpy)(NO 3) 2·2H 2O (bpy = 2,2′-bipyridine) of composition [Zn(bpy) 2(H 2O) 2] 2+·(LH 3) 2·7H 2O ( 2). In this structure, the ligand exists as anion ( LH 3 - ) as revealed by its single crystal X- ray crystallographic study. Packing studies of 2 provide three dimensional helical chains as secondary structure. However, its PXRD pattern shows that 2 does not retain its structure upon dehydration. Recrystallization of 2 from DMSO results in complex 3 with the composition [Zn(bpy) 2(H 2O) 2] 2+·( LH 3 - ) 2·DMSO·9H 2O assigned on the basis of its elemental analysis and its X-ray crystallography. In order to understand the role of the 2,2′-bipyridine ring in the construction of supramolecular architecture, the reaction of zinc(II) nitrate salt with 1 is carried out to provide complex 4 of composition [Zn(LH 3) 2(H 2O) 4]·4H 2O characterized by its single crystal X-ray diffraction study. Thermal, emission and photoluminescence properties of the complexes are also studied.

An intriguing hydrogen bond arrangement of polymeric 1D chains of 4,4′-bipyridine coordinated to Co2+, Ni2+, Cu2+ and Zn2+ ions having barbiturate as counterions in a 3D network

In this work, four new transition metal complexes of general formula {[M(bipy)(H2O)4](B)2}(H2O)6 (where MCo2+, Ni2+, Cu2+ and Zn2+, B is barbiturate anion and bipy is 4,4′-bipyridine) were obtained and investigated by means of single crystal X-ray diffraction and vibrational (Raman and infrared) spectroscopic techniques. The CoB2bipy·10H2O, NiB2bipy·10H2O and CuB2bipy·10H2O basic units give rise to polymers, where Ni(II) and Cu(II) compounds are isomorphous, however all compounds present the same molecular structure. The metal sites appear in a distorted octahedral geometry coordinated by two pyridine nitrogen atoms and four oxygen atoms from coordinated water molecules. Each structure showing one covalent linear [M(bipy)(H2O)4]2+ chains one-dimensional, which interact by hydrogen-bonding with the barbiturate anion and crystallization water resulting in a 3D arrangement. The analysis of the Ni2+ and Cu2+ complexes’ structures show flexible 2D hydrogen bonded networks being constructed by the barbiturate anions and the crystallization waters molecules, which may be deemed to be the host, while the robust 1D [M(bipy)(H2O)4]2+ chains may be deemed to be the guest, in a very intriguing and interesting structure. The vibrational spectra of the compounds are very similar, in agreement to the crystal data. In all infrared spectra a medium band at 1690 cm−1 has been observed, assigned to the CO stretch of the barbiturate anion. In the Raman spectra the most important bands referring to 4,4′-bipyridine are the ones at 1616, 1290 and 1020 cm−1, assigned to δCC/CN, δring and δCH modes, respectively. The vibrational results strongly suggest the Zn2+ compound presents the same molecular arrangement, based on the fact that the spectra of all complexes are similar.

(Barbiturato-κO)[hydridotris(5-methyl-3-phenyl-1-pyrazolyl)borato-κ3 N 2,N 2′,N 2′′]zinc(II) methanol solvate

In the title compound, [Zn(C30H28BN6)(C4H3N2O3)]·CH3OH, the ZnII atom is coordinated in a tetrahedral geometry by three N atoms from the pyrazolyl groups of the tripodal ligand, and by one O atom from a barbiturate anion.

Hydrogen-bonded supramolecular motifs in crystal structures of trimethoprim barbiturate monohydrate (I) and 2-amino-4,6-dimethylpyrimidinium barbiturate trihydrate (II)

In the present study, we report the crystal structures of two organic salts, namely 2,4-diamino-5-(3′,4′,5′-trimethoxybenzyl)pyrimidinium (TMP) barbiturate monohydrate (TMPBAR) (I), 2-amino-4,6-dimethylpyrimidinium (AMPY) barbiturate trihydrate (AMPYBAR) (II). In both complexes, one ring nitrogen of TMP and AMPY are protonated as a result of proton transfer from the−CH2 group of barbituric acid. In compound (I), the TMP cation and barbiturate anion are paired through twoN−H···O and one N−H···N hydrogen bonds. This pair further self-organizes through N−H···O hydrogen bonds to generate an array of six hydrogen bonds. These arrays are further cross-linked by N−H···O hydrogen bonds forming a sheet-like structure. The water molecule is also embedded in the sheet via O−H···O and C−H···O hydrogen bonds, forming a rosette-like supramolecular motif. TMP cations are also bridged by alternating water molecules via C−H···O and O−H···N hydrogen bonds. In compound (II), the symmetrical barbiturate anions self-organize on both sides through N−H···O hydrogen bonds generating a supramolecular chain. These chains are cross-linked by the three water molecules. A pair of barbiturate anions and two water molecules constitute an array of four hydrogen bonds (DADA quadruple array). These arrays are cross-linked by another water molecule. 2-Amino-4,6-dimethylpyrimidine cations are paired through N−H···N hydrogen bonds. These pairs are bridged by three water molecules generating a supramolecular ribbon. The barbiturate chains and base pairs are arranged in an alternate manner via N−H···O and O−H···O hydrogen bonds to generate a 3-D network.

Redox active macrocyclic receptors for neutral guests.

A novel series of triazine-appended macrocyclic complexes has been investigated as potential hydrogen bonding receptors for complementarily disposed heterocycles. Cocrystallization of a melamine-appended azacyclam complex of Cu(II) has been achieved with barbitone, the barbiturate anion and thymine. In each case, a complementary DAD/ADA hydrogen bonding motif between the melamine group and the heterocycle has been identified by X-ray crystallography. Electrochemical studies of the copper macrocycles in both nonaqueous and aqueous solution show anodic shifts of the Cu(II/)(I) redox couple of more than 60 mV upon addition of guest molecules with matching H-bonding motifs. The Zn(II) analogues have been synthesized via transmetalation of the Cu(II) complex, and their guest binding properties investigated by NMR spectroscopy. (1)H NMR shifts of up to 0.8 ppm were observed upon addition of guest, and stability constants are similar to those obtained electrochemically.