Showing papers in "Dalton Transactions in 2006"

Metal-based antitumour drugs in the post genomic era.

Abstract: The discovery of new metal-based antitumour drugs, whether cisplatin derivatives or those based on other metals, has been largely based on cell viability assays (IC50 values) and compounds that bind to DNA. This approach has been applied for more than 30 years during which time very few new drugs have entered clinical use. In this article we discuss what the future holds for metal-based drugs, in particular anti-metastasis drugs, in these enlightened times of the post genomic era.

A unifying mechanism for all high-temperature Heck reactions. The role of palladium colloids and anionic species

Abstract: The Heck reaction has been the subject of intense investigation in the past decade. Many new types of catalysts have been developed in addition to the existing palladium/phosphine complexes. Prominent among these are palladacycles, pincers, several types of heterogeneous palladium catalysts, colloids and ligand-free palladium, usually in the form of Pd(OAc)2. Most of the newer types function only at higher temperatures, typically between 120 and 160 °C. It has been shown that irrespective of the catalyst precursor, none of these catalysts are stable at these high temperatures. They all have a tendency to form soluble palladium(0) colloids or nanoparticles, certainly with less reactive substrates such as aryl bromides or chlorides. The Heck reaction takes place by attack of the arylating agent on the palladium atoms in the outer rim of the nanoparticles. This leads to formation of monomeric or dimeric anionic palladium complexes that undergo the usual steps of the Heck mechanism as described by Amatore and Jutand.

Application of neutral amidines and guanidines in coordination chemistry

Abstract: The aim of this perspective is to highlight the continued development and application of amidines and guanidines as neutral, N-based donor ligands in coordination chemistry. From a sporadic interest dating back to the 1980s, work on these two closely related classes of compounds has steadily grown, with examples of evermore sophisticated substitution about the common ligand framework being developed to address specific problems in a number of fields of chemistry. These compounds have shown varied coordination modes at metals from across the periodic table, and examples of their application in a number of catalytic processes are emerging as the field matures.

Engineering emissive europium and terbium complexes for molecular imaging and sensing

Abstract: Emissive f-block coordination complexes constitute an important class of optical probes, with applications ranging from sensing of bioactive species, high throughput assays and screening protocols in vitro, to time-resolved imaging studies in cellulo or in vivo. The key chemistry issues to be addressed in complex design and characterisation are defined, with an emphasis on the use of emissive europium and terbium complexes and their conjugates in molecular imaging. Both luminescent ‘tags’ useful in energy transfer studies and ‘responsive’ systems for sensing are discussed.

The vitality of uranium molecular chemistry at the dawn of the XXIst century.

Abstract: The intent of this Dalton Perspective is to highlight the recent advances in uranium molecular chemistry, with the results reported during the 2000–2006 period. This discipline is currently witnessing an impressive development, together with the theoretical chemistry and solid-state chemistry of the f-elements, and its face has profoundly changed, revealing unsuspected structural and reactivity features. This progress required and was facilitated by the use of new precursors. Studies of low-valent compounds gave a better insight into lanthanide(III)/actinide(III) differentiation and led to the discovery of unusual reactions, including activation of small molecules. A number of tetravalent uranium complexes, in particular polynuclear compounds, have been synthesized, which exhibit exciting structures and physicochemical properties. The potential of uranium(III) and uranium(IV) complexes in catalysis has been confirmed. The uranyl complexes, from mononuclear species to supramolecular assemblies, reveal a variety of novel structures, changing the generally accepted ideas on the coordination geometry and the stability of the UO22+ ion.

Metal complexes in medicinal chemistry: new vistas and challenges in drug design

Abstract: An overview is presented of selected metal-based pharmaceuticals, either diagnostic or therapeutic, with emphasis on specific attributes and in vivo interactions of these compounds relevant to their use in medicinal applications. Both the advantages and the challenges of this approach are outlined, with possibilities for future developments accentuated.

Mechanochemical preparation of molecular and supramolecular organometallic materials and coordination networks.

