Selective Generation of Aldimine and Ketimine Tautomers of the Schiff Base Condensates of Amino Acids with Imidazole Aldehydes or of Imidazole Methanamines with Pyruvates-Isomeric Control with 2- vs. 4-Substituted Imidazoles.

The Schiff base condensation of 5-methyl-4-imidazole carboxaldehyde, 5Me4ImCHO, and the anion of an amino acid, H2N-CH(R)CO2- (R = -CH3, -CH(CH3)2 and -CH2CH(CH3)2), gives the aldimine tautomer, Im-CH=N-CH(R)CO2-, while that of 5-methylimidazole-4-methanamine, 5MeIm-4-CH2NH2, with a 2-oxocarboxylate anion, R-C(O)-CO2-, gives the isomeric ketimine tautomer, Im-CH2-N=C(R)CO2-. All are isolated as the neutral nickel(II) complexes, NiL2, and are characterized by single crystal structure determination, IR, and positive ion ESI MS. In the cases of the 4 substituted imidazoles, either 5MeIm-4-CHO or 5MeIm-4-CH2NH2, both the aldimine and ketimine complexes are isolated cleanly with no evidence of an equilibrium between the two tautomers under the experimental conditions. The aldimines are blue while the tautomeric ketimines are green. In contrast, for the 2-substituted imidazoles, with either Im-2-CHO or Im-2-CH2NH2, the isolated product from the Schiff base condensation is the ketimine, which in the solid is green, as observed for the 4-isomer. These results suggest that for the 2-substituted imidazoles, there is a facile equilibrium between the aldimine and ketimine tautomers, and that the ketimine form is the thermodynamically favored tautomer. The aldimine tautomers of the 4-substituted imidazoles have three stereogenic centers, the nickel (Δ or Ʌ) and the two alpha carbon atoms (R or S). The observed pair of enantiomers is the ɅRR/ΔSS enantiomeric pair, suggesting that this pair is lower in energy than the others and that this is in general the preferred chiral correlation in these complexes.

Structural Diversity of Lanthanide 3-Nitrotrispyrazolylborates: Tunable Nuclearity and Intra-Ligand Charge Transfer Sensitization of Visible and NIR Ln3+ Emission.

Reported are the syntheses, crystal structures, and photophysical properties of 28, novel lanthanide compounds across five structural types, [Ln(3-NO2Tp)2(NO3)] (1-Ln, Ln = La-Tm, except Pm), [Bu4N][Ln(3-NO2Tp)(NO3)3] (2-Ln, Ln = Yb, Lu), [Eu(3-NO2Tp)2Cl(H2O)]·2iPrOH (3-Eu), [{Ln(3-NO2Tp)2}2(µ2-CO3)]·MeOH (4-Ln, Ln = La-Gd, except Pm), and [{Ln(3-NO2Tp)}4(µ2-OMe)6(µ4-O)] (5-Ln, Ln = Pr-Tb, except Pm) with the 3-nitrotrispyrazolylborate (3-NO2Tp-) ligand. The reaction of methanol or isopropanol solutions of LnX3 (X = Cl, NO3) with the tetrabutyl ammonium salt of the flexidentate 3-NO2Tp- ([Bu4N][3-NO2Tp]) yields Ln(3-NO2Tp)x complexes of various nuclearities as either monomers (1-Ln, 2-Ln, 3-Eu), dimers (4-Ln), or tetramers (5-Ln) owing to the efficient conversion of atmospheric CO2 to CO32- (dimers) or ligand controlled solvolysis of lanthanide ions (tetramers). 3-NO2Tp- is an efficient sensitizer for both the visible and near-IR (NIR) emissions of most of the lanthanide series, except thulium. Optical measurements, supported by density functional theory calculations, indicate that the dual visible and NIR Ln3+ emission arises from two intraligand charge transfer (ILCT) transitions of 3-NO2Tp-. This is the first report of lanthanide complexes with a nitro-functionalized pyrazolylborate ligand. The derivatization of the known Tp- ligand results in new coordination chemistry governed by the increased denticity of 3-NO2Tp-, imparting remarkable structural diversity and charge transfer properties to resultant lanthanide complexes.

