Imidazole Nitrogen Atoms Research Articles

Fluorescent behavior of 2‐(3,4,5,6‐tetrafluoro‐2‐hydroxyphenyl)imidazo‐[1,2‐a]pyridine in the presence of metal perchlorate

Abstractmagnified image2‐(3,4,5,6‐Tetrafluoro‐2‐hydroxyphenyl)imidazo[1,2‐a]pyridine (1) emits long wavelength light around 540 nm both in polar and in nonpolar solvents. Zn2+ perchlorate in acetonitrile causes the intermediate wavelength emission around 430 nm, which is ascribed to the species where the imidazole nitrogen atom and the phenolate oxygen atom bridge Zn2+. In the presence of Hg2+ and Al3+ perchlorates, short wavelength emission around 370 nm is strongly increased and this fluorescent enhancement is attributable not to the coordination of Hg2+ and Al3+ to 1 but to the formation of the salt of perchloric acid of 1.

Tautomerism of Histidine 64 Associated with Proton Transfer in Catalysis of Carbonic Anhydrase

The imidazole (15)N signals of histidine 64 (His(64)), involved in the catalytic function of human carbonic anhydrase II (hCAII), were assigned unambiguously. This was accomplished by incorporating the labeled histidine as probes for solution NMR analysis, with (15)N at ring-N(delta1) and N(epsilon2), (13)Cat ring-Cepsilon1, (13)C and (15)N at all carbon and nitrogen, or (15)N at the amide nitrogen and the labeled glycine with (13)C at the carbonyl carbon. Using the pH dependence of ring-(15)N signals and a comparison between experimental and simulated curves, we determined that the tautomeric equilibrium constant (K(T)) of His(64) is 1.0, which differs from that of other histidine residues. This unique value characterizes the imidazole nitrogen atoms of His(64) as both a general acid (a) and base (b): its epsilon2-nitrogen as (a) releases one proton into the bulk, whereas its delta1-nitrogen as (b) extracts another proton from a water molecule within the water bridge coupling to the zinc-bound water inside the cave. This accelerates the generation of zinc-bound hydroxide to react with the carbon dioxide. Releasing the productive bicarbonate ion from the inside separates the water bridge pathway, in which the next water molecules move into beside zinc ion. A new water molecule is supplied from the bulk to near the delta1-nitrogen of His(64). These reconstitute the water bridge. Based on these features, we suggest here a catalytic mechanism for hCAII: the tautomerization of His(64) can mediate the transfers of both protons and water molecules at a neutral pH with high efficiency, requiring no time- or energy-consuming processes.

Molecular dynamics studies of the inhibitory mechanism of copper(II) on aggregation of amyloid β-peptide

The inhibitory mechanism of copper(II) on the aggregation of amyloid β-peptide (Aβ) was investigated by molecular dynamics simulations. The binding mode of copper(II) with Aβ is characterized by the imidazole nitrogen atom, Nπ, of the histidine residue H13, acting as the anchoring site, and the backbone's deprotoned amide nitrogen atoms as the main binding sites. Drove by the coordination bonds and their induced hydrogen bond net, the conformations of Aβ converted from β-sheet non-β-sheet conformations, which destabilized the aggregation of Aβ into fibrils.

Metal Phosphonates Based on Bis(benzimidazol-2-ylmethyl)imino Methylenephosphonate: From Discrete Dimer to Two-Dimensional Network Containing Metallomacrocycles

Hydrothermal reactions of bis(benzimidazol-2-ylmethyl)imino(methylenephosphonic acid) {[(C(7)H(5)N(2))CH(2)]2NCH(2)PO(3)H(2), bbimpH2} with metal salts result in four new compounds, namely, Mn2{[(C(7)H(5)N(2))CH2]2NCH(2)PO(3)}2(H2O)2.2H2O (1), Cd2{[(C(7)H(5)N(2))CH2]2NCH(2)PO(3)}2.2H2O (2), Fe2{[(C(7)H(5)N(2))CH2]2NCH(2)PO(3)}2.H2O (3), and CuI(2){[(C(7)H(5)N(2))CH2]2NCH(2)P(OH)O2}2 (4). Compounds 1 and 2 have dinuclear structures in which two {MN(3)O(3)} octahedra are linked through edge sharing. In compound 3, a chain structure is observed where the {FeN(3)O(2)} trigonal bipyramids are linked by {CPO(3)} tetrahedra through corner-sharing. The structure of compound 4 is unique. The monovalent Cu(I) ions are connected by the imidazole nitrogen atoms from the bbimp(2-) ligands forming a 16-member metallomacrocycle. These metallomacrocycles are further connected by the phosphonate oxygen atoms, leading to a two-dimensional net containing 16- and 32-member rings. Magnetic studies of 1 and 3 reveal that weak ferromagnetic interactions are mediated between magnetic centers in compound 1, while antiferromagnetic interactions were observed in compound 3.