Abstract: This Dalton Perspective deals with solvent-free reactions taking place within solids or between solids or involving a solid and a vapour. The focus is on reactions involving organometallic and coordination compounds and occurring via reassembling of non-covalent bonding, e.g. hydrogen bonds, and/or formation of ligand-metal coordination bonds. It is argued that reactions activated by mechanical mixing of solid reactants as well as those obtained by exposing a crystalline solid to a vapour can be exploited to "make crystals", which is the quintessence of crystal engineering. It is demonstrated through a number of examples that solvent-free methods, such as co-grinding, kneading, milling of molecular solids, or reactions of solid with vapours represent viable alternative, when not unique, routes for the preparation of novel molecular and supramolecular solids as well as for the preparation of polymorphic or solvate modifications of a same species. The structural characterization of the products requires the preparation of single crystals suitable for X-ray diffraction, a goal often achieved by seeding.

Ruthenium polypyridyl chemistry; from basic research to applications and back again

Abstract: Since the mid 1970's interest in the chemistry and applications of ruthenium polypyridyl complexes has increased steadily. In this perspective, the development of this area is tracked and discussed taking into account new scientific developments as well as novel applications. The interaction between basic and applied research is of particular importance and selected examples are highlighted.

Ligand-centred reactivity in diiminepyridine complexes.

Abstract: The diiminepyridine ligand, made famous by Brookhart and Gibson a decade ago through its use in the first iron-containing olefin polymerization catalyst, also displays an amazing variety of ligand-centred reactions, including electron transfer, alkylation, dimerization and deprotonation. The present Perspective summarizes these reactions and also speculates on the links between ligand- and metal-centred reactivity.

Revisiting ceramics for medical applications

Abstract: The most significant demand for biomaterials has emerged as a consequence of the need to provide clinical treatment to a large number of patients. The search for potential solutions produces a large demand for materials suitable for bone repair or replacement. Calcium phosphates, bio-glasses, bio-glass ceramics and ordered silica mesoporous materials, among other types of materials, will be reviewed and studied from the point of view of their potential applications as replacement materials in bone repair and regeneration, as potential substrates in tissue engineering, and also as drug delivery systems. An overview on the present achievements, but also on the "missing links" will be presented.

Cationic, linear Au(I) N-heterocyclic carbene complexes: synthesis, structure and anti-mitochondrial activity

Abstract: Six linear, two-coordinate cationic Au(I) N-heterocyclic carbene complexes of the form [(R2Im)2Au]+ (R = Me 1, Me, Et 2, i-Pr 3, n-Bu 4, t-Bu 5 and Cy 6) have been prepared by the reaction of two equivalents of the appropriate dialkylimidazol-2-ylidene (R2Im) with (Me2S)AuCl in dmf Single crystal structural studies for 1·PF6, 2·PF6, 3·Cl and 4–6·PF6 show that for all six complexes the gold(I) centres have quasi-linear C–Au–C coordination, with quasi-parallel pairs of aromatic imidazole planes, except in 5·PF6 where they are quasi-normal; in the latter, Au–C are 2038(3), 2033(3) A, cf (eg) 3 2027(2) A Inter-cation Au⋯Au are close at 3487(2), 3525(2) A in 1·PF6 and 2·PF6 The structural studies and low temperature NMR experiments provide no supportive evidence for the presence of π back-bonding within this series of complexes The lipophilicities of the six compounds, as estimated from the logarithm of the n-octanol–water partition coefficients (log P), varied across the series within the range −109 to 173 To investigate their potential as possible anti-mitochondrial anti-tumour agents, five of the compounds have been evaluated for their propensities to induce mitochondrial membrane permeabilization (MMP) in isolated rat liver mitochondria At concentrations between 1–10 µM compounds 1·Br and 3–6·Cl induced dose-dependent, Ca2+-sensitive mitochondrial swelling at rates that increased with the lipophilicities of the complexes, with the most lipophilic compounds inducing the most rapid onset of swelling The swelling was completely inhibited by cyclosporin A, the specific inhibitor of the mitochondrial permeability transition pore

Metal ion selectivity of oligopeptides

Abstract: Metal binding affinity and selectivity of peptides are reviewed with a special emphasis on the high structural variety of peptide complexes. The most common structural type of these complexes is built up by the deprotonation and metal ion coordination of subsequent amide groups in the form of fused five-membered chelate rings. The metal ion selectivity of this process and the role of various anchoring groups are discussed in detail. The highest metal binding affinity of peptides is connected to the presence of two anchoring groups in appropriate location (the “double anchor”): e.g. the NH2-Xaa-Xaa-His/Cys/Asp/Met-Xaa sequence. Among the side chain donor functions, the imidazole of histidyl and thiolate of cysteinyl residues are the most effective ligating groups and their involvement in metal binding results in a great variety of different macrochelate or loop structures and/or formation of various polynuclear complexes. Examples of these structural motifs and their possible applications have been thoroughly discussed.

