Influence Of Ligand Structure Research Articles

High molar mass ethene/1‐olefin copolymers synthesized with acenaphthyl substituted metallocene catalysts

AbstractThe influence of ligand structure on copolymerization properties of metallocene catalysts was elucidated with three C1‐symmetric methylalumoxane (MAO) activated zirconocene dichlorides, ethylene(1‐(7, 9)‐diphenylcyclopenta‐[a]‐acenaphthadienyl‐2‐phenyl‐2‐cyclopentadienyl)ZrCl2 (1), ethylene(1‐(7, 9)‐diphenylcyclopenta‐[a]‐acenaphthadienyl‐2‐phenyl‐2‐fluorenyl)ZrCl2 (2), and ethylene(1‐(9)‐fluorenyl‐(R)1‐phenyl‐2‐(1‐indenyl)ZrCl2 (3). Polyethenes produced with 1/MAO had considerable, ca. 10% amount of trans‐vinylene end groups, resulting from the chain end isomerization prior to the chain termination. When ethene was copolymerized with 1‐hexene or 1‐hexadecene using 1/MAO, molar mass of the copolymers varied from high to moderate (531–116 kg/mol) depending on the comonomer feed. At 50% comonomer feed, ethene/1‐olefin copolymers with high hexene or hexadecene content (around 10%) were achievable. In the series of catalysts, polyethenes with highest molar mass, up to 985 kg/mol, were obtained with sterically most crowded 2/MAO, but the catalyst was only moderately active to copolymerize higher olefins. Catalyst 3/MAO produced polyethenes with extremely small amounts of trans‐vinylene end groups and relatively low molar mass 1‐hexene copolymers (from 157 to 38 kg/mol) with similar comonomer content as 1. These results indicate that the catalyst structure, which favors chain end isomerization, is also capable to produce high molar mass 1‐olefin copolymers with high comonomer content. In addition, an exceptionally strong synergetic effect of the comonomer on the polymerization activity was observed with catalyst 3/MAO. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 373–382, 2008

Influence of ligand structure on the stability and oxidation of copper nanoparticles

The stability and oxidation of copper nanoparticles stabilized with various ligands have been studied. Lauric acid-capped copper nanoparticles were prepared by a modified Brust–Schiffrin method. Then, ligand exchange with an excess of different capping agents was performed. Oxidation and stability were studied by UV–vis, XRD, and TEM. Alkanethiols and oleic acid were found to improve air stability. The oxidation resistance of thiol-capped copper nanoparticles was found to increase with the chain length of the thiol. However, excess thiol caused etching of the particles under nitrogen. With oleic acid no etching was observed under nitrogen. After oxidation, no traces of the ligand-exchanged particles were found, suggesting their dissolution due to excess ligand. Oleic acid protected the particles against oxidation better than the tested thiols at large excess (ligand–copper ratio 20:1).

Influence of ligand structure on Fe(II) spin-state and redox rate in cytotoxic tripodal chelators

The Fe coordination chemistry of several tripodal aminopyridyl hexadentate chelators is reported along with cytotoxicity toward cultured Hela cells. The chelators are based on cis, cis-1,3,5-triaminocyclohexane (tach) with three pendant –CH 2–2-pyridyl groups where 2-pyridyl is R-substituted thus are named tach- x-Rpyr where x = 3, R = Me; x = 3, R = MeO; x = 6; R = Me. The structures of [Fe(tach-3-Mepyr)]Cl 2 and [Fe(tach-3-MeOpyr)](FeCl 4) are reported and their metric parameters indicate strongly bound, low-spin Fe(II). The structure of [Fe(tach-6-Mepyr)](ClO 4) 2 implies steric effects of 6-Me groups push donor N py’s away so one Fe–N py bond is substantially longer at 2.380(3) Å vs. 2.228(3) Å for the others, and Fe(II) in the high-spin-state. Accordingly, anions X − = Cl or SCN afford [Fe(tach-6-Mepyr)(X)] + from [Fe(tach-6-Mepyr)] 2+ (UV–vis spectroscopy). Consistent with a biological cytotoxicity involving Fe chelation, chelators of low-spin Fe(II) have greater toxicity in the order [IC 50(72 h) is in parentheses then the spin-state SS = H (high) or L (low)]: tachpyr = tach-3-Mepyr (6 μM, SS = L) ≳ tach-3-MeOpyr (12 μM, SS = L) ≫ tach-6-Mepyr (>200 μM, SS = H). Iron-mediated oxidative dehydrogenation with O 2 oxidant removes hydrogens from coordinated nitrogen and the adjacent CH 2, converting aqueous [Fe(tach-3-Rpyr)] 2+ (R = H, Me and MeO) into a mix of low-spin imino- and aminopyridyl-armed complexes, but [Fe(tach-6-Mepyr)] 2+ does not react (NMR and ESI-MS spectroscopies). The difference of IC 50 for chelators at different time points (ΔIC 50 = [IC 50(24 h) − IC 50(72 h)]) is used to compare rate of cytotoxic action to qualitative rate of oxidation in the Fe-bound chelator, giving the order, from rapid to slow oxidation and cell killing of: [Fe(tach-3-Mepyr)] 2+ (ΔIC 50 = 5 μM) > [Fe(tachpyr)] 2+ (ΔIC 50 = 16 μM) > [Fe(tach-3-MeOpyr)] 2+ (ΔIC 50 = 118 μM). Thus, those chelators whose Fe(II) complexes undergo rapid oxidation kill cells faster, and those that bind Fe(II) as low-spin are far more cytotoxic.

