Positive Spin Density Research Articles

Understanding the role of nitrogen and sulfur doping in promoting kinetics of oxygen reduction reaction and sodium ion battery performance of hollow spherical graphene

Here, a simple, economical one-step doping strategy is adopted to prepare N, S co-doped graphene hollow spheres (NSGHS). The NSGHS exhibits remarkable oxygen-reduction-reaction (ORR) activity with a high half-wave potential (E1/2) of 0.847 V vs. RHE and considerable long-term stability. When applied as anode materials for sodium-ion batteries (SIBs), it could deliver a high reversible capacity of 385 mAh g−1 at 500 mA g−1, excellent rate performance of 308 mAh g−1 at even 20 A g−1 and superior cycling performance at 10 A g−1 during 5000 cycles with negligible capacity fade. DFT calculation reveals that the doping of N and S endows the surrounding carbon with high positive charge and spin density, respectively, making them be highly active ORR catalytic sites except for S itself. Despite the possible oxidization of C–S–C active sites, the rest C atoms of NSGHS will still hold a considerably high catalytic activity superior to those of mono-doped or undoped cases. In addition, S doping will improve the binding between Na and graphene substrate to get a high capacity and N doping effectively curbs the rising of oxygen levels and oxidization of S in C–S–C active sites, thus the excellent stability for ORR and SIBs.

Scalable synthesis of 2D hydrogen-substituted graphdiyne on Zn substrate for high-yield N2 fixation

Hydrogen-substituted graphdiyne (HsGDY), as a new rising star of carbon materials family, demonstrates high conjugation, robust chemical stability, and versatility for modification. However, grand challenges, including low production rate, disordered topology, and amorphous structure, greatly hinder its large-scale applications. Herein, we report, for the first time, the scalable synthesis (up to gram-level) of two-dimensional (2D) crystalline HsGDY nanosheets with Zn as a substrate. Moreover, as a metal-free catalyst for electrochemical N2 fixation, 2D HsGDY achieves an ultrahigh yield of 103 μg h−1 mg−1cat. with a potential of −0.2 V vs reversible hydrogen electrode (RHE), which is comparable with that of noble metals and single-atom catalysts. Different from the heteroatom active sites in carbon-based catalysts reported before, the inner alkynyl C itself in 2D HsGDY was identified as the active site, which adsorbs and activates NN due to the positive charge and high spin density triggered by the slight O doping in the form of CO at the outer alkynyl C. We believe that this Zn-templated scalable production of high-quality HsGDY paves the way for its large-scale production and provides a new playground for the multiple research fields.

Spin Density Topology.

Despite its role in spin density functional theory and it being the basic observable for describing and understanding magnetic phenomena, few studies have appeared on the electron spin density subtleties thus far. A systematic full topological analysis of this function is lacking, seemingly in contrast to the blossoming in the last 20 years of many studies on the topological features of other scalar fields of chemical interest. We aim to fill this gap by unveiling the kind of information hidden in the spin density distribution that only its topology can disclose. The significance of the spin density critical points, the 18 different ways in which they can be realized and the peculiar topological constraints on their number and kind, arising from the presence of positive and negative spin density regions, is addressed. The notion of molecular spin graphs, spin maxima (minima) joining paths, spin basins and of their valence is introduced. We show that two kinds of structures are associated with a spin–polarized molecule: the usual one, defined through the electron density gradient, and the magnetic structure, defined through the spin density gradient and composed in general by at least two independent spin graphs, related to spin density maxima and minima. Several descriptors, such as the spin polarization index, are introduced to characterize the properties of spin density critical points and basins. The study on the general features of the spin density topology is followed by the specific example of the water molecule in the 3B1 triplet state, using spin density distributions of increasing accuracy.

In situ molecular-level synthesis of N, S co-doped carbon as efficient metal-free oxygen redox electrocatalysts for rechargeable Zn–Air batteries