An improved method for the computation of ligand steric effects based on solid angles

Abstract: An improved algorithm has been designed to characterize ligand interactions in organometallic and coordination complexes in terms of the percentage of the metal coordination sphere shielded by a given ligand. The computations for ligand solid angles are performed numerically and employ introduced atomic radii that are larger than covalent but smaller than van der Waals radii. This approach enables facile evaluation of steric congestion in the metal coordination sphere, quantification of unfavorable interligand contacts, and in some cases prediction of the complex composition or ligand coordination on purely geometrical grounds.

Selective deoxygenation of sugar polyols to α,ω-diols and other oxygen content reduced materials—a new challenge to homogeneous ionic hydrogenation and hydrogenolysis catalysis

Abstract: An oxygen atom on every carbon--this is the problem! While nature provides linear C(3) to C(6) building blocks in the form of sugar alcohols in large and renewable abundance, they are overfunctionalized for the purpose of most chemical applications. Selective deoxygenation by anthropogenic catalyst systems may be one answer to this challenge.

Redox behavior of tumor-inhibiting ruthenium(III) complexes and effects of physiological reductants on their binding to GMP

Abstract: Biotransformation of ruthenium(III) anticancer complexes as hypothesized in the activation-by-reduction theory is the central topic of the present paper. The redox behavior of tetrachlorobis(azole)ruthenate(III)-type complexes was studied by NMR spectroscopy and square wave voltammetry. The influence of reducing agents on the binding behavior toward the DNA-modeling nucleotide GMP was determined by capillary electrophoresis, accompanied by identification of arising peaks by online coupling to electrospray ionization mass spectrometry. The determination of redox potentials revealed that the biologically relevant reductants ascorbic acid and glutathione are capable of reducing the studied Ru(III) complexes under physiological conditions. Characteristic differences in reduction kinetics dependent on the pH value can be explained by higher reduction strength of ascorbic acid and glutathione at higher pH compared to the pH-independent redox response of ruthenium(III) complexes. Binding behavior of (H2ind)[trans-RuCl4(Hind)2] (Hind = 1H-indazole) toward GMP was found to be increased upon addition of two equivalents of glutathione but not of ascorbic acid. In contrast, only a minor influence on the GMP-binding under reductive conditions was found for (H2im)[trans-RuCl4(Him)2] (KP418, Him = 1H-imidazole).

Mechanistic investigation of CO2 hydrogenation by Ru(II) and Ir(III) aqua complexes under acidic conditions: two catalytic systems differing in the nature of the rate determining step

Abstract: Ruthenium aqua complexes [(η6-C6Me6)RuII(L)(OH2)]2+ {L = bpy (1) and 4,4′-OMe-bpy (2), bpy = 2,2′-bipyridine, 4,4′-OMe-bpy = 4,4′-dimethoxy-2,2′-bipyridine} and iridium aqua complexes [Cp*IrIII(L)(OH2)]2+ {Cp* = η5-C5Me5, L = bpy (5) and 4,4′-OMe-bpy (6)} act as catalysts for hydrogenation of CO2 into HCOOH at pH 3.0 in H2O. The active hydride catalysts cannot be observed in the hydrogenation of CO2 with the ruthenium complexes, whereas the active hydride catalysts, [Cp*IrIII(L)(H)]+ {L = bpy (7) and 4,4′-OMe-bpy (8)}, have successfully been isolated after the hydrogenation of CO2 with the iridium complexes. The key to the success of the isolation of the active hydride catalysts is the change in the rate-determining step in the catalytic hydrogenation of CO2 from the formation of the active hydride catalysts, [(η6-C6Me6)RuII(L)(H)]+, to the reactions of [Cp*IrIII(L)(H)]+ with CO2, as indicated by the kinetic studies.