Enantioselective Hydrogenation with Self‐Assembling Rhodium Phosphane Catalysts: Influence of Ligand Structure and Solvent

Three sets of new and related chiral phospholane and phosphepine ligands have been prepared for Rh-catalyzed enantioselective hydrogenation. The size and substitution pattern of the cyclic monophosphanes were varied. More importantly, the ligands differ in the nature of the heterocyclic group linked to the trivalent phosphorus atom: 2-pyridone or 2-alkoxypyridine. In the corresponding Rh complexes, the pyridone units of two monodentate P ligands can assemble by hydrogen bonding and form chelates. In contrast, synthetic precursors bearing alkoxypyridine appendages are not able to aggregate via intramolecular hydrogen bonds. The nature of self-assembly is dependent on the nature of the P ligand and the solvent used for the hydrogenation (CH2Cl2 vs. MeOH). These features affect the rate of the reaction as well as the enantioselectivity, which varied in the range of 0-99 % ee Complexation studies and DFT calculations were performed to explain these differences.

Influence of ligand structure and molecular geometry on the properties of d 6 polypyridinic transition metal complexes

Different strategies to improve the excited state properties of polypyridinic complexes by varying ligand structure and molecular geometry are described. Bidentate and tetradentate ligands based on fragments as dipyrido[3,2- a:2′,3′- c]phenazine, dppz, and pyrazino[2,3- f][1,10]-phenanthroline, ppl, have been used. Quinonic residues were fused to these basic units to improve acceptor properties. Photophysical studies were performed in order to test theoretical predictions.

Influence of ligand structures on methanol electro-oxidation by mixed catalysts based on platinum and organic metal complexes for DMFC

In order to investigate the influence of mixing organic metal complexes with various metal coordination structure on Pt/C catalyst for methanol oxidation reaction (MOR) in acidic media, N, N′-mono-8-quinolyl- o-phenylenediamine (mqph), N, N′-bis(anthranilidene)ethylenediamine (anthen) and N, N′-bis(salicylidene)ethylenediamine (salen), coordinating to Co(II) and Ni(II), were examined as the model compounds. M(mqph), M(anthen) and M(salen) have MN 3, MN 4 and MN 2O 2 moiety as the catalytic active sites, respectively. Pt–M(complex)/C mixed catalysts were prepared by depositing the mixture of Pt tetra-ammine chloride and each organic metal complex with various mixing ratio (w/o) on graphite powder and then heat-treating at 600 °C in Ar atmosphere. The Pt–M(complex)/C samples were put on a glassy carbon disk electrode, and tested for electrochemical MOR in 1 mol dm −3 CH 3OH + 0.05 mol dm −3 H 2SO 4 aqueous solutions at 25 °C. The mixed catalysts, especially Pt–Ni(mqph)/C and Pt–Co(mqph)/C, were found to enhance the MOR and exhibited higher catalytic activities than Pt/C, although each organic metal complex solely showed no catalytic activity. The catalytic ability was enhanced by mixing up to 50/50 (Pt–M(complex)) mixing ratio for the Co(mqph) and up to 80/20 mixing ratio for the Ni(mqph). Larger amounts of M(complex) resulted in a decrease in the catalytic activities. These results indicate that the organic metal complexes, especially M(mqph), promote effectively the electrochemical MOR on Pt/C catalyst, the degree of which depends strongly on the ligand structure. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectra (XAS) for the Pt–M(complex)/C mixed catalysts showed that the metal coordination structures of organic metal complexes are partially retained on the graphite powder even after the severe heat-treatment at 600 °C in Ar atmosphere. Ab initio calculations for the organic metal complexes exhibited that the Ni metal site of the Ni(mqph) interact with OH − group more strongly than those of the other organic metal complexes. This fact suggests that the Pt–M(mqph)/C electro-oxidizes a byproduct CO absorbed on Pt by “bifunctional effect” in a similar way as Pt–Ru alloy catalysts do in promoting the effective electrochemical MOR.

