Vinca Institute of Nuclear Sciences, Laboratory of Theoretical Physics and Condensed Matter Physics, Belgrade, Yugoslavia. email@example.com
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Transition metal complexes with thiosemicarbazide-based ligands. XV. A square-pyramidal NiII complex with an asymmetric coordination of 2, 6-diacetylpyridine bis(S-methylisothiosemicarbazone)
Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovica 3, 21000 Novi Sad, Yugoslavia.
The title compound,[diacetylpyridine bis(S-methylisothiosemicarbazonato)]iodonickel(II),[Ni(C(13)H(18)N(7)S(2))I], is the first example of a complex involving the (2)N coordination of the isothiosemicarbazide moiety. 2,6-Diacetylpyridine bis(S-methylisothiosemicarbazone), as a potentially pentadentate ligand (N(5)), is coordinated as a tetradentate species, whereby one (deprotonated) isothiosemicarbazide moiety is coordinated in the usual way ((1)N(4)N), but the other (neutral) is bonded via the (2)N atom only, the fourth ligator being the pyridine nitrogen. The difference in coordination mode of the isothiosemicarbazide moiety is reflected in the (1)N-(2)N bond lengths of 1.359 (4) and 1.379 (3) A in the deprotonated and undeprotonated moieties, respectively. The structure contains three fused chelate rings in a 5:5:6 arrangement. The six-membered ring has a non-planar conformation.
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Elzbieta Budzisz, Urszula Krajewska, Marek Rozalski, Agata Szulawska, Malgorzata Czyz, Barbara Nawrot
Department of Bioinorganic Chemistry, Faculty of Pharmacy, Medical University of Lodz, Muszyńskiego 1, 90-151 Lodz, Poland. firstname.lastname@example.org
The new platinum (II) and palladium (II) complexes (2-4) with ligands 5-(2-hydroxyphenyl)-1,3-dimethyl-4-(dimethoxy)phosphonyl-1H]-pyrazole (1a) and 5-(2-hydroxyphenyl)-1,3-dimethyl-4-methoxycarbonyl-1H]-2-pyrazole (1b) were screened in a search for novel anticancer agents. Thus, alkylating activity, cytotoxicity, ability for induction of apoptosis and binding to DNA were tested. The cis-[Pt(1b)2Cl2] complex (3b) was the most potent alkylating agent in a Preussmann test, in comparison with the other test compounds and cis-platin. The highest cytotoxicity against the HL-60 and NALM-6 leukemia cell lines was observed for complexes 3b and 4b (trans-[Pd(1b)2Cl2]), although the extent of the effect was lower relative to cis-platin. Moreover, both complexes were remarkably less toxic to human umbilical vein endothelial cells (HUVECs) with IC50 values of 3b 14 and 20 times higher than that ones for HL-60 and NALM-6 cells, respectively. Complexes 3b and 4b induced caspase-3 activity. Apoptosis occurred in a strictly dose-dependent manner and required only low concentrations of 4b. However, compounds 3b and 4b showed lower binding affinity to double-stranded DNA than cis-platin.
Byelorussian State University, Minsk, Belarus. email@example.com
Radical scavenging activities of flavonoids rutin, taxifolin,(-)-epicatechin, luteolin, and their complexes with transition metal (Fe2+, Fe3+, and Cu2+) towards superoxide were determined using illumination of riboflavin as source and NBT as detector of O*2-. The scavenger potencies of flavonoid metal complexes were significantly higher than those of the parent flavonoids. To elucidate the mechanism of this phenomenon, the rates of superoxide-dependent oxidation of flavonoids and their metal complexes in photochemical system with riboflavin were examined. It was found for the first time that flavonoids bound to metal ions were much less subjected to oxidation compared with those of free compounds. The findings directly demonstrate superoxide scavenging activity of metal ions in complexes with flavonoids and support earlier suggestions that flavonoid metal complexes may exhibit superoxide dismuting activity.
