Multicomponent metal-ligand self-assembly

Self-assembly of pre-designed organic ligands with transition metal atoms is a powerful method for construction of novel supramolecular architectures. Particularly, various discrete 3-D hollow structures such as cages, cones, capsules and boxes have been obtained by multicomponent self-assembly of exo-multidentate ligands with cis-protected square planar metal complexes, [(L)M](NO(3))(2) (where L is ethylenediamine or 2,2'-bipyridine and M is Pd or Pt). Furthermore, these hollow structures act as molecular flasks to encapsulate guest molecules and regulate/promote specific reactions; for example, oligomerization of silanetriols and [2+2] intermolecular photodimerization of olefins.     

Synthesis, structure and ethylene oligomerization behavior of non-symmetric bidentate neutral arylnickel(II) phosphine complexes

A series of new neutral arylnickel(II) phosphine complexes bearing non-symmetric bidentate chelate ligands, 3-aryliminomethyl-5-chloro-2-hydroxybenzaldehyde, have been synthesized, and the structure of representative complexes (2b and 2d) has been confirmed by X-ray crystallographic analysis. These neutral arylnickel(II) phosphine complexes have been investigated as catalysts for ethylene oligomerization. Using methylaluminoxane (MAO) as a cocatalyst, these complexes display high ethylene oligomerization activities. A catalytic activity of up to 4.6 × 10⁶ g mol⁻¹ h⁻¹ has been observed. The influence of Al/Ni molar ratio, reaction temperature, reaction period and pressure on catalytic activity was investigated.     

Group-10 metal complexes of biological molecules and related ligands: structural and functional properties

The complexes of group-10 metals, Ni, Pd, and Pt, with biological molecules and related ligands have been attracting increasing attention in recent years due to their reactivities and functions, such as catalysts and drugs, and their biological relevance. The well-defined structures and kinetic inertness especially of Pt complexes have been used as the sites for weak interactions with other molecules. The Ni complexes have been reported as models not only for Ni enzymes but also for other metalloenzyme active sites for deeper understanding of the reactivities such as oxygen activations and detailed electronic structures. Pd Complexes are widely known for their catalytic activities in conversions of various organic molecules including useful biological molecules, such as Suzuki?Miyaura cross-coupling, while Pt complexes have been intensively studied for their antitumor activities. We focus in this review on our recent results on weak interactions and reactivities of the group-10 metal complexes with biological molecules and related compounds, and discuss their structural features and some new properties pointing to functional possibilities.     

Homonuclear and heteronuclear transition metal complexes by syn-proportionation reactions

Redox processes consisting of disproportionation and syn-proportionation are reviewed with special attention to metal complexes containing carbon-based ligands, i.e. carbon monoxide or unsaturated hydrocarbons. An introduction and a survey of reactions aimed to show the large applicability of syn-proportionation reactions in the field of coordination chemistry, is followed by examples of the use of these redox processes for the preparation of catalytic precursors. The latter studies derive from the idea that if a syn-proportionation reaction can be carried out between two complexes containing different metals in different oxidation states, inter-metallic systems could be formed which may act as active catalysts, e.g. for polymerization reactions.     

Ethylene oligomerization using nickel dithiolene complexes Ni(S2C2R2)2 (R=Ph, CF3) and the crystal structure of Ni[S2C2(CF3)2]2

Nickel bis(dithiolene) complexes have been known for over four decades, yet little is known regarding the chemistry of this important subclass of inorganic coordination complexes in olefin oligomerization or polymerization. We report here that Ni(S(2)C(2)R(2))(2) (R=Ph, CF(3)) are converted to active catalysts for ethylene oligomerization when activated with methylaluminoxane (MAO). The catalyst activity is comparable to some nickel coordination complexes with N-donor ligands under similar conditions. The products are mainly butenes and hexenes, with small amounts of higher oligomers. The product distribution pattern is consistent with a nickel hydride species being the active center, where fast beta-hydride elimination limits the products to mostly butenes and hexenes. The exact nature of the active center and the reaction mechanism remain to be investigated. In addition, we determined the crystal structure for Ni[S(2)C(2)(CF(3))(2)](2). The molecule crystallizes in the P2(1)/n space group and adopts a planar geometry with expected bond lengths and angles. Comparing this structure with that for the donor-acceptor complex with perylene reveals elongation of both the Ni-S and the S-C bonds in the latter, suggesting reduction of Ni[S(2)C(2)(CF(3))(2)](2) may have occurred in the latter.     

