Effect Of Spin State Research Articles

Computational Study of CO2 Reduction Catalyzed by Iron(I) Complex at Different Spin States: Cooperativity of Hydrogen Bonding and Auxiliary Group Effect

To explore alternative approaches to the CO2 reduction to formate and provide an insight into the spin state effect on the CO2 reduction, we theoretically designed a kind of low-valence iron(I) model complex, whose doublet, quartet, and sextet states are denoted as 2Fe(I), 4Fe(I), and 6Fe(I), respectively. This complex is featured with an iron(I) center, which bonds to a 1,2-ethanediamine (en) and a 2-hydroxy-biphenyl group. Reaction mechanisms for the CO2 reduction to formate catalyzed by this iron(I) model complex were explored using density functional theory (DFT) computations. Studies showed that the univalent iron(I) compound can efficiently fix and activate a CO2 molecule, whereas its oxidized forms with trivalent iron(III) or bivalent iron(II) cannot activate CO2. For the iron(I) compound, it was found that the lowest spin state 2Fe(I) is the most favorable for the CO2 reduction as the reactions barriers involving 2Fe(I), 4Fe(I), and 6Fe(I) are 25.6, 37.2, and 35.9 kcal/mol, respectively. Yet, a photosensitizer-free visible-light-mediated high–low spin shift from 4Fe(I) and 6Fe(I) to 2Fe(I) is likely through the reverse intersystem crossing (RIC) because the 4Fe(I) and 6Fe(I) compounds have strong absorption in the visible-light range. Notably, the synergistic interaction between the hydrogen bonding from the auxiliary hydroxyl group in the 2-hydroxy-biphenyl moiety to CO2 and an intermediate five-membered ring promotes the proton transfer, leading to the formation of the −COOH moiety from CO2 and the Fe–O bond. With the addition of H2, one H2 molecule is split by the Fe–O bond and thus serves as H atom sources for both the CO2 reduction and the recovery of the auxiliary hydroxyl group. The present theoretical study provides a novel solution for the challenging CO2 reduction, which calls for further experimental verifications.

Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System.

Bimetallic nitrogen (N2) splitting to form metal nitrides is an attractive method for N2 fixation. Although a growing number of pincer-supported systems can bind and split N2, the precise relationship between the ligand properties and N2 binding/splitting remains elusive. Here we report the first example of an N2-bridged rhenium(III) complex, [(trans-P2tBuPyrr)ReCl2]2(μ-η1:η1-N2) (P2tBuPyrr = [2,5-(CH2PtBu2)2C4H2N]-). In this case, N2 binding occurs at a higher oxidation level than that in other reported pincer analogues. Analysis of the electronic structure through computational studies shows that the weakly π-donor pincer ligand stabilizes an open-shell electronic configuration that leads to enhanced binding of N2 in the bridged complex. Utilizing SQUID magnetometry, we demonstrate a singlet ground state for this Re-N-N-Re complex, and we offer tentative explanations for antiferromagnetic coupling of the two local S = 1 sites. Reduction and subsequent heating of the rhenium(III)-dinitrogen complex leads to chloride loss and cleavage of the N-N bond with isolation of the terminal rhenium(V) nitride complex (P2tBuPyrr)ReNCl.

Electronic and Structural Properties of the Double Cubane Iron-Sulfur Cluster

The double-cubane cluster (DCC) refers to an [Fe8S9] iron-sulfur complex that is otherwise only known to exist in nitrogenases. Containing a bridging µ2-S ligand, the DCC in the DCC-containing protein (DCCP) is covalently linked to the protein scaffold via six coordinating cysteine residues. In this study, the nature of spin coupling and the effect of spin states on the cluster’s geometry are investigated computationally. Using density functional theory (DFT) and a broken symmetry (BS) approach to study the electronic ground state of the system, we computed the exchange interaction between the spin-coupled spins of the four FeFe dimers contained in the DCC. This treatment yields results that are in excellent agreement with both computed and experimentally determined exchange parameters for analogously coupled di-iron complexes. Hybrid quantum mechanical (QM)/molecular mechanical (MM) geometry optimizations show that cubane cluster A closest to charged amino acid side chains (Arg312, Glu140, Lys146) is less compact than cluster B, indicating that electrons of the same spin in a charged environment seek maximum separation. Overall, this study provides the community with a fundamental reference for subsequent studies of DCCP, as well as for investigations of other [Fe8S9]-containing enzymes.