Electronic structure control of the nucleophilicity of transition metal-thiolate complexes: an experimental and theoretical study.
Department of Chemistry, University of Wisconsin-Madison, 1101 West University Avenue, Madison, Wisconsin 53706, USA.
New metal(II)-thiolate complexes supported by the tetradentate ligand 1,5-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)py(2)) have been synthesized and subjected to physical, spectroscopic, structural, and computational characterization. The X-ray crystal structures of these complexes,[L(8)py(2)M(S-C(6)H(4)-p-CH(3))]BPh(4)(M = Co, Ni, Zn), reveal distorted square-pyramidal divalent metal ions with four equatorial nitrogen donors from L(8)py(2) and axial p-toluenethiolate ligands. The reactions of the complexes with benzyl bromide produce isolable metal(II)-bromide complexes (in the cases of Co and Ni) and the thioether benzyl-p-tolylsulfide. This reaction is characterized by a second-order rate law (nu = k(2)[L(8)py(2)M(SAr)(+)][PhCH(2)Br]) for all complexes (where M = Fe, Co, Ni, or Zn). Of particular significance is the disparity between k(2) for M = Fe and Co versus k(2) for M = Ni and Zn, in that k(2) for M = Ni and Zn is ca. 10 times larger (faster) than k(2) for M = Fe and Co. An Eyring analysis of k(2) for [L(8)py(2)Co(SAr)](+) and [L(8)py(2)Ni(SAr)](+) reveals that the reaction rate differences are not rooted in a change in mechanism, as the reactions of these complexes with benzyl bromide exhibit comparable activation parameters (M = Co: DeltaH()= 45(2) kJ mol(-)(1), DeltaS()=-144(6) J mol(-)(1) K(-)(1); M = Ni: DeltaH()= 43(3) kJ mol(-)(1), DeltaS()=-134(8) J mol(-)(1) K(-)(1)). Electronic structure calculations using density functional theory (DFT) reveal that the enhanced reaction rate for [L(8)py(2)Ni(SAr)](+) is rooted in a four-electron repulsion (or a "filled/filled interaction") between a completely filled nickel(II) d(pi) orbital and one of the two thiolate frontier orbitals, a condition that is absent in the Fe(II) and Co(II) complexes. The comparable reactivity of [L(8)py(2)Zn(SAr)](+) relative to that of [L(8)py(2)Ni(SAr)](+) arises from a highly ionic zinc(II)-thiolate bond that enhances the negative charge density on the thiolate sulfur. DFT calculations on putative thioether-coordinated intermediates reveal that the Co(II)- and Zn(II)-thioethers exhibit weaker M-S bonding than Ni(II). These combined results suggest that while Ni(II) may serve as a competent replacement for Zn(II) in alkyl group transfer enzymes, turnover may be limited by slow product release from the Ni(II) center.
Structural analysis of five-coordinate transition metal boryl complexes with different d-electron configurations.
Department of Chemistry and Open Laboratory of Chirotechnology, Institute of Molecular Technology for Drug Discovery and Synthesis, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon.
The site preference of boryl ligands in five-coordinate transition metal boryl complexes has been investigated with the aid of density functional theory calculations. The preferred site for a boryl ligand depends on the electron count of the complex under consideration. Our studies show that the very strong sigma-donating boryl ligands choose to occupy coordination sites such that those orbitals accommodating metal d electrons have minimal metal-boryl sigma-antibonding character.
Metal vs Ligand Reduction in Complexes of Dipyrido[3,2-a:2',3'-c]phenazine and Related Ligands with [(C(5)Me(5))ClM](+)(M = Rh or Ir): Evidence for Potential Rather Than Orbital Control in the Reductive Cleavage of the Metal-Chloride Bond.
Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany, and J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejuskova 3, CZ-18223 Prague, Czech Republic.
