Moving protons with pendant amines: proton mobility in a nickel catalyst for oxidation of hydrogen

Proton transport is ubiquitous in chemical and biological processes, including the reduction of dioxygen to water, the reduction of CO(2) to formate, and the production/oxidation of hydrogen. In this work we describe intramolecular proton transfer between Ni and positioned pendant amines for the hydrogen oxidation electrocatalyst [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+) (P(Cy)(2)N(Bn)(2) = 1,5-dibenzyl-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane). Rate constants are determined by variable-temperature one-dimensional NMR techniques and two-dimensional EXSY experiments. Computational studies provide insight into the details of the proton movement and energetics of these complexes. Intramolecular proton exchange processes are observed for two of the three experimentally observable isomers of the doubly protonated Ni(0) complex, [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+), which have N-H bonds but no Ni-H bonds. For these two isomers, with pendant amines positioned endo to the Ni, the rate constants for proton exchange range from 10(4) to 10(5) s(-1) at 25 °C, depending on isomer and solvent. No exchange is observed for protons on pendant amines positioned exo to the Ni. Analysis of the exchange as a function of temperature provides a barrier for proton exchange of ΔG(?) = 11-12 kcal/mol for both isomers, with little dependence on solvent. Density functional theory calculations and molecular dynamics simulations support the experimental observations, suggesting metal-mediated intramolecular proton transfers between nitrogen atoms, with chair-to-boat isomerizations as the rate-limiting steps. Because of the fast rate of proton movement, this catalyst may be considered a metal center surrounded by a cloud of exchanging protons. The high intramolecular proton mobility provides information directly pertinent to the ability of pendant amines to accelerate proton transfers during catalysis of hydrogen oxidation. These results may also have broader implications for proton movement in homogeneous catalysts and enzymes in general, with specific implications for the proton channel in the Ni-Fe hydrogenase enzyme.     

Synthesis and reactivity of molybdenum and tungsten bis(dinitrogen) complexes supported by diphosphine chelates containing pendant amines

Molybdenum and tungsten bis(dinitrogen) complexes of the formula M(N(2))(2)(PNP)(2) (M = Mo and W) and W(N(2))(2)(dppe)(PNP), supported by diphosphine ligands containing a pendant amine of the formula (CH(2)PR(2))(2)NR' = P(R)N(R')P(R) (R = Et, Ph; R' = Me, Bn), have been prepared by Mg reduction of metal halides under an N(2) atmosphere. The complexes have been characterized by NMR and IR spectroscopy, X-ray crystallography, and cyclic voltammetry. Reactivity of the target Mo and W bis(dinitrogen) compounds with CO results in the formation of dicarbonyl complexes.     

Air oxidation of ethylbenzene: catalysis with polymer supported nickel

The air oxidation of ethylbenzene in liquid phase at atmospheric pressure catalyzed by nickel acetylacetonate was investigated. The catalyst was supported on a variety of polyvinylpyridines as homopolymers or crosslinked with divinylbenzene. In addition, a co-catalyst based on N-n-butylpolyvinylpyridinium hexafluorophosphate was employed. Results show that the poly-2-vinylpyridine-divinylbenzene based catalyst gives the best results in terms of both ethylbenzene conversion and ethylbenzene hydroperoxide selectivity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanism of the catalysis by aliphatic amines of the oxidation of anthrone with molecular oxygen

Using anthrone as an example, the oxidation of aromatic ketones with molecular oxygen catalyzed by aliphatic amines of various structures has been studied. Analysis of kinetic and spectroscopic data permits the prediction of the mechanism of the reaction, the establishment of the equilibrium constants, and the rate constants for the separate stages.     

