Introduction to Electron Paramagnetic Resonance

Title

Conformational Flexibility of Hybrid [3]- and [4]-Rotaxanes

Abstract

The synthesis, structures, and properties of [4]- and [3]-rotaxane complexes are reported where [2]-rotaxanes, formed from heterometallic {Cr7Ni} rings, are bound to a fluoride-centered {CrNi2} triangle. The compounds have been characterized by single-crystal X-ray diffraction and have the formulas [CrNi2(F)(O2Ct Bu)6]{(BH)[Cr7NiF8(O2Ct Bu)16]}3 (3) and [CrNi2(F)(O2Ct Bu)6(THF)]{(BH)[Cr7NiF8(O2Ct Bu)16]}2 (4), where B = pyCH2CH2NHCH2C6H4SCH3. The [4]-rotaxane 3 is an isosceles triangle of three [2]- rotaxanes bound to the central triangle while the [3]-rotaxane 4 contains only two [2]- rotaxanes bound to the central triangle. Studies of the behaviour of 3 and 4 in solution by small-angle X-ray scattering and atomistic molecular dynamic simulations show that the structure of 3 is similar to that found in the crystal but that 4 has a different conformation to the crystal. Continuous wave and pulsed electron paramagnetic resonance spectroscopy was used to study the structures present and demonstrate that in frozen solutions (at 5 K) 4 forms more extended molecules than 3 and with a wider range of conformations

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Title:

In situ NMR metrology reveals reaction mechanisms in redox flow batteries

Abstract:

Large-scale energy storage is becoming increasingly critical to balancing renewable energy production and consumption. Organic redox flow batteries, made from inexpensive and sustainable redox-active materials, are promising storage technologies that are cheaper and less environmentally hazardous than vanadium based batteries, but they have shorter lifetimes and lower energy density. Thus, fundamental insight at the molecular level is required to improve performance. Here we report two in situ nuclear magnetic resonance (NMR) methods of studying redox flow batteries, which are applied to two redox-active electrolytes: 2,6-dihydroxyanthraquinone (DHAQ) and 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy) dibutyrate (DBEAQ). In the first method, we monitor the changes in the 1 H NMR shift of the liquid electrolyte as it flows out of the electrochemical cell. In the second method, we observe the changes that occur simultaneously in the positive and negative electrodes in the full electrochemical cell. Using the bulk magnetization changes (observed via the 1 H NMR shift of the water resonance) and the line broadening of the 1 H shifts of the quinone resonances as a function of the state of charge, we measure the potential differences of the two single-electron couples, identify and quantify the rate of electron transfer between the reduced and oxidized species, and determine the extent of electron delocalization of the unpaired spins over the radical anions. These NMR techniques enable electrolyte decomposition and battery self-discharge to be explored in real time, and show that DHAQ is decomposed electrochemically via a reaction that can be minimized by limiting the voltage used on charging. We foresee applications of these NMR methods in understanding a wide range of redox processes in flow and other electrochemical systems.

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Title:

Isolation and electronic structures of derivatized manganocene, ferrocene and cobaltocene anions

Abstract:

The discovery of ferrocene nearly 70 years ago marked the genesis of metallocene chemistry. Although the ferrocenium cation was discovered soon afterwards, a derivatized ferrocenium dication was only isolated in 2016 and the monoanion of ferrocene has only been observed in low-temperature electrochemical studies. Here we report the isolation of a derivatized ferrocene anion in the solid state as part of an isostructural family of 3d metallocenates, which consist of anionic complexes of a metal centre (manganese, iron or cobalt) sandwiched between two bulky Cpttt ligands (where Cpttt is {1,2,4-C5H2 t Bu3}). These thermally and air-sensitive complexes decompose rapidly above −30 °C; however, we were able to characterize all metallocenates by a wide range of physical techniques and ab initio calculations. These data have allowed us to map the electronic structures of this metallocenate family, including an unexpected high-spin S= 3/2 ground state for the 19e− derivatized ferrocene anion.

