138 research outputs found
Ligandenumlagerungen an Fe/S-Cofaktoren: langsame Isomerisierung eines biomimetischen [2Fe-2S]-Clusters
No full textElectrocatalytic Ammonia Oxidation with a Highly Preorganized Metal-Metal Cooperative Diruthenium Complex
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Organometallic μ-Nitridodiiron Complexes in Oxidation States Ranging from (III/III) to (IV/IV)
No full textLigand Rearrangements at Fe/S Cofactors: Slow Isomerization of a Biomimetic [2Fe-2S] Cluster
No full textLigand exchange plays an important role in the biogenesis of Fe/S clusters, most prominently during cluster transfer from a scaffold protein to its target protein. Although in vivo and in vitro studies have provided some insight into this process, the microscopic details of the ligand exchange steps are mostly unknown. In this work, the kinetics of the ligand rearrangement in a biomimetic [2Fe-2S] cluster with mixed S/N capping ligands have been studied. Two geometrical isomers of the cluster are present in solution, and mechanistic insight into the isomerization process was obtained by variable-temperature H-1 NMR spectroscopy. Combined experimental and computational results reveal that this is an associative process that involves the coordination of a solvent molecule to one of the ferric ions. The cluster isomerizes at least two orders of magnitude faster in its protonated and mixed-valent states. These findings may contribute to a deeper understanding of cluster transfer and sensing processes occurring in Fe/S cluster biogenesis
Bis(pyrazolato) Bridged Diiron Complexes: Ferromagnetic Coupling in a Mixed‐Valent HS‐Fe<sup>II</sup>/LS‐Fe<sup>III</sup> Dinuclear Complex
No full textUsing a new bis(tridentate) compartmental pyrazolate‐centered ligand HL, the bis(pyrazolato)‐bridged diiron complex [L2FeII2][OTf]2 (1) and its singly oxidized mixed‐valent congener [L2FeIIFeIII][OTf]3 (2) have been synthesized and structurally characterized. While 1 features two HS‐FeII ions coordinated to two cis‐axial pyridine moieties in a highly distorted octahedral environment, the metal ions in 2 are coordinated by the ligand strand in a trans‐axial configuration. Very different Fe–N bond lengths and distinctly different coordination polyhedra are associated with pronounced valence localization in the case of 2. The electronic structures and magnetic properties of 1 and 2 have been further investigated by Mössbauer spectroscopy and variable temperature magnetic susceptibility measurements. In the case of 1, weak antiferromagnetic coupling is observed between the two HS‐FeII ions (J = –0.6 cm–1), while the HS‐FeII and LS‐FeIII ions in 2 are ferromagnetically coupled (J = +5.2 cm–1) to give an ST = 5/2 ground state with significant zero‐field splitting (DFe(II) = 2.3 cm–1). The findings are rationalized with the help of DFT computations
A μ‐Phosphido Diiron Dumbbell in Multiple Oxidation States
No full textReaction of the ferrous complex [LFe(NCMe)2](OTf)2 (1) containing a macrocyclic tetracarbene as ligand with Na(OCP) generates OCP− ligated complex [LFe(PCO)(CO)]OTf (2) together with dinuclear μ‐phosphido complex [(LFe)2P](OTf)3 (3) featuring an unprecedented linear Fe‐(μ‐P)‐Fe motif and a "naked" P‐atom bridge that appears at +1480 ppm in the 31P NMR spectrum. 3 is shown to exhibit rich redox chemistry, and both singly and doubly oxidized species 4 and 5 could be isolated and fully characterized. X‐ray crystallography, UV‐vis, EPR and 57Fe Mößbauer spectroscopies in combination with DFT computations provide a comprehensive electronic structure description and evidence that the Fe‐(μ‐P)‐Fe core is highly covalent and structurally invariant over the series of oxidation states that are formally described as ranging from FeIIIFeIII to FeIVFeIV. 3 – 5 now add a higher homologue set of complexes to many systems with Fe‐(μ‐O)‐Fe and Fe‐(μ‐N)‐Fe core structures that are prominent in bioinorganic chemistry and catalysis
Nonclassical Single-State Reactivity of an Oxo-Iron(IV) Complex Confined to Triplet Pathways