Complexes formed between the quadridentate, heterocyclic molecules 6,6′-bis-(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-bipyridine (BTBP) and lanthanides(III): implications for the partitioning of actinides(III) and lanthanides(III)

Abstract: New hydrophobic, tetradentate nitrogen heterocyclic reagents, 6,6′-bis-(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-bipyridines (BTBPs) have been synthesised. These reagents form complexes with lanthanides and crystal structures with 11 different lanthanides have been determined. The majority of the structures show the lanthanide to be 10-coordinate with stoichiometry [Ln(BTBP)(NO3)3] although Yb and Lu are 9-coordinate in complexes with stoichiometry [Ln(BTBP)(NO3)2(H2O)](NO3). In these complexes the BTBP ligands are tetradentate and planar with donor nitrogens mutually cisi.e. in the cis, cis, cis conformation. Crystal structures of two free molecules, namely C2-BTBP and CyMe4-BTBP have also been determined and show different conformations described as cis, trans, cis and trans, trans, trans respectively. A NMR titration between lanthanum nitrate and C5-BTBP showed that two different complexes are to be found in solution, namely [La(C5-BTBP)2]3+ and [La(C5-BTBP)(NO3)3]. The BTBPs dissolved in octanol were able to extract Am(III) and Eu(III) from 1 M nitric acid with large separation factors.

Titanium, zinc and alkaline-earth metal complexes supported by bulky O,N,N,O-multidentate ligands: syntheses, characterisation and activity in cyclic ester polymerisation.

Abstract: The reactions of the bulky amino-bis(phenol) ligand Me2NCH2CH2N{CH2-3,5-But2-C6H2OH-2}2 (1-H2) with Zn[N(SiMe3)2]2 (4), [Mg{N(SiMe3)2}2]2 (5) and Ca[N(SiMe3)2]2(THF)2 (6) yield the complexes 1-Zn, 1-Mg and 1-Ca in good yields. The X-ray structure of 1-Ca showed the complex to be dimeric, with calcium in a distorted octahedral coordination geometry. Five of the positions are occupied by an N2O3 donor set, while the sixth is taken up by an intramolecular close contact to an o-But substituent, a rare case of a Ca⋯H–C agostic interaction (Ca⋯H distances of 2.37 and 2.41 A). Another sterically hindered calcium complex, Ca{2-But-6-(C6F5NCH)C6H3O}2(THF)2·(C7H8)2/3 (7), was prepared by reaction of 6 with the iminophenol 2-But-6-(C6F5NCH)C6H3OH (3-H). According to the crystal structure, 7 is monomeric and octahedral, with trans THF ligands. The complex Ti[N{CH2-3-But-5-Me-C6H2O-2}2{CH2CH2NMe2}](OPri)2 (2-Ti) was prepared by treatment of Ti(OPri4) with the new amino-bis(phenol) Me2NCH2CH2N{CH2-3-But-5-Me-C6H2OH-2}2 (2-H2). The reduction of 2-Ti with sodium amalgam gave the titanium(III) salt Ti[N{CH2-3-But-5-Me-C6H2O-2}2{CH2CH2NMe2}](OPri)2·Na(THF)2 (8). A comparison of the X-ray structures of 2-Ti and 8 showed that the additional electron in 8 significantly reduced the intensity of the π-bonding from the oxygen atoms of the isopropoxide groups to titanium. 1-Ca and 8 were active initiators for the ring-opening polymerisation of e-caprolactone (up to 97% conversion of 200 equivalents in 2 hours) and yielded polymers with narrow molecular weight distributions.

Metal complex SERMs (selective oestrogen receptor modulators). The influence of different metal units on breast cancer cell antiproliferative effects

Abstract: The selective oestrogen receptor modulator tamoxifen is a leading agent in the adjuvant treatment of breast cancer. Several organometallic moieties have been vectorised with tamoxifen, in order to improve on the latter's antiproliferative properties by the addition of a potentially cytotoxic moiety, and have been evaluated versus both oestrogen receptor positive (MCF7) and oestrogen receptor negative (MDA-MB231) breast cancer cells. For tamoxifen analogues with ((R,R)-trans-1,2-diaminocyclohexane)platinum(II), cyclopentadienyl rhenium tricarbonyl, and ruthenocene tethers, there was no enhancement of the antiproliferative effect on oestrogen receptor positive cells, nor any cytotoxic effect on oestrogen receptor negative cells, while those containing cyclopentadienyl titanium dichloride showed an oestrogenic effect. However, compounds where ferrocene replaces tamoxifen's phenyl ring were strongly cytotoxic against both cell lines. The synthesis and biological results of these compounds is reviewed and placed in the historic context of inorganic compounds in therapy.

