Silane complexes of electrophilic metal centers

Photolysis of solutions of M(CO)6 (M = Cr, Mo, and W) in the presence of Et3SiH affords the silane complexes Cr(CO)5(eta2-HSiEt3), Mo(CO)5(eta2-HSiEt3), and W(CO)5(eta2-HSiEt3). Observed values of J(SiH) in these complexes are consistent with modest elongation of the Si-H bond. With Ph3SiH, complexes of Cr(CO)5 and W(CO)5 were obtained, but no complex with Mo was observed. When Ph2SiH2 was employed, only one Si-H bond interacts with the metal center. A dynamic exchange process observable on the magnetic resonance time scale exchanges the pendant and coordinated Si-H bonds of the coordinated diphenylsilane. Silanes bound to M(CO)5 are activated with respect to reaction with nucleophiles. With methanol, catalytic methanolysis of HSiEt3 has been observed in the presence of Cr(CO)5(eta2-HSiEt3), affording Et3SiOMe.     

具有生理活性甾体金属配合物的研究进展

按照金属离子与甾体配体之间的连接方式对甾体金属配合物进行分类。综述了近年来合成的甾体金属配合物及其抗肿瘤、抗菌等多种生理活性研究进展,并对此类化合物发展趋势、应用前景作出展望。     

Properties of metal centers in low-symmetry complexes of p-azacyclophanes

Abstract

The properties of metal centers in low-symmetry complexes of p-azacyclophanes are reviewed.     

Kinetics and mechanisms of substitution at paramagnetic metal centers in organometallic complexes

The mechanism of ligand substitution in 17- and 19-electron organometallic radicals is discussed. These species substitute ligands by an associative process some 10⁶ to 10¹⁰ faster than analogous 18-electron complexes. When 17-electron species can be generated by bond homolysis or electron transfer reactions of 18-electron complexes, they can act as intermediates in radical chain reactions of 18-electron complexes. A 17–19 electron rule is proposed to explain transformations of organometallic radicals just as the 16–18 electron rule finds use for closed shell organometallic complexes. The origin of this rule is the favorable two-center three-electron bond that can form when an odd electron in a sterically accessible metal d-orbital interacts with an electron pair on an entering nucleophile. Besides simple substitution, these radicals can disproportionate, dimerize, and undergo insertion or atom abstraction reactions.     

Review: recent advances and future development of metal complexes as anticancer agents

Since the initial discovery of applications of platinum complexes in the clinical treatment of many kinds of cancers, the efficiency of platinum complexes in inhibiting the proliferation of various types of tumors surprised researchers working on the development of anticancer drugs. Meanwhile, despite the potent clinical treatment patients get from platinum complexes, there are also disadvantages including limited solubility in aqueous media and side effects like ototoxicity, myelosuppression, nephrotoxicity, and poor selectivity toward healthy cells. For this reason, efforts have been made to search for novel solutions. Non-platinum complexes (like Fe, Pd, Ru, Cu, Bi, Zn, etc.) were found with potential anticancer activities. We here review the properties of five metal complexes as anticancer agents and make comparisons among them in biological features and cytotoxic activity. Seeking the interrelation between microstructure and mechanism of anticancer, we hope this review provides distinct insights into future study of anticancer agents.     

Early-late heterobimetallic complexes as initiator for ethylene polymerization. Cooperative effect of two metal centers to afford highly branched polyethylene

Ethylene polymerization initiated by early-late transition-metal complexes afforded a polymer with different branched structures and properties depending on the type of late transition-metal.     

Some uses of transition metal complexes as anti-cancer and anti-HIV agents

The success of the clinical uses of cisplatin, cis-[Pt(II)(NH(3))(2)Cl(2)], has stimulated considerable interest in using other metal complexes as new therapeutic agents. This perspective describes our recent work on several classes of gold(III), platinum(II), ruthenium(II, III, IV), iron(II) and vanadium(IV) complexes for anti-cancer and anti-HIV treatments.     