Construction and structure studies of DNA-bipyridine complexes as versatile scaffolds for site-specific incorporation of metal ions into DNA

The facile construction of metal–DNA complexes using ‘Click’ reactions is reported here. A series of 2′-propargyl-modified DNA oligonucleotides were initially synthesized as structure scaffolds and were then modified through ‘Click’ reaction to incorporate a bipyridine ligand equipped with an azido group. These metal chelating ligands can be placed in the DNA context in site-specific fashion to provide versatile templates for binding various metal ions, which are exchangeable using a simple EDTA washing-and-filtration step. The constructed metal–DNA complexes were found to be thermally stable. Their structures were explored by solving a crystal structure of a propargyl-modified DNA duplex and installing the bipyridine ligands by molecular modeling and simulation. These metal–DNA complexes could have wide applications as novel organometallic catalysts, artificial ribonucleases, and potential metal delivery systems.     

Structural and microbial studies of some transition metal complexes

N-pyridinobenzamide-2-carboxylic acid has been synthesized. Its binary and ternary (using 8-hydroxy-quinoline as the other ligand) Cu(II), Ni(II), Co(II), and Zn(II) complexes have been synthesized and characterized by their elemental analysis, molecular weight determination, molar conductance, infrared and electronic spectral data, and magnetic measurements. Antibacterial activity of these ligands and their metal complexes has been determined on gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) bacteria at 37 degrees C, and antifungal activity has been determined on common fungi viz. Aspergillus niger, Aspergillus nidulense, and Candida albicans at 28 degrees C. A considerable increase in the biocidal activity of these ligands on being coordinated with metal ions has been reported.     

Valence-tautomerism in high-valent iron and manganese porphyrins

Iron and manganese hemes are "high-valent" when the valence state of the metal exceeds III. Redox chemistry of the high valent metal complexes involves redistribution of holes and electrons over the metal ion and the porphyrin and axial ligands, defined as valence tautomerism. Thus, catalytic pathways of heme-containing biomolecules such as peroxidases, catalases and cytochromes P450 involve valence tautomerism, as do pathways of biomimetic oxygen transfer catalysis by manganese porphyrins, robust catalysts with potential commercial value. Determinants of the site of electron abstraction are key to understanding valence tautomerism. In model systems, metal-centered oxidation is supported by hard anionic axial ligands that are also strongly pi-donating, such as oxo, aryl, bix-methoxy and bis-fluoro groups. Manganese(IV) is more stable than iron(IV) and metal-centered one-electron oxidations occur with weaker pi-donating axial ligands such as bisazido, -isocyanato, -hypochlorito and bis chloro groups. Virtually all known high-valent iron porphyrin complexes oxidized by two-electrons above the ferric state are coordinated by the strongly pi-donating oxo or nitrido ligands. In all well-characterized oxo complexes, iron is in the ferryl state and the second oxidizing equivalent resides on the porphyrin. Complexes with iron(V) have not been definitively characterized. One-electron oxidation of oxomanganese(IV) porphyrin complexes gives the oxomanganese(IV) porphyrin pi-cation redicals. In aqueous solution, oxidation of Mn(III) complexes of tetra cationic N-methylpyridiniumylporphyrin isomers by monooxygen donors yields a transient oxomanganese(V) species.     

