Hydrogen Atom Abstraction Thermodynamics of a µ-1,2-Superoxo Dicopper(II) Complex.

Pyrazolate-based µ-1,2-peroxo dicopper(II) complex 1 undergoes clean 1e- oxidation at low potential (-0.59 V vs Fc/Fc+) to yield the rather stable µ-1,2-superoxo dicopper(II) complex 3, which was characterized by spectroscopic methods (ν̃(O-O) = 1070 cm-1, Δ(18O-16O) = -59 cm-1) and analyzed by DFT calculations. 3 is also formed via H-atom abstraction from the corresponding µ-1,1-hydroperoxo dicopper(II) complex 2, while 3 itself is able to abstract H-atoms from weaker X-H bonds such as TEMPO-H to re-form 2. Kinetic and thermodynamic analyses evidence a concerted proton-electron transfer pathway for these processes. The thermodynamic square scheme reveals a bond dissociation free energy of 71.7 ± 1.1 kcal mol-1 for the hydroperoxo OO-H bond of 2.

Reductive Transformations of a Pyrazolate-Based Bioinspired Diiron-Dinitrosyl Complex.

Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to N2O, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L-, which serve as model systems for nonheme diiron active sites. Their ferrous nitrile precursors [L{Fe(R'CN)}2(µ-OOCR)](X)2 (1) can be readily converted into the corresponding nitrosyl adducts ([L{Fe(NO)}2(µ-OOCR)](X)2, 2). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe(NO)}7 (Enemark-Feltham notation) units. The nitrosyl adducts 2 can be (electro)chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of N2O. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of 2a (R = Me, X = ClO4). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L{Fe(NO)2}2](ClO4), 3a') and a diacetato-bridged ferrous complex [LFe2(µ-OAc)2](ClO4) (3a″). Especially 3a' parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which N2O release is much less efficient than for FNORs.

Proton-Induced, Reversible Interconversion of a µ-1,2-Peroxo and a µ-1,1-Hydroperoxo Dicopper(II) Complex.

The µ-1,2-peroxo dicopper(II) complex (2) of a compartmental bis(tetradentate) pyrazolate-based ligand is shown to convert, upon protonation, to the corresponding µ-1,1-hydroperoxo dicopper(II) complex (3). The transformation is cleanly reversed with base, and an apparent pK(a) = 22.2 ± 0.3 for the Cu2OOH unit in MeCN has been determined. The unprecedented stability of 3 (t(1/2) = 9 h in nitrile solvents at room temperature, giving the hydroxo-bridged dicopper complex) has allowed for its structural characterization by X-ray diffraction. While the O-O bond length (1.462(3) Å) barely changes upon protonation from 2 to 3, the O-O stretching frequency is much higher in the hydroperoxo complex 3 (860 cm(-1)). 3 mediates 2e(-) oxo transfer to the nucleophilic substrate PPh3 but is not activated for H-atom abstraction.

Resting-state networks in healthy adult subjects: a comparison between a 32-element and an 8-element phased array head coil at 3.0 Tesla.

BACKGROUND: Little research exists on the influence of a magnetic resonance imaging (MRI) head coil's channel count on measured resting-state functional connectivity. PURPOSE: To compare a 32-element (32ch) and an 8-element (8ch) phased array head coil with respect to their potential to detect functional connectivity within resting-state networks. MATERIAL AND METHODS: Twenty-six healthy adults (mean age, 21.7 years; SD, 2.1 years) underwent resting-state functional MRI at 3.0 Tesla with both coils using equal standard imaging parameters and a counterbalanced design. Independent component analysis (ICA) at different model orders and a dual regression approach were performed. Voxel-wise non-parametric statistical between-group contrasts were determined using permutation-based non-parametric inference. RESULTS: Phantom measurements demonstrated a generally higher image signal-to-noise ratio using the 32ch head coil. However, the results showed no significant differences between corresponding resting-state networks derived from both coils (p < 0.05, FWE-corrected). CONCLUSION: Using the identical standard acquisition parameters, the 32ch head coil does not offer any significant advantages in detecting ICA-based functional connectivity within RSNs.

A ferromagnetically coupled (S = 1) peroxodicopper(II) complex.

Copper enzymes play important roles in the binding and activation of dioxygen in biological systems. Key copper/dioxygen intermediates have been identified and studied in synthetic analogues of the metalloprotein active sites, including the µ-η(2):η(2)-peroxodicopper(II) motif relevant to type III dicopper proteins. Herein, we report the synthesis and characterization of a bioinspired dicopper system that forms a stable µ-η(1):η(1)-peroxo complex whose Cu-O-O-Cu torsion is constrained to around 90° by ligand design. This results in sizeable ferromagnetic coupling between the copper(II) ions, which is detected by magnetic measurements and HF-EPR spectroscopy. The new dicopper peroxo system is the first with a triplet ground state, and it represents a snapshot of the initial stages of O2 binding at type III dicopper sites.

Crowning of coinage metal pyrazolates: double-decker homo- and heteronuclear complexes with synergic emissive properties.

A new pyrazole ligand with flexible thioether chelate arms was synthesized and was used to obtain an unprecedented class of hexanuclear coinage metal complexes of general formula [MM'L]3Y3 (M, M' = Cu, Ag, Au; Y = OTf, BF4). Three of them were characterized by X-ray crystallography, namely, homometallic [Ag2L]3(OTf)3 and [Ag2L]3(BF4)3 as well as heterometallic [CuAgL]3(OTf)3, revealing that the classical [M(µ-pz)]3 core is crowned by a second deck of S-bound M' ions. Depending on the solvent, these oligonuclear systems undergo rapid dynamics and show cation-anion aggregation in solution, which has been investigated by DOSY and temperature dependent NMR spectroscopy. Preliminary luminescence data for selected hexametallic [MM'L]3Y3 complexes show that the combination of ligand-directed intramolecular and supramolecular d(10) metal ion interactions in the solid state gives rise to synergic emissive properties that allow for a selective addressing of different emission wavelengths.

The impact of cardiac perception on emotion experience and cognitive performance under mental stress.

Mental stress evokes several physiological responses such as the acceleration of heart rate, increase of electrodermal activity and the release of adrenaline. Moreover, physiological stress responses interact with emotional and behavioral stress responses. In the present study we provide evidence that viscero-sensory feedback from the heart (cardiac perception) is an important factor modulating emotional and cognitive stress responses. In our study, we compared participants with high versus low cardiac perception using a computerized mental stress task, in which they had to respond to rapidly presented visual and acoustic stimuli. Additionally, we assessed physiological responses (heart rate, skin conductance). Participants high in cardiac perception reported more negative emotions and showed worse task performance under the stressor than participants low in cardiac perception. These results were not moderated by physiological responses. We conclude that cardiac perception modulates stress responses by intensifying negative emotions and by impairing cognitive performance.

Synthesis of Carbonates from Alcohols and CO2.

Alcohols are ubiquitous compounds in nature that offer modular building blocks for synthetic chemistry. Here we discuss the most recent development of different classes of alcohols and their coupling chemistry with carbon dioxide as to afford linear and cyclic carbonates, the challenges associated with their formation, and the potential of this chemistry to revive a waste carbon feed stock.