Multiple forms of copper (II) co-ordination occur throughout the disordered N-terminal region of the prion protein at pH 7.4

Although the physiological function of the prion protein remains unknown, in vitro experiments suggest that the protein may bind copper (II) ions and play a role in copper transport or homoeostasis in vivo. The unstructured N-terminal region of the prion protein has been shown to bind up to six copper (II) ions, with each of these ions co-ordinated by a single histidine imidazole and nearby backbone amide nitrogen atoms. Individually, these sites have micromolar affinities, which is weaker than would be expected of a true cuproprotein. In the present study, we show that with subsaturating levels of copper, different forms of co-ordination will occur, which have higher affinity. We have investigated the copper-binding properties of two peptides representing the known copper-binding regions of the prion protein: residues 57-91, which contains four tandem repeats of the octapeptide GGGWGQPH, and residues 91-115. Using equilibrium dialysis and spectroscopic methods, we unambiguously demonstrate that the mode of copper co-ordination in both of these peptides depends on the number of copper ions bound and that, at low copper occupancy, copper ions are co-ordinated with sub-micromolar affinity by multiple histidine imidazole groups. At pH 7.4, three different modes of copper co-ordination are accessible within the octapeptide repeats and two within the peptide comprising residues 91-115. The highest affinity copper (II)-binding modes cause self-association of both peptides, suggesting a role for copper (II) in controlling prion protein self-association in vivo.

Structural, Spectroscopic, and Magnetic Study of Bis(9,10-dihydro-9-oxo-10-acridineacetate)bis(imidazole)bis(methanol) Nickel(II)

The mixed ligand complex [Ni(CMA)2(im)2(MeOH)2] (where CMA = 9,10-dihydro-9-oxo-10-acridineacetate ion, im = imidazole) was prepared, and its crystal and molecular structure were determined. The nickel ions are hexa-coordinated by four oxygen atoms of the carboxylate and hydroxyl groups and by two imidazole nitrogen atoms, to form a distorted octahedral arrangement. The structure consists of a one-dimensional network of the complex molecules connected by strong intermolecular hydrogen bonds. The weak intermolecular C-H...X hydrogen bonds and stacking interactions make up the 2-D structure. Very strong intramolecular hydrogen bonds significantly affect the geometry and vibrational characteristics of the carboxylate group. The UV-vis-NIR electronic spectrum was deconvoluted into Gaussian components. Electronic bands of the Ni(II) ion were assigned to suitable spin-allowed transitions in the D4h symmetry environment. The single ion zero-field splitting (ZFS) parameters for the S = 1 state of Ni(II), as well as the g components, have been determined by high-field and high-frequency EPR (HF-HFEPR) spectroscopy over the frequency range of 52-432 GHz and with the magnetic fields up to 14.5 T: D = 5.77(1) cm-1, E = 1.636(2) cm-1, gx = 2.29(1), gy = 2.18(1), and gz = 2.13(1). These values allowed us to simulate the powder magnetic susceptibility and field-dependent magnetization of the complex.

Low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp-[Fe(OEP)(2-MeHIm)(2)]Cl): a consequence of hydrogen bonding?

The synthesis and characterization of low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp[Fe(OEP)(2-MeHIm)2]Cl) is reported. The structure shows that the cation is a low-spin species with two imidazole ligands having a relative perpendicular orientation. The porphyrin core is very ruffled, which leads to shortened equatorial bonds of 1.974(4) A and slightly elongated axial Fe-N bond lengths of 2.005(10) A that are about 0.02 A shorter and 0.03 A longer, respectively, in comparison to bis-imidazole ligated iron(III) species with parallel oriented axial ligands. A one-dimensional hydrogen-bond chain is formed between chloride anions and uncoordinated imidazole nitrogen atoms. Compared with paral-[Fe(OEP)(2-MeHIm)2]ClO4, hydrogen bonding may play an important role in the differences in the two structures. Mössbauer spectra show broadened quadrupole doublets with quadrupole splittings of 1.81 mm/s at RT and 1.94 mm/s at 20 K. The isomer shift ranges from 0.26 to 0.36 mm/s. These confirm that the title complex is a low-spin iron(III) species with the ground state (dxy)2(dxz,dyz)3. Crystal data: monoclinic, space group P2(1)/c, a = 14.066(3) A, b, 20.883(4) A, c = 19.245(4) A, beta = 109.67 degrees , and Z = 4.