Synthesis and Structure of Sn14Cl6(CH2SiMe3)12: Toward Nanoclusters of 4-Coordinate α-Sn.

Orange crystals of a Sn14 cluster have been isolated in up to 22% yield from a reaction between Me3SiCH2SnCl3, SnCl4, and LiAlH4. The structure determined by single crystal X-ray diffraction shows three unique Sn atoms in a 6:6:2 ratio, with all Sn atoms 4-coordinate, similar to the tetrahedral bonding in elemental gray Sn. The solid state 117Sn MAS NMR spectrum shows the three types of distinct Sn atoms in the expected 3:3:1 intensity ratio with respective chemical shifts of 87.9, -66.6, and -607.1 ppm relative to Me4Sn. The chemical shift of the two Sn atoms without ligands (bonded only to Sn), at -607.1 ppm, is the most upfield, and is the closest to the chemical shift, reported here, of bulk gray tin (-910 ppm). First-principles density functional theory calculations of the chemical shielding tensors corroborate this assignment. While the core coordination is distorted from the ideal tetrahedral arrangement in the diamond structure of gray tin, this Sn14 cluster, as the largest reported cluster with all 4-coordinate Sn, represents a major incremental step toward being able to prepare atomically precise nanoparticles of gray tin.

Responsive nature of 1,2,4,5-tetrakis(2-pyridylthiomethyl)benzene toward group 12 metal nitrates: activity of coordinated nitrate in metal complexes.

This study provides a detailed analysis on the responsive behavior of 1,2,4,5-tetrakis(2-pyridylthiomethyl)benzene (L) toward group 12 metal nitrates in both aqueous and nonaqueous media. The ligand L proved to be an environmentally responsive species, and structural investigations of its complexes with respective M(NO3)2 (M = Zn, Cd, and Hg) allowed one to remark on the inherent activity of the nitrate ion, resulting in a distinctively higher coordination number and dimensionality to the metal cations.

Di-aqua-bis-(nicotinamide-κN (1))bis-(thio-cyanato-κN)nickel(II).

In the title complex, [Ni(NCS)2(C6H6N2O)2(H2O)2], the Ni(II) ion is located on an inversion center and is coordinated in a distorted octa-hedral environment by two N atoms from two nicotinamide ligands and two water mol-ecules in the equatorial plane, and two N atoms from two thio-cyanate anions in the axial positions, all acting as monodentate ligands. In the crystal, weak N-H⋯S hydrogen bonds between the amino groups and the thio-cyanate anions form an R 4 (2)(8) motif. The complex mol-ecules are linked by O-H⋯O, O-H⋯S, and N-H⋯S hydrogen bonds into a three-dimensional supra-molecular structure. Weak π-π inter-actions between the pyridine rings is also found [centroid-centroid distance = 3.8578 (14) Å].

Carbonate-templated self-assembly of an alkylthiolate-bridged cadmium macrocycle.

In the presence of Cd(ClO4)2 and a base, a new mixed N,S-donor alkylthiolate ligand supported both carbonate formation from atmospheric CO2 and the self-assembly of a novel bicapped puckered (CdS)6 molecular wheel. The remarkable stability of the complex was demonstrated by slow intermolecular ligand exchange on the (2)J(HH) and J((111/113)Cd(1)H) time scales at elevated temperature. Both CO2 and the base were required to convert amorphous "CdLClO4" precipitated in the absence of air to the carbonate complex. The complex shares structural features with the ζ-carbonic anhydrase class associating cadmium(II) with the biogeochemical cycling of carbon and is the first structurally characterized carbonate complex of any metal involving an alkylthiolate ligand.

Enaminones 11. An examination of some ethyl ester enaminone derivatives as anticonvulsant agents.