Developing metal-free oxygen redox catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) to eliminate the risk caused by transition metal based catalysts is highly desirable as promising substitutes for precious metal catalysts for widespread adoption of rechargeable Zn-Air batteries. Herein, N, S co-doped carbon (NS/C) was developed by a molecular-level doping strategy using a new precursor of N and S co-containing bis(imino)-pyridine based polymer, copolymerized by three monomers of 2, 6-Diacetylpyridine (DAP), 1, 5-naphthalenediamine (NDA) and 2, 5-dithiobiurea (DBU) with Schiff base reaction. The results demonstrated that the ratio between the three monomers strongly affects the texture characteristics of the corresponding carbon materials, doping types and contents of N and S, as well their synergetic effects in carbon matrix. The NS/C obtained by the copolymer of DAP, NDA and DBU with their corresponding molar ratio of 1: 0.5: 0.5 exhibits a small overpotential difference of 0.72 V vs RHE and a remarkable stability in 0.1 M KOH for overall ORR/OER activity, outperforming most of metal-free catalysts reported. Meanwhile, a single rechargeable Zn-Air battery test further identified its excellent activity, in which the battery delivers a maximum power density output of 149 mW·cm−2, a specific capacity of 769 mAh·gZn−1, and long charge-discharge capability at 5 mA·cm−2 with 600 cycles in 100 h, surpassing that with the mixed Pt/C and IrO2 (1:1 wt.%) catalysts under the same mass loading. The theoretical calculation results show that the oxygen redox activity of NS/C critically roots in C atoms with positive charge and spin density, induced by the strong interaction between the specific N and S dopants derived from the new designed precursors.

C59N Heterofullerene: A Promising Catalyst for NO Conversion into N2O

AbstractWe report, for the first time, the possibility of using N‐doped C60 fullerene (C59N) as a novel and highly active metal‐free catalyst for reduction of NO molecules to N2O. First‐principles density functional theory calculations are used to find the most energetically favorable adsorbed/coadsorbed configurations of NO molecules over C59N. Our results indicate that introducing a N impurity in C60 can induce a positive charge and large spin density over the carbon atoms nearest to the dopant atom. Consequently, these carbon atoms are identified as the most reactive sites to interact with the NO molecule. According to our results, the dissociation of NO molecule over C59N is almost impossible, due to its relatively high activation energy (1.35 eV). The reduction of NO over C59N starts with the coadsorption of two NO molecules to form (NO)2 species, followed by the dissociation of (NO)2 into N2O and an active atomic oxygen (Oads). The energy barrier to convert NO molecules into N2O ranges from 0.33 to 0.75 eV, which indicates this process is likely to proceed at normal temperature. Besides, by overcoming a negligible energy barrier (0.18 eV), the remaining Oads moiety can be easily eliminated by a CO molecule. To further understand the role of nitrogen atoms in the NO reduction process, the catalytic performance of high percentage N‐doped fullerene (C48N12) is also studied.

A Model Exact Study of the Properties of Low-Lying Electronic States of Perylene and Substituted Perylenes.

There is a resurgence of interest in the electronic structure of perylene for its applications in molecular devices such as organic photovoltaics and organic light-emitting diodes. In this study, we have obtained the low-lying singlet states of perylene by exactly solving the Parisar-Parr-Pople model Hamiltonian of this system with 20 sites and 20 electrons in the VB basis where dimensionality is ∼5.92 billion. The triplet states of perylene are obtained using a DMRG scheme with symmetry adaptation. The one- and two-photon states are very close in energy ∼3.2 eV while the lowest triplet state is slightly below 1.6 eV indicating that perylene is a good candidate for singlet fission. To explore the tunability of the electronic states, we have studied donor-acceptor substituted perylenes. The two donors and two acceptors are substituted symmetrically at either the four bay sites or four peri sites. In all the bay substitution and one peri substitution at moderate D/A strength, the optical gap is lowered to about 2.8 eV. These molecules can be used as blue emitters. We have also reported bond orders in all the cases, and perylene as well as substituted perylenes can be viewed as two weakly coupled naphthalenes in the singlet states, but in triplets these bonds tend to be comparable to other bonds in strength. The charge densities in substituted perylenes are mostly localized around the substitution sites in the ground state. The positive spin densities in triplets are concentrated around the peri and bay sites with the remaining sites having small spin densities of either sign.

1H NMR Spectroscopy of [FeFe] Hydrogenase: Insight into the Electronic Structure of the Active Site.

The [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii has been studied using 1H NMR spectroscopy identifying the paramagnetically shifted 1H resonances associated with both the [4Fe-4S]H and the [2Fe]H subclusters of the active site “H-cluster”. The signal pattern of the unmaturated HydA1 containing only [4Fe-4S]H is reminiscent of bacterial-type ferredoxins. The spectra of maturated HydA1, with a complete H-cluster in the active Hox and the CO-inhibited Hox–CO state, reveal additional upfield and downfield shifted 1H resonances originating from the four methylene protons of the azadithiolate ligand in the [2Fe]H subsite. The two axial protons are affected by positive spin density, while the two equatorial protons experience negative spin density. These protons can be used as important probes sensing the effects of ligand-binding to the catalytic site of the H-cluster.