Complexes between the chlorometal(III) cations [(C(5)Me(5))ClM](+), M = Rh or Ir, and the 1,10-phenanthroline-derived alpha-diimine (N wedge N) ligands dipyrido[3,2-a:2',3'-c]phenazine (dppz), 1,4,7,10-tetraazaphenanthrene (tap), or 1,10-phenanthroline-5,6-dione (pdo) were investigated by cyclic voltammetry, EPR, and UV-vis-NIR spectroelectrochemistry with respect to either ligand-based or metal-centered (and then chloride-dissociative) reduction. Two low-lying unoccupied molecular orbitals (MOs) are present in each of these three N wedge N ligands; however, their different energies and interface properties are responsible for different results. Metal-centered chloride-releasing reduction was observed for complexes of the DNA-intercalation ligands dppz and tap to yield compounds [(N wedge N)(C(5)Me(5))M] in a two-electron step. The separation of alpha-diimine centered optical orbitals and phenazine-based redox orbitals is apparent from the EPR and UV-vis-NIR spectroelectrochemistry of [(dppz)(C(5)Me(5))M](0/)(*)(-)(/2-). In contrast, the pdo complexes undergo a reversible one-electron reduction to yield o-semiquinone radical complexes [(pdo)(C(5)Me(5))ClM](*) before releasing the chloride after the second electron uptake. The fact that the dppz complexes undergo a Cl(-)-dissociative two-electron reduction despite the presence of a lowest lying pi MO (b(1)(phz)) with very little overlap to the metal suggests that an unoccupied metal/chloride-based orbital is lower in energy. This assertion is confirmed both by the half-wave reduction potentials of the ligands (tap,-1.95 V; dppz,-1.60 V; pdo,-0.85 V) and by the typical reduction peak potentials of the complexes [(L)(C(5)Me(5))ClM](PF(6))(tap,-1.1 V; dppz,-1.3 V; pdo,-0.6 V; all values against Fc(+/0)).
Structure, energetics, and bonding of first row transition metal pentazolato complexes: a DFT study.
Department of General and Inorganic Chemistry, Faculty of Chemistry, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece. firstname.lastname@example.org
Quantum chemical calculations with gradient-corrected (B3LYP) density functional theory for the mono- and bispentazolato complexes of the first row transition metals (V, Cr, Mn, Fe, Co, and Ni), the all-nitrogen counterparts of metallocenes, were performed, and their stability was investigated. All possible bonding modes (e.g. eta1, eta2, eta3, and eta5) of the pentazolato ligand to the transition metals have been examined. The transition metal pentazolato complexes are predicted to be strongly bound molecules. The computed total bond dissociation enthalpies that yield free transition metal atoms in their ground states and the free pentazolato ligands were found in the range of 122.0-201.9 (3.7-102.3) kcal mol(-1) for the bispentazolato (monopentazolato) complexes, while those yielding M2+ and anionic pentazolato ligands were found in the range of 473.2-516.7 (273.6-353.5) kcal mol(-1). The electronic ground states of azametallocenes along with their spectroscopic properties (IR, NMR, and UV-vis) obtained in a consistent manner across the first transition metal series provide means for discussion of their electronic and bonding properties, the identification of the respective azametallocenes, and future laboratory studies. Finally, exploring synthetic routes to azametallocenes it was found that a [2 + 3] cycloaddition of dinitrogen to a coordinated azide ligand with nickel(II) does not seem to provide a promising synthetic route for transition metal pentazolato complexes while the oxidative addition of phenylpentazole and fluoropentazole to Ni(0) bisphosphane complexes merits attention for the experimentalists.
van't Hoff Institute of Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands.