Non-heme perferryl intermediates: Effect of spin state on the epoxidation enantioselectivity

• Chiral non-heme iron complexes catalyze epoxidation of olefins with H 2 O 2 in up to 96 % ee . • The active oxoiron(V) intermediates have been detected by EPR spectroscopy. • The Fe V =O intermediates adopt either low-spin ( S = 1/2) or high-spin ( S = 3/2) ground state. • The spin state is governed by the electron-donating ability of p -substituents at the pyridine rings. • The high-spin Fe V =O intermediates are more enantioselective than their low-spin counterparts. The catalyst systems Fe 2 (PDP NR1R2 ) 2 /H 2 O 2 /RCOOH (PDP = N , N′ -bis(pyridyl-2-methyl)-( S , S )-2,2′-bipyrrolidine, NR 1 R 2 = NMe 2 , NEt 2 , NMe i Pr or N(CH 2 ) 4 at the para -position in the pyridine rings, RCOOH = acetic or 2-ethylhexanoic acid) generate the high-spin ( S = 3/2) oxoiron(V) oxidizing intermediates with proposed structure [(PDP NR1R2 )Fe V =O(OC(O)R)] 2+ and the EPR spectrum g 1 , = 4.30, g 2 = 3.69, g 3 = 1.96. In contrast, the catalyst systems Fe 2 (PDP Me2OMe ) 2 /H 2 O 2 /RCOOH and Fe 2 (PDP MeOCH2CF3 ) 2 /H 2 O 2 /RCOOH, based on similar iron complexes, containing 3,5-Me 2 -4-OMe and 3-Me-4-OCH 2 CF 3 substituents at the pyridine rings, generate the low-spin ( S = 1/2) oxoiron(V) intermediates with characteristic EPR spectrum g 1 = 2.07, g 2 = 2.01, g 3 = 1.96. Both high and low-spin intermediates directly epoxidize cyclohexene, with the low-spin species being much more reactive than the high-spin congeners; the corresponding second-order rate constants have been evaluated. Crucially, the catalyst systems exhibiting the high-spin oxidizing intermediates demonstrate higher enantioselectivity in the epoxidation of prochiral olefinic substrates, compared with the catalyst systems featuring the low-spin intermediates. This enhanced enantioselectivity is explained in terms of attenuated reactivity of the high-spin intermediates, leading to more product-like transition state with tighter substrate-catalyst interactions.

Kβ X-Ray Emission Spectroscopic Study of a Second-Row Transition Metal (Mo) and Its Application to Nitrogenase-Related Model Complexes.

In recent years, X‐ray emission spectroscopy (XES) in the Kβ (3p‐1s) and valence‐to‐core (valence‐1s) regions has been increasingly used to study metal active sites in (bio)inorganic chemistry and catalysis, providing information about the metal spin state, oxidation state and the identity of coordinated ligands. However, to date this technique has been limited almost exclusively to first‐row transition metals. In this work, we present an extension of Kβ XES (in both the 4p‐1s and valence‐to‐1s [or VtC] regions) to the second transition row by performing a detailed experimental and theoretical analysis of the molybdenum emission lines. It is demonstrated in this work that Kβ2 lines are dominated by spin state effects, while VtC XES of a 4d transition metal provides access to metal oxidation state and ligand identity. An extension of Mo Kβ XES to nitrogenase‐relevant model complexes shows that the method is sufficiently sensitive to act as a spectator probe for redox events that are localized at the Fe atoms. Mo VtC XES thus has promise for future applications to nitrogenase, as well as a range of other Mo‐containing biological cofactors. Further, the clear assignment of the origins of Mo VtC XES features opens up the possibility of applying this method to a wide range of second‐row transition metals, thus providing chemists with a site‐specific tool for the elucidation of 4d transition metal electronic structure.