Interaction of Cr3+ with valence and conduction bands in the long persistent phosphor ZnGa2O4:Cr3+, studied by ENDOR spectroscopy

Cr3+-doped zinc gallate ZnGa2O4 is a red-near infrared (IR) long persistent phosphor that can be excited by orange-red light, in the transparency window of living tissues. With this property, persistent luminescence nanoparticles were recently used for in vivo optical imaging of tumors in mice. In order to understand the origin of the excitability of persistent luminescence by visible light in this material, a Q-band ENDOR investigation of 71/69Ga and 53Cr nuclei was performed in ZnGa2O4:Cr3+ to get information on the interaction of Cr3+ with valence and conduction bands. The positive electron spin density at Ga nuclei revealed a dominant interaction of the 4A2 ground state of Cr3+ with the valence band, and a weaker interaction with the conduction band. The latter may occur only in the excited 2E and 4T2 states of Cr3+. It is proposed that when these two interactions are present, pairs of electrons and holes can be generated from excited Cr3+ in distorted sites undergoing local electric field produced by neighboring defects with opposite charges.

Quantum entanglement in manganese(II) hexakisimidazole nitrate: on electronic structure imaging - A polarized neutron diffraction and DFT study

Quantum entanglement has been visualized for the first time, in view of the spin density distribution and electronic structure for manganese in manganese(II)hexakisimidazole nitrate. Using polarized neutron diffraction and density functional theory modelling we have found for the complex, which crystallizes in the R3̄ spacegroup, a = b = 12.4898(3) Å, c = 14.5526(4) Å, α = γ = 90°, β = 120°, Z = 3, that spatially antisymmetric and spatially symmetric shaped regions of negative spin density, in the spin density map for manganese, are a result of quantum entanglement of the high spin d5 configuration due to dative imidazole- manganese π- donation and σ-bonding interactions respectively. We have found leakage of the entangled states for manganese observed as regions of positive spin density with spherical (3.758(2) μB) and non-spherical (1.242(3) μB) contributions. Our results, which are supportive of Einstein's theory of general relativity, provide evidence for the existence of a black hole spin density distribution at the origin of an electronic structure and also address the paradoxical views of entanglement and quantum mechanics. We have also found the complex, which is an insulator, to be suitable for spintronic studies.

Boosting activation of oxygen molecules on C60 fullerene by boron doping.

The activation of oxygen molecules on boron-doped C60 fullerene (C59 B) and the subsequent water formation reaction are systematically investigated by using hybrid density functional calculations. Results indicate that C59 B shows a favorable ability to activate oxygen molecules both kinetically and thermodynamically. The oxygen molecule is first adsorbed on the boron atom, which is identified to be the most reactive site in C59 B for O2 adsorption because of its high positive charge and spin density. The adsorption structure C59 BO2 can further isomerize to form two products with small reaction barriers. Water formation reactions upon these two structures are energetically favorable and suggest a four-electron mechanism for the oxygen reduction reaction catalyzed by C59 B. This work provides a reliable theoretical insight into the catalytic properties of boron-doped fullerene, which is believed to be helpful to explore fullerene catalysts.

Axial thiophenolate coordination on diiron(III)bisporphyrin: influence of heme-heme interactions on structure, function and electrochemical properties of the individual heme center.

The binding of a series of substituted thiophenols as axial ligands on a highly flexible ethane-bridged diiron(III)bisporphyrin framework has been investigated as a model of diheme proteins. Spectroscopic characterization reveals a high-spin (S = 5/2) state of iron for all of the pentacoordinate thiophenolato complexes. In the UV-visible spectra of the complexes, the positions of the Soret and band I have been found to be dependent on the pKa of thiophenols. The alternating shift pattern, which has opposite sign of the chemical shifts for meta- vs. ortho- and para-protons in the (1)H NMR spectra, is attributed to negative and positive spin densities, respectively, on thiophenolate carbon atoms and is indicative of π-spin delocalization to the bound thiophenolate ligand. The Fe(III)/Fe(II) redox couple of the complexes bears a linear relationship with the pKa of thiophenol and is found to be positively shifted with decreasing pKa. The effect of the electronic nature of the substituent on the thiophenolate ring has also been demonstrated in which a large potential range of 540 mV was observed (in contrast to the value of only 270 mV in case of monoheme analogues) for the Fe(III)/Fe(II) redox couple on going from monoheme to diheme and is attributed to the interheme interaction. Also, the Fe(III)/Fe(II) redox potential of the thiophenolato complexes has been found to be more positively shifted compared to their phenolato analogues, which was further supported by DFT calculation. The addition of another thiophenol at the sixth axial position of the five-coordinate thiophenolato complex causes a change in iron spin from high (S = 5/2) to low (S = 1/2) along with a large positive shift of 490 mV for the Fe(III)/Fe(II) redox couple.

