Ligand-exchange processes on solvated lithium cations: DMSO and water/DMSO mixtures.

Solutions of LiClO(4) in solvent mixtures consisting of dimethylsulfoxide (DMSO) and water, or DMSO and gamma-butyrolactone, were studied by (7)Li NMR spectroscopy (for complexation by cryptands in gamma-butyrolactone as a solvent, see: E. Pasgreta, R. Puchta, M. Galle, N. J. R. van Eikema Hommes, A. Zahl, R. van Eldik, J. Incl. Phen., 2007, 58, 81-88). Chemical shifts indicate that the Li(+) ion is coordinated by four DMSO molecules. In the binary solvent mixture of water and DMSO, no selective solvation is detected, thus indicating that on increasing the water content of the solvent mixture, DMSO is gradually displaced by water in the coordination sphere of Li(+). The ligand-exchange mechanism of Li(+) ions solvated by DMSO and water/DMSO mixtures was studied using DFT calculations. Ligand exchange on [Li(DMSO)(4)](+) was found to follow a limiting associative (A) mechanism. The displacement of coordinated H(2)O by DMSO in [Li(H(2)O)(4)](+) follows an associative interchange mechanism. The suggested mechanisms are discussed in reference to available experimental and theoretical data.     

Reduction pathway of end-on terminally coordinated dinitrogen. V. N-N bond cleavage in Mo/W hydrazidium complexes with diphosphine coligands. Comparison with triamidoamine systems

N-N cleavage of the dialkylhydrazido complex [W(dppe)2(NNC5H10)] (B(W)) upon treatment with acid, leading to the nitrido/imido complex and piperidine, is investigated experimentally and theoretically. In acetonitrile and at room temperature, B(W) reacts orders of magnitude more rapidly with HNEt3BPh4 than its Mo analogue, [Mo(dppe)2(NNC5H10)] (B(Mo)). A stopped-flow experiment performed for the reaction of B(W) with HNEt3BPh4 in propionitrile at -70 degrees C indicates that protonation of B(W) is completed within the dead time of the stopped-flow apparatus, leading to the primary protonated intermediate B(W)H+. Propionitrile coordination to this species proceeds with a rate constant k(obs(1)) of 1.5 +/- 0.4 s(-1), generating intermediate RCN-B(W)H+ (R = Et) that rapidly adds a further proton at Nbeta and then mediates N-N bond splitting in a slower reaction (k(obs(2)) = 0.35 +/- 0.08 s(-1), 6 equiv of acid). k(obs(1)) and k(obs(2)) are found to be independent of the acid concentration. The experimentally observed reactivities of B(Mo) or B(W) with acids in nitrile solvents are reproduced by DFT calculations. In particular, geometry optimization of models of solvent-coordinated, Nbeta-protonated intermediates is found to lead spontaneously to separation into the nitrido/imido complexes and piperidine/piperidinium, corresponding to activationless heterolytic N-N bond cleavage processes. Moreover, DFT indicates a spontaneous cleavage of nonsolvated B(W) protonated at Nbeta. In the second part of this article, a theoretical analysis of the N-N cleavage reaction in the Mo(III) triamidoamine complex [HIPTN3N]Mo(N2) is presented (HIPTN3N = hexaisopropylterphenyltriamidoamine). To this end, DFT calculations of the Mo(III)N2)triamidoamine complex and its protonated and reduced derivatives are performed. Calculated structural and spectroscopic parameters are compared to available experimental data. N-N cleavage most likely proceeds by one-electron reduction of the Mo(V) hydrazidium intermediate [HIPTN3N]Mo(NNH3)+, which is predicted to have an extremely elongated N-N bond. From an electronic-structure point of view, this reaction is analogous to that of Mo/W hydrazidium complexes with diphos coligands. The general implications of these results with respect to synthetic N2 fixation are discussed.     

Kinetics and mechanism of the cobalt phthalocyanine catalyzed reduction of nitrite and nitrate by dithionite in aqueous solution

The reaction of sodium nitrite with sodium dithionite was studied in the presence of cobalt(II) tetrasulfophthalocyanine, COII(TSPc)4-, in aqueous alkaline solution. The overall mechanism comprises the reduction of CoII(TSPc)4- by dithionite, followed by the formation of an intermediate complex between COI(TSPc)5- and nitrite, which undergoes two parallel subsequent reactions with and without nitrite as a reagent. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction was found to be ammonia. Contrary to those found for the catalytic reduction of nitrite, the products of the catalytic reduction of nitrate were found to be dinitrogen and nitrous oxide. The possible catalytic reduction of nitrous oxide was confirmed by independent experiments. The striking differences in the reduction products of nitrite and nitrate are explained in terms of different structures of the intermediate complex between CoI(TSPc)5- and substrate, in which nitrite and nitrate are suggested to coordinate via nitrogen and oxygen, respectively.     