Metallabenzenes and metallabenzenoids

Abstract: Metallabenzenes are analogues of benzene in which one of the benzene CH groups has been formally replaced by a transition metal and its ancillary ligands. This article summarises some of the important recent highlights of metallabenzene chemistry including the synthesis of the first platinabenzene, the synthesis of the first metallanaphthalene (an iridanaphthalene) and the development of new synthetic routes to metallabenzenes. The results of a number of theoretical studies that provide important insights into the nature and reactivity of metallabenzenes are discussed and in the concluding section some of the immediate goals and challenges in this field are identified.

Influence of gamma irradiation on hydrophobic room-temperature ionic liquids [BuMeIm]PF6 and [BuMeIm](CF3SO2)2N

Abstract: Stability of neat hydrophobic Room-Temperature Ionic Liquids (RTIL) [BuMeIm]X, where [BuMeIm]+ is 1-butyl-3-methylimidazolium and X− is PF6−, and (CF3SO2)2N−, was studied under gamma radiolysis (137Cs) in an argon atmosphere and in air. It was found that the density, surface tension, and refraction index of RTILs are unchanged even by an absorbed dose of approximately 600 kGy. Studied RTILs exhibit considerable darkening when subjected to gamma irradiation. The light absorbance of ionic liquids increases linearly with the irradiation dose. Water has no influence on radiolytic darkening. A comparative study of [BuMeIm]X and [Bu4N][Tf2N] leads to the conclusion that the formation of colored products is related to gamma radiolysis of the [BuMeIm]+ cation. The radiolytic darkening kinetics of RTILs is influenced by the anions as follows: Cl− < (CF3SO2)2N− < PF6−. Electrospray ionization mass spectrometry and NMR analysis reveal the presence of nonvolatile radiolysis products at concentrations below 1 mol% for an absorbed dose exceeding 1200 kGy. Initial step of BuMeIm+ cation radiolysis is the loss of the Bu˙ group, the H˙ atom from the 2 position on the imidazolium ring, and the H˙ atom from the butyl chain. Radiolysis of ionic liquid anions yields F˙ and CF3˙ from PF6− and [Tf2N]−, respectively. Recombinations of these primary products of radiolysis lead to various polymeric and acidic species.

Coordination frameworks--where next?

Abstract: A personal perspective on the developing field of coordination frameworks, of where the challenges currently lie, where the field is heading and where the next breakthroughs will be made.

Porous coordination polymers with zeolite topologies constructed from 4-connected building units

Abstract: Two novel 3D coordination polymers, Cd(CTC)(H2O)·(H2PIP)0.5(H2O) (1) with zeolite ABW topology and Cd(CTC)·(HIPA) (2) with zeolite BCT topology, have been synthesized by constructing inorganic and organic 4-connected building units and using the organic bases as templates, and the frameworks of 1 and 2 not only expand the original structures of zeolites ABW and BCT, but also exhibit significant advantages over them in terms of thermal stability, ion exchange and adsorption.

