Mimicking the CO-Bound State of the [Ni,Fe]-CO Dehydrogenase.

Complexes, where a doubly reduced CO (carbonite) ligand is spanned between a nickel(II) centre and a transition metal(II) ion (TM=Fe, Co, Zn) have been accessed. In non-coordinating solvents the carbonite ligand exhibits a flexible coordination behaviour as observed by NMR spectroscopy and supported by DFT calculations. In particular the [Ni-CO-Fe] representative replicates the respective entity in an intermediate formed during CO-conversion by the enzyme [Ni,Fe]-CODH in many ways (structure, spectroscopic properties, reactivity).

THz-EPR-based Magneto-Structural Correlations for Cobalt(II) Single-Ion Magnets With Bis-Chelate Coordination.

New cobalt(II)-based complexes with [NO] coordination formed by two bis-chelate ligands were synthesized and characterized by a multi-technique approach. The complexes possess an easy-axis anisotropy (D<0) and magnetic measurements show a field-induced slow relaxation of magnetization. The spin-reversal barriers, i.

A Bioinspired Nonheme Fe-(O)-Cu Complex with an = 1 Ground State.

Cytochrome oxidase (CcO) is a heme copper oxidase (HCO) that catalyzes the natural reduction of oxygen to water. A profound understanding of some of the elementary steps leading to the intricate 4e/4H reduction of O is presently lacking. A total spin = 1 Fe-(O)-Cu () intermediate is proposed to reduce the overpotentials associated with the reductive O-O bond rupture by allowing electron transfer from a tyrosine moiety without the necessity of any spin-surface crossing.

Trapping of a phenoxyl radical at a non-haem high-spin iron(II) centre.

The activation of dioxygen at haem and non-haem metal centres, and subsequent functionalization of unactivated C‒H bonds, has been a focal point of much research. In iron-mediated oxidation reactions, O binding at an iron(II) centre is often accompanied by an oxidation of the iron centre. Here we demonstrate dioxygen activation by sodium tetraphenylborate and protons in the presence of an iron(II) complex to form a reactive radical species, whereby the iron oxidation state remains unaltered in the presence of a highly oxidizing phenoxyl radical and O.

Kink distortion of the pseudo- axis in pseudotetrahedral [NO] bis-chelate cobalt(II) single-ion magnets leads to increased magnetic anisotropy.

Two new mononuclear cobalt(II) complexes with the general formula [Co(L)] (1 and 2) were synthesized using bidentate Schiff base ligands with NO donor set, namely, 2-(benzothiazole-2-ylimino)methyl-5-(diethylamino)phenol (HL1) and its methyl substituted derivative 2-(6-methylbenzothiazole-2-ylimino)methyl-5-(diethylamino)phenol (HL2). X-ray structure analysis reveals a distorted pseudotetrahedral coordination sphere at the cobalt(II) ion, that cannot be described by a simple twisting of the two ligand chelate planes with respect to each other, which would imply a rotation about the pseudo- axis of the complex. Such a pseudo-rotation axis would approximately be colinear with the two vectors defined by the cobalt ion and the two centroids of the chelate ligands, where the angle between the two vectors would be 180° in an ideal pseudotetrahedral arrangement.

Weakening the Interchain Interactions in One Dimensional Cobalt(II) Coordination Polymers by Preventing Intermolecular Hydrogen Bonding.

The reaction of Co(NCS) with -methylaniline leads to the formation of [Co(NCS)(-methylaniline)] (), in which the cobalt(II) cations are octahedrally coordinated and linked into linear chains by pairs of thiocyanate anions. In contrast to [Co(NCS)(aniline)] () reported recently, in which the Co(NCS) chains are linked by strong interchain N-H···S hydrogen bonding, such interactions are absent in . Computational studies reveal that the cobalt(II) ions in compound show an easy-axis anisotropy that is lower than in , but with the direction of the easy axis being similar in both compounds.

Experimental assignment of long-range magnetic communication through Pd & Pt metallophilic contacts.

Record-breaking magnetic exchange interactions have previously been reported for 3d-metal dimers of the form [M(Pt(SAc))(pyNO)] (M = Ni or Co) that are linked in the solid state metallophilic Pt⋯Pt bridges. This contrasts the terminally capped monomers [M(Pt(SAc))(py)], for which neither metallophilic bridges nor magnetic exchange interactions are found. Computational modeling has shown that the magnetic exchange interaction is facilitated by the pseudo-closed shell d⋯d metallophilic interaction between the filled Pt 5d orbitals.