Iridium(III) Tris-cyclometalated Complexes with Diphenyloxazolic Ligands: Influence of Ligand Structure on Electrochemical and Photophysical Properties

Two luminescent iridium(III) tris-cyclometalated complexes with isomeric 2,4-diphenyloxazolato and 2,5-diphenyloxazolato ligands, fac-Ir(24dpo)3 and fac-Ir(25dpo)3, are reported. The molecular structure for Ir(24dpo)3 was determined by X-ray diffraction. Electrochemical and spectroscopic studies supported by structures are discussed so as to demonstrate how the ligand structure influences the luminescent properties.

Amine Bis(phenolate) Zirconium Complexes: Influence of Ligand Structure and Cocatalyst on Copolymerization Behavior

Five amine bis(phenolate)zirconium dibenzyl complexes, [ONDO]ZrBn2, were investigated in ethylene/1-hexene copolymerization. The copolymerization behavior is sensitive both to the symmetry and connectivity of the ligand framework and to the nature of the cocatalyst. The Cs-symmetric [ONDO]ZrBn2 complexes were more active and exhibited higher comonomer incorporations than the C2-symmetric [ONDO]ZrBn2 complexes. The nature of the cocatalyst was observed to have a significant effect on the amount of hexene incorporated into the copolymers. Activation of the Cs-symmetric [ONDO]ZrBn2 complexes with MMAO yielded copolymers with 10% higher hexene incorporation than that observed upon activation with [PhNMe2H][B(C6F5)4], [B(C6F5)3], or [Ph3C][B(C6F5)4].

The Catalytic Diastereo‐ and Enantioselective Claisen Rearrangement of 2‐Alkoxycarbonyl‐Substituted Allyl Vinyl Ether

AbstractThe catalytic asymmetric Claisen rearrangement of 2‐alkoxycarbonyl‐substituted allyl vinyl ethers that contain two stereogenic double bonds is described. A combination of the highly Lewis acidic [Cu{(S,S)‐tert‐Bu‐box}](H2O)2(SbF6)2 complex and molecular sieves served as catalyst and afforded the Claisen rearrangement products, substituted and functionalized α‐keto esters, in high yield with a remarkable diastereo‐ and enantioselectivity. The influence of ligand structure, counterion and allyl vinyl ether double bond configuration on the stereoselectivity of the rearrangement was briefly investigated. We propose an explanation for the rate accelerating effect of the Lewis acid as well as a stereochemical model which serve to explain and predict the stereochemical course of the copper bis(oxazoline) catalyzed Claisen rearrangement.

Asymmetric reactions catalyzed by chiral titanium complexes

This presentation reviewed some of our recent progress in asymmetric reactions catalyzed by chiral Ti(IV) complexes, including asymmetric oxa-Diels-Alder reaction(up to 99%ee), asymmetric oxidation of sulfides for sulfoxides (up to 96% ee), enantioselective silylcyanation of aldehydes (up to 87% ee) and ketones (up to 69% ee). The influence of ligand structure, substrate, amount of catalyst, counter ion, solvent, temperature, and other conditions on the reactivity and selectivity were discussed. Some mechanistic insights were also presented.

Control of multivalent interactions by binding epitope density.

Receptor clustering by multivalent ligands can activate signaling pathways. In principle, multivalent ligand features can control clustering and the downstream signals that result, but the influence of ligand structure on these processes is incompletely understood. Using a series of synthetic polymers that vary systematically, we studied the influence of multivalent ligand binding epitope density on the clustering of a model receptor, concanavalin A (Con A). We analyze three aspects of receptor clustering: the stoichiometry of the complex, rate of cluster formation, and receptor proximity. Our experiments reveal that the density of binding sites on a multivalent ligand strongly influences each of these parameters. In general, high binding epitope density results in greater numbers of receptors bound per polymer, faster rates of clustering, and reduced inter-receptor distances. Ligands with low binding epitope density, however, are the most efficient on a binding epitope basis. Our results provide insight into the design of ligands for controlling receptor-receptor interactions and can be used to illuminate mechanisms by which natural multivalent displays function.