Transfer of chirality from ligands to metal centers: recent examples

Recent examples of "chiral at metal" complexes and assemblies that are produced by chirality transfer of information from enantiopure ligands to metal ions are reported. They highlight the new progress that has been made in this area in the last two years and the new tendencies that such a topic is following. Besides the fundamental aspects related to the stereoselective synthesis of chiral complexes, the progress in diverse classes of chiral complexes for applications in materials science (chiral switches and molecular machines, biological mimics, CPL probes, etc.) is reported.     

Fine tuning of the electronic coupling between metal centers in cyano-bridged mixed-valent trinuclear complexes

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-), trans-[Ru(II)L(4)[CNFe(III)(CN)(5)](2)](4-), and cis-[Ru(II)(bpy)(2)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P2(1)/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (epsilon approximately 2000-9000 M(-1) cm(-1)) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-,5-,6-) were characterized in methanolic solution. The mixed-valent Fe(II)-Ru(II)-Fe(III) system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers.     

Mechanistic insight on the catecholase activity of dinuclear copper complexes with distant metal centers

The catecholase activity of two dinuclear Cu(II) complexes with distant metal centers is discussed together with solid state and solution studies. The crystal structure for one of them, [Cu(2)(diep)(H(2)O)(4)](ClO(4))(4)·2H(2)O, is described, showing the two copper ions are 7.457 ? apart and in a square pyramidal coordination. Both complexes display a weak antiferromagnetic coupling in the solid state that is manifest in the dimer EPR spectra obtained in frozen solution. The pH-potentiometric speciation performed in 1:1 MeOH-H(2)O allowed the assignment of hydrolyzed copper species as those catalytically active in the oxidation of 3,5-di-tert-butylcatechol (DTBC). The kinetic measurements led us to propose behavior consistent with Michaelis-Menten plus a linear dependence of the initial rate on [DTBC]. This can be associated with the presence of more than one catalytically active species, which is consistent with the evidence of several differently hydrolyzed species shown in the predominance diagrams. Product characterization studies led to establishing the formation of hydrogen peroxide during the catalytic cycle, while semiquinone and superoxide radicals were detected by EPR spectroscopy, supporting one-electron transference at each of the copper centers.     

Silver(I)-carbene complexes/ionic liquids: novel N-heterocyclic carbene delivery agents for organocatalytic transformations

[reaction: see text] N-Heterocyclic carbene (NHC) complexes with silver were investigated as sources of unsaturated NHC carbene catalysts via thermal decomposition. The NHC complex (1-ethyl-3-methylimidazol-2-ylidene)silver(I) chloride is an ionic liquid, and was found to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight. Silver-carbene complexes can also be used for the catalysis of small molecule transesterification reactions. Thermolysis of the silver complexes in the presence of CS(2) yielded the zwitterionic CS(2) adducts of the carbene, implicating the intermediacy of the free carbene in these reactions.     

High-pressure pulse-radiolysis study of the formation and decomposition of complexes with iron-carbon sigma bonds: mechanistic comparison for different metal centers

Volumes of activation for the formation and homolysis of the transient complexes (hedta)Fe(III)-CO(2)(2-) and (hedta)Fe(III)-CH(3)(-) (HOCH(2)CH(2)N(CH(2)CO(2-))CH(2)CH(2)N(CH(2)CO(2-))(2) = hedta) were determined using high-pressure pulse-radiolysis techniques. A comparison of the results with those for analogous complexes with other central transition-metal cations (M(n+)) and ligands (L) points out that (i) the reaction of M(n)L(m) with aliphatic radicals (R(*)) proceeds via an interchange ligand substitution mechanism, i.e. M(n)L(m) + R(*) --> L(m-1)M(n+1)-R + L, (ii) the homolysis of the metal-carbon bonds naturally follows the same mechanism, and (iii) the volume of activation for the homolysis reaction depends strongly on the nature of the central cation, i.e. larger for M(n+1) = Cr(III), Co(III), Ni(III) and smaller for Fe(III). The volume of activation for the reaction (hedta)Fe(III)-CO(2)(2-) + CO(2)(*-) + 2H(+) --> Fe(II)(hedta)(H(2)O)(-) + CO + CO(2) was measured, and the results enable a tentative proposal for the nature of the transition state of this interesting reaction.     