Cyclometalated analogues of platinum terpyridine complexes: kinetic study of the strong sigma-donor cis and trans effects of carbon in the presence of a pi-acceptor ligand backbone

The substitution kinetics of the complexes [Pt(N-N-C)Cl] (N-N-CH = 6-phenyl-2,2'-bipyridine), [Pt(N-C-N)Cl] (N-CH-N = 1,3-di(2-pyridyl)benzene), and [Pt(N-N-N)Cl]Cl (N-N-N = 2,2':6',2' '-terpyridine) with the nucleophiles Br(-), I(-), and, for the first two complexes, also thiourea, N,N-dimethylthiourea, and N,N,N',N'-tetramethylthiourea, have been studied in methanol as solvent. In case of the thioureas, the activation parameters DeltaH, DeltaS, and DeltaV were also determined from the temperature and pressure dependence of the reactions. Two crystal structures of [Pt(N-N-C)Cl] were determined (yellow and red polymorphs); the intense red color of the latter polymorph results from Pt-Pt interactions (Pt-Pt distance = 3.366 A). The data enable an analysis of the cis and trans effects and the influence of the strong sigma-donor carbon in the presence of an electron withdrawing pi-acceptor ligand backbone. The results indicate that the intrinsic reactivity is enhanced greatly by the labilizing effect of the trans carbon donor, but the nucleophilic discrimination is dramatically reduced due to the decrease in electrophilicity on the metal center. However, although the electron withdrawing pi-acceptor effect is partly counteracted by the sigma-donor effect, the complex still benefits from a higher nucleophilic discrimination than in the comparable Pt(II) trans carbon donor complexes, where no or fewer pi-acceptors are present. In the case of the cis carbon donor complex, the intrinsic reactivity remains unchanged, but the nucleophilic discrimination is reduced and leads to a reduced reactivity of the [Pt(N-N-C)Cl] complex in comparison to [Pt(N-N-N)Cl]Cl. On the basis of these results, a more detailed treatment of the nature of the cis effect is offered.     

7-hydroxy (and acetoxy)-α-mullolene from the soft-coral heteroxenia fuscescens

The isolation and structural determination of three cadalane type sesquiterpenes from a soft coral is reported. The ¹³C NMR of these compounds namely (+)-α-murrolene, 7-hydroxy, and 7-acetoxy-α-murrolene is discussed.     

Engineered and artificial metalloenzymes for selective C–H functionalization

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Ligand‐Exchange Processes on Solvated Zinc Cations: Water Exchange on [Zn(H2O)4(L)]2+⋅2 H2O (L=Heterocyclic Ligand)

The water‐exchange mechanisms of [Zn(H₂O)₄(L)]²⁺?2 H₂O (L=imidazole, pyrazole, 1,2,4‐triazole, pyridine, 4‐cyanopyridine, 4‐aminopyridine, 2‐azaphosphole, 2‐azafuran, 2‐azathiophene, and 2‐azaselenophene) have been investigated by DFT calculations (RB3LYP/6‐311+G**). The results support limiting associative reaction pathways that involve the formation of six‐coordinate intermediates [Zn(H₂O)₅(L)]²⁺?H₂O. The basicity of the coordinated heterocyclic ligands shows a good correlation with the activation barriers, structural parameters, and stability of the transition and intermediate states.     

Impact of the tick-size on financial returns and correlations

We demonstrate that the lowest possible price change (tick-size) has a large impact on the structure of financial return distributions. It induces a microstructure as well as possibly altering the tail behavior. On small return intervals, the tick-size can distort the calculation of correlations. This especially occurs on small return intervals and thus contributes to the decay of the correlation coefficient towards smaller return intervals (Epps effect). We study this behavior within a model and identify the effect in market data. Furthermore, we present a method to compensate this purely statistical error.     

Linear fractional transformations of Stieltjes functions

Linear fractional transformations of Stieltjes (and inverse Stieljes) functions, which appear naturally in the extension theory of nonnegative symmetric operators with defect one in Hilbert spaces, are investigated. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)