Kβ X‐Ray Emission Spectroscopic Study of a Second‐Row Transition Metal (Mo) and Its Application to Nitrogenase‐Related Model Complexes

AbstractIn recent years, X‐ray emission spectroscopy (XES) in the Kβ (3p‐1s) and valence‐to‐core (valence‐1s) regions has been increasingly used to study metal active sites in (bio)inorganic chemistry and catalysis, providing information about the metal spin state, oxidation state and the identity of coordinated ligands. However, to date this technique has been limited almost exclusively to first‐row transition metals. In this work, we present an extension of Kβ XES (in both the 4p‐1s and valence‐to‐1s [or VtC] regions) to the second transition row by performing a detailed experimental and theoretical analysis of the molybdenum emission lines. It is demonstrated in this work that Kβ2 lines are dominated by spin state effects, while VtC XES of a 4d transition metal provides access to metal oxidation state and ligand identity. An extension of Mo Kβ XES to nitrogenase‐relevant model complexes shows that the method is sufficiently sensitive to act as a spectator probe for redox events that are localized at the Fe atoms. Mo VtC XES thus has promise for future applications to nitrogenase, as well as a range of other Mo‐containing biological cofactors. Further, the clear assignment of the origins of Mo VtC XES features opens up the possibility of applying this method to a wide range of second‐row transition metals, thus providing chemists with a site‐specific tool for the elucidation of 4d transition metal electronic structure.

C-H Insertions by Iron Porphyrin Carbene: Basic Mechanism and Origin of Substrate Selectivity.

Recent experimental reports of heme carbene C-H insertions show promising results for sustainable chemistry due to good yield and selectivity, low cost of iron, and low/no toxicity of hemes. But mechanistic details are mostly unknown. Despite structural similarity and isoelectronic nature between heme carbene and the FeIV =O intermediate, our quantum chemical studies with detailed geometric and electronic information for the first time reveal an FeII -based, concerted, hydride-transfer mechanism, which is different from the FeIV -based stepwise hydrogen atom transfer mechanism for C-H functionalization by native heme enzymes. A trend of broad range experimental C-H insertion yields (0-88 %) of five different C-H bonds, including mostly non-functionalized moieties, was well reproduced. Results suggest that the substrate selectivity originates from the hydride formation capability. The predicted kinetic isotope effects were also in excellent agreement with experiment. Useful geometry, charge, and energy parameters well correlated with barriers were reported. These results provide the first theoretical evidence that carbene formation is the overall rate-limiting step, and suggest a key role of the formation of strong electrophilic heme carbene in developing heme-based C-H insertion catalysts and biocatalysts.

The Effects of Oxidation States, Spin States and Solvents on Molecular Structure, Stability and Spectroscopic Properties of Fe-Catechol Complexes: A Theoretical Study

In this study, in order to explain the solvent and spin state effects on the molecular structure of catechol-Fe complex [Fe(cat)3]n﹣ where n = 2 and 3, Hartree Fock (HF)-Density Functional Theory (DFT) hybrid calculations are performed at the B3LYP/6-311g(d,p) level of theory. The binding energies of Fe2+ and Fe3+ in high-spin state are higher than intermediate and low-spin states which show that the complex formation in a high spin state is more favorable. The calculated binding energies at different solvents indicate that the binding energies in polar solvents are lower than non-polar solvents. Furthermore, spectroscopic studies including FTIR and Raman spectrum in various solvents reveal that the formation of intermolecular bonds between the oxygen atom of carbonyl group and the hydrogen atom of solvent causes a spectral red shift. The calculated FTIR and geometry parameters are in good agreement with previous experimental data. Donor-acceptor interaction energies are evaluated due to the importance of the charge transfer in the complex formation. It is observed that the free electrons of oxygen atom interact with the antibonding orbitals of the iron. Finally, some correlations between the quantum chemical reactivity indices of the complexes and solvent polarity are considered. The study indicates a linear correlation between chemical hardness and binding energies of [Fe(cat)3]3﹣ complex.

