Effect of Heme–Heme Interactions and Modulation of Metal Spins by Counter Anions in a Series of Diiron(III)‐μ‐hydroxo Bisporphyrins: Unusual Stabilization of Two Different Spins in a Single Molecular Framework

A new family of five ethene‐bridged diiron(III)‐μ‐hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe₂(bisporphyrin)]OH ? X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo‐bridged dimer 1 by strong Brønsted acids such as HI, HBF₄, HPF₆, HSbF₆, and HClO₄ produces the μ‐hydroxo complexes with I₅^? ( 2 ), BF₄^? ( 3 ), PF₆^? ( 4 ), SbF₆^? ( 5 ), and ClO₄^? ( 6 ) as counter anions, respectively. The X‐ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed‐intermediate spins in both the solid state and solution. Moreover, the two different Fe^III centers of 3 – 5 are best described as having admixed‐high and admixed‐intermediate spins with variable contributions of S=5/2 and 3/2 for each state in the solid, but two different admixed‐intermediate spins in solution. In contrast, the two Fe^III centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X‐ray structures reveal that the Fe? O bond length increases on going from the μ‐oxo to the μ‐hydroxo complexes, and the Fe‐O(H)‐Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6 , respectively. Variable‐temperature magnetic data have been subjected to a least‐squares fitting using the expressions derived from the spin Hamiltonians H=?2JS₁?S₂?μ?B+D[${S{{2\hfill \atop z\hfill}}}$ ?1/3S(S+1)] (for 2 , 3 , 4 , and 5 ) and H=?2JS₁?S₂ (for 6 ). The results show that strong antiferromagnetic coupling between the two Fe^III centers in 1 is attenuated to nearly zero (?2.4 cm^?1) in 2 , whereas the values are ?46, ?32.6, ?33.5, and ?34 cm^?1 for 3 , 4 , 5 , and 6 , respectively.     

Thermoelectric properties of unoxidized graphene/Bi2Te2.7Se0.3 composites synthesized by exfoliation/re‐assembly method

Nanocomposites of n‐type thermoelectric Bi₂Te_2.7Se_0.3 (BTS) and unoxidized graphene (UG) were prepared from the exfoliated BTS and UG nanoplatelets. Polycrystalline BTS ingots were exfoliated into nanoscoll‐type crystals by chemical exfoliation, and were re‐assembled with UG nanoplatelets. The composites were chemically reduced by hydrazine hydrate and sintered by a spark‐plasma‐sintering method. The thermoelectric properties of the sintered composites were evaluated and exhibited decreased carrier concentration and increased thermal conductivity due to the embedded graphene. The peak ZT values for the UG/BTS‐US and UG/BTS‐EX composites were ～0.8 at the UG concentration of 0.05 wt%. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of hyperstoichiometric manganese on the structure and transport, magnetic, and magnetoresistance properties of manganite-lanthanum (La0.7Ca0.3)1 ? x Mn1 + x O3 perovskites

Ceramic (La_0.7Ca_0.3)_{1 ? x} Mn_{1 + x} O₃ samples are studied by X-ray diffraction, resistive, magnetic, ⁵⁵Mn NMR, and magnetoresistance methods. The concentration changes of lattice parameter a of the cubic perovskite structure and its average ionic radius are in good agreement if the concentrations of anion and cation vacancies and nanostructured clusters with Mn²⁺ in the A positions increase with x. Phase transition temperatures T _ms and T _c weakly depend on x, and the electrical resistivity and the activation energy decrease substantially with increasing x due to a change in the imperfection of the perovskite structure. An analysis of the broad asymmetric ⁵⁵Mn NMR spectra of the samples indicate a high-frequency Mn³⁺? Mn⁴⁺ electron superexchange and nonuniform magnetic and valence states of these ions because of a nonuniform distribution of ions and defects, which decrease the amplitude resonance frequency with increasing x. The magnetoresistive (MR) effect near phase-transition temperatures T _ms and T _c increases substantially with x and is caused by the effect of a magnetic field on the scattering of charge carriers by intracrystallite nanostructured heterogeneities of an imperfect perovskite structure. The second MR effect is located in the low-temperature range, is related to tunneling through mesostructural crystallite boundaries, and decreases weakly with increasing x. A correlation is found between the hyperstoichiometric manganese content, the imperfection of the perovskite structure, and the magnitude of the MR effect.     

A Porphyrin Iron(III) π-Dication Species and its Relevance in Catalyst Design for the Umpolung of Nucleophiles

Isoporphyrins have recently been identified as remarkable species capable of turning the nucleophile attached to the porphyrin ring into an electrophile, thereby providing umpolung of reactivity (Inorg. Chem. 2022 , 61, 8105–8111). They are generated by nucleophilic attack on an iron(III) π-dication, a class of species that has received scant attention. Here, we explore the effect of the porphyrin meso-substituent and report a iron(III) π-dication bearing the meso-tetraphenylporphyrin (TPP) ligand. We provide an extensive study of the species by UV/Vis absorption, ²H NMR, EPR, applied field Mössbauer, and resonance Raman spectroscopy. We further explore the system's highly dynamic and tunable properties and address the nature of the axial ligands as well as the conformation of the porphyrin ring. The insights presented are essential for the rational design of catalysts for the umpolung of nucleophiles. Such catalytic avenues could for example provide a novel method for electrophilic chlorinations. We further examine the importance of electronic tuning of the porphyrin by nature of the meso-substituent as a factor in catalyst design.     

Understanding the Surprising Oxidation Chemistry of Au−OH Complexes

Au is known to be fairly redox inactive (in catalysis) and bind oxygen adducts only quite weakly. It is thus rather surprising that stable Au−OH complexes can be synthesized and used as oxidants for both one- and two-electron oxidations. A charged Au^III−OH complex has been shown to cleave C−H and O−H bonds homolytically, resulting in a one-electron reduction of the metal center. Contrasting this, a neutral Au^III−OH complex performs oxygen atom transfer to phosphines, resulting in a two-electron reduction of the hydroxide proton to form a Au^III−H rather than causing a change in oxidation state of the metal. We explore the details of these two examples and draw comparisons to the more conventional reactivity exhibited by Au^I−OH. Although the current scope of known Au−OH oxidation chemistry is still in its infancy, the current literature exemplifies the unique properties of Au chemistry and shows promise for future findings in the field.