In this paper, we investigated the previously synthesized anticonvulsant enaminone ethyl ester analogs using the computational gaussian 03 programs. The significant chemical features of the enaminone compounds that lead to positive anticonvulsant activity were identified. From our analyses, we believe that the neutrality of the phenyl ring may be important for binding in the hydrophobic pocket of the active site and that the binding of the phenyl substituent is the main reason why some analogs are active and others are inactive.

Controlled synthesis of photoresponsive bismuthinite (Bi2S3) nanostructures mediated through a new 1D bismuth-pyrimidylthiolate coordination polymer as a molecular precursor.

Bismuthinite (Bi2S3) nanostructures have garnered significant interest due to their appealing photoresponsivity which has positioned them as an attractive choice for energy conversion applications. However, to utilize their full potential, a simple and economically viable method of preparation is highly desirable. Herein, we present the synthesis and characterization including structural elucidation of a new air- and moisture-stable bismuth-pyrimidylthiolate complex. This complex serves as an efficient single-source molecular precursor for the facile preparation of phase-pure Bi2S3 nanostructures. Powder X-ray diffraction (PXRD), Raman spectroscopy, electron dispersive spectroscopy (EDS) and electron microscopy techniques were used to assess the crystal structure, phase purity, elemental composition and morphology of the as-prepared nanostructures. This study also revealed the profound effects of temperature and growth duration on the crystallinity, phase formation and morphology of nanostructures. The optical band gap of the nanostructures was tuned within the range of 1.9-2.3 eV, which is blue shifted with respect to the bulk bandgap and suitable for photovoltaic applications. Liquid junction photo-electrochemical cells fabricated from the as-prepared Bi2S3 nanostructure exhibit efficient photoresponsivity and good photo-stability, which project them as promising candidates for alternative low-cost photon absorber materials.

Synthesis and conformation of fluorinated ß-peptidic compounds.

Experimental and theoretical data indicate that, for α-fluoroamides, the F-C-C(O)-N(H) moiety adopts an antiperiplanar conformation. In addition, a gauche conformation is favoured between the vicinal C-F and C-N(CO) bonds in N-ß-fluoroethylamides. This study details the synthesis of a series of fluorinated ß-peptides (1-8) designed to use these stereoelectronic effects to control the conformation of ß-peptide bonds. X-ray crystal structures of these compounds revealed the expected conformations: with fluorine ß to a nitrogen adopting a gauche conformation, and fluorine α to a C=O group adopting an antiperiplanar conformation. Thus, the strategic placement of fluorine can control the conformation of a ß-peptide bond, with the possibility of directing the secondary structures of ß-peptides.

Tandem chain extension-Mannich reaction: an approach to ß-proline derivatives.

A zinc carbenoid-initiated chain extension reaction provides access to an organometallic intermediate, which can be used to capture activated imines. Deprotection of the nitrogen and reduction provides access to racemic derivatives of ß-proline. The relative stereochemistry of the ß-proline can be controlled through use of different activating groups on the imine nitrogen.

Hydrogen-bonding 2D metal-organic solids as highly robust and efficient heterogeneous green catalysts for Biginelli reaction.

Two new Zn(II) and Cd(II) MOFs have been synthesized. These MOFs have been applied as heterogeneous catalysts for the green synthesis of a variety of dihydropyrimidinone derivatives through the Biginelli reaction and the desired products were obtained in high yields with short reaction time under mild solvent- free conditions. Moreover, the MOF catalysts may be readily recovered after the reaction and reused for many cycles.

1-Nonyl-1H-benzimidazol-2(3H)-one.

The crystal structure of the title compound, C(16)H(24)N(2)O, is built up from two fused six- and five-membered rings linked to C(9)H(19) chains. The fused-ring system is essentially planar, the largest deviation from the mean plane being 0.009 (2) Å. The chain is nearly perpendicular to this plane [dihedral angle = 80.27 (17)°]. In the crystal, inter-molecular N-H⋯O hydrogen bonds form dimers with an R(2) (2)(8) graph-set motif. These dimers are further connected through C-H⋯O hydrogen bonds, building sheets parallel to (100).