Solvent and spin state effects on molecular structure, IR spectra, binding energies and quantum chemical reactivity indices of deferiprone–ferric complex: DFT study

In this work, in order to determine the solvent and spin state effects on the molecular structure of deferiprone–ferric complex ([Fe(DFP)3]), density functional theory (DFT) calculations were performed at the CAM-B3LYP/6-31G(d) level of the theory. The results indicate that the stability and binding energy of [Fe(DFP)3] in the polar solvents are higher and lower, respectively, than non-polar ones. The analysis of the spin state energies of the complexes showed that the corresponding stability constants increase with the spin states increment. The binding energies in high-spin state is higher than intermediate- and low-spin states which showed that the complex formation in a high-spin state is more favorable. Furthermore, the evaluation of the IR spectrum in the various solvents reveals that the intermolecular hydrogen bond formation between the oxygen atom of the carbonyl group and the hydrogen atom of the solvent causes a spectral red shift. Because of the importance of the charge transfer in the complex formation, donor–acceptor interaction energies were evaluated. Based on this analysis, lone pair electrons of the oxygen atom interact with the antibonding orbitals of the iron, more than the other possibilities. Finally, some probable correlations between the quantum chemical reactivity indices of the complexes and solvent polarity were considered. According to the obtained results, an enhancement in the dielectric constant of the solvent decreases the electrophilicity index while the electronic chemical hardness increases. Recent study indicates a linear correlation between chemical hardness and binding energies of [Fe(DFP)3] complex.

An atomically thin ferromagnetic half-metallic pyrazine-fused Mn-porphyrin sheet: a slow spin relaxation system

The effect of spin states (high or low) on magnetic anisotropy energy.

Spin-State Effects on the Thermal Dihydrogen Release from Solid-State [MH(η2-H2)dppe2]+ (M = Fe, Ru, Os) Organometallic Complexes for Hydrogen Storage Applications.

Mössbauer spectroscopy, experimental thermodynamic measurements, and computational studies were performed to investigate the properties of molecular hydrogen binding to the organometallic fragments [MHdppe2]+ (M = Fe, Ru, Os; dppe =1,2-bis(diphenylphosphino)ethane) to form the dihydrogen complex fragments [MH(η2-H2)dppe2]+. Mössbauer spectroscopy showed that the dehydrogenated complex [FeHdppe2]+ adopts a geometry consistent with the triplet spin state, transitioning to a singlet state complex upon addition of the dihydrogen molecule in a manner similar to the previously studied dinitrogen complexes. From simulations, this spin transition behavior was found to be responsible for the strong binding behavior experimentally observed in the iron complex. Spin-singlet to spin-singlet transitions were found to exhibit thermodynamics consistent with the 5d > 3d > 4d binding trend observed for other transition metal dihydrogen complexes. Finally, the method for distinguishing between dihydrogen and dihydride complexes based on partial quadrupole splittings observed in Mössbauer spectra was confirmed, providing a tool for further characterization of these unique species for Mössbauer active compounds.

Spin entropy enhancement tuned by spin state transition in La0.7−xYxCa0.3CoO3

The effect of spin states on the spin entropy has been studied in La0.7−xYxCa0.3CoO3 (x=0.0, 0.05, 0.1 and 0.15). Magnetothermopower results reveal that Y-doping improves the spin entropy of La0.7Ca0.3CoO3. The magnetic results indicate that the Co3+ ions undergo a transition from an intermediate spin state (t52ge1g; S=1) to a low spin state (t62ge0g; S=0), which results in the enhancement of the spin entropy. A suitable theoretical model is suggested to explain well the enhancement of the spin entropy induced by spin state transition. This investigation gives strong evidence that the enhanced spin entropy contribution to the thermopower is tuned by the spin state transition.

