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Model Complexes (model + complex)
Selected AbstractsHigh-Turnover Photochemical Hydrogen Production Catalyzed by a Model Complex of the [FeFe]-Hydrogenase Active SiteCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2010Daniel Streich Lifting the diiron curtain: 200,equivalents of H2 per catalyst at a maximum turnover rate of 3.7,H2 per minute can be obtained in a light-driven proton reduction process in near-neutral H2O/DMF (pH,5.5) by using an [FeFe]-hydrogenase active-site model complex as the catalyst (see scheme). [source] An Iron(II) Carbonyl Thiolato Complex Bearing 2-Methoxy-Pyridine: A Structural Model of the Active Site of [Fe] HydrogenaseCHEMISTRY - AN ASIAN JOURNAL, Issue 9 2010Soichiro Tanino Dr. Model complex: An FeII complex bearing a thiolate, 2-methoxy-pyridine, and three facially arranged CO ligands was synthesized as a structural analogue of the active site of a CO-inhibited form of [Fe] hydrogenase, from the reaction of [FeBr2(CO)4] with NaS{2,6-(mesityl)2C6H3} and the successive treatment with 2-methoxy-pyridine. [source] The N -Acylated Derivatives of Parent Complex [{(,-SCH2)2NH}Fe2(CO)6] as Active Site Models of Fe-Only Hydrogenases: Synthesis, Characterization, and Related PropertiesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 2 2008Li-Cheng Song Abstract A series of N -acylated diiron azadithiolate complexes as H-cluster models was synthesized and structurally characterized. Treatment of parent complex [{(,-SCH2)2NH}Fe2(CO)6] (A) with 2-chloroacetic acid in the presence of dicyclohexylcarbodiimide or with 2-chloroacetyl chloride in the presence of Et3N gave N -chloroacetyl complex [{(,-SCH2)2NC(O)CH2Cl}Fe2(CO)6] (1). Further treatment of 1 with MeC(O)SK afforded N -acetylthioacetyl complex [{(,-SCH2)2NC(O)CH2SC(O)Me}Fe2(CO)6] (2). N -Ethoxylcarbonylacetyl complex [{(,-SCH2)2NC(O)CH2CO2Et}Fe2(CO)6] (3) and N-heterocyclic complexes [{(,-SCH2)2NC(O)C4H3Y-2}Fe2(CO)6] (4, Y = O; 5, Y = S) were produced by reactions of A with EtO2CCH2C(O)Cl, 2-furancarbonyl chloride, and 2-thiophenecarbonyl chloride in the presence of pyridine or Et3N. Similarly, N -malonyl complex [{Fe2(CO)6(,-SCH2)2NC(O)}2CH2] (6) and N -carbonylbenzaldehyde complex [{(,-SCH2)2NC(O)C6H4CHO- p}Fe2(CO)6] (7) could be obtained by reaction of A with malonyl dichloride in the presence of pyridine and with p -CHOC6H4C(O)Cl in the presence of Et3N. More interestingly, further reaction of 7 with PhCHO and pyrrole in a 1:3:4 molar ratio in the presence of BF3·OEt2 followed by p -chloranil yielded the first light-driven type of model complex containing an N -carbonylphenylporphyrin moiety [{(,-SCH2)2NC(O)(TPP)}Fe2(CO)6] (8, TPP = tetraphenylporphyrin group). Whereas the molecular structures of 2, 5, and 7 were established by X-ray crystallography, the electrochemical properties of 2,5 as well as the proton reduction to hydrogen gas catalyzed by 2 and 3 were studied by CV techniques.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] High-Turnover Photochemical Hydrogen Production Catalyzed by a Model Complex of the [FeFe]-Hydrogenase Active SiteCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2010Daniel Streich Lifting the diiron curtain: 200,equivalents of H2 per catalyst at a maximum turnover rate of 3.7,H2 per minute can be obtained in a light-driven proton reduction process in near-neutral H2O/DMF (pH,5.5) by using an [FeFe]-hydrogenase active-site model complex as the catalyst (see scheme). [source] Luminescent Molecular Copper(I) Alkynyl Open Cubes: Synthesis,,Structural,Characterization, Electronic Structure, Photophysics, and PhotochemistryCHEMISTRY - AN ASIAN JOURNAL, Issue 1-2 2006Chui-Ling Chan Dr. Abstract A novel class of tetranuclear copper(I) alkynyl complexes with an "open-cube" structure was synthesized. The crystal structure of [Cu4{P(p -MeC6H4)3}4(,3 -,1,,1,,2 -C,C- p -MeOC6H4)3]PF6 was determined. These complexes were found to display dual emission behavior. Through systematic comparison studies on the electronic absorption and photoluminescence properties of a series of [Cu4(PR3)4(,3 -,1,,1,,2 -C,CR,)3]+ complexes, together with density functional theory (DFT) calculations at the PBE1PBE level on the model complex [Cu4(PH3)4(,3 -,1,,1,,2 -C,C- p -MeOC6H4)3]+, the nature of