Novel ternary cobalt(II) and nickel(II) complexes of N-phthaloylglycinate. Synthesis, characterization and X-ray crystal structure

A new series of N-phthaloylglycineate (N-phthgly) ternary complexes of cobalt(II) and nickel(II) with imidazole (imi), N-methylimidazole (mimi) and 2,2′-bipyridyl (bipy) have been synthesized and characterized by elementary analyses, IR spectroscopy, thermogravimetric analysis. X-ray crystal structure analyses of the three complexes of [Co(mimi)2(N-phthgly)2] (1), [Co(bipy)(OH2)4](N-phthgly)2(2) and [Ni(imi)2(N-phthgly)2(OHCH3)2] (3) were also carried out. In complex (1), the Co(II) exists in a distorted tetrahedral enviroment, where two nitrogen atoms of two methylimidazole molecules and two oxygen atoms of the carboxylate group of two N-phthaloylglycinate molecules are coordinated. On the other hand, in complex (2) the cobalt atom coordinates a 2,2′-bipyridine molecule and four water oxygen atoms forming a distorted octahedral conformation. A molecule of N-phthaloylglycinate is connected by van der waals contact and H-bonds. For complex (3), the nickel atom is surrounded by four oxygens (two oxygens of two different N-phthaloylglycinate molecules and two of methanol ligand) in the basal plane of octahedron along with two imidazole nitrogen atoms at the apical positions. Strong intramolecular H-bond exists between the uncoordinated carboxylic oxygen of the N-phthaloylglycinate ligand and the O–H of the methanol group.

A Novel 1‐D Stacking Z‐Type Supramolecular Complex, [Ni(sphs)(iz)] · EtOH (sphs=4‐(N‐Salicylaldehyde Schiff Base) Benzeneformylhydrazone‐N′‐salicylaldehyde, iz=Imidazole), Constructed by Hydrogen Bonds

A new ligand, 4‐(N‐salicylaldehyde Schiff base) benzeneformylhydrazone‐N′‐salicylaldehyde (H3L), and a new quaternary complex 1, [Ni(sphs)(iz)] · EtOH (sphs=H3L, iz=imidazole), has been designed and synthesized. The structural unit of complex 1 consists of the following groups: one Ni(II), one molecular H3L, one imidazole and one ethanol. It belongs to the orthorhombic system and has a Pbca space group. One deprotonated oxygen in enol‐form, one deprotonated oxygen of hydroxyl and one nitrogen of metheleneimine for H3L and one nitrogen atom of imidazole coordinated to central Ni with two oxygen and two nitrogen in anti‐form, respectively, constructed distorted square geometry, an ethanol existed in crystal lattice. There are intramolecular and intermolecular hydrogen bonds in the crystal structure.

A Ruthenium(II) Complex with a Tripodal Ligand Containing Three Imidazole Groups

AbstractThe orange [RuII(H3L)](ClO4)2·3H2O (1) complex was synthesized from cis‐[RuCl2(dmso)4] and H3L, where H3L (tris{[2‐{(imidazol‐4‐yl)methylidene}amino]ethyl}amine) is a tripodal ligand obtained by condensation of tris(2‐aminoethyl)amine and 4‐formylimidazole in a 1:3 mol ratio. The X‐ray crystal structure analysis revealed that the complex has an octahedral structure coordinated by three imidazole nitrogen atoms and three Schiff base (imine) nitrogen atoms. The uncoordinated NH groups of the imidazole moieties of 1 are easily deprotonated by the action of a base. The effect of deprotonation on the RuIII/RuII redox potential was studied by cyclic voltammetry in methanol containing 0.1 M LiClO4. Complete deprotonation shifts the RuIII/RuII potential to a much more negative value from –0.18 to –0.72 V vs. Ag/Ag+, making it easier to oxidize. The bluish‐purple RuIII complex, [RuIII(H3L)]3+, was formed by controlled‐potential electrolysis of 1, and the electronic spectrum is reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Transition metal complexes of terminally protected peptides containing histidyl residues