The Origins of Dramatic Axial Ligand Effects: Closed‐Shell MnVO Complexes Use Exchange‐Enhanced Open‐Shell States to Mediate Efficient H Abstraction Reactions

Spin-state effects: In [(Cz)MnVO] complexes, even with singlet closed-shell ground states, the reactive state is the open-shell triplet B state (SB=1) that has a high oxyl radical character and a transition state (TS) stabilized by exchange-enhanced reactivity (EER; see picture). The dramatic axial ligand effect originates in the EER of this state.

Cobalt(ii), nickel(ii) and copper(ii) complexes of a hexadentate pyridine amide ligand. Effect of donor atom (ether vs. thioether) on coordination geometry, spin-state of cobalt and MIII–MII redox potential

Using an acyclic hexadentate pyridine amide ligand, containing a -OCH(2)CH(2)O- spacer between two pyridine-2-carboxamide units (1,4-bis[o-(pyrydine-2-carboxamidophenyl)]-1,4-dioxabutane (H(2)L(9)), in its deprotonated form), four new complexes, [Co(II)(L(9))] (1) and its one-electron oxidized counterpart [Co(III)(L(9))][NO(3)]·2H(2)O (2), [Ni(II)(L(9))] (3) and [Cu(II)(L(9))] (4), have been synthesized. Structural analyses revealed that the Co(II) centre in 1 and the Ni(II) centre in 3 are six-coordinate, utilizing all the available donor sites and the Cu(II) centre in 4 is effectively five-coordinated (one of the ether O atoms does not participate in coordination). The structural parameters associated with the change in the metal coordination environment have been compared with corresponding complexes of thioether-containing hexadentate ligands. The μ(eff) values at 298 K of 1-4 correspond to S = 3/2, S = 0, S = 1 and S = 1/2, respectively. Absorption spectra for all the complexes have been investigated. EPR spectral properties of the copper(II) complex 4 have been investigated, simulated and analyzed. Cyclic voltammetric experiments in CH(2)Cl(2) reveal quasireversible Co(III)-Co(II), Ni(III)-Ni(II) and Cu(II)-Cu(I) redox processes. In going from ether O to thioether S coordination, the effect of the metal coordination environment on the redox potential values of Co(III)-Co(II) (here the effect of spin-state as well), Ni(III)-Ni(II) and Cu(II)-Cu(I) processes have been systematically analyzed.

Effect of Spin State on the Dihydrogen Binding Strength to Transition Metal Centers in Metal-Organic Frameworks

In several recent experimental studies, it has been found that exposed transition metal (TM) sites in metal-organic frameworks (MOFs) yield the largest binding to H2 by about 10 kJ/mol. However, this binding is significantly smaller than what has been observed in the so-called Kubas molecules, also with exposed metal sites. Our ab initio calculations quantitatively account for the experimental findings and further show that the splitting and occupation of the spin orbitals in the MOF systems are the reasons for their unexpected small binding energies. We predict that the binding energy to H2 in the MOF systems can be tuned between 10 and 50 kJ/mol by using early TM elements, namely, Sc, Ti, V, Cr, and Mn.

Electronic spin transitions in iron-bearing MgSiO 3 perovskite

The electronic spin state of iron in perovskites with the chemical composition Mg 0.9375Fe 0.0625SiO 3, Mg 0.8750Fe 0.1250SiO 3 and Mg 0.9375Fe 0.0625Si 0.9375Fe 0.0625O 3, have been determined at 0 K and various pressures between 40–160 GPa, using ab initio methods. The results indicate that ferric iron exhibits a wide range of spin transition pressures between about 60–160 GPa, while ferrous iron has a much narrower span, between about 130–145 GPa. In general, the lowest spin transition pressures are associated with the most energetically favorable substitution configurations, where iron atoms are closest together. The effect of spin state on calculated elastic properties is found to be small.

