Low-temperature Magnetic Circular Dichroism Spectroscopy Research Articles

Anion Exchange and Lateral Heterostructure Formation in Ferromagnetic PEA2Cr(Cl,Br)4 Two-Dimensional Perovskites.

Postsynthetic vapor-phase anion exchange in the ferromagnetic two-dimensional (2D) hybrid metal-halide perovskite, PEA2CrCl4 (PEA+ = phenethylammonium), is reported. Anion exchange using vaporous trimethylsilyl bromide (TMS-Br) is shown to drive complete conversion of solution-processed PEA2CrCl4 polycrystalline thin films to PEA2CrBr4. Low-temperature magnetic circular dichroism spectroscopy indicates ferromagnetic ordering in these PEA2CrCl4 and PEA2CrBr4 films. Via partial anion exchange of exfoliated flakes of PEA2CrCl4 single crystals, we demonstrate that it is possible to generate abrupt lateral PEA2CrCl4/PEA2CrBr4 magneto-heterointerfaces. Kinetic studies reveal that lateral heterostructure formation is dictated by rapid edge-site halide exchange followed by slower intralayer bromide diffusion, and there is negligible interlayer (3D) bromide or TMS-Br diffusion. The importance of the bulky PEA+ interlayer cation in suppressing 3D diffusion is highlighted by parallel anion-exchange experiments on MA2CrCl4 (MA+ = methylammonium), which instead show 3D exchange. Comparison of anion-exchange reactions in PEA2CrCl4, PEA2MnCl4, and PEA2PbCl4 shows that 2D bromide diffusion is slowest in PEA2CrCl4, attributed to the antiferrodistortive ordering found in this composition. In addition to demonstrating both postsynthetic composition control and heterostructure formation in ferromagnetic Cr-based 2D perovskites for the first time, these results also advance our fundamental understanding of ion-exchange processes in this relatively unexplored family of 2D perovskites, broadening opportunities for investigation and control of novel spin effects in low-dimensional metal-halide perovskites.

Spectroscopic and Computational Studies of a Small-Molecule Functional Mimic of Iron Superoxide Dismutase, Iron 2,6-Diacetylpyridinebis(semioxamazide)

Iron 2,6-diacetylpyridinebis(semioxamazide) (Fe(dapsox)) is a heptacoordinate pentagonal bipyramidal, functional mimic of iron-dependent superoxide dismutase that has been well-characterized on the basis of kinetics and mechanistic studies; however, prior to our studies, its electronic structure had yet to be examined. This paper details our initial characterization of Fe(dapsox) in both its reduced and oxidized states, by electronic absorption (Abs) and low-temperature magnetic circular dichroism spectroscopies. Density functional theory (DFT) geometry optimizations have yielded models in good agreement with the published crystal structures. Time-dependent DFT and INDO/S-CI calculations performed on these models successfully reproduce the experimental Abs spectra and identify intense, low-energy transitions in the reduced complex (Fe(II)(H(2)dapsox)) as metal-to-ligand charge transfer transitions, suggesting the presence of π-backbonding in this complex. This backbonding, along, with the proton uptake accompanying metal ion reduction, provides a compelling mechanism by which the metal-centered redox potential is correctly tuned for catalytic superoxide disproportionation.

Elucidating second coordination sphere effects in heme proteins using low-temperature magnetic circular dichroism spectroscopy

This paper reviews recent findings on how the second coordination sphere of heme proteins fine-tunes the properties of the heme active site via hydrogen bonding. This insight is obtained from low-temperature magnetic circular dichroism (MCD) spectroscopy. In the case of high-spin ferric hemes, MCD spectroscopy allows for the identification of a multitude of charge-transfer (CT) transitions. Using optically-detected magnetic saturation curves, out-of-plane polarized CT transitions between the heme and its axial ligand(s) can be identified. In the case of ferric Cytochrome P450cam, the corresponding S(σ)→Fe(III) CT transition can be used as a probe for the {Fe(III)–axial ligand} interaction, indicating that the hydrogen bonding network of the proximal Cys only plays a limited role for fine-tuning the Fe(III)–S(Cys) interaction. In the case of high-spin ferrous hemes with axial His/imidazole coordination, our MCD-spectroscopic investigations have uncovered a direct correlation between the strength of the hydrogen bond to the proximal imidazole ligand and the ground state of the complexes. With neutral imidazole coordination, the doubly occupied d-orbital of high-spin iron(II) is of dπ character, located orthogonal to the heme plane. As the strength of the hydrogen bond increases, this orbital rotates into the heme plane, changing the ground state of the complex.