Enantiopure platinum(II) complexes with chiral diphosphine and diphosphinite ligands derived from 2,2-biphosphole: Synthesis, crystal structure and catalysis

Enantiopure platinum(II) complexes have been synthetized with chiral stereodynamic diphosphine and diphosphinite ligands derived from 2,2-biphosphole through a dynamic chirality control upon coordination. Catalytic performances of these platinum complexes have been explored in asymmetric hydroformylation. All complexes proved to be effective catalysts with respect to chemoselectivity and regioselectivity but induced only low enantioselectivities.     

Synthesis, characterization and ethylene oligomerization studies of nickel(II) based new formazane derivatives

A series of nickel(II) complexes based novel mono- and bis-formazan ligands has been synthesized and characterized with IR, UV, NMR spectroscopies, CHN analysis, mass spectrometry, and single crystal X-ray diffraction analysis. The prepared complexes have been tested for catalytic activity in ethylene oligomerization.     

Nickel-quinolones interaction. Part 4. Structure and biological evaluation of nickel(II)-enrofloxacin complexes compared to zinc(II) analogues

The nickel(II) complexes with the second-generation quinolone antibacterial agent enrofloxacin in the presence or absence of the nitrogen-donor heterocyclic ligands 1,10-phenanthroline, 2,2′-bipyridine or pyridine have been synthesized and characterized. Enrofloxacin acts as bidentate ligand coordinated to Ni(II) ion through the ketone oxygen and a carboxylato oxygen. The crystal structure of (1,10-phenanthroline)bis(enrofloxacinato)nickel(II) has been determined by X-ray crystallography. UV study of the interaction of the complexes with calf-thymus DNA (CT DNA) has shown that they bind to CT DNA and bis(pyridine)bis(enrofloxacinato)nickel(II) exhibits the highest binding constant to CT DNA. The cyclic voltammograms of the complexes have shown that in the presence of CT DNA the complexes can bind to CT DNA by the intercalative binding mode which has also been verified by DNA solution viscosity measurements. Competitive study with ethidium bromide (EB) has shown that the complexes can displace the DNA-bound EB indicating that they bind to DNA in strong competition with EB. The complexes exhibit good binding propensity to human or bovine serum albumin protein having relatively high binding constant values. The biological properties of the complexes have been evaluated in comparison to the corresponding Zn(II) enrofloxacinato complexes as well as Ni(II) complexes with the first-generation quinolone oxolinic acid.     

Synthesis, characterization and ethylene polymerization activity of titanium aminopyridinato complexes

Three titanium complexes derived from 2-(2,6-difluoroanilino)pyridine and 2-(2-chloroanilino)pyridine were synthesized and characterized by X-ray diffraction or spectroscopic methods. All titanium complexes have been used to catalyze the polymerization of ethylene in the presence of MAO as cocatalyst. The mono(2,6-difluorophenylaminopyridinato) titanium catalyst was found to be more active in ethylene polymerization than the bis(2,6-difluorophenylaminopyridinato) and bis(2-chlorophenylaminopyridinato) titanium catalysts. ortho-Halogens disturbed the β-elimination transition state of ethylene polymerization and formed higher molar mass polyethylene than their unhalogenated congener. Due to fluxionality, the bis(2-chlorophenylaminopyridinato) titanium catalyst formed broader molar mass distribution than the bis(2,6-difluorophenylaminopyridinato) titanium catalysts.     

Heterogenized polymetallic catalysts Part II. Oxidation of 3,5-di-t-butylphenol by Cu(II), Fe(III) and Pd(II) complexed to a polyphenylene polymer containing ß-di- and triketone surface ligands; an improved catalyst system