Encapsulated transition metal catalysts are presented that are formed by templated self-assembly processes of simple building blocks such as porphyrins and pyridylphosphine and phosphite ligands, using selective metal-ligand interactions. These ligand assemblies coordinate to transition metals, leading to a new class of transition metal catalysts. The assembled catalyst systems were characterized using NMR and UV-vis spectroscopy and were identified under catalytic conditions using high-pressure infrared spectroscopy. Tris-3-pyridylphosphine binds three mesophenyl zinc(II) porphyrin units and consequently forms an assembly with the phosphorus donor atom completely encapsulated. The encapsulated phosphines lead exclusively to monoligated transition metal complexes, and in the rhodium-catalyzed hydroformylation of 1-octene the encapsulation of the catalysts resulted in a 10-fold increase in activity. In addition, the branched aldehyde was formed preferentially (l/b = 0.6), a selectivity that is highly unusual for this substrate, which is attributed to the encapsulation of the transition metal catalysts. An encapsulated rhodium catalyst based on ruthenium(II) porphyrins and tris-meta-pyridyl phosphine resulted in an even larger selectivity for the branched product (l/b = 0.4). These encapsulated catalysts can be prepared easily, and various template ligands and porphyrins, such as tris-3-pyridyl phosphite and ruthenium(II) porphyrins, have been explored, leading to catalysts with different properties.
Chemistry. 2004 Jan 5;10 (1):190-207 14695564
Departament de Química Inorgànica, and Centre de Recerca en Química Teòrica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain, Fax:(+34) 93-4907725.
A continuous shape and symmetry study of tetracoordinate transition-metal complexes is presented, in an attempt to provide a systematic description of the stereochemistry of the metal coordination sphere in this important family of compounds. A tetrahedron/square-planar symmetry map has been developed, the main distortion paths of the ideal geometries are presented, and the applicability of a sawhorse shape measure is discussed. More than 13,000 structural data sets have been analyzed and the corresponding stereochemistries assigned from the values of their tetrahedral and square-planar symmetry measures. A good number of structures that are quite distant from the two ideal geometries can be adequately described as snapshots along the spread pathway for their interconversion, making use of the corresponding path deviation function. Further analysis of the structural data by metal electron configuration or by the denticity and conformation of the ligands provide general rules to describe the stereochemical preferences of tetracoordinate transition metal centers.
Derivatives of phosphate Schiff base transition metal complexes: synthesis, studies and biological activity.
Chemistry Department, Faculty of Science (Girl's), Al-Azhar University Nasr-City, P.O. Box 11754, Cairo, Egypt. email@example.com
We report the synthesis and structural characterization of series of tetra- and hexacoordinate metal chelate complexes of phosphate Schiff base ligands having the general composition LMX(n).H(2)O and L(2)MX(n)(L=phosphate Schiff base ligand; M=Ag(+), Mn(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), or Fe(3+) and X=NO(3)(-), Br(-) or Cl(-)). The structure of the prepared compounds was investigated using elemental analysis, IR, 1H and 31P NMR, UV-vis, mass spectra, solid reflectance, magnetic susceptibility and conductance measurements as well as conductometric titration. In all the complexes studied, the ligands act as a chelate ligand with coordination involving the phosphate-O-atom and the azomethine-N-atom. IR, solid reflectance spectra and magnetic moment measurement are used to infer the structure and to illustrate the coordination capacity of ligand. IR spectra show the presence of coordinated nitrate and water molecule, the magnetic moments of all complexes show normal magnetic behavior and the electronic spectra of the metal complexes indicate a tetra- and octahedral structure for Mn(2+), octahedral structure of Fe(3+) and both square-planar and distorted octahedral structure for Cu(2+) complexes. Antimicrobial activity of the ligands and their complexes were tested using the disc diffusion method and the chosen strains include Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Microsporum canis, Trichophyton mentagrophyte and Trichophyton rubrum. Some known antibiotics are included for the sake of comparison and the chosen antibiotic are Amikacin, Doxycllin, Augmantin, Sulperazon, Unasyn, Septrin, Cefobid, Ampicillin, Nitrofurantion, Traivid and Erythromycin.