What Can We Learn from a Biomimetic Model of Nature's Oxygen-Evolving Complex?

A recently reported synthetic complex with a Mn4CaO4 core represents a remarkable structural mimic of the Mn4CaO5 cluster in the oxygen-evolving complex (OEC) of photosystem II (Zhang et al., Science 2015, 348, 690). Oxidized samples of the complex show electron paramagnetic resonance (EPR) signals at g ≈ 4.9 and 2, similar to those associated with the OEC in its S2 state (g ≈ 4.1 from an S = 5/2 form and g ≈ 2 from an S = 1/2 form), suggesting similarities in the electronic as well as geometric structure. We use quantum-chemical methods to characterize the synthetic complex in various oxidation states, to compute its magnetic and spectroscopic properties, and to establish connections with reported data. Only one energetically accessible form is found for the oxidized "S2 state" of the complex. It has a ground spin state of S = 5/2, and EPR simulations confirm it can be assigned to the g ≈ 4.9 signal. However, no valence isomer with an S = 1/2 ground state is energetically accessible, a conclusion supported by a wide range of methods, including density matrix renormalization group with full valence active space. Alternative candidates for the g ≈ 2 signal were explored, but no low-spin/low-energy structure was identified. Therefore, our results suggest that despite geometric similarities the synthetic model does not mimic the valence isomerism that is the hallmark of the OEC in its S2 state, most probably because it lacks a coordinatively flexible oxo bridge. Only one of the observed EPR signals can be explained by a structurally intact high-spin one-electron-oxidized form, while the other originates from an as-yet-unidentified rearrangement product. Nevertheless, this model provides valuable information for understanding the high-spin EPR signals of both the S1 and S2 states of the OEC in terms of the coordination number and Jahn-Teller axis orientation of the Mn ions, with important consequences for the development of magnetic spectroscopic probes to study S-state intermediates immediately prior to O-O bond formation.Financial support by the Max Planck Society and by Project MANGAN (03EK3545) funded by the Bundesministeriums fü r Bildung und Forschung is gratefully acknowledged. N.C. acknowledges the support of the Australian Research Council (Grant FT140100834). D.A.P. acknowledges network support by the COST action CM1305 “Explicit Control Over SpinStates in Technology and Biochemistry (ECOSTBio)”

Charge-Transfer-Induced Magnetism in Mixed-Stack Complexes

Explanation of the ferromagnetic anomaly in two recently synthesized mixed-stack charge-transfer (CT) complexes (1) (HMTTF)­[Ni­(mnt)₂] (HMTTF = bis­(trimethylene)-tetrathiafulvalene, mnt = maleonitrile dithiolate) and (2) (ChSTF)­[Ni­(mnt)₂] (ChSTF = 2,3-cyclohexylenedithio-1,4-dithia-5,8-diselenafulvalene) is the cornerstone of this investigation. Because these systems are reported to achieve magnetic properties through charge transfer from the neutral organic donor to the neutral organometallic acceptor stack, their magnetic interaction is assessed through the charge-transfer energy and the spin densities on the concerned sites following one of our recent formalisms. The positive value of <i>J</i> obtained in this way is found to be in good agreement with that evaluated through ab<i> </i>initio and density functional theory (DFT). In DFT framework, broken symmetry (BS) approach is adopted to evaluate <i>J</i> using spin-projection technique. No overlap between singly occupied molecular orbitals (SOMOs) suggests a through-space ferromagnetic interaction between the donor and the acceptor in the ground state of the complexes. Apart from the ground state, the magnetic status of the molecules is studied by varying interlayer distance <i>d</i>, the extent of slippage (slipping distance <i>r</i>, <i>r</i>^/, and deviation angle α), and rotational angle θ, which play a crucial role in magneto-structural correlation. Furthermore, it is categorically observed that the ferromagnetic interaction reaches its zenith at minimum energy crystallographic stacking mode resulting in maximum value of coupling constant in the ground state

What Can We Learn from a Biomimetic Model of Nature’s Oxygen-Evolving Complex?

A recently reported synthetic complex with a Mn₄CaO₄ core represents a remarkable structural mimic of the Mn₄CaO₅ cluster in the oxygen-evolving complex (OEC) of photosystem II (Zhang et al., <i>Science</i> <b>2015</b>, 348, 690). Oxidized samples of the complex show electron paramagnetic resonance (EPR) signals at <i>g</i> ≈ 4.9 and 2, similar to those associated with the OEC in its <i>S</i>₂ state (<i>g</i> ≈ 4.1 from an <i>S</i> = ⁵/₂ form and <i>g</i> ≈ 2 from an <i>S</i> = ¹/₂ form), suggesting similarities in the electronic as well as geometric structure. We use quantum-chemical methods to characterize the synthetic complex in various oxidation states, to compute its magnetic and spectroscopic properties, and to establish connections with reported data. Only one energetically accessible form is found for the oxidized “<i>S</i>₂ state” of the complex. It has a ground spin state of <i>S</i> = ⁵/₂, and EPR simulations confirm it can be assigned to the <i>g</i> ≈ 4.9 signal. However, no valence isomer with an <i>S</i> = ¹/₂ ground state is energetically accessible, a conclusion supported by a wide range of methods, including density matrix renormalization group with full valence active space. Alternative candidates for the <i>g</i> ≈ 2 signal were explored, but no low-spin/low-energy structure was identified. Therefore, our results suggest that despite geometric similarities the synthetic model does not mimic the valence isomerism that is the hallmark of the OEC in its <i>S</i>₂ state, most probably because it lacks a coordinatively flexible oxo bridge. Only one of the observed EPR signals can be explained by a structurally intact high-spin one-electron-oxidized form, while the other originates from an as-yet-unidentified rearrangement product. Nevertheless, this model provides valuable information for understanding the high-spin EPR signals of both the <i>S</i>₁ and <i>S</i>₂ states of the OEC in terms of the coordination number and Jahn–Teller axis orientation of the Mn ions, with important consequences for the development of magnetic spectroscopic probes to study <i>S</i>-state intermediates immediately prior to O–O bond formation