Purely organic magnets: How to get ferromagnetic interactions

Purely organic magnets, materials with spontaneous magnetization despite no containing magnetic ions, are very promising for technological applications due to their peculiar properties as flexibility, lightness or even biocompatibility. Most of the published purely organic magnets are free radical compounds. One of the most important difficulties in order to get spontaneous magnetization in such materials is that magnetic interactions among free radicals are usually antiferromagnetic and, in addition, ferromagnetic interactions are weaker than the antiferromagnetic ones. A possible strategy to overcome this issue is the use of triradical molecules with total spin S=1/2. In this case, an adequate packing of the triradical molecules can give place to antiferromagnetic interactions between regions with positive spin density and regions with negative spin density of two close molecules. This antiferromagnetic interaction between regions with opposite spin density would result in an overall ferromagnetic interaction between the two close triradicals. With this idea in mind we have performed an study of the spin density distribution and of the intramolecular and intermolecular magnetic interactions of the triradical compound 2-[3',5'-bis(Ntert-butylaminoxyl)phenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1-Himidazol-1-oxyl-3-oxide, containing two N-tert-butyl aminoxyls and a nitronyl nitroxide groups. Combination of experimental data from a polarized neutron diffraction experiment and ab initio calculations (DFT) has allowed us to obtain the spin density distribution. In addition, the intramolecular and intermolecular magnetic interactions have been computed by ab initio quantum chemistry methods. The values for the intramolecular interactions confirm the S=1/2 ground state of the triradical. As for the intermolecular interactions, the two strongest ones are ferromagnetic, what is in agreement with the overlapping of regions with opposite spin density of the two interacting triradicals. These results support the strategy of using triradical molecules for obtaining purely organic magnets with higher magnetic transition ordering temperatures since is easier to obtain ferromagnetic interactions between the radicals and these interactions, having an antiferromagnetic origin, can be stronger than typical ferromagnetic interactions between radicals.

Paramagnetic Hexacyanometalates. The Diversity of Spin Distribution Studied by 13C and 15N MAS NMR Spectroscopy

With the aim of probing the spin density distribution in the open-shell cyanometallates Cs2K[M(CN)6] (M = Cr, Mn, Fe), K3[M(CN)6] (M = Mn, Fe), K4[M(CN)6] (M = Cr, Mn), and K4[V(CN)7] have been studied by solid-state (13)C and (15)N NMR spectroscopy. The signals appear in strongly shifted and broad ranges ((13)C, -2100 to -8900 ppm; (15)N, -1900 to 2400 ppm) except K4[V(CN)7], which is NMR-silent. Analysis of the isotropic signal shifts yields negative spin density in all carbon 2s orbitals (up to 12.2% at the six ligands of [Mn(CN)6](3-)) and positive spin density in all nitrogen 2s orbitals (up to 1.1% at the six ligands of [Mn(CN)6](4-) and [Fe(CN)6](3-)). This is in accord with the induction of alternating spin at the CN ligands by successive polarization of σ bonds triggered by the spin center M(n+). The signal shift anisotropies are related to spin in the carbon and nitrogen 2pπ and 2pσ orbitals. In the case of Cs2K[Cr(CN)6] and K4[Cr(CN)6] much positive spin is found in the nitrogen 2pπ orbitals, which corresponds to direct M → N spin transfer. On Cs2K[M(CN)6] (M = Mn, Fe), the 2pπ spin density at nitrogen is negative. The results are in accord with and extend the data of polarized neutron diffraction and EPR spectroscopy. Owing to high signal resolution, small deviations of the [M(CN)6](n-) ions from octahedral symmetry and disorder of crystal layers have been detected. This corresponds to the crystal symmetry and to Jahn-Teller distortion. The disorder entails a scatter of spin densities. In the case of K4[Mn(CN)6] it reaches 19% for the C 2s orbitals and 80% for the N 2s orbitals with regard to the respective smallest spin population.

Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

The binding of a series of substituted phenols as axial ligands onto a diiron(III)bisporphyrin framework have been investigated. Spectroscopic characterization revealed high-spin states of the iron centers in all of the phenolate complexes, with one exception in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin, which only stabilized the pure intermediate-spin (S=3/2) state of the iron centers. The average FeN (porphyrin) and FeO (phenol) distances that were observed with the 2,4,6-trinitrophenolate complex were 1.972(3) Å and 2.000(2) Å, respectively, which are the shortest and longest distances reported so far for any Fe(III) porphyrin with phenoxide coordination. The alternating shift pattern, which shows opposite signs of the chemical shifts for the meta versus ortho/para protons, is attributed to negative and positive spin densities on the phenolate carbon atoms, respectively, and is indicative of π-spin delocalization onto the bound phenolate. Electrochemical data reveals that the E1/2 value for the Fe(III) /Fe(II) couple is positively shifted with increasing acidity of the phenol. However, a plot of the E1/2 values for the Fe(III) /Fe(II) couple versus the pKa values of the phenols shows a linear relationship for all of the complexes, except for the 2,4,6-trinitrophenolate complex. The large deviation from linearity is probably due to the change of spin for the complex. Although 2,4,6-trinitrophenol is the weakest axial ligand in the series, its similar binding with the corresponding Fe(III) monoporphyrin only results in stabilization of the high-spin state. The porphyrin macrocycle in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin is the most distorted, whilst the "ruffling" deformation affects the energy levels of the iron d orbitals. The larger size and weaker binding of 2,4,6-trinitrophenol, along with hemeheme interactions in the diiron(III)bisporphyrin, are responsible for the larger ring deformations and eventual stabilization of the pure intermediate-spin states of the iron centers in the complex.

Syntheses and properties of 3,4-diaryldithieno [2,3-b;3′,2′-d]thiophenes

Dithieno[2,3-b;3′,2′-d]thiophenes (DTT), having substituted phenyl groups at 3- and 4-carbons, have been synthesized, applying 2,5-diketone ring closure reaction using P4S10. Cyclic voltammetry studies indicated that all the DTTs had the oxidation potentials between 1.19 and 1.70 V. Computational calculations displayed positive radical spin densities at C2 positions for DTTs and negative for their dimers.

Effect of Microstructure of Nitrogen-Doped Graphene on Oxygen Reduction Activity in Fuel Cells

The development of fuel cells as clean-energy technologies is largely limited by the prohibitive cost of the noble-metal catalysts needed for catalyzing the oxygen reduction reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structures to the catalytic activity can accelerate the search for highly active and abundant nonmetal catalysts to replace platinum. Here, we present a first-principles study of ORR on nitrogen-doped graphene in acidic environment. We demonstrate that the ORR activity primarily correlates to charge and spin densities of the graphene. The nitrogen doping and defects introduce high positive spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.

Selective syntheses of vinylenedithiathiophenes (VDTTs) and dithieno[2,3- b;2′,3′- d]thiophenes (DTTs); building blocks for π-conjugated systems

Selective syntheses of 3,4-vinylenedithiathiophenes (VDTTs) and dithieno[2,3- b;2′,3′- d]thiophenes (DTTs), having Ph, 4-CH 3OC 6H 4, 4-BrC 6H 4, 4-NO 2C 6H 4 and 4-(CH 3) 2NC 6H 4 groups, were achieved through the reaction of 1,8-diketone with phosphorus decasulfide (P 4S 10). The reaction could be shifted between VDTT and DTT by adding base (sodium bicarbonate) or acid ( para-toluenesulfonic acid), respectively, to the reaction mixture. While the VDTTs were obtained in moderate yields, an important achievement has been made with the syntheses of the DTTs, obtaining them in higher yields compared with the previous report. Polymers of the VDTTs, which are the analogues of ethylenedioxythiophene, EDOT, were prepared using FeCl 3. Unfortunately, all attempts for their electropolymerization failed. Spin density calculations revealed that none of the VDTTs had a significant positive spin density at the ‘α’ carbon atoms of the thiophene ring. Considering their solubility and functional groups, which could be further derivatized, they are useful building blocks for the preparation of new organic materials.