No full textC-H bond activation mediated by oxo-iron (IV) species represents the key step of many heme and nonheme O-2-activating enzymes. Of crucial interest is the effect of spin state of the Fe-IV(O) unit. Here we report the C-H activation kinetics and corresponding theoretical investigations of an exclusive tetracarbene ligated oxo-iron(IV) complex, [(LFeIV)-Fe-NHC(O)(MeCN)](2+) (1). Kinetic traces using substrates with bond dissociation energies (BDEs) up to 80 kcal mol(-1) show pseudo-first-order behavior and large but temperature-dependent kinetic isotope effects (KIE 32 at -40 degrees C). When compared with a topologically related oxo-iron(IV) complex bearing an equatorial N-donor ligand, [(LFeIV)-Fe-TMC(O) (MeCN)](2+) (A), the tetracarbene complex 1 is significantly more reactive with second order rate constants k'(2) that are 2-3 orders of magnitude higher. UV-vis experiments in tandem with cryospray mass spectrometry evidence that the reaction occurs via formation of a hydroxo-iron(III) complex (4) after the initial H atom transfer (HAT). An extensive computational study using a wave function based multireference approach, viz. complete active space self-consistent field (CASSCF) followed by N-electron valence perturbation theory up to second order (NEVPT2), provided insight into the HAT trajectories of 1 and A. Calculated free energy barriers for 1 reasonably agree with experimental values. Because the strongly donating equatorial tetracarbene pushes the Fe-d(x2-y2) orbital above d(z2), 1 features a dramatically large quintet-triplet gap of similar to 18 kcal/mol compared to similar to 2-3 kcal/mol computed for A. Consequently, the HAT process performed by 1 occurs on the triplet surface only, in contrast to complex A reported to feature two-state-reactivity with contributions from both triplet and quintet states. Despite this, the reactive Fe-IV(O) units in 1 and A undergo the same electronic-structure changes during HAT. Thus, the unique complex 1 represents a pure "triplet-only" ferryl model
A μ‐Phosphido Diiron Dumbbell in Multiple Oxidation States
No full textThe reaction of the ferrous complex [LFe(NCMe)2](OTf)2 (1), which contains a macrocyclic tetracarbene as ligand (L), with Na(OCP) generates the OCP−‐ligated complex [LFe(PCO)(CO)]OTf (2) together with the dinuclear μ‐phosphido complex [(LFe)2P](OTf)3 (3), which features an unprecedented linear Fe‐(μ‐P)‐Fe motif and a “naked” P‐atom bridge that appears at δ=+1480 ppm in the 31P NMR spectrum. 3 exhibits rich redox chemistry, and both the singly and doubly oxidized species 4 and 5 could be isolated and fully characterized. X‐ray crystallography, spectroscopic studies, in combination with DFT computations provide a comprehensive electronic structure description and show that the Fe‐(μ‐P)‐Fe core is highly covalent and structurally invariant over the series of oxidation states that are formally described as ranging from FeIIIFeIII to FeIVFeIV. 3–5 now add a higher homologue set of complexes to the many systems with Fe‐(μ‐O)‐Fe and Fe‐(μ‐N)‐Fe core structures that are prominent in bioinorganic chemistry and catalysis
Ambiphilicity of a mononuclear cobalt(III) superoxo complex
No full textAddition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1
Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene–Oxoiron(IV) Complex
No full textIn biology, high
valent oxo–iron(IV) species have been shown
to be pivotal intermediates for functionalization of C–H bonds
in the catalytic cycles of a range of O2-activating iron
enzymes. This work details an electronic-structure investigation of
[FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene,
complex 1) using helium tagging infrared photodissociation
(IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy,
coupled with DFT and highly correlated wave function based multireference
calculations. The IRPD spectrum of complex 1 reveals
the Fe–O stretching vibration at 832 ± 3 cm–1. By analyzing the Franck–Condon progression, we can determine
the same vibration occurring at 616 ± 10 cm–1 in the E(dxy → dxz,yz) excited state. Both values are similar to those
measured for [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane).
The low-temperature MCD spectra of complex 1 exhibit
three pseudo A-term signals around 12 500,
17 000, and 24 300 cm–1. We can unequivocally
assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2‑y2, respectively, through direct calculations of MCD spectra
and independent determination of the MCD C-term signs
from the corresponding electron donating and accepting orbitals. In
comparison with the corresponding transitions observed for [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine),
the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation
energy for dxz,yz → dx2‑y2 is significantly higher
(∼12 000 cm–1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that
the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2‑y2 orbital to eventually lift it above dz2. As a consequence, this unusual electron configuration leads to
an unprecedentedly larger quintet–triplet energy separation
for complex 1, which largely rules out the possibility
that the H atom transfer reaction may take place on the quintet surface
and hence quenches two-state reactivity. The resulting mechanistic
implications are discussed
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