A New Thiolate-Bound Dimanganese Cluster as a Structural and Functional Model for Class Ib Ribonucleotide Reductases.

In class Ib ribonucleotide reductases (RNRs) a dimanganese(II) cluster activates superoxide (O ⋅ ) rather than dioxygen (O ), to access a high valent Mn -O -Mn species, responsible for the oxidation of tyrosine to tyrosyl radical. In a biomimetic approach, we report the synthesis of a thiolate-bound dimanganese complex [Mn (BPMT)(OAc) ](ClO) (BPMT=(2,6-bis{[bis(2-pyridylmethyl)amino]methyl}-4-methylthiophenolate) (1) and its reaction with O ⋅ to form a [(BPMT)MnO Mn] complex 2. Resonance Raman investigation revealed the presence of an O-O bond in 2, while EPR analysis displayed a 16-line S =1/2 signal at g=2 typically associated with a Mn Mn core, as detected in class Ib RNRs.

Spectroscopic Properties of a Biologically Relevant [Fe (μ-O) ] Diamond Core Motif with a Short Iron-Iron Distance.

Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe-Fe distances of 2.5-3.

Magnetic Anisotropy and Relaxation of Pseudotetrahedral [N O ] Bis-Chelate Cobalt(II) Single-Ion Magnets Controlled by Dihedral Twist Through Solvomorphism.

The methanol solvomorph 1 ⋅ 2MeOH of the cobalt(II) complex [Co(L ) ] (1) with the sterically demanding Schiff-base ligand 2-(([1,1'-biphenyl]-2-ylimino)methyl)phenol (HL ) shows the thus far largest dihedral twist distortion between the two chelate planes compared to an ideal pseudotetrahedral arrangement. The cobalt(II) ion in 1 ⋅ 2MeOH exhibits an easy-axis anisotropy leading to a spin-reversal barrier of 55.3 cm , which corresponds to an increase of about 17 % induced by the larger dihedral twist compared to the solvent-free complex 1.

Evidence of Sulfur Non-Innocence in [Co (dithiacyclam)] -Mediated Catalytic Oxygen Reduction Reactions.

In many metalloenzymes, sulfur-containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S-ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four-nitrogen bound [Co(cyclam)] (1; cyclam=1,5,8,11-tetraaza-cyclotetradecane) and the S-containing analog [Co(S N -cyclam)] (2; S N -cyclam=1,8-dithia-5,11-diaza-cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere.

Antisymmetric Spin Exchange in a μ-1,2-Peroxodicopper(II) Complex with an Orthogonal Cu-O-O-Cu Arrangement and = 1 Spin Ground State Characterized by THz-EPR.

A unique type of Cu/O adduct with orthogonal (close to 90°) Cu-O-O-Cu arrangement has been proposed for initial stages of O binding at biological type III dicopper sites, and targeted ligand design has now allowed us to emulate such an adduct in a pyrazolate-based μη η-peroxodicopper(II) complex () with Cu-O-O-Cu torsion φ of 87°, coined intermediate. Full characterization of , including X-ray diffraction ( = 1.452 Å) and Raman spectroscopy (ν̃ = 807 cm), completes a series of closely related Cu/O intermediates featuring μη η-peroxodicopper(II) cores with φ ranging from 55° (, -peroxo ; Brinkmeier A.

Sequential Hydrogen Tunneling in o-Tolylmethylene.

o-Tolylmethylene 1 is a metastable triplet carbene that rearranges to o-xylylene 2 even at temperatures as low as 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopical techniques were used to identify two conformers of 1 (anti and syn) in noble gas matrices and in frozen organic solutions.

Single-Ion Magnetic Behaviour in an Iron(III) Porphyrin Complex: A Dichotomy Between High Spin and 5/2-3/2 Spin Admixture.

A mononuclear iron(III) porphyrin compound exhibiting unexpectedly slow magnetic relaxation, which is a characteristic of single-ion magnet behaviour, is reported. This behaviour originates from the close proximity (≈550 cm ) of the intermediate-spin S=3/2 excited states to the high-spin S=5/2 ground state. More quantitatively, although the ground state is mostly S=5/2, a spin-admixture model evidences a sizable contribution (≈15 %) of S=3/2 to the ground state, which as a consequence experiences large and positive axial anisotropy (D=+19.

Influence of the Coligand onto the Magnetic Anisotropy and the Magnetic Behavior of One-Dimensional Coordination Polymers.