Axial phenoxide coordination on di-iron(III) bisporphyrin: Insights from experimental and DFT studies

Synthesis, structure and properties of new five-coordinate phenolate complexes of di-iron(III) bisporphyrin are reported here, in which phenol binds in η 1-fashion as an axial ligand. The solid and solution EPR at 120 K and 1H NMR spectral pattern in solution provide unequivocal evidence for the high spin (S = 5/2) nature of the complex. Mulliken spin density calculation using DFT demonstrates the positive spin densities at the meso carbons and negative spin densities at the methylene carbons and, as a result, the meso and methylene protons are shifted in the upfield and down field regions, respectively in the 1H NMR spectra of the molecule. Also, the ortho- and para-protons of the phenolate ligands are observed to be shifted in the upfield region while meta-protons are shifted downfield. The alternating shift pattern, which is the opposite sign of the chemical shifts for meta-versus ortho- and para-protons, was also explained due to negative and positive spin densities, respectively on the carbons and indicative of π spin delocalization on the phenolate ligand. Thus, the calculated spin density maps accounted for the essential 1H NMR spectroscopic features that are observed here for the phenolate complexes of di-iron(III) bisporphyrin. The temperature dependence of the signals follows the Curie law which is indicative of single spin state throughout the temperature range of −40 to +40°C. The single crystal X-ray structure of the corresponding chloro derivative, trans 1,2-bis(chloroiron(III) octaethyl porphyrinyl)ethene, has also been reported here which authenticates the high-spin nature of the complex. The synthesis of a family of five-coordinate phenolate complexes of diiron(III)bisporphyrin is reported. Mulliken spin density calculations using DFT demonstrate π-spin delocalization on the phenolate ligand and also explain the essential 1H NMR spectroscopic features observed.

Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells

Graphene and its derivatives are attractive for electrocatalytical application in fuel cells because of their unique structures and electronic properties. The electrocatalytical mechanism of nitrogen doped graphene in acidic environment was studied by using density functional theory (DFT). The simulations demonstrate that the oxygen reduction reaction (ORR) on N-doped graphene is a direct four-electron pathway, which is consistent with the experimental observations. The energy calculated for each ORR step shows that the ORR can spontaneously occur on the N-graphene. The active catalytical sites on single nitrogen doped graphene are identified, which have either high positive spin density or high positive atomic charge density. The nitrogen doping introduces asymmetry spin density and atomic charge density, making it possible for N-graphene to show high electroncatalytic activities for the ORR.

Binding of Catechols to Iron(III)–Octaethylporphyrin: An Experimental and DFT Investigation

AbstractThe synthesis, X‐ray structure, and properties of high‐spin, five‐coordinate catecholate complexes of (octaethylporphyrinato)iron(III), FeIII(OEP)(L) (L: catecholate monoanion), are reported here for the first time. In these complexes, catechol binds in an η1‐fashion as an axial ligand, which is supported by DFT calculations. The Fe–O–C angles of FeIII(OEP)(Hcat), FeIII(OEP)(4‐tBu‐Hcat), FeIII(OEP)(4‐NO2‐Hcat), and FeIII(OEP)(sal) are 119.5, 125.1, 122.2, and 124.3°, respectively. FeIII(OEP)(Hcat) has the smallest Fe–O–C angle in addition to the smallest dihedral angle of 26.2° between the planes of the porphyrin and axial catechol ligand among all phenolate complexes of iron(III)–porphyrins. In comparison to those of FeIII(OEP)(OPh) and FeIII(OEP)(4‐tBu‐Hcat), the Fe–O bond in FeIII(OEP)(Hcat) is elongated by 0.064 and 0.038 Å, respectively. This is due to the H‐bonding interactions in FeIII(OEP)(Hcat) and not caused by steric hindrance. In the 1H NMR spectra of the complexes, the signals of the ortho‐ and para‐protons of catechol are shifted upfield, whereas those of the meta‐protons are shifted downfield. The alternating shift pattern observed is due to negative and positive spin densities on the catechol carbon atoms and is indicative of π‐spin delocalization on the catecholato ligand. Electrochemical data reveal that the complexes undergo three one‐electron oxidations and a single one‐electron reduction. Based on spectroelectrochemical and DFT studies, the first oxidation is assigned to a catechol‐to‐semiquinone transformation and the second and third oxidations are found to be porphyrin‐ring‐centered.