Polyphenylene polymer preparation involves the cyclic trimerization polymerization of acetylated methyl benzoate with diacetyl benzene. Since the methyl benzoate groups do not take part in the polymerization they are present in high concentration. The ß-diketone ligands were placed on the surface by reaction of the methylbenzoate group with base and a methyl ketone and the triketone by reaction with base to give the ß-triketone. The ß-triketones can bind two metal ions in a known geometry that is suitable for bimetallic catalysis of the rapid polyelectron oxidation of catechols. The final catalytic surfaces were prepared by treating the chemically modified polymer with copper(II), iron(II) and palladium(II) acetonitrile complexes with non-coordinating BF₄⁻ as the anion. Since the metal ions contain no strongly coordinating ligand, they are very reactive species. These surfaces catalyzed the rapid air oxidation of 3,5-di-tert-butylcatechol (DTBC). The diketone surfaces gave only 3,5-di-tert-butyl-o-quinone (DTBQ) while the triketone surfaces gave ring-cleaved products, confirming the special catalytic effect of the triketone surface. Also, only the triketone catalysts showed any activity for ring cleavage oxidation of DTBQ. These catalysts were much more reactive than previous ones using the same polyphenylene polymer but without the methyl benzoate groups. With these polymers the di- and triketone groups were placed on the surface by chemical modification of the unpolymerized acetyl groups.     

New cobalt-mediated radical polymerization (CMRP) of methyl methacrylate initiated by two single-component dinuclear β-diketone cobalt (II) catalysts

Two dinuclear cobalt complexes based on bis-diketonate ligands (ligand 1: 3,3'-(1,3-phenylene)bis(1-phenylpropane-1,3-dione); ligand 2: 3,3'-(1,4-phenylene)bis(1-phenylpropane-1,3-dione)) were successfully synthesized. The two neutral catalysts all showed satisfactory activities in the cobalt-mediated radical polymerization (CMRP) of methyl methacrylate (MMA) with the common initiator of azodiisobutyronitrile (AIBN). The resulting polymerizations have all of the characteristics of a living polymerization and displayed linear semilogarithmic kinetic plots, a linear correlation between the number-average molecular weight and the monomer conversion, and low polydispersities. Mono- or dicomponent low polydispersity polymers could be obtained by using the two dinuclear catalysts under proper reaction conditions. All these improvements facilitate the implementation of the acrylate CMRP and open the door to the scale-up of the syntheses and applications of the multicomponent low polydispersity polymers.     

Effective new palladium catalysts for the Suzuki coupling of various haloxylenes to prepare sterically hindered bi- and triaryls

Several new ortho-alkyl and heteroalkyl substituted aryl and aryl alkyl phosphanes and their palladium complexes have been selectively prepared, characterized and compared as potential catalysts for the Suzuki coupling reaction. The modification of the structures of the palladium complexes were made in search of the best possible catalytic activity. The novel catalysts were subsequently used to synthesize sterically hindered bi- and triaryls by coupling various bulky, unactivated bromoxylenes and chloroxylenes with a range of phenyl boronic acids under microwave irradiation. We showed that under optimized reaction conditions, very good results can be obtained with a selection of the new phosphanes and their mononuclear palladium complexes.     

Antibacterial and antifugal mono- and di-substituted symmetrical and unsymmetrical triazine-derived Schiff-bases and their transition metal complexes

A new series of antibacterial and antifungal triazine-derived mono- and di-substituted (symmetrical and unsymmetrical) Schiff-bases and their cobalt(II), copper(II), nickel(II) and zinc(II) metal complexes have been synthesized and characterized by their elemental analyses, molar conductances, magnetic moments and IR and electronic spectral measurements. IR spectra indicated the ligands to act as tridentate towards divalent metal ions via a trazine-N, the azomethine-N and, indole-NH and deprotonated-O of salicylaldehyde. The magnetic moments and electronic spectral data suggest octahedral geometry for the Co(II), Ni(II) and Zn(II)complexes and square-pyramid for Cu(II) complexes. NMR spectral data of the ligands and their diamagnetic zinc(II) complexes well-define their proposed structures/geometries. Elemental analyses data of the ligands and metal complexes agree with their proposed structures/geometries. The synthesized ligands, along with their metal complexes were screened for their antibacterial activity against Escherichia coli, Bacillus subtillis, Shigella flexneri, Staphylococcus aureus, Pseudomonas aeruginosa and Salmonella typhi and for antifungal activity against Trichophyton longifusus, Candida albicans, Aspergillus flavus, Microsporum canis, Fusarium solani and Candida glaberata. The results of these studies show the metal complexes to be more antibacterial/ antifungal against two or more species as compared to the uncomplexed Schiff-base ligands.     