Cucurbit[n]uril binding of platinum anticancer complexes

Abstract: The encapsulation of cisplatin by cucurbit[7]uril (Q[7]) and multinuclear platinum complexes linked via a 4,4′-dipyrazolylmethane (dpzm) ligand by Q[7] and cucurbit[8]uril (Q[8]) has been studied by NMR spectroscopy and molecular modelling. The NMR studies suggest that some cisplatin binds in the cucurbituril cavity, while cis-[PtCl(NH3)2(H2O)]+ only binds at the portals. Alternatively, the dpzm-linked multinuclear platinum complexes are quantitatively encapsulated within the cavities of both Q[7] and Q[8]. Upon encapsulation, the non-exchangeable proton resonances of the multinuclear platinum complexes show significant upfield shifts in 1H NMR spectra. The H3/H3* resonances shift upfield by 0.08 to 0.55 ppm, the H5/H5* shift by 0.9 to 1.6 ppm, while the methylene resonances shift by 0.74 to 0.88 ppm. The size of the resonance shift is dependent on the cavity size of the encapsulating cucurbituril, with Q[7] encapsulation producing larger shifts than Q[8]. The upfield shifts of the dpzm resonances observed upon cucurbituril encapsulation indicate that the Q[7] or Q[8] is positioned directly over the dpzm linking ligand. The terminal platinum groups of trans-[{PtCl(NH3)2}2μ-dpzm]2+ (di-Pt) and trans-[trans-{PtCl(NH3)2}2-trans-{Pt(dpzm)2(NH3)2}]4+ (tri-Pt) provide a barrier to the on and off movement of cucurbituril, resulting in binding kinetics that are slow on the NMR timescale for the metal complex. Although the dpzm ligand has relatively few rotamers, encapsulation by the larger Q[8] resulted in a more compact di-Pt conformation with each platinum centre retracted further into each Q[8] portal. Encapsulation of the hydrolysed forms of di-Pt and tri-Pt is considerably slower than for the corresponding Cl forms, presumably due to the high-energy cost of passing the +2 platinum centres through the cucurbituril portals. The results of this study suggest that cucurbiturils could be suitable hosts for the pharmacological delivery of multinuclear platinum complexes.

Synthesis, structure and fluorescent studies of novel uranium coordination polymers in the pyridinedicarboxylic acid system

Abstract: Self-assembly under hydrothermal conditions has been employed to synthesize several novel uranium-containing polymeric materials in the pyridinedicarboxylic acid (pydc) system. Uranium containing coordination polymers were synthesized utilizing 2,3-pyridinedicarboxylic acid (2,3-pydc), 2,4-pyridinedicarboxylic acid (2,4-pydc) and 2,6-pyridinedicarboxylic acid (2,6-pydc) as the organic linker. Furthermore, several bimetallic compounds were also synthesized, U–M–2,6-pydc (M = Cu, Ag, Pb). A new secondary building unit for uranium(VI) compounds has also been realized in compound 4 [(UO2)2(C7H3NO4)(O)(H2O)] through tetramer building units edge shared to form one-dimensional chains. Presented herein will be the syntheses, crystal structures and fluorescent properties of these uranium-containing compounds.

Metallasupramolecular architectures, an overview of functional properties and applications

Abstract: The synthesis of metallasupramolecular architectures, such as two-dimensional squares, triangles and polygons, and three-dimensional cages and polyhedra, has attracted much interest in the past decade. These structures are designed to have novel specific shapes and dimensions with interesting functional properties. In this overview the functional properties of metallasupramolecular architectures are highlighted with emphasis on potential applications such as catalysis, cavity-directed synthesis and sensing, that can be performed with these materials.

Bis(imidazolylidene)s as modular building blocks for monomeric and macromolecular organometallic materials

Abstract: The synthetic and structural progression surrounding N-heterocyclic carbenes has given rise to great functional and architectural diversity in organometallic chemistry, catalysis, and materials science. The development of new, modular scaffolds for bridging transition metals is essential in order to expand the boundaries of these scientific areas. This Frontier article summarizes recent advances in the synthesis and study of ditopic ligands displaying two linearly opposed carbene moieties and emphasizes their versatility in the preparation of new organometallic and macromolecular materials. The conclusion previews their utility in conjugated organic/inorganic hybrid materials with potential applications in the emerging fields of molecular- and nanoelectronics.