Advancing insights towards electrocatalytic activity of La/ Ba-Sr-Co-Fe-O-based perovskites for oxygen reduction & evolution process in reversible solid oxide cell

Electrocatalytic activity of La/Ba-Sr-Co-Fe-O-based mixed ionic and electronically conducting (MIEC) perovskites has been studied for selective oxygen reduction (ORR) and evolution (OER) processes applicable in Reversible Solid Oxide Cell (R-SOC). XPS study establishes scavenging of oxygen vacancy in LSCF and generation of the same in BSCF. BSCF enables faster oxygen ion transport and is correlated with lower frontier molecular orbital (FMO) energy gap of 1.18 eV derived from density functional theory (DFT). Relatively higher AEFMO(Absolute) 1.75 eV in LSCF accounts for higher charge transfer. Amperometric measurements @800celcius in asymmetric cell configuration exhibit lowest time-dependent current loss of 0.019 mA.h-1 & 0.035 mA.h-1 for BSCF & LSCF under applied anodic (+0.8 V) and cathodic potentials (-0.8 V) for 200 h with respective surface resistances (Rs) of 0.19 l.cm2 and 0.081 l.cm2. H2 flux of 0.4Nl.h-1.cm-2 obtained with BSCF, establishes its effectivity as OER whereas LSCF is found to be more selective in ORR

Electrochemical Generation of High-Valent Oxo-Manganese Complexes Featuring an Anionic N5 Ligand and Their Role in O―O Bond Formation

Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help to understand the mechanism of O―O bond formation presumably occurring at the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscores the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species, [(dpaq)MnIV(O)]+ (2), through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability and could be reverted to the starting complex (1) by switching the potential to 0.25 V vs. NHE. The formation of 2 via PCET oxidation of 1 demonstrates an alternate pathway for the generation of the oxomanganese(IV) transient (2) without the requirement of redox-inactive metal ions or acid additives as proposed earlier. Theoretical studies predict that one-electron oxidation of [(dpaq)MnIV(O)]+ (2) forms a manganese(V)-oxo (3) species, which can be oxidized further by one-electron to a formally manganese(VI)-oxo transient (4). Theoretical analyses suggest that the first oxidation event (2 to 3) takes place at the metal-based d-orbital whereas, in the second oxidation process (3 to 4), the electron eliminates from an orbital composed of equitable contribution from metal and ligand, leaving a single electron in the quinoline-dominated orbital in the doublet ground spin state of the manganese(VI)-oxo species (4). This mixed metal- ligand (quinoline)-based oxidation is proposed to generate a formally Mn(VI) species (4), a non-heme analogue of the species ‘compound I’, formed in the catalytic cycle of cytochrome P-450. We propose that the highly electrophilic species 4 catches water during cyclic voltammetry experiments and results in O―O bond formation leading to electrocatalytic oxidation of water to hydrogen peroxide

Synthesis, crystal structure, Hirshfeld surface, and DFT studies of a Copper(II) complex of 5,5′-dimethyl-2,2′-bipyridine and 1,2,2-trimethylcyclopentane-1,3-dicarboxylic acid

A new metal-organic hybrid complex [Cu(5,5′-dmbipy) (D-cam) (H2O)]n (1), (5,5′-dmbipy = 5,5′-dimethyl-2,2′-bipyridine, D-cam = D-camphoric acid anion) was hydrothermally synthesized. This complex was characterized by FTIR spectroscopy, TGA, and single-crystal X-ray diffraction. Crystallographic studies show that the title complex 1 crystallizes in an orthorhombic system with a P212121 space group with a = 06.9518(05) Ǻ, b = 13.5516(13) Ǻ, c = 22.6380(02)Ǻ; V = 2132.7(3) Ǻ3. The title CuII complex adopts a square pyramidal configuration. DFT study and Hirshfeld topology analysis of complex 1 was also done. The crystal achieves its three-dimensional structure and stability through polymeric chains having helical motifs of arrangement in between moieties and interconnected through hydrogen bonding interactions between the apical water molecule and non-coordinated oxygen atoms of the D-cam2- ligands. TGA, DFT calculations and Hirshfeld topology analysis revealed that the title complex 1 was stable