• Read the full article at Nature Chemistry

Title:

Electro-reduction of carbon dioxide at low over-potential at a metal–organic framework decorated cathode

Abstract:

The discovery of ferrocene nearly 70 years ago marked the genesis of metallocene chemistry. Although the ferrocenium cation was discovered soon afterwards, a derivatized ferrocenium dication was only isolated in 2016 and the monoanion of ferrocene has only been observed in low-temperature electrochemical studies. Here we report the isolation of a derivatized ferrocene anion in the solid state as part of an isostructural family of 3d metallocenates, which consist of anionic complexes of a metal centre (manganese, iron or cobalt) sandwiched between two bulky Cpttt ligands (where Cpttt is {1,2,4-C5H2 t Bu3}). These thermally and air-sensitive complexes decompose rapidly above −30 °C; however, we were able to characterize all metallocenates by a wide range of physical techniques and ab initio calculations. These data have allowed us to map the electronic structures of this metallocenate family, including an unexpected high-spin S= 3/2 ground state for the 19e− derivatized ferrocene anion.

• Read the full article at Nature Communications

Title:

Mechanistic basis of substrate–O2 coupling within a chitin-active lytic polysaccharide monooxygenase: An integrated NMR/EPR study

Abstract:

Lytic polysaccharide monooxygenases (LPMOs) have a unique ability to activate molecular oxygen for subsequent oxidative cleavage of glycosidic bonds. To provide insight into the mode of action of these industrially important enzymes, we have performed an integrated NMR/electron paramagnetic resonance (EPR) study into the detailed aspects of an AA10 LPMO–substrate interaction. Using NMR spectroscopy, we have elucidated the solutionphase structure of apo-BlLPMO10A from Bacillus licheniformis, along with solution-phase structural characterization of the Cu(I)-LPMO, showing that the presence of the metal has minimal effects on the overall protein structure. We have, moreover, used paramagnetic relaxation enhancement (PRE) to characterize Cu(II)-LPMO by NMR spectroscopy. In addition, a multifrequency continuous-wave (CW)-EPR and 15N-HYSCORE spectroscopy study on the uniformly isotope-labeled 63Cu(II)-bound 15N-BlLPMO10A along with its natural abundance isotopologue determined copper spin-Hamiltonian parameters for LPMOs to markedly improved accuracy. The data demonstrate that large changes in the Cu(II) spinHamiltonian parameters are induced upon binding of the substrate. These changes arise from a rearrangement of the copper coordination sphere from a five-coordinate distorted square pyramid to one which is four-coordinate near-square planar. There is also a small reduction in metal–ligand covalency and an attendant increase in the d(x2 −y2 ) character/energy of the singly occupied molecular orbital (SOMO), which we propose from density functional theory (DFT) calculations predisposes the copper active site for the formation of a stable Cu–O2 intermediate. This switch in orbital character upon addition of chitin provides a basis for understanding the coupling of substrate binding with O2 activation in chitin-active AA10 LPMOs.

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Title:

Synthesis of nitro compounds from nitrogen dioxide captured in a metal-organic framework

Abstract:

Increasing levels of air pollution are driving the need for the development of new processes that take “waste-to-chemicals”. Herein, we report the capture and conversion under ambient conditions of a major air pollutant, NO₂, using a robust metal-organic framework (MOF) material, Zr-bptc (H₄bptc = 3,3′,5,5′-biphenyltetracarboxylic acid), comprising {Zr₆(μ₃-O)₄(μ₃-OH)₄(COO)₁₂} clusters linked by 4-connected bptc^4– ligands in an ftw topology. At 298 K, Zr-bptc shows exceptional stability and adsorption of NO₂ at both low (4.9 mmol g^–1 at 10 mbar) and high pressures (13.8 mmol g^–1 at 1.0 bar), as measured by isotherm experiments. Dynamic breakthrough experiments have confirmed the selective retention of NO₂ by Zr-bptc at low concentrations under both dry and wet conditions. The immobilized NO₂ can be readily transformed into valuable nitro compounds relevant to construction, agrochemical, and pharmaceutical industries. In situ crystallographic and spectroscopic studies reveal strong binding interactions of NO₂ to the {Zr₆(μ₃-O)₄(μ₃-OH)₄(COO)₁₂} cluster node. This study paves a circular pathway to enable the integration of nitrogen-based air pollutants into the production of fine chemicals.

• Read the full article at JACS