The role of pendant amines in the breaking and forming of molecular hydrogen catalyzed by nickel complexes

We present the results of a comprehensive theoretical investigation of the role of pendant amine ligands in the oxidation of H(2) and formation of H(2) by [Ni(P(R)(2)N(R')(2))(2)](2+) electrocatalysts (P(R)(2)N(R')(2) is the 1,5-R'-3,7-R derivative of 1,5-diaza-3,7-diphosphacyclooctane, in which R and R' are aryl or alkyl groups). We focus our analysis on the thermal steps of the catalytic cycle, as they are known to be rate-determining for both H(2) oxidation and production. We find that the presence of pendant amine functional groups greatly facilitates the heterolytic H(2) bond cleavage, resulting in a protonated amine and a Ni hydride. Only one single positioned pendant amine is required to serve this function. The pendant amine can also effectively shuttle protons to the active site, making the redistribution of protons and the H(2) evolution a very facile process. An important requirement for the overall catalytic process is the positioning of at least one amine in close proximity to the metal center. Indeed, only protonation of the pendant amines on the metal center side (endo position) leads to catalytically active intermediates, whereas protonation on the opposite side of the metal center (exo position) leads to a variety of isomers, which are detrimental to catalysis.     

Efficient and green oxidation of alcohols with tert-butyl hydrogenperoxide catalyzed by a recyclable magnetic core-shell nanoparticle-supported oxo-vanadium ephedrine complex

A novel method for the oxidation of alcohols to the corresponding carbonyl compounds has been successfully developed using tert-butyl hydrogenperoxide (TBHP) in the presence of a catalytic amount of recyclable magnetic nanoparticle-supported oxo-vanadium ephedrine complex (VO(ephedrine)₂@MNPs) in PEG as a green solvent at 80 °C. The catalyst can be magnetically recycled and successfully reused in six subsequent reaction cycles with only slight decreases of its catalytic activity.     

Oxygen evolution catalysis by a dimanganese complex and its relation to photosynthetic water oxidation

[Mn2(III/IV)(mu-O) 2(terpy)2(OH 2)2](NO3)3 (1, where terpy = 2,2':6'2'-terpyridine) acts as a water-oxidation catalyst with HSO5(-) as the primary oxidant in aqueous solution and, thus, provides a model system for the oxygen-evolving complex of photosystem II (Limburg, J.; et al. J. Am. Chem. Soc. 2001, 123, 423-430). The majority of the starting [Mn2(III/IV)(mu-O)2](3+) complex is converted to the[Mn2(IV/IV)(mu-O)2](4+) form (2) during this reaction (Chen, H.; et al. Inorg. Chem. 2007, 46, 34-43). Here, we have used stopped-flow UV-visible spectroscopy to monitor UV-visible absorbance changes accompanying the conversion of 1 to 2 by HSO5(-). With excess HSO5(-), the rate of absorbance change was found to be first-order in [1] and nearly zero-order in [HSO5(-)]. At relatively low [HSO5(-)], the change of absorbance with time is distinctly biphasic. The observed concentration dependences are interpreted in terms of a model involving the two-electron oxidation of 1 by HSO5(-), followed by the rapid reaction of the two-electron-oxidized intermediate with another molecule of 1 to give two molecules of 2. In order to rationalize biphasic behavior at low [HSO5(-)], we propose a difference in reactivity of the [Mn2(III/)(IV)(mu-O)2](3+) complex upon binding of HSO5(-) to the Mn(III) site as compared to the reactivity upon binding HSO5(-) to the Mn(IV) site. The kinetic distinctness of the Mn(III) and Mn(IV) sites allows us to estimate upper limits for the rates of intramolecular electron transfer and terminal ligand exchange between these sites. The proposed mechanism leads to insights on the optimization of 1 as a water-oxidation catalyst. The rates of terminal ligand exchange and electron transfer between oxo-bridged Mn atoms in the oxygen-evolving complex of photosystem II are discussed in light of these results.     

Charge ordering and carrier generation by air oxidation of a nickel complex with selenium-containing extended-tetrathiafulvalenedithiolate ligands

The structures and electrical properties of ((n)Bu(4)N)[Ni(dmstfdt)(2)] (1), ((n)Bu(4)N)(2)[Ni(dmstfdt)(2)] (2), and ((n)Bu(4)N)(3)[Ni(dmstfdt)(2)](2) (3), where dmstfdt = extended-tetrathiafulvalenedithiolate ligand, were examined. The fresh crystal of 1 was found to be a Mott insulator, but the crystal gradually became highly conducting because of air oxidation. Compound 3 exhibited a semiconducting charge-ordering state.     