Histidine-containing peptide fragments of prion protein are efficient ligands to bind various transition metal ions and they have high selectivity in metal binding. The metal ion affinity follows the order: Pd(II) ≫ Cu(II) ≫ Ni(II) ⩾ Zn(II) > Cd(II) ∼ Co(II) > Mn(II). The high selectivity of metal binding is connected to the involvement of both imidazole and amide nitrogen atoms in metal binding for Pd(II), Cu(II) and Ni(II), while only the monodentate N im-coordination is possible with the other metal ions. The stoichiometry and binding mode of palladium(II) complexes show great variety depending on the metal ion to ligand ratio, pH and especially the presence of coordinating donor atoms in the side chains of peptide fragments. It is also clear from our data that the peptide fragments containing histidine outside the octarepeat (His96, His111 and His187) are more efficient ligands than the monomer peptide fragments of the octarepeat domain.

Phenoxyl radicals: H-bonded and coordinated to Cu(ii) and Zn(ii)

Two pro-ligands ((R)LH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole ((Bz)LH) or a 4,5-di-p-methoxyphenyl substituted imidazole ((PhOMe)LH), have been structurally characterised. Each possesses an intramolecular O-H[dot dot dot]N hydrogen bond between the phenolic O-H group and an imidazole nitrogen atom and (1)H NMR studies show that this bond is retained in solution. Each (R)LH undergoes an electrochemically reversible, one-electron, oxidation to form the [(R)LH] (+) radical cation that is considered to be stabilised by an intramolecular O...H-N hydrogen bond. The (R)LH pro-ligands react with M(BF(4))(2).H(2)O (M = Cu or Zn) in the presence of Et(3)N to form the corresponding [M((R)L)(2)] compound. [Cu((Bz)L)(2)] (), [Cu((PhOMe)L)(2)] (), [Zn((Bz)L)(2)] and [Zn((PhOMe)L)(2)] have been isolated and the structures of .4MeCN, .2MeOH, .2MeCN and .2MeCN determined by X-ray crystallography. In each compound the metal possesses an N(2)O(2)-coordination sphere: in .4MeCN and .2MeOH the {CuN(2)O(2)} centre has a distorted square planar geometry; in .2MeCN and .2MeCN the {ZnN(2)O(2)} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both and , in CH(2)Cl(2)-DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(ii) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)((R)L)((R)L )](+), comprising a M(ii) centre bound to a phenoxide ((R)L) and a phenoxyl radical ((R)L ) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF(4)]. The second oxidation produces [M(II)((R)L )(2)](2+). The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [(R)LH] (+) (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)((R)L)((R)L )](+) (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(ii) is slightly more effective than Zn(II) in this respect.

Crystal and Molecular Structure of Bis(1-Vinylimidazole)diacetatozinc

The crystal and molecular structure of bis(1-vinylimidazole)diacetatozinc (C14H18N4O4Zn), a highly effective antidote and antihypoxic drug, was determined [R1 0.0335 for 8902 unique reflections with I > 2σ(I) and wR2 0.0931 for all 10 752 unique reflections]. The triclinic unit cell contains two independent molecules of the complex, A and B. These molecules have short contacts O⋯H-C. The zinc atoms in molecules A and B have distorted tetrahedral coordination with the coordination sites occupied by the imidazole nitrogen atoms and acetate oxygen atoms. The lengths of the Zn-N bonds (2.019–2.050 A) and Zn-O bonds with three acetate groups (1.956–1.958 A) are typical for zinc complexes, whereas the Zn-O3 bond with one of the acetate groups in molecule A is somewhat longer, 2.009(1) A; also, there is an additional contact of the zinc atom with the carbonyl oxygen atom of this group (Zn-O4 2.498 A).

A multi-approach study of the interaction of the Cu(II) and Ni(II) ions with alanylglycylhistamine, a mimicking pseudopeptide of the serum albumine N-terminal residue

The protonation equilibria of alanylglycylhistamine (Ala–Gly–Ha) and the complexation of this ligand with Cu(II) and Ni(II) have been studied by pH-potentiometry, 1 H and 14N NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), circular dichroism (CD), UV–Vis spectrophotometry and electron paramagnetic resonance (EPR). From pH ≈ 2–12, the following complexes: MLH, MLH −1, MLH −2 and MLH −3 are successively formed in aqueous solutions, the ligand under its neutral form being noted L. At physiological pH, the MLH −2 complex is predominant. The coordination in this complex is assumed by one amino, two deprotonated peptide and one imidazole nitrogen atoms. The ESI-MS study confirmed the formation of the MLH −1, MLH −2 and MLH −3 complexes. The structure of MLH −2 was determined by single crystal X-ray analysis. CD and UV–Vis techniques allowed us to propose that the imidazole-N3 nitrogen acts as the anchor group for the coordination to the metal(II) ions rather than the amino group. At high pH values, the further deprotonation of the N–H imidazole group, leading to the formation of MLH −3, occurs, as revealed by 1H NMR spectroscopy.