their emission origins was probed. Their photochemical properties were also investigated by oxidative quenching experiments and transient absorption spectroscopy. [source] Strong Evidence for an Unprecedented Borderline Case of Dissociation and Cycloaddition in Open-Shell 1,3-Dipole Chemistry: Transient Nitrilium Phosphane-Ylide Complex Radical CationsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 22 2009Holger Helten Abstract The reaction of 3-ferrocenyl-substituted 2H -azaphosphirene complexes 1a,c in the presence of substoichiometric amounts of ferrocenium hexafluorophosphate yields 3,5-diferrocenyl-substituted 2H -1,4,2-diazaphosphole complexes 3a,c and difluoro(organo)phosphane complexes 4a,c. The reaction of 1a,c and [FcH]PF6 with cyanoferrocene yields 3a,c in a straightforward way. The molecular structures of 3a,c were unambiguously identified by multinuclear NMR spectroscopic experiments, mass spectrometry, and single-crystal X-ray diffraction studies. DFT calculations on model complexes 1d,m and 3d,m reveal a close similarity of Mo and W complexes (vs. Cr) and a strong influence of the ferrocenyl substituent on the geometry, spin, and charge distribution of reactive intermediates and the reaction course. Strong support for the assumption of a dissociation,cycloaddition reaction sequence leading to 3 and thus a surprising "cannibalistic" reaction was obtained.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Isostructural Potassium and Thallium Salts of Sterically Crowded Thio- and Selenophenols: A Structural and Computational StudyEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 36 2008Denis Bubrin Abstract Because of their similar cationic radii, potassium and thallium(I) compounds are usually regarded as closely related. Homologous molecular species containing either K+ or Tl+ are very rare, however. We have synthesized potassium and thallium salts MEAr* [M, E = K, S (2a); K, Se (2b); Tl, S (3a); Tl, Se (3b); Ar* = 2,6-Trip2C6H3, Trip = 2,4,6- iPr3C6H2] derived from terphenyl-substituted thio- and selenophenols. In the solid-state structures of dimeric 2a, 2b, 3a, and 3b additional metal-,n,,-arene interactions to the flanking arms of the terphenyl substituents of different hapticity n are observed. Remarkably, the homologous potassium and thallium complexes 2b and 3b crystallize in isomorphous cells. For 2a, 3a, and model complexes of the composition METph (Tph = C6H4 -2-Trip) the nature of the M,E and M···C(arene) bonding was studied by density functional theory calculations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Luminescent Gold(I) and Copper(I) Phosphane Complexes Containing the 4-Nitrophenylthiolate Ligand: Observation of ,,,* Charge-Transfer EmissionEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2008Cheng-Hui Li Abstract Gold(I) and copper(I) phosphane complexes containing the 4-nitrophenylthiolate ligand, namely [(PCy3)Au(SC6H4NO2 -4)] (1) (PCy3 = tricyclohexylphosphane), [Au2(,-dcpm)(SC6H4NO2 -4)2] (2) [dcpm = bis(dicyclohexylphosphanyl)methane], [Au2(,-dppm)(SC6H4NO2 -4)2] (3) [dppm = bis(diphenylphosphanyl)methane], and [(,2 -SC6H4NO2 -4)2(,3 -SC6H4NO2 -4)2(CuPPh3)4] (4), were prepared and characterized by X-ray crystal analysis. All of these complexes show an intense absorption band with ,max at 396,409 nm attributed to the intraligand (IL) ,(S),,*(C6H4NO2 -4) charge-transfer transition. The assignment is supported by the results of DFT and TDDFT calculations on the model complexes [PH3Au(SC6H4NO2 -4)] and [(,2 -SC6H4NO2 -4)2(,3 -SC6H4NO2 -4)2(CuPH3)4]. The emissions of solid samples and glassy solutions (methanol/ethanol, 1:4, v/v) of 1,4 at 77 K are assigned to the [,(S),,*(C6H4NO2 -4)] charge-transfer excited state. Metallophilic interactions are not observed in both solid state and solutions of complexes 1,3. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Tetrahedral CoII Complexes with CoI2O2 and CoO2S2 Cores , Crystal Structures of [Co{HN(OPPh2)(SPPh2)- O}2I2] and [Co{N(OPPh2)(SPPh2)- O,S}2]EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 29 2007M. Carla Aragoni Abstract The compound [CoII{HN(OPPh2)(SPPh2)- O}2I2] (1) was synthesised by the reaction of cobalt in powder with the iodine adduct of tetraphenylthiooxoimidodiphosphinic acid, HN(OPPh2)(SPPh2), in Et2O; treatment of compound 1 with NaOH resulted in deprotonation of the ligands bound to the metal ion and a separation of [CoII{N(OPPh2)(SPPh2)- O,S}2] (2). Molecular structures of complexes 1 and 2 were elucidated by X-ray diffraction analysis, which revealed a CoI2O2 tetrahedral core for compound 1 in which two neutral ligands bind through the oxygen atoms the CoII ion, and a tetrahedral CoO2S2 core for compound 2 with the oxygen and sulfur atoms of each anionic ligand chelating a CoII centre. Variable-temperature magnetic susceptibility measurements are consistent with tetrahedral high-spin (S = 3/2) CoII that possesses a 4A2 ground state with best fit parameters g = 2.25, |D| = 12.0 cm,1 and g = 2.37, |D| = 11.9 cm,1 for complexes 1 and 2, respectively. The compounds were further characterised by UV/Vis and IR spectroscopy. DFT calculations were performed on model complexes [CoII{N(OPH2)(SPH2)- O,S}2] (3) and [CoII{N(SPH2)2 - S,S,}2] (4) to compare the electronic properties of the CoO2S2 and CoS4 cores. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Sodiated ,-Diphosphonate Carbanions: Characterization of the Tetrameric Cubane and the Hexameric Ladder Complexes [{(iPrO)2P(O)}2CHNa]4 and [{(EtO)2P(O)}2CHNa]6EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 18 2003Roger Nassar Abstract Reaction of the methylene diphosphonates [(RO)2P(O)]2CH2, where R = iPr or Et, with the metalating reagents NaH, nBuNa or the superbasic mixture tBuONa/nBuLi leads to monodeprotonation, and formation of the complexes [{(iPrO)2P(O)}2CHNa]4 (1) and [{(EtO)2P(O)}2CHNa]6 (2). Single-crystal X-ray diffraction studies revealed that 1 is tetrameric in the solid state, with a central Na4O4 cubane core. Each metal in 1 is rendered pentacoordinate by binding to three ,3 -O phosphoryl bridges within the cubane, a chelating phosphoryl unit, and also a single iPrO group. In contrast, the structure of 2 is hexameric, and is composed of an unusual six-rung Na6O6 ladder core. Three distinct sodium environments as well as ,1 -, ,2 - and ,3 -O phosphoryl units are present within the ladder framework. No Na,C interactions are present in either 1 or 2. All of the six-membered NaOPCPO chelate rings display localized P=O double bonds and partial multiple bonding on the P,C,P linkages (mean P,O: 1.49 Å, and mean P,C: 1.69 Å). Ab initio molecular orbital calculations (HF/6,31G*) on model complexes indicate that the hexameric arrangement found for 2 is thermodynamically more stable than the cubane structure of 1, in the absence of steric effects. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Alkylzinc Complexes with Achiral and Chiral Monoanionic N,N,O Heteroscorpionate LigandsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 2 2003Ina Hegelmann Abstract The synthesis of the new chiral ligand (3,5-di- tertbutylpyrazol-1-yl)(3,,5,-dimethylpyrazol-1-yl)aceticacid (bpaHtBu2,Me2) (4) has been achieved. Two different synthetic routes to its precursor 3,5-di- tert -butyl-1-[(3,5-dimethyl-1H -pyrazol-1-yl)methyl]-1H -pyrazole (bpmtBu2,Me2) (3) are reported. Deprotonation at the methylene group, followed by reaction with carbon dioxide, yielded a racemic mixture of 4. The chemical behaviour of bis(3,5-di- tert -butylpyrazol-1-yl)acetic acid (bdtbpzaH) (2) and the new chiral N,N,O scorpionate ligand 4 involving their coordination to zinc ions was studied. [Zn(bpatBu2,Me2)Cl] (5) was formed from a mixture of ZnCl2, 4 and base. Reaction of bis(3,5-di- tert -butylpyrazol-1-yl)acetic acid (bdtbpzaH) (2) with Zn(CH3)2 or Zn(CH2CH3)2 gave the alkylzinc complexes [Zn(bdtbpza)(CH3)] (6) and [Zn(bdtbpza)(CH2CH3)] (7). [Zn(bpatBu2,Me2)(CH3)] (8) was obtained from a synthesis analogous to that of 6 with 4. The further reactions of 6 and 8 with acetic acid resulted in the acetato complexes [Zn(OAc)(bdtbpza)] (9) and [Zn(OAc)(bpatBu2,Me2)] (10). The chiral methyl complex 8 may serve as a precursor for structural model complexes of the active sites of zinc enzymes, such as thermolysin or carboxypeptidase A. [Zn(bpatBu2,Me2)2] (11) was formed from a side reaction. Crystal structures of 4, 5, 8 and 11 were obtained; 5 crystallised as the dimer [Zn(bpatBu2,Me2)Cl]2; 11 presents an unusual zinc binding geometry. (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2003) [source] Structure and hydrolysis of the