Hydrogen bonding by alcohols and amines

The pure base calorimetric method has been used to determine the enthalpies of hydrogen bond complex formation between aliphatic amines and alcohols. The enthalpies of complexation for the series methanol-n-butanol bonding with triethylamine increase with decreasing alkyl chain length in accordance with the electron donating properties of alkyl groups. Unexpectedly, the enthalpies for the complexes of n-butanol with tributylamine, tripropylamine, and triethylamine increase with decreasing alkyl chain length.Primary and secondary amines form hydrogen bonded complexes with n-butanol in which the amine protons form an NH···O bond with the alcohol and the alcohol hydroxyl proton donates a proton to the amine nitrogen. The difference in enthalpy of complex formation between tertiary amines and secondary amines is largely accounted for by the involvement of the amine proton of the secondary amine. Primary amines, like secondary amines, donate only one proton to the complex with n-butanol but have a larger complex enthalpy than secondary amines probably because of steric hindrance and differences in basicity.     

Heterochiral triangulo nickel complex as evidence of a large positive nonlinear effect in catalysis

A novel triangulo complex was generated by heating, in vacuo, a racemic C2-symmetric octahedral nickel(II) diamine complex. The trinuclear species was identified by single-crystal X-ray diffraction, which revealed a 2:1 ligand stereochemistry relationship in each unit. This solid-state structure evidences the hypothesis that a 1:2 stereochemical relationship lies at the heart of the strong positive nonlinear effect observed in enecarbamate aldol-like reactions catalyzed by nickel(II) diamine complexes.     

Combined polymerization catalysis of nickel complex and zirconocene for branched polyethylene

The polymerization behavior of 2-(2′-pyridyl) quinoxaline nickel dibromide/Cp₂ZrCl₂/MAO system was investigated in three ways: the Ni catalyst was added first, followed by addition of Zr catalyst (method I); the Ni and Zr catalysts were added simultaneously (method II); and the Zr catalyst was added first, followed by addition of Ni catalyst (method III). Results of GC-MS, GPC,¹³C NMR and DSC investigations indicated that the properties of resulting polyethylene were greatly varied by changing feeding orders of the two catalysts. Decreasing Ni/Zr molar ratio or increasing polymerization temperature gave corresponding polyethylenes with less branches and higher melting point. Compared to the procedure using Cp₂ZrCl₂ catalyst only, the activity of Zr catalyst in those combined system decreased because of the competition of ethylene between the [Ni−C] and [Zr−C] active centers. In addition, other zirconocenes were also employed as copolymerization catalysts in the combined system with nickel complex. compared to Cp₂ZrCl₂ case, the ethyl-bridged Zr catalyst performed better for polymerization of ethylene while the Si-bridged Zr catalyst showed better copolymerization ability.     

Selective oxidation of ethylbenzene by dioxygen. The effect of chelate center on catalysis by bicyclic nickel complexes

The effect of the nature of the chelate center in Ni^II complexes on their catalytic activity in the selective oxidation of ethylbenzene by dioxygen to α-phenylethyl hydroperoxide in the presence of nickel bis(acetylacetonate) (chelate center Ni(O,O)₂) and nickel bis(enaminoacetonate) (chelate center Ni(O,NH)₂) was studied. The efficiency of selective oxidation of ethylbenzene increases substantially in the presence of the chelate with the Ni(O,NH)₂ active center as a catalyst, which is mainly due to the transformation of the catalyst into more active species during the oxidation process. The mechanism of transformation of nickel bis(enaminoacetonate) under the action of dioxygen was suggested. The sequence of formation of the reaction products at different stages of the catalytic process was determined. The activity of the nickel complex with the Ni(O,NH)₂ chelate center and the products of its transformation in the elementary stages of chain oxidation of ethylbenzene is discussed. Translated fromIzvestiya Akedemii Nauk. Seriya Khimicheskaya, No. 1, pp. 55–60, January, 1999.