Beyond the active-electron approximation: Origin of ferromagnetic exchange in donor–acceptor heterospin biradicals

The complex, ( NNSQ)CuTp Cum,Me, is comprised of three different spin carrying units. These three S = 1/2 sites are comprised of Cu(II), nitronyl nitroxide ( NN), and 5- tert-butyl-semiquinone ( SQ). Both the Cu(II)– SQ and SQ– NN pairwise interactions are strongly exchange coupled resulting an S T = 3/2 ground state due to ferromagnetic interactions. Low-temperature EPR spectroscopy confirms the S T = 3/2 ground state determined from magnetic susceptibility studies and indicates that the quartet is zero-field split by | D| ∼ 0.5 cm −1. We have probed the origin of the strong ferromagnetic exchange within the NNSQ heterospin biradical ligand by a combination of electronic absorption, and low-temperature magnetic circular dichroism spectroscopies. The origin of the strong intraligand NNSQ ferromagnetic exchange has been suggested to result from an intraligand charge transfer (ILCT) transition transition that possesses appreciable SQ π → NN π* charge transfer character. The magnitude of the ferromagnetic exchange has been related to the energy of this ILCT transition, the magnitude of the NN single site exchange integral, and the magnitude of the electronic coupling matrix element that connects the SQ SOMO with the NN LUMO.

Y25S Variant of Paracoccus pantotrophus Cytochrome cd1 Provides Insight into Anion Binding by d1 Heme and a Rare Example of a Critical Difference between Solution and Crystal Structures

Tyr25 is a ligand to the active site d1 heme in as isolated, oxidized cytochrome cd1 nitrite reductase from Paracoccus pantotrophus. This form of the enzyme requires reductive activation, a process that involves not only displacement of Tyr25 from the d1 heme but also switching of the ligands at the c heme from bis-histidinyl to His/Met. A Y25S variant retains this bis-histidinyl coordination in the crystal of the oxidized state that has sulfate bound to the d1 heme iron. This Y25S form of the enzyme does not require reductive activation, an observation previously interpreted as meaning that the presence of the phenolate oxygen of Tyr25 is the critical determinant of the requirement for activation. This interpretation now needs re-evaluation because, unexpectedly, the oxidized as prepared Y25S protein, unlike the wild type, has different heme iron ligands in solution at room temperature, as judged by magnetic circular dichroism and electron spin resonance spectroscopies, than in the crystal. In addition, the binding of nitrite and cyanide to oxidized Y25S cytochrome cd1 is markedly different from the wild type enzyme, thus providing insight into the affinity of the oxidized d1 heme ring for anions in the absence of the steric barrier presented by Tyr25.

Nonoxovanadium(IV) and Oxovanadium(V) Complexes with Mixed O, X, O-Donor Ligands (X = S, Se, P, or PO)

Ligating properties of four potentially tridentate bisphenol ligands containing [O, X, O] donor atoms (X = S 1, Se 2, P 3, or P=O 4) toward the vanadium ions in +IV or +V oxidation states have been studied. Each ligand with different heterodonor atoms yields as expected nonoxovanadium(IV) complexes, V(IV)L(2), whose structures have been determined by X-ray diffraction methods as having six-coordinate V(IV), VO(4)X(2), core. Compounds 1-4 have also been studied with electrochemical methods, variable-temperature (2-295 K) magnetic susceptibility measurements, X-band electron paramagnetic resonance (EPR) (2-60 K) spectroscopy, and magnetic circular dichroism (MCD) (5 K) measurements. Electrochemical results suggest metal-centered oxidations to V(V) (i.e., no formation of phenoxyl radicals from the coordinated phenolates). A combination of density functional theory calculations and experimental EPR investigations indicates a dramatic effect of the heteroatoms on the electronic structure of 1-4 with consequent reordering of the energy levels; 1 and 3 possess a trigonal ground state (d(z)()(2))(1), but 4 with the phosphoryl oxygen as the heterodonor atom in contrast exhibits a tetragonal ground state, (d(xy)())(1). On the basis of the intense electronic transitions in absorption spectra, all electronic transitions observed for 4 have been assigned to ligand-to-metal charge-transfer transitions, which have been confirmed by preliminary resonance Raman measurements and C/D ratios obtained from low-temperature MCD spectroscopy. Moreover, diamagnetic complexes 5 and 6 containing mononuclear and dinuclear oxovanadium(V) units have also been synthesized and structurally and spectroscopically ((51)V NMR) characterized.

Electron paramagnetic resonance and magnetic circular dichroism studies of electron-transfer flavoprotein-ubiquinone oxidoreductase from pig liver

Pig liver electron-transfer flavoprotein-ubiquinone oxidoreductase has been investigated by room temperature UV-visible, low-temperature electron paramagnetic resonance and low-temperature magnetic circular dichroism spectroscopies. The results provide unambiguous evidence for the presence of a single [4Fe-4S] cluster that is diamagnetic in the isolated enzyme and becomes paramagnetic with an S = 1 2 , ground state on reduction with dithionite or enzymatically with the physiological electron donor. The EPR data for samples at pH 7.8 indicate that FAD is reduced by one electron to the anionic semiquinone form in the enzymatically reduced enzyme, and by two electrons to the hydroquinone form by excess dithionite. The possibility of weak spin-spin interaction between the FAD semiquinone and the [4Fe-4S] 1+ center is discussed in the light of the observation of a small increase in the linewidth of the Fe-S EPR in enzymatically reduced samples.