Sulfide oxidation by hydrogen peroxide catalyzed by iron complexes: two metal centers are better than one

Peroxoiron species have been proposed to be involved in catalytic cycles of iron-dependent oxygenases and in some cases as the active intermediates during oxygen-transfer reactions. The catalytic properties of a mononuclear iron complex, [Fe(II)(pb)(2)(CH(3)CN)(2)] (pb=(-)4,5-pinene-2,2'-bipyridine), have been compared to those of its related dinuclear analogue. Each system generates specific peroxo adducts, which are responsible for the oxidation of sulfides to sulfoxides. The dinuclear catalyst was found to be more reactive and (enantio)selective than its mononuclear counterpart, suggesting that a second metal site affords specific advantages for stereoselective catalysis. These results might help for the design of future enantioselective iron catalysts.     

Metal-based anticancer agents: targeting androgen-dependent and androgen-independent prostate and COX-positive pancreatic cancer cells by phenanthrenequinone semicarbazone and its metal complexes

A planar, polycyclic and aromatic hydrocarbon ligand, namely 9,10-phenanthrenequinone semicarbazone, and its transition metal complexes have been synthesized and structurally characterized. The in vitro antiproliferative activity of these compounds against five human cancer cell lines revealed that they were effective against androgen receptor-positive/negative prostate cancer cells as well as COX-positive pancreatic BxPC-3 cancer cell line. The driving force behind such antiproliferative activity seems to be the up-regulated COX expression in these cells, which was amenable for targeting through metal complexation. These structural motifs can, therefore, serve as a starting point for developing novel cytotoxic agents against the growing number of prostate and pancreatic cancers.     

Reactivity and electrochemical behavior of ruthenium dithiolene complexes with coordinatively unsaturated metal centers: cycloaddition and dimerization reactions

The novel ruthenium dithiolene complexes [(arene)Ru{S₂C₂(COOMe)₂}] (arene = C₆H₆ (1a), C₆H₄(Me)(iPr) (1b), C₆Me₆ (1c)) were synthesized. The equilibrium between complex 1a and the corresponding dimer [(C₆H₆)Ru{S₂C₂(COOMe)₂}]₂ (1a′) was confirmed in solution. The reaction of complex 1a with dimethyl- or diethylacetylene dicaboxylate gave the alkene-bridged adducts [(C₆H₆)Ru{S₂C₂(COOMe)₂}{C₂(COOR)₂}] (R = Me (2a), Et (3a)) as [2 + 2] cycloaddition products formally. The reactions of complex 1a with diazo compounds also gave the alkylidene-bridged adducts [(C₆H₆)Ru{S₂C₂(COOMe)₂}(CHR)] (R = H (4a), SiMe₃ (5a), COOEt (6a)) as [2 + 1] cycloaddition products. The electrochemical behavior of complex 1a was investigated. The reductant of complex 1a was a stable species for several minutes. The oxidant of complex 1a was very unstable; the cation 1a⁺ formed was immediately converted to the corresponding cationic dimer 1a′⁺. The cationic dimer 1a′⁺ was stable for several minutes, and it was rapidly and quantitatively converted to the neutral complex 1a when it was reduced.     

An empirical potential function for metal centers: Application to molecular mechanics calculations on metalloproteins

At present, most molecular mechanics programs that model metalloproteins do not allow for specific geometric requirements at metal centers. An analysis of small-molecule crystal structures containing four-, five-, and six-coordinated zinc, retrieved from the Cambridge Structural Database (CSD), leads to a new metal-center potential function for use in molecular mechanisc programs. This potential function includes as variables the metal-ligand separations and the angles subtended at the metal and allows specifically for distortions from frequently occurring types of coordination geometries (e.g. tetrahedron square pyramid, trigonal bipyramid, and octahedron). The combination of such a metal-center potential function and one for hydrogen bonds allowing for Ione-pair directionality makes monopole electrostatic contributions to the force-field energy superfluous, thus circumventing many problems associated with the assignment to the force-field energy superfluous, thus circumventing many problems associated with the assignment of atomic apartial charges and a dielectric constant. The molecular mechanics program ‘YETI’, containing both types of potential functions, has been used to refine details of substrate binding of 16 complexes of human carbonic anhydrase II with small molecules. The stereochemistry of the refined complexes is in good agreement with data retrived from the CSD and hence allows realistic structure activity relationships.