Influence of the electrochemical conversion of [(LH)Mn(II)Cl2] into [(L)Mn(III)Cl]+ on the protonic state of a phenol-containing ligand

A bischloromanganese(II) complex [(LH)MnCl2] (1), where LH is the pentadentate ligand N,N-bis(2-pyridylmethyl)-N'-salicylidene- ethane-1,2-diamine, has been synthesized. Elemental analysis, UV-visible, and cyclic voltammetry experiments showed that the phenol function of the ligand LH remains protonated. Exhaustive electrolysis at 1.0 V vs SCE led to the formation of the Mn(III) derivative [(L)MnCl]+ (3) with the concomitant expulsion of H+ and Cl-. The formation of the Mn(III) species was confirmed by UV-visible spectroscopy and X-ray crystallography. Complex 1 could be regenerated by the reduction of complex 3 in the presence of H+ and Cl-.     

Spectroscopic Characterisation of a Bio-Inspired Ni-Based Proton Reduction Catalyst Bearing a Pentadentate N2S3 Ligand with Improved Photocatalytic Activity

Inspired by the sulfur-rich environment found in active hydrogenase enzymes, a Ni-based proton reduction catalyst with pentadentate N₂S₃ ligand was synthesised. When coupled with [Ru(bpy)₃]²⁺ (bpy=2,2′-bipyridine) as photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared with a homologous counterpart bearing a tetradentate N₂S₂ ligand. The mechanistic pathway of photoinduced hydrogen evolution was comprehensively analysed through optical transient absorption and time-resolved X-ray absorption spectroscopy, which revealed important electronic and structural changes in the catalytic system during photoirradiation. The Ni^II catalyst undergoes a photoinduced metal-centred reduction to form a Ni^I intermediate with distorted square-bipyramidal geometry. Further kinetic analyses revealed differences in charge-separation dynamics between the pentadentate and tetradentate forms.     

Click chemistry on a ruthenium polypyridine complex. An efficient and versatile synthetic route for the synthesis of photoactive modular assemblies

In this Communication, we present the synthesis and use of [Ru(bpy)(2)(bpy-CCH)](2+), a versatile synthon for the construction of more sophisticated dyads by means of click chemistry. The resulting chromophore-acceptor or -donor complexes have been studied by flash photolysis and are shown to undergo efficient electron transfer to/from the chromophore. Additionally, the photophysical and chemical properties of the original chromophore remain intact, making it a very useful component for the preparation of visible-light-active dyads.     

Molecular and electronic structure of square-planar nickel II, nickel III and nickel III pi-cation radical complexes with a tetradentate o-phenylenedioxamidate redox-active ligand

The molecular and electronic structures of the electron transfer series of four-coordinate square-planar nickel complexes with the ligand o-phenylenebis(N'-methyloxamidate), [NiL]z (z = 2-, 1-, 0), have been evaluated by DFT and TDDFT calculations, and most of their experimentally available structural and spectroscopic properties (X. Ottenwaelder et al., Dalton Trans., 2005, DOI: 10.1039/b502478a) have been reasonably reproduced at the B3LYP level of theory. The anionic species [NiL]2- and [NiL]- are genuine low-spin nickel II and nickel III complexes with diamagnetic singlet (S = 0) and paramagnetic doublet (S = 1/2) states, respectively. The nickel III complex presents shorter Ni-N(amidate) bond distances (1.85-1.90 A) than the parent nickel II complex (1.88-1.93 A) and characteristic LMCT bands in the NIR region (lambda max = 794 and 829 nm) while the analogous MLCT bands for the nickel(II) complex are in the UV region (lambda max = 346 and 349 nm). The neutral species [NiL] is a nickel III o-benzosemiquinonediimine pi-cation radical complex with a diamagnetic singlet (S = 0) and a paramagnetic triplet (S = 1) states fairly close in energy but fundamentally different in orbital configuration. The singlet metal-radical ground state results from the antiferromagnetic coupling between the 3d(yz) orbital of the Ni III ion (S(M) = 1/2) and the pi(b) orbital of the benzosemiquinone-type radical ligand (S(L) = 1/2), which have a large overlap and thus strong covalent bonding. The triplet metal-radical excited state involves the ferromagnetic coupling between the Ni III 3d(zx) orbital and the benzosemiquinone-type pi(b) orbital, which are orthogonal to each other. The singlet and triplet states of the nickel III pi-cation radical complex possess characteristic quinoid-type short-long-short alternating sequence of C-C bonds in the benzene ring, as well as intense MLCT transitions in the VIS (lambda max = 664 nm) and NIR (lambda max = 884 nm) regions, respectively.     