Bis[methyl 3-(propyl-amino)-but-2-eno-ato]zinc.

The title compound, [Zn(C(8)H(14)NO(2))(2)], represents a zinc complex with the Zn(2+) cation coordinated by two O and two N atoms in a distorted tetrahedral geometry.

1,3-Bis(prop-2-yn-yl)-1H-1,3-benzimid-azol-2(3H)-one.

In the title compound, C(13)H(10)N(2)O, the fused-ring system is essentially planar, the largest deviation from the mean plane being 0.015 (1) Å. The two propynyl groups are nearly perpendicular to the benzimidazole plane, making dihedral angles of 85 (3) and 80 (2) °, and point in opposite directions. There are two short inter-molecular C-H⋯O contacts to the carbonyl O atom, one involving the acetyl-enic H atom and the other a H atom of the methyl-ene group.

3-[2-(6-Bromo-2-phenyl-3H-imidazo[4,5-b]pyridin-3-yl)eth-yl]-1,3-oxazolidin-2-one.

In the title mol-ecule, C(17)H(15)BrN(4)O(2), the fused-ring system is essentially planar, the largest deviation from the mean plane being 0.015 (2) Å, and forms dihedral angles of 37.8 (2) and 35.5 (2)° with the phenyl and oxazolidine rings, respectively. The conformation adopted by the mol-ecule is stabilized by an intra-molecular π⋯π inter-action [centroid-centroid distance = 3.855(2) Å] between oxazolidine and phenyl rings. The crystal packing features inter-molecular C-H⋯N and C-H⋯O inter-actions.

Bis(maltolato)vanadium(III)-polypyridyl complexes: synthesis, characterization, DNA cleavage, and insulin mimetic activity.

Four vanadium(III) complexes of the general formula [V(maltol)(2)(N-N)]ClO(4), where N-N is 2,2'-bipyridine (bpy) (1); 1,10-phenanthroline (phen) (2); dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) (3), and dipyrido[3,2-a:2',3'-c]phenazine (dppz) (4), have been synthesized and characterized by IR, UV-visible, NMR spectroscopies, and electrospray ionization mass spectra (ESI-MS). The complexes exhibit the typical (1)H NMR spectra for paramagnetic V(III) species. The structures of complexes 1, 2, and 3 were characterized by single crystal X-ray diffraction. All complexes are monomeric and cationic containing V(III) species ligated to one neutral polypyridyl ligand and two monoanionic bidentate maltolate ligands with a distorted octahedral geometry. The complexes show an irreversible redox peak around +0.80 V versus Ag/AgCl corresponding to one-electron oxidation of V(III) to V(IV). The time-resolved UV-visible spectral changes for the complexes during the electrolysis in acetonitrile solution at +1.0 V are consistent with one-electron oxidation of the complexes to yield the stable V(IV) species. All complexes cleave plasmid pBR322 DNA without the addition of any external agents. In vitro insulin mimetic activity against insulin responsive RIN 5f cells indicates that complex 1 has similar activity to insulin while the others have moderate insulin mimetic activity.

Use of Pyrazole Hydrogen Bonding in Tripodal Complexes to Form Self Assembled Homochiral Dimers.

The 3:1 condensation of 5-methyl-1H-pyrazole-3-carboxaldehyde (MepyrzH) with tris(2-aminoethyl)amine (tren) gives the tripodal ligand tren(MePyrzH)3. Aerial oxidation of a solution of cobalt(II) with this ligand in the presence of base results in the isolation of the insoluble Co(tren)(MePyrz)3. This complex reacts with acids, HCl/NaClO4, NH4ClO4, NH4BF4, and NH4I to give the crystalline compounds Co(tren)(MePyrzH)3(ClO4)3, {[Co(tren)(MePyrzH0.5)3](ClO4)1.5}2 {[Co(tren)(MePyrzH0.5)3](BF4)1.5}2 and [Co(tren)(MePyrzH)3][Co(tren)(MePyrzH)3]I2. The latter three complexes are dimeric, held together by three Npyrazole -H…Npyrazolate hydrogen bonds. The structures and symmetries of these homochiral dimers or pseudodimers are discussed in terms of their space group. Possible applications of these complexes by incorporation into new materials are mentioned.