Synthesis, characterization and biological studies of sulfonamide Schiff's bases and some of their metal derivatives

A new series of Schiff base ligands derived from sulfonamide and their metal(II) complexes [cobalt(II), copper(II), nickel(II) and zinc(II)] have been synthesized and characterized. The nature of bonding and structure of all the synthesized compounds has been explored by physical, analytical and spectral data of the ligands and their metal(II) complexes. The authors suggest that all the prepared complexes possess an octahedral geometry. The ligands and metal(II) complexes have been screened for their in vitro antibacterial activity against bacterial strains, Escherichia coli, Shigella flexneri, Pseudomonas aeruginosa, Salmonella typhi and for antifungal activity against fungal strains, Trichophyton longifusus, Candida albicans, Aspergillus flavus, Microsporum canis, Fusarium solani and Candida glabrata. These assays enabled the identification of the metal complexes as an effective antimicrobial agent with low cytotoxicity.     

Organometallic heterocyclic chemistry I. some group VI transition metal complexes containing thiazole,isothiazole, and related ligands

The heterocycles thiazole, isothiazole, and 4(5) methyl-isothiazole have been used as ligands in reaction with the group VI metal hexacarbonyls. The resulting pentacarbonyl derivatives are coordinated to the metal via the nitrogen atom exclusively; however, it is clear from mass spectral data that migration of the metal from nitrogen to sulfur is an important process in the fragmentation of the complexes as well as formation of pi-bonded species. The electronic effects of the ligands are very similar and related to those of pyridine.     

Transporters of ligands for essential metal ions in plants

Essential metals are required for healthy plant growth but can be toxic when present in excess. Therefore plants have mechanisms of metal homeostasis which involve coordination of metal ion transporters for uptake, translocation and compartmentalization. However, very little metal in plants is thought to exist as free ions. A number of small, organic molecules have been implicated in metal ion homeostasis as metal ion ligands to facilitate uptake and transport of metal ions with low solubility and also as chelators implicated in sequestration for metal tolerance and storage. Ligands for a number of essential metals have been identified and proteins involved in the transport of these ligands and of metal-ligand complexes have been characterized. Here we review recent advances in understanding the role of mugineic acid, nicotianamine, organic acids (citrate and malate), histidine and phytate as ligands for iron (Fe), zinc (Zn), copper (Cu), manganese (Mn) and nickel (Ni) in plants, and the proteins identified as their transporters.     

Directing iridium-catalyzed C-C bond formation by selection of the ancillary ligands: Polymerization and cyclotrimerization of alkynes

The ability of organoiridium derivatives of catalyzing oligomerization and polymerization of terminal alkynes is markedly influenced by the nature of non-participative ligands coordinated to the metal. The dimeric species [Ir(cod)Cl]₂ and [Ir(cod)(OMe)]₂ (cod = 1,5-cyclooctadiene) as well as the phosphine complexes HIr(cod)(PR₃)₂ (PR₃ = PPh₃, P(p-MeOC₆H₄)₃, P(o-MeOC₆H₄)Ph₂, PCyPh₂) catalyze the polymerization reaction, whereas the diphosphine derivatives HIr(cod)(P-P) (P-P = Ph₂P(CH₂)_nPPh₂ (n = 1-4), o-C₆H₄(PPh₂)₂) promote the regioselective formation of 1,2,4-trisubstituted benzenes. On the other hand, the iridium complexes with nitrogen chelating ligands Ir(cod)(N-N)X and Ir(hd)(N-N)X (hd = 1,5-hexadiene; N-N = 1,10-phenanthroline and substituted derivatives; X = halogen) catalyze alkynes polymerization. In most cases one catalytic reaction predominates over the other possible routes, so that polymerization often takes place in the absence of oligomerization side reactions, and conversely cyclotrimerization is rarely accompanied by formation of either polyene or dimers.