Interactions among Endothelial Nitric Oxide Synthase,Cardiovascular System,and Nociception during Physiological and Pathophysiological States

Nitric oxide synthase (NOS) plays important roles within the cardiovascular system in physiological states as well as in pathophysiologic and specific cardiovascular (CV) disease states, such as hypertension (HTN), arteriosclerosis, and cerebrovascular accidents. This review discusses the roles of the endothelial NOS (eNOS) and its effect on cardiovascular responses that are induced by nociceptive stimuli. The roles of eNOS enzyme in modulating CV functions while experiencing pain will be discussed. Nociception, otherwise known as the subjective experience of pain through sensory receptors, termed “nociceptors”, can be stimulated by various external or internal stimuli. In turn, events of various cascade pathways implicating eNOS contribute to a plethora of pathophysiological responses to the noxious pain stimuli. Nociception pathways involve various regions of the brain and spinal cord, including the dorsolateral periaqueductal gray matter (PAG), rostral ventrolateral medulla (RVLM), caudal ventrolateral medulla, and intermediolateral column of the spinal cord. These pathways can interrelate in nociceptive responses to pain stimuli. The alterations in CV responses that affect GABAergic and glutamatergic pathways will be discussed in relation to mechanical and thermal (heat and cold) stimuli. Overall, this paper will discuss the aggregate recent and past data regarding pain pathways and the CV system.     

Pillared salicylaldehyde derivatives as building blocks for the design of cofacial salen-type ligands

The syntheses of pillared salicylaldehyde derivatives on a naphthalene block are reported. These compounds can be considered as building blocks for the elaboration of cofacial salen-typed ligands. The X-ray structure of a dinuclear manganese(III) complex is described, supporting our strategy to build topologically controlled multinuclear metal complexes.     

Hydroxide ion versus chloride and methoxide as an exogenous ligand reveals the influence of hydrogen bonding with second-sphere coordination water molecules in the electron transfer kinetics of Mn complexes

We recently reported on the synthesis of a new pentadentate N(4)O ligand, tBuL(-), together with the X-ray diffraction structure of the corresponding mononuclear manganese(III)-hydroxo complex namely [(tBuL)Mn(III)OH](ClO(4)), (1 (ClO(4))). [El Ghachtouli et al. Energy Environ. Sci. 2011, 4, 2041.] In the present work, we evidence through electrochemical analysis that complex 1(+), in the presence of water, shows a peculiar behavior toward electron-transfer kinetics. The synthesis, single-crystal X-ray diffraction, and EPR spectroscopic characterization of two other mononuclear manganese(III)-chlorido and methoxo complexes-namely, [(tBuL)Mn(III)Cl](PF(6)), (2(PF(6))) and [(tBuL)Mn(III)OMe](ClO(4)), (3(ClO(4)))-are also reported. 2(PF(6)) and 3(ClO(4)) compounds will serve as reference complexes for the electron-transfer kinetics investigation. The peculiar behavior of 1(ClO(4)) is attributed to the specificity of hydroxide anion as ligand presumably allowing intermolecular hydrogen-bonding interactions and thus affecting electron-transfer properties.     

A hybrid strategy for correcting geometric distortion in echo-planar images

A hybrid strategy for geometric distortion correction of echo-planar images is demonstrated. This procedure utilizes standard field mapping for signal displacement correction and the so-called reverse gradient acquisition for signal intensity correction. (The term reverse gradient refers to an acquisition of two sets of echo-planar images with phase encoding gradients of opposite polarity.) The hybrid strategy is applied to human brain echo-planar images acquired with and without diffusion-weighting. A comparison of the hybrid distortion corrected images to those corrected with standard field mapping only demonstrates much better performance of the hybrid method. A variant of the hybrid method is also demonstrated which requires the acquisition of only one pair of opposite polarity images within a set of images.