Combination of single-molecule magnet behaviour and luminescence properties in a new series of lanthanide complexes with tris(pyrazolyl)borate and oligo(ß-diketonate) ligands.

A series of tris(pyrazolyl)borate mono-, di- and trinuclear complexes, [Tp2Ln]nX (Ln = Eu, Tb, Gd, Dy, Xn- = various mono-, bis- and tris(ß-diketonates) has been prepared. The Tb3+ and Dy3+ complexes are luminescent single molecular magnets (SMM) and exhibit luminescence quantum efficiencies up to 73% for the Tb3+ and 4.4% for the Dy3+ compounds. Similar Eu3+ complexes display bright emission only at lower temperatures. The Dy3+ and Tb3+ complexes possess SMM behavior in a non-zero dc field at low temperatures, while the polynuclear Dy3+ complexes also show slow magnetic relaxation even in zero dc field up to 8 K. Ueff-values determined from dynamic magnetic measurements were up to 31 and 6 cm-1 for the Dy3+ and Tb3+ complexes, respectively. It was found that within a series of Dy3+ and Tb3+ compounds, Ueff and luminescence quantum yields decreased with increasing nuclearity of the compounds and a shortening of the intramolecular Ln-Ln distance. ΔOrbach-values estimated from low-temperature luminescence spectra were significantly higher than those obtained from ac magnetic data, which may be due to involvement of additional processes in the relaxation mechanism (quantum tunneling, Raman, direct) reducing the energy barrier. Some of the Tb3+-compounds also display metal-centred electroluminescence, giving them potential as emitting layers in LEDs.

Potassium chlorido-tris-(hypersil-oxy)aluminate dimer.

The tris-(tri-methyl-silylsiloxide) ligand, also known as hypersiloxide, is an extremely bulky group. In an attempt to make the monomeric Al(OSi(SiMe3)3)3, AlCl3 was combined with 3 equiv. of potassium hypersiloxide. The crystalline product isolated (40% yield), namely di-µ3-chlorido-bis-[µ2-tris-(tri-methyl-sil-yl)silanolato]tetra-kis-[tris-(tri-methyl-sil-yl)silanolato]dialuminium-dipotassium, [Al2K2Cl2(C19H27OSi4)6], is a KCl adduct of aluminium tris(hypersilyloxide) that is dimerized through a planar K2Cl2 ring, with K-Cl distances of 3.1131 (8) and 3.319 (3) Å, and ring angles of 77.41 (2) and 102.60 (2)°. This ring is on an inversion center, and there is no supra-molecular coordination.

Bis-tridentate Chelates of an Asymmetric Ligand: X-ray Structures and Solution NMR Characterization of Divalent Zinc Triad Metal Ion Complexes of N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine.

Divalent zinc triad metal ion complexes of type M(L)(2)(ClO(4))(2) (L = N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine) with N(4)S(2) metal coordination spheres were isolated and characterized by X-ray crystallography and variable temperature proton NMR. Although bis-tridentate chelates have nine geometric isomers, the crystallographically characterized complexes of all three metal ions had trans facial octahedral coordination geometry with C(i) symmetry. Despite the low coordination number and geometric preferences of d(10) metal ions, which facilitate inter- and intramolecular exchange processes, dilute solutions of these bis-tridentate chelates exhibited slow geometric isomerization. Symmetry, sterics and shielding arguments supported specific isomeric assignments for the major and minor chemical shift environments observed at low temperature. At elevated temperature, rapid intramolecular exchange occurred for all three complexes but slow intermolecular exchange on the coupling constant time scale was evidenced through detection of J(HgH) interactions for Hg(L)(2) (2+). These unusual observations are discussed in the context of the zinc triad metal ion coordination chemistry of related bis-tridentate chelates.