heavy alkaline earth cations: Relativistic studiesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 12 2006Maria Barysz Abstract The relativistic spin-averaged Douglas,Kroll theory at the level of the MP2 and CCSD(T) approximations has been applied to explain the hydrolysis trends for the Group 2 dications through the investigation of the structure and energetics of the model complexes with the water molecule. The parallel nonrelativistic calculations have been performed to show the possible role of the relativistic effect on the structure and hydrolysis of the heaviest metal cations. The calculated data are compared with the recent experimental results. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Hydrogen-bonding effects on electronic g -tensors of semiquinone anion radicals: Relativistic density functional investigationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2002Konstantin M. Neyman Abstract We report results of systematic g -tensor calculations of hydrogen-bonded complexes of two benchmark semiquinone anion radicals, 1,4 -benzoquinone and tetramethyl- 1,4 -benzoquinone (duroquinone), with water and methanol molecules. The calculations have been carried out with the help of a recently developed g -tensor module that is based on a relativistic density functional method that takes spin,orbit interaction self-consistently into account. We demonstrate the applicability of this new computational scheme to describe quantitatively delicate effects of hydrogen bonding on electronic g -tensor values. Also, we explored general trends of how g -tensors depend on the structure and stoichiometry of hydrogen-bonded semiquinone complexes. Complexes exhibiting one hydrogen bond per oxygen atom of the quinones with a linear arrangement of the CO , H moieties were shown to feature g-shifts induced by these hydrogen bonds that are in close agreement with measured electron paramagnetic resonance data. Based on deviations of calculated and measured g-components, we classify all other model complexes studied as less probable under the experimental conditions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] A CSOV study of the difference between HF and DFT intermolecular interaction energy values: The importance of the charge transfer contributionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2005Jean-Philip Piquemal Abstract Intermolecular interaction energy decompositions using the Constrained Space Orbital Variation (CSOV) method are carried out at the Hartree,Fock level on the one hand and using DFT with usual GGA functionals on the other for a number of model complexes to analyze the role of electron correlation in the intermolecular stabilization energy. In addition to the overall stabilization, the results provide information on the variation, with respect to the computational level, of the different contributions to the interaction energy. The complexes studied are the water linear dimer, the N -methylformamide dimer, the nucleic acid base pairs, the benzene,methane and benzene-N2 van der Waals complexes, [Cu+ -(ImH)3]2, where "ImH" stands for the Imidazole ligand, and ImH-Zn++. The variation of the frozen core energy (the sum of the intermolecular electrostatic energy and the Pauli repulsion energy) calculated from the unperturbed orbitals of the interacting entities indicates that the intramolecular correlation contributions can be stabilizing as well as destabilizing, and that general trends can be derived from the results obtained using usual density functionals. The most important difference between the values obtained from HF and DFT computations concerns the charge transfer contribution, which, in most cases, undergoes the largest increase. The physical meaning of these results is discussed. The present work gives reference calculations that might be used to parametrize new correlated molecular mechanics potentials. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1052,1062, 2005 [source] Humic acid metal cation interaction studied by spectromicroscopy techniques in combination with quantum chemical calculationsJOURNAL OF SYNCHROTRON RADIATION, Issue 2 2010M. Plaschke Humic acids (HA) have a high binding capacity towards traces of toxic metal cations, thus affecting their transport in aquatic systems. Eu(III),HA aggregates are studied by synchrotron-based scanning transmission X-ray microscopy (STXM) at the carbon K -edge and laser scanning luminescence microscopy (LSLM) at the 5D0,7F1,2 fluorescence emission lines. Both methods provide the necessary spatial resolution in the sub-micrometre range to resolve characteristic aggregate morphologies: optically dense zones embedded in a matrix of less dense material in STXM images correspond to areas with increased Eu(III) luminescence yield in the LSLM micrographs. In the C 1s -NEXAFS of metal-loaded polyacrylic acid (PAA), used as a HA model compound, a distinct complexation effect is identified. This effect is similar to trends observed in the dense fraction of HA/metal cation aggregates. The strongest complexation effect is observed for the Zr(IV),HA/PAA system. This effect is confirmed by quantum chemical calculations performed at the ab initio level for model complexes with different metal centres and complex geometries. Without the high spatial resolution of STXM and LSLM and without the combination of molecular modelling with experimental results, the different zones indicating a `pseudo'-phase separation into strong complexing domains and weaker complexing domains of HA would never have been identified. This type of strategy can be used to study metal interaction with other organic material. [source] Thermal stability of some self-assembling hydrogen-bonded polymers and related model complexes,POLYMER INTERNATIONAL, Issue 11 2002Gordon Armstrong Abstract The thermal stability of polymers is of fundamental importance both in processing and in many applications, eg, injection moulding, hot melts. As part of an investigation to determine the suitability of supramolecular polymers for novel applications in materials science, the thermal behaviour of two model compounds representing the principal classes of supramolecular polymer has been studied in some detail. p -Methoxybenzoic acid was complexed with 1,2-di(4-pyridyl)ethylene in 2:1 ratio as a model compound representing liquid,crystal association chain supramolecular polymers. It is proposed that the model compound degrades as a single species obeying first-order kinetics; the activation energy (Eact) of the degradation process was calculated to be 127,kJ,mol,1. A model ureidopyrimidinone dimer degraded in two steps, also following first order kinetics, with Eact,=,71.5,kJ,mol,1. The dimer was unaffected by annealing, suggesting that related polymers may be used at elevated temperatures. Polymer analogues of both model compounds were synthesized and their thermal behaviour was found to parallel that of the models. In light of these results, the implications for processing both supramolecular polymers are also considered. © 2002 Society of Chemical Industry [source] Modulation of Stacking Interactions by Transition-Metal Coordination: Ab Initio Benchmark StudiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2010Shaun Abstract A series of ab initio calculations are used to determine the CH,,,, and ,,,,,-stacking interactions of aromatic rings coordinated to transition-metal centres. Two model complexes have been employed, namely, ferrocene and chromium benzene tricarbonyl. Benchmark data obtained from extrapolation of MP2 energies to the basis set limit, coupled with CCSD(T) correction, indicate that coordinated aromatic rings are slightly weaker hydrogen-bond acceptors but are significantly stronger hydrogen-bond donors than uncomplexed rings. It is found that ,,,,, stacking to a second benzene is stronger than in the free benzene dimer, especially in the chromium case. This is assigned, by use of energy partitioning in the local correlation method, to dispersion interactions between metal d and benzene , orbitals. The benchmark data is also used to test the performance of more efficient theoretical methods, indicating that spin-component scaling of MP2 energies performs well in all cases, whereas various density functionals describe some complexes well, but others with errors of more than 1,kcal,mol,1. [source] Metallomacrocycles with a Difference: Macrocyclic Complexes with Exocyclic Ruthenium(II)-Containing DomainsCHEMISTRY - A EUROPEAN JOURNAL, Issue 43 2009Edwin Abstract The templated synthesis of organic macrocycles containing rings of up to 