Mutational analysis of the estrogen receptor ligand-binding domain: influence of ligand structure and stereochemistry on transactivation.

The mouse estrogen receptor (mER) exhibits ligand stereochemical specificity for indenestrol A (IA), a stilbestrol estrogen. IA has a chiral C3 methyl group, and the mER preferentially binds the S-enantiomer (IA-S), resulting in elevated biological activity when compared with the IA-R enantiomer. To elucidate the mechanisms for this stereochemical recognition, we have constructed a series of mERs with individual amino acid substitutions at Met521, His528, Met532, and Val537. The abilities of yeast-expressed wild-type and mutant mERs to transactivate an estrogen-responsive reporter gene construct were measured in the presence of diethylstilbestrol (DES) and IA enantiomers. The concentration of IA-S required to induce half-maximal transactivation by wild-type mER was 10-fold lower than IA-R, which is attributed to the 15-fold greater binding affinity for IA-S. Wild-type mER displayed similar dose-response curves for IA-R and demethyl IA, which lacks a C3 methyl group, demonstrating that the presence and correct orientation of the C3 methyl group on the IA compound is required for high-affinity ligand binding and transcriptional activity. Each mutant exhibited a reduced preference for IA-S enantiomer with respect to transactivation, suggesting that this region of the mER functions in ligand stereochemical recognition and activation. A mutation at Met532 diminished DES- and IA-S-induced transactivation by 7.5-fold and 40-fold respectively, with minimal change on their binding affinity. These data suggest that Met532 is required for transactivation induced by the potent agonist, IA-S, and the M532G mutation effectively uncouples IA-S ligand binding from transactivation. Use of these stereochemically different ligands in combination with mutagenesis of the mER demonstrates that ligand structure could influence transactivation by specifically altering the conformation of the mER AF-2 region.

The interaction of intercalators and groove‐binding agents with DNA triple‐helical structures: The influence of ligand structure, DNA backbone modifications and sequence

The effects of ligand structure and properties, DNA backbone modifications and DNA sequence on the interaction of a variety of well-known groove-binding agents and intercalators with DNA duplexes and triplexes have been evaluated by thermal melting experiments and molecular modeling. Both methylphosphonate and phosphorothioate substitutions generally destabilize DNA duplexes and triplexes. Modified duplexes can be strongly stabilized by both groove-binding agents and intercalators whereas triplexes are primarily stabilized by intercalators. Of the compounds tested, the intercalators coralyne and quinacrine provide the largest stabilization of the triplex dT19.dA19.dT19. Molecular modeling studies suggest that the large intercalating ring system of coralyne stacks well with the triplex bases whereas the alkylamino side chain of quinacrine fits snugly into the remaining space of the major groove of dT19.dA19.dT19 triplex and forms extensive van der Waals contacts with the thymine methyl groups that line the groove. Converting some of the T.A.T base triples to C+.G.C (e.g. dT19.dA19.dT19 to d(T4C+)3T4.d(A4G)3A4.(T4C)3T4) causes very significant decreases in observed Tm increases for compounds such as quinacrine and coralyne. Although removal of thymine methyl groups and addition of positive charge on substitution of C+.G.C for T.A.T should reduce binding of cationic intercalators, the large difference observed between the pure AT and the mixed sequence triplexes suggest that they may also have differences in structure and properties.

Influence of ligand structure and solvent effects on complexation of neutral guests by several crown ethers

The formation of complexes between crown ethers and acetonitrile, chloroform, and nitromethane were investigated in carbon tetrachloride at 25°C. A significant influence of the ring size on the selectivity of the host is evident. The host 18-crown-6 forms complexes for which the reaction enthalpy and entropy are quite high. Host molecules with benzene side groups form complexes of lower reaction enthalpy and entropy and therefore the complexes formed are less stable than that of the analogous crown ethers without aromatic groups. Solvent effects on the stability constant K, the reaction enthalpy ΔH, and the reaction entropy ΔS were studied for the complexation of malonitrile by 18-crown-6. The reaction enthalpy and entropy values change in accordance with the dielectric constant of the solvent used, but no overall effect on complex stability with change in solvent dielectric constant was observed.