Synthesis, Structure and Characterization of a Series of Transition Metal Complexes with Tripodal Polyimidazole Ligand

Five new metal transition metal complexes formed with tripodal polyimidazole ligand tri{2-[2-(1-methyl)imi-dazoly](methylimino)ethyl}amine ((min)3tren), [Zn(min)3tren](ClO4)2 (1) [Cu(min)3tren](ClO4)2 (2), [Ni(min)3tren]-(ClO4)2 (3), [Co(min)3tren](ClO4)2 (4), and [Mn(min)3tren](ClO4)2·CH3CN (5) were synthesized and characterized by elemental analysis, molar conductances, IR and electronic spectra. Analytical results show 1:1 metal-ligand stoichiometry and 2:1 type of electrolyte in all metal complexes. The crystal structures of 4 and 5 have been determined. The metal atoms in 4 and 5, being in distorted [MN6] octahedra, are coordinated with three imine nitrogen atoms and three imidazole nitrogen atoms.

Bis(1‐methylimidazol‐2‐yl)propionates and Bis(1‐methylbenzimidazol‐2‐yl)‐propionates: A New Family of Biomimetic N,N,O Ligands – Synthesis, Structures and CuII Coordination Complexes

AbstractA general synthetic route towards new tridentate, tripodal N,N,O ligands, bearing one carboxylate group and two of the biologically relevant imidazole moieties, is presented. The parent compounds of this new family of ligands, A[MIm2Pr] (1) and A[BenzMIm2Pr] (2) (A = H, K, Bu4N) were synthesized in two steps and high overall yield. The structures of HMIm2Pr (1a) and HBenzMIm2Pr (2a) were confirmed by X‐ray crystal structure determination, both molecules forming infinite one‐dimensional hydrogen bonded chains. The synthetic route allows for the fine‐tuning of the physical properties of both ligand systems, e.g., solubility and ease of crystallization. The capability of the two ligands to coordinate facially through all donor atoms was investigated and two isostructural mononuclear copper(II) complexes of the type [CuL2] were obtained. Both [Cu(MIm2Pr)2]·2H2O (3) and [Cu(BenzMIm2Pr)2]·3.12(H2O)·1.74(C2H5OH) (4) have been characterized by X‐ray crystallography, ESI‐MS, elemental analysis, UV/Vis, IR, and EPR spectroscopy. In both structures the copper atoms are on an inversion center, which results in a tetragonally distorted octahedral coordination geometry. The CuN4O2 chromophores consist of four equatorially coordinated imidazole nitrogen atoms and two axially coordinated carboxylate groups. The non‐coordinated water molecules of both crystal structures are involved in a one‐dimensional hydrogen‐bonded network. Two neighboring octahedrons are connected by two water molecules, each water molecule forming two hydrogen bonds with the non‐coordinated oxygen atoms of the carboxylate group. This results in the formation of infinite one‐dimensional chains. EPR spectroscopy and ESI‐MS measurements provide evidence for the integrity of the complexes in solution. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

A New Trinuclear Linear Copper(II) Complex: Unusual Crystal Structure with Semi‐Coordinated Thiophene Moieties and Weak Antiferromagnetic Coupling Through the Bridging Imidazolate Rings

AbstractA trinuclear copper(II) complex, [Cu3(imthio)2(NO3)4(MeOH)2] [where Himthio is 1,1‐bis(imidazol‐2‐yl)‐3‐(thiophen‐2‐yl)‐2‐azapropane], has been synthesized by the reaction of Himthio with Cu(NO3)2·3H2O in MeOH. The structure of the copper complex is centrosymmetric and contains a linear trinuclear array of copper atoms. The central copper(II) ion, Cu1, which lies on an inversion centre, is equatorially coordinated by four nitrogen atoms from two anionic imthio ligands and axially by two semicoordinated thiophene sulfur atoms to give an octahedral environment (Cu1−S1 3.136 Å). The Himthio ligand bridges Cu1 and the terminal copper(II) ions through imidazole nitrogen atoms. The coordination around the external ions is completed by the oxygen atoms from two nitrate groups and by a methanol molecule in a distorted square‐pyramidal geometry. The two terminal copper(II) complexes can be considered to be chelating ligands for the central CuII ion. The EPR spectrum of the complex in MeOH at 77 K shows two mononuclear species, as is fully confirmed by EPR simulation. Magnetic susceptibility of the complex shows weak antiferromagnetic behaviour (J = −71.37 cm−1) caused by the overlapping of the magnetic orbitals of the copper atoms and the bridging imidazolate rings. (© Wiley‐VCH Verlag GmbH & Co.. KGaA, 69451 Weinheim, Germany, 2004)