96 atoms and three 2,2,-bipyridine (bpy) units is described. Starting with the bpy-centred ligands 5,5,-bis[3-(1,4-dioxahept-6-enylphenyl)]-2,2,-bipyridine and 5,5,-bis[3-(1,4,7-trioxadec-9-enylphenyl)]-2,2,-bipyridine, we have applied Grubbs' methodology to couple the terminal alkene units of the coordinated ligands in [FeL3]2+ complexes. Hydrogenation and demetallation of the iron(II)-containing macrocyclic complexes results in the isolation of large organic macrocycles. The latter bind {Ru(bpy)2} units to give macrocyclic complexes with exocyclic ruthenium(II)-containing domains. The complex [Ru(bpy)2(L)]2+ (isolated as the hexafluorophosphate salt), in which L=5,5,-bis[3-(1,4,7,10-tetraoxatridec-12-enylphenyl)]-2,2,-bipyridine, undergoes intramolecular ring-closing metathesis to yield a macrocycle which retains the exocyclic {Ru(bpy)2} unit. The poly(ethyleneoxy) domains in the latter macrocycle readily scavenge sodium ions, as proven by single-crystal X-ray diffraction and atomic absorption spectroscopy data for the bulk sample. In addition to the new compounds, a series of model complexes have been fully characterized, and representative single-crystal X-ray structural data are presented for iron(II) and ruthenium(II) acyclic and macrocyclic species. [source] Metal Ion Coordination to Azole NucleosidesCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2005Jens Müller Dr. Abstract To evaluate the possibility of introducing azole nucleosides as building blocks for metal-mediated base pairs in artificial oligonucleotides, imidazole nucleoside, 1,2,4-triazole nucleoside and tetrazole nucleoside have been synthesized and characterized. The X-ray crystal structures of p -toluoyl-protected 1,2,4-triazole and tetrazole nucleosides are reported. Contrary to the situation primarily found for deoxyribonucleosides, the sugar moieties adopt C3,- endo conformations. The acidity of the , nucleosides increases with increasing number of nitrogen ring atoms, giving pKa values of 6.01±0.05, 1.32±0.05 and <,3, respectively. This decrease in basicity results in a decreasing ability to form 2:1 complexes with linearly coordinating metal ions such as Ag+ and Hg2+. In all cases, the Ag+ complexes are of higher stability than the corresponding Hg2+ complexes. Whereas imidazole nucleoside forms highly stable 2:1 complexes with both metal ions (estimated log ,2 values of >10), only Ag+ is able to reach this coordination pattern in the case of triazole nucleoside (log ,2 = 4.3±0.1). Tetrazole nucleoside does not form 2:1 complexes at all under the experimental conditions used. These data suggest that imidazole nucleoside, and to a lesser extent 1,2,4-triazole nucleoside, are likely candidates for successful incorporation as ligands in oligonucleotides based on metal-mediated base pairs. DFT calculations further corroborate this idea, providing model complexes for such base pairs with glycosidic bond distances (10.8,11.0 Å) resembling those in idealized B-DNA (10.85 Å). [source] Metal-Induced Tautomerization of p - to o -Quinone Compounds: Experimental Evidence from CuI and ReI Complexes of Azophenine and DFT StudiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2004Stéphanie Frantz Abstract Azophenine (7,8-diphenyl-2,5-bis(phenylamino)- p -quinonediimine, Lp) reacts with [Cu(PPh3)4](BF4) or [Re(CO)5Cl] to yield the (Ph3P)2Cu+ or [(OC)3ClRe] complex of the tautomeric form 7,8-diphenyl-4,5-bis(phenylamino)- o -quinonediimine, Lo, as evident from structure determinations and from very intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. Time-dependent DFT (TD-DFT) calculations on model complexes [(N,N)Re(CO)3Cl] confirm the spectroscopic results, showing considerably higher oscillator strengths of the MLCT transition for the o -quinonediimine complexes in comparison to compounds with N,N=1,4-dialkyl-1,4-diazabutadiene. The complexes are additionally stabilized through hydrogen bonding between two now ortho -positioned NHPh substituents and one fluoride of the BF4, anion (Cu complex) or the chloride ligand (Re complex). DFT Calculations on the model ligands p -quinonediimine or 2,5-diamino- p -quinonediimine and their ortho -quinonoid forms with and without Li+ or Cu+ are presented to discuss the relevance for metal-dependent quinoproteins. [source] | ||||||||||