Synthesis of a series of 4-pyridyl-1,2,4-triazole-containing cadmium(II) luminescent complexes

Abstract: Using two 4-substitued triazole ligands, 4-(pyrid-2-yl)-1,2,4-triazole (L1) and 4-(pyrid-3-yl)-1,2,4-triazole (L2), a series of novel triazole–cadmium(II) complexes varying from zero- to three-dimensional have been prepared and their crystal structures determined via single-crystal X-ray diffraction. [Cd2(μ2-L1)3(L1)2(NO3)(μ2-NO3)(H2O)2](NO3)2·1.75H2O (1) is a binuclear complex containing bidendate, monodedate and free nitrate anions. When the bridging anions SCN− and dca (dca = N(CN)2−) were added to the reaction system of 1, one-dimensional (1D) [Cd(L1)2(NCS)2]n (2) and two-dimensional (2D) [Cd(L1)2(dca)2]n (3) were isolated, respectively. When L2 instead of L1 was used, [Cd(L2)2(NCS)2(H2O)2] (4) and 1D [Cd(L2)2(dca)2]n (5) were obtained. When the ratio of Cd to L2 was changed from 1 : 2 to 1 : 1 in the reaction system of 5, three-dimensional (3D) {[Cd3(μ2-L2)3(dca)6]·0.75H2O}n (6) with 1D microporous channels along the a direction was isolated. Further investigations on other Cd(II) salts and the L2 ligand in a Cd to L2 ratio of 1 : 1, an unexpected complex [Cd(μ2-L2)(μ3-SO4)(H2O)]n (7) with a 3D open framework was obtained. All of the complexes exhibit strong blue fluorescence emission bands in the solid state at ambient temperature, of which the excitation and emission maxima are red-shifted to longer wavelength as compared to those in water. Powder X-ray diffraction and thermal studies were used to investigate the bulk nature of the 3D coordination polymers 6 and 7.

Syntheses and structures of metal tetrazole coordination polymers.

Abstract: Two salts and seven copper(I/II) and silver(I) coordination polymers containing tetrazolyl ligands have been hydro(solvo)thermal synthesized by metal salts, NaN3 and various nitriles generated via [2 + 3] cycloaddition reactions of organonitriles and sodium azide. The study also shows that in some cases the azide can play a dual role in the in situ syntheses of metal tetrazole complexes, namely, starting material for tetrazole ligand and co-ligand in the tetrazole-based coordination complexes. Compounds 1 and 2 are simple salts of ammonium and sodium 5-methyltetrazolate. Compound 3 has a 3-D framework with intersecting channel and unprecedented (49.66) topology constructed from mixed-valent Cu8 clusters. Compounds 4 and 5 are isomorphous, and have 3-D organic–inorganic frameworks constructed by [M2(mtta)]+ (Hmtta = 5-methyltetrazole) ribbon and [M2(N3)]+ (M = Cu, Ag) layer two types of structural motifs, which contains an µ4-1,1,1,3 azide. Compound 6 is a 3-D four-connected chiral complex with (42.84)Cu(42.82.102)tta topology. The structure of 7 consists of 2-D three-connected layers that are linked by ligand-unsupported Ag(I)⋯Ag(I) interactions to form a 3-D supramolecular array. Compound 8 shows a 3-D chiral framework containing tetrahedrally and linearly coordinated Ag(I) ions and µ3- and µ4-two types of 5-propyltetrazolate. Compound 9 has a 2-D layered structure formed by linkage of [Ag(tetrazolyl)] ribbons via C–C and N–Ag bonds. Magnetic measurement confirmed that there are two Cu(II) ions and six Cu(I) ions per Cu8 unit consistent with a mixed-valent Cu(I,II) complex.

Breaking the N2 triple bond : insights into the nitrogenase mechanism

Abstract: Nitrogenase is the metalloenzyme that performs biological nitrogen fixation by catalyzing the reduction of N2 to ammonia. Understanding how the nitrogenase active site metal cofactor (FeMo-cofactor) catalyzes the cleavage of the N2 triple bond has been the focus of intense study for more than 50 years. Goals have included the determination of where and how substrates interact with the FeMo-cofactor, and the nature of reaction intermediates along the reduction pathway. Progress has included the trapping of intermediates formed during turnover of non-physiological substrates (e.g., alkynes, CS2) providing insights into how these molecules interact with the nitrogenase FeMo-cofactor active site. More recently, substrate-derived species have been trapped at high concentrations during the reduction of N2, a diazene, and hydrazine, providing the first insights into binding modes and possible mechanisms for N2 reduction. A comparison of the current state of knowledge of the trapped species arising from non-physiological substrates and nitrogenous substrates is beginning to reveal some of the intricacies of how nitrogenase breaks the N2 triple bond.