RK‐805, an Endothelial‐Cell‐Growth Inhibitor Produced by Neosartorya sp., and a Docking Model with Methionine Aminopeptidase‐2.

Abstract We found that a fungus Neosartorya sp. produced an angiogenesis inhibitor, RK-805. By spectroscopic analyses and semi-synthetic methods from fumagillin, the structure of RK-805 was identified as 6-oxo-6-deoxyfumagillol, which has not been reported as a natural product. RK-805 preferentially inhibited the growth of human umbilical vein endothelial cells (HUVECs) rather than that of human normal fibroblast in cell proliferation assays and blocked endothelial cell migration induced by vascular endothelial growth factor (VEGF). Moreover, RK-805 selectively inhibited methionine aminopeptidase-2 (MetAP2), but not methionine aminopeptidase-1 (MetAP1). The docked structure of RK-805 complexed with human MetAP2 indicated that not only a covalent bond between a nucleophilic imidazole nitrogen atom of His231 and the carbon of the reactive spirocyclic epoxide of RK-805, but also a hydrogen bond between NH (Asn329) and the carbonyl group of RK-805 at C-6 promote close contact in the binding pocket of the enzyme. Taken together, these results suggest that structure activity relationships of RK-805 derivatives at both C-4 and C-6, in comparison with ovalicin and TNP-470, would be useful for development of new angiogenesis inhibitors.

Spectral characteristics of 2-(2′-hydroxy-3′-pyridyl)benzimidazole: effects of solvents and acid or base concentrations

Absorption, fluorescence and fluorescence excitation spectroscopy and time dependent fluorimetry have been used to study the spectral characteristics of 2-(2′-hydroxy-3′-pyridyl)benzimidazole (2-H3PyBI) in different solvents and at different acid–base concentrations. Observation of only one small Stokes shifted fluorescence band reveals the absence of excited state intramolecular proton transfer (ESIPT). 2-H3PyBI is mainly present as the keto form even in cyclohexane. First site of protonation is imidazole nitrogen atom in both ground (S 0) and first excited singlet state (S 1). Dication in S 0 state is formed by protonating exo carbonyl group and pyridine nitrogen in S 1 state. Deprotonation occurs from >N–H moiety of pyridone, followed by structural reorganization to yield enolate ion. AM1 semi-empirical quantum mechanical and density functional theory (DFT) calculations have been used to assign the structure of neutral species and the site of protonation and deprotonation of 2-H3PyBI. Dissociation constants of various prototropic equilibria were determined and discussed. Theoretical calculations have been carried out to find the cause for the absence of ESIPT.

Stabilization of a Copper(II) Complex by Hydrogen Bonds between the Axial Tetrafluoroborate Anion and the Ammonium Group from the Coordinated Cationic Ligand Bis(imidazol‐2‐yl)methyl(isopropyl)ammonium

AbstractA new mononuclear copper complex [Cu(HIm‐iPr)2(BF4)2](BF4)2·2CH3OH {where Im‐iPr is [bis(imidazol‐2‐yl)methyl](isopropyl)amine} has been obtained by diethyl ether diffusion into a methanol solution of Im‐iPr and Cu(BF4)2. The complex has been characterized by X‐ray analysis, UV/Vis, IR and EPR spectroscopy. Its crystal structure is built up by centrosymmetric [Cu(HIm‐iPr)2(BF4)2]2+ complex cations, BF4− counterions and methanol lattice molecules. In the cation, the Cu atom exhibits a distorted octahedral CuN4F2 chromophore coordinated by two axial BF4− ions and by four equatorial imidazole nitrogen atoms from two cationic HIm‐iPr ligands. The complex is stabilized by two intramolecular pseudo‐chelate hydrogen bonds between the axial BF4− ions and the ammonium nitrogen atoms of the HIm‐iPr ligand [N5···F8 = 2.761(5) Å]. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)