Electronic spin transitions and the seismic properties of ferrous iron‐bearing MgSiO3 post‐perovskite

The elastic constants of post‐perovskite of chemical composition Mg0.9375Fe0.0625SiO3 and Mg0.8750Fe0.1250SiO3 have been calculated at 0 K and 136 GPa using ab initio methods. For both compositions studied, iron remains in a high‐spin state below 180 GPa at 0 K. The effect of spin state on elastic properties is small. Logarithmic derivations of isotropic wave velocities and density with respect to ferrous iron content are similar to those predicted from pure end‐members.

Spin-state effects on the outer core-level multiplet structures for high-spin Mn molecular clusters

Oxo-bridged manganese polynuclear complexes have applications in a variety of technologies, such as single-molecule nanomagnets, catalysis, and photosynthetic redox chemistry. X-ray photoemission spectroscopy was used to study the Mn 3p, 3s, and valence band electronic behavior as a function of Mn cluster structural properties, where the cluster size and nuclearity are systematically varied. Results show a chemical shift of the Mn 3p3/2,1/2 spin–orbit pair related to the cluster size and nuclearity. In addition, the Mn 3s S7 and S5 final state multiplet components shift since they involve the binding energy of a ligand valence electron. In addition, the branching ratio of the S7:S5 states is related to the 3s–3d electron correlation. Specifically, in the S7 state, the remaining 3s electron is well correlated with 3d electrons of parallel spin, while in the S5 state, the two spins are antiparallel. Changes in this electron correlation are clearly observed in the S7:S5 branching ratio as a function of cluster size and ligand electronegativity.

Macrocyclic ligand ring size effects. Part 2. Synthetic, structural and electrochemical studies of the interaction of Ni II with 14-membered tetraaza macrocycles containing fused dibenzo substituents

A comparative study of the high- and low-spin complexes of NiII with two tetraaza dibenzo-substituted macrocycles incorporating 14-membered inner rings has been carried out. The results obtained are also compared with those for the analogous nickel(II) derivatives of 1,4,8,11-tetraazacyclotetradecane. Four X-ray structures of nickel complexes as well as the structure of one free ligand have been determined. These structures taken together with previous X-ray structural data and selected molecular mechanics calculations have enabled a detailed assessment of the effects of spin state and ligand substituent on the hole size and ring conformations adopted by individual complexes. A comparative electrochemical investigation of individual high- and low-spin complexes in the series has been performed.

Effect of spin state on reduction of cytochrome P-450 ( P-450 c21) from bovine adrenocortical microsomes

The effect of spin state on cytochrome P-450 reduction was studied with a reconstituted system consisting of P-450 c21 and NADPH-cytochrome P-450 reductase (NADPH:ferricytochrome oxidoreductase, EC 1.6.2.4) purified from bovine adrenocortical microsomes. The absolute high spin contents of substrate-free, progesterone-bound and 17α-hydroxyprogesterone-bound P-450 c21 were estimated from the analysis of thermally induced difference spectra to be 25, 78 and 94% at 25°C, respectively, in 50 mM potassium phosphate buffer (pH 7.2) containing 20% glycerol, 0.1 mM EDTA and 0.5% Emulgen 913. The effect of the high spin content on P-450 c21 reduction by NADPH in the reconstituted system was analyzed by a steady-state method and by a stopped-flow method at 25°C. The steady-state results showed that the rate of P-450 c21 reduction was not affected by the high spin content of substrate-bound P-450 c21 but was very slow without a steroid substrate. Biphasic reduction of P450 c21 containing two first-order processes was observed in the stopped-flow experiment in the presence of either of the steroid substrates, but the reduction was very slow without the substrate. There were no significant differences in the rate and the amount of the fast phase of reduction between 17α-hydroxyprogesterone-bound and progesterone-bound P-450 c21. Both kinetic studies indicate that the spin state does not control the electron transfer from NADPH to P-450 c21 via NADPH-cytochrome P-450 reductase but the presence of substrate is essential for the reduction of P-450 c21.