Spectral Characteristics and Kinetic Features of Redox Reactions of Nickel(II) And Nickel(III) Complexes with Dioxotetraaza Macrocyclic Ligands

Abstract Spectral characteristics of nickel(II) and nickel(III) complexes with unsubstituted and benzyl-substituted 13- and 14-membered dioxotetraaza macrocyclic ligands have been studied. The interaction of nickel(II) complexes with peroxodisulfate was shown to yield two different products depending on the ring size of the macrocycle: [Ni(III)L]' in case of 14-membered ligands and NiL-2H, i.e. nickel(II) complexes with the oxidized macrocycle, in case of 13-membered ligands. The [Ni(III)L]+ complexes decompose in neutral solutions via an intramolecular redox reaction to yield Ni(II)L-2H compounds. The presence of a new C˭N bond in the products of redox reaction was established by means of electronic spectroscopy and 1H and 13C NMR data. Rate constants and activation parameters for oxidation of Ni(II)L by peroxodisulfate and decomposition reactions of [Ni(III)L]+ in aqueous solution at pH 5.7 have also been measured. The influence of ligand structure on the kinetic parameters of redox reactions studied was investigated. Comparison of data obtained with the results of the study of redox reactions of similar copper complexes has been made and possible reaction mechanisms discussed.

Low-potential nickel(III,II) complexes: new systems based on tetradentate amidate-thiolate ligands and the influence of ligand structure on potentials in relation to the nickel site in [NiFe]-hydrogenases

In a continuing study of Ni II,III complexes with low redox potentials that are of possible relevance to the nickel site in [NiFe]-hydrogenases, Ni II complexes of the tetraanions of the amide-thiol ligands N,N'-ethylenebis (2-mercaptoacetamide) N,N'-1,2-phenylenebis (2-mercaptoacetamide), and N,N'-ethylenebis(2-mercaptoisobutyramide) have been prepared

Control of asymmetric induction in the organometallic synthesis of 3-methyl-(E)-4-hexen-2-one

Bis(μ-methyl-1,3-dimethyl-η3-allylnickel) reacts in the presence of phosphorus(III) ligands (phosphines, phosphites) with CO to give 3-methyl-(E)-4-hexen-2-one. Systematic experiments were performed using chiral ligands containing menthyl or menthoxy groups and different achiral groups on the P atom to determine the influence of ligand structure and concentration on the direction and extent of chiral induction in this ketone synthesis. It could be shown that reversion of enantioselectivity can be obtained not only by the obvious method of reversing the absolute configuration of the ligand (“parity control”) but also by modifying the nonchiral substituents of the phosphorus ligand (“complementarity control”). The synthesis, physical, and spectroscopic properties of several new menthyl phosphines and phosphites are reported.

ChemInform Abstract: Stabilization of Trivalent Nickel in Tetragonal NiS4N2 and NiN6 Environments: Synthesis, Structures, Redox Potentials, and Observations Related to (NiFe)‐Hydrogenases.

AbstractThe influence of ligand structure on Ni(III)/Ni(II) redox potentials is examined in order to elucidate those factors which lead to unusually low values for this couple, such as are found in (NiFe)‐hydrogenases.

Stabilization of trivalent nickel in tetragonal NiS4N2 and NiN6 environments: synthesis, structures, redox potentials and observations related to [NiFe]-hydrogenases

The influence of ligand structure on Ni(III)/Ni(II) redox potentials has been examined in order to elucidate those factors that lead to unusually low values for this couple, such as are found in [NiFe]-hydrogenases. The compound (Et 4 N) 2 [Ni(pdtc) 2 ] [3, pdtc=pyridine-2,6-bis(monothiocarboxylate) (2−)] is readily prepared and crystallizes in monoclinic space group C2/c. Reaction of 3 with iodine in methanol and addition of (Ph 3 PCH 2 Ph) Br affords green (Ph 3 PCH 2 Ph) [Ni(pdtc) 2 ] (4), which crystallizes in monoclinic space group P2 1 /n. Also prepared was (Ph 3 PCH 2 Ph) [Co(pdtc) 2 ], which is isomorphous with 4

Reactions of Copper(II) Complexes of Polydentate 8-Mercapto-Quinoline Derivatives with Phenols

Abstract The reactions of copper complexes with chelate thiaaza ligands as models of the active centres of ‘blue’ copper proteins have been studied. The overall reaction stoichiometry for the reduction of copper(II) complexes by 4-methyl-2,6-di-tert-butyIphenol as well as hydroquinone in acetonitrile was found to be 2:1. The reactions are first-order with respect to the concentration of both complex and phenol. The second-order rate constants and activation energies have been determined and the influence of ligand structure on these parameters discussed. The effect of pyridine in the reaction mixture on the redox potential and catalytic properties of some complexes on the oxidation of phenols by dioxygen was observed.