Reaction of Co(NCS) with different coligands leads to the formation of three compounds with the general composition [Co(NCS)(L)] (L = aniline (), morpholine (), and ethylenethiourea ()). In all of these compounds the cobalt(II) cations are octahedrally coordinated by two thiocyanate N and S atoms and the apical donor atoms of the coligands and are linked into linear chains by pairs of anionic ligands. The magnetic behavior was investigated by a combination of static and dynamic susceptibility as well as specific-heat measurements, computational studies, and THz-EPR spectroscopy.

Variation of the Chain Geometry in Isomeric 1D Co(NCS) Coordination Polymers and Their Influence on the Magnetic Properties.

Two different isomers of [Co(NCS)(4-chloropyridine)] ( and ) were synthesized from solution and by thermal decomposition of Co(NCS)(4-chloropyridine)(HO) (), which show a different metal coordination leading to corrugated chains in and to linear chains in . Solvent mediated conversion experiments prove that is thermodynamically stable at room temperature where is metastable. Magnetic measurements reveal that the magnetic exchange in is comparable to that observed for previously reported related chain compounds, whereas in with corrugated chains, the ferromagnetic interaction within the chains is strongly suppressed.

Single-Chain Magnet Based on Cobalt(II) Thiocyanate as XXZ Spin Chain.

Invited for the cover of this issue is the group of Michał Rams at Jagiellonian University (Kraków, Poland) and colleagues at Christian-Albrechts-Universität zu Kiel, Friedrich-Schiller-Universität Jena, and Helmholtz-Zentrum Berlin. The image represents a 1D coordination polymer with Co(II) spins that are flipped by photons during an EPR experiment. Read the full text of the article at 10.

Single-Chain Magnet Based on Cobalt(II) Thiocyanate as XXZ Spin Chain.

The cobalt(II) in [Co(NCS) (4-methoxypyridine) ] are linked by pairs of thiocyanate anions into linear chains. In contrast to a previous structure determination, two crystallographically independent cobalt(II) centers have been found to be present. In the antiferromagnetic state, below the critical temperature (T =3.

A Two-Coordinate Iron(II) Imido Complex with NHC Ligation: Synthesis, Characterization, and Its Diversified Reactivity of Nitrene Transfer and C-H Bond Activation.

Iron terminal imido species are typically implicated as reaction intermediates in iron-catalyzed transformations. While a large body of work has been devoted to mid- and high-valent iron imidos, to date the chemistry of iron(II) imidos has remained largely unexplored due to the difficulty in accessing them. Herein, we present a study on the two-coordinate iron(II) imido complex [(IPr)Fe(NAr)] (3; IPr = 1,3-bis(2',6'-diisopropylphenyl)imidazol-2-ylidene; Ar = 2,6-bis(2',4',6'-triisopropylphenyl)phenyl) prepared from the reaction of an iron(0) complex with the bulky azide ArN.

Light-controlled switching of the spin state of iron(III).

Controlled switching of the spin state of transition metal ions, particularly of Fe and Fe, is a prerequisite to achieve selectivity, efficiency, and catalysis in a number of metalloenzymes. Here we report on an iron(III) porphyrin with a photochromic axial ligand which, upon irradiation with two different wavelengths reversibly switches its spin state between low-spin (S = /) and high-spin (S = /) in solution (DMSO-acetone, 2:598). The switching efficiency is 76% at room temperature.

Reactions of Cyclopentadienylidenes with CFI: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom.

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CFI has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CFI at 3 K. Bimolecular reactions between the carbenes and CFI were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species.

The First State in the Catalytic Cycle of the Water-Oxidizing Enzyme: Identification of a Water-Derived μ-Hydroxo Bridge.

Nature's water-splitting catalyst, an oxygen-bridged tetramanganese calcium (MnOCa) complex, sequentially activates two substrate water molecules generating molecular O. Its reaction cycle is composed of five intermediate (S) states, where the index i indicates the number of oxidizing equivalents stored by the cofactor. After formation of the S state, the product dioxygen is released and the cofactor returns to its lowest oxidation state, S.

Light and Temperature Control of the Spin State of Bis(p-methoxyphenyl)carbene: A Magnetically Bistable Carbene.

Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon matrices by IR, UV-vis, and X-band EPR spectroscopy and in MTHF glasses by W-band EPR and Q-band ENDOR spectroscopy.

Removal of Ca(2+) from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn4O5 Core Structure: A Polarized Mn X-ray Absorption Spectroscopy Study.

Ca(2+)-depleted and Ca(2+)-reconstituted spinach photosystem II was studied using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca(2+) ion in the Mn4O5Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca(2+)-depleted S1 (S1') and S2 (S2') states, the S2'YZ(•) state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca(2+)-reconstituted S1 state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism.