Catalytic, Spectroscopic, and Theoretical Studies of Fe4S4-Based Coordination Polymers as Heterogenous Coupled Proton-Electron Transfer Mediators for Electrocatalysis.

Iron-sulfur clusters play essential roles in biological systems, and thus synthetic [Fe4S4] clusters have been an area of active research. Recent studies have demonstrated that soluble [Fe4S4] clusters can serve as net H atom transfer mediators, improving the activity and selectivity of a homogeneous Mn CO2 reduction catalyst. Here, we demonstrate that incorporating these [Fe4S4] clusters into a coordination polymer enables heterogeneous H atom transfer from an electrode surface to a Mn complex dissolved in solution. A previously reported solution-processable Fe4S4-based coordination polymer was successfully deposited on the surfaces of different electrodes. The coated electrodes serve as H atom transfer mediators to a soluble Mn CO2 reduction catalyst displaying good product selectivity for formic acid. Furthermore, these electrodes are recyclable with a minimal decrease in activity after multiple catalytic cycles. The heterogenization of the mediator also enables the characterization of solution-phase and electrode surface species separately. Surface enhanced infrared absorption spectroscopy (SEIRAS) reveals spectroscopic signatures for an in situ generated active Mn-H species, providing a more complete mechanistic picture for this system. The active species, reaction mechanism, and the protonation sites on the [Fe4S4] clusters were further confirmed by density functional theory calculations. The observed H atom transfer reactivity of these coordination polymer-coated electrodes motivates additional applications of this composite material in reductive H atom transfer electrocatalysis.

Aliovalent Substitution Tunes Physical Properties in a Conductive Bis(dithiolene) Two-Dimensional Metal-Organic Framework.

Two-dimensional conductive metal-organic frameworks have emerged as promising electronic materials for applications in (opto)electronic, thermoelectric, magnetic, electrocatalytic, and energy storage devices. Many bottom-up or postsynthetic protocols have been developed to isolate these materials or further modulate their electronic properties. However, some methodologies commonly used in classic semiconductors, notably, aliovalent substitution, are conspicuously absent. Here, we demonstrate how aliovalent Fe(III) to Ni(II) substitution enables the isolation of a Ni bis(dithiolene) material from a previously reported Fe analogue. Detailed characterization supports the idea that aliovalent substitution of Fe(III) to Ni(II) results in an in situ oxidation of the organic dithiolene linker. This substitution-induced redox tuning modulates the electronic properties in the system, leading to higher electrical conductivity and Hall mobility but slightly lower carrier densities and weaker antiferromagnetic interactions. Moreover, this aliovalent substitution improves the material's electrochemical stability and thus enables pseudocapacitive behavior in the Ni material. These results demonstrate how classic aliovalent substitution strategies in semiconductors can also be leveraged in conductive MOFs and add further support to this class of compounds as emerging electronic materials.

Intervalence Charge Transfer in Nonbonding, Mixed-Valence, Homobimetallic Ytterbium Complexes.

There are several reports of compounds containing lanthanide ions in two different formal oxidation states; however, there are strikingly few examples of intervalence charge transfer (IVCT) transitions observed for these complexes, with those few occurrences limited to extended solids rather than molecular species. Herein, we report the synthesis, characterization, and computational analysis for a series of ytterbium complexes including a mixed-valence Yb25+ complex featuring a remarkably short Yb···Yb distance of 2.9507(8) Å. In contrast to recent reports of short Ln···Ln distances attributed to bonding through 5d orbitals, the formally Yb25+ complex presented here displays clear localization of Ln2+ and Ln3+ character and yet still displays an IVCT in the visible spectrum. These results demonstrate the ability to tune the electronic structure of formally mixed oxidation state lanthanide complexes: the high exchange stabilization of the Yb2+ 4f14 configuration disfavors the formation of a 5d1 bonding configuration, and the short metal-metal distance enforced by the ligand framework allows for the first observed lanthanide IVCT in a molecular system.

Coordination Modes and Binding Patterns in Lanthanum Phosphoramide Complexes.

A monoanionic phosphoramide ligand is introduced, which forms a series of lanthanum complexes with the ligand in both anionic and neutral forms. Stoichiometric control alone provides monometallic complexes with either two or three phosphoramide ligands. Alternatively, a combination of anionic and neutral proteo ligands featuring intramolecular hydrogen bonding can be obtained. The anionic form of the ligand binds lanthanum as a bi- or monodentate ligand, depending on the steric demand at the metal center, while the protonated ligand binds exclusively through the phosphoramide oxygen donor.

Ground-State Spin Dynamics in d1 Kagome-Lattice Titanium Fluorides.

Many quantum magnetic materials suffer from structural imperfections. The effects of structural disorder on bulk properties are difficult to assess systematically from a chemical perspective due to the complexities of chemical synthesis. The recently reported S = 1/2 kagome lattice antiferromagnet, (CH3NH3)2NaTi3F12, 1-Ti, with highly symmetric kagome layers and disordered interlayer methylammonium cations, shows no magnetic ordering down to 0.1 K. To study the impact of structural disorder in the titanium fluoride kagome compounds, (CH3NH3)2KTi3F12, 2-Ti, was prepared. It presents no detectable structural disorder and only a small degree of distortion of the kagome lattice. The methylammonium disorder model of 1-Ti and order in 2-Ti were confirmed by atomic-resolution transmission electron microscopy. The antiferromagnetic interactions and band structures of both compounds were calculated based on spin-polarized density functional theory and support the magnetic structure analysis. Three spin-glass-like (SGL) transitions were observed in 2-Ti at 0.5, 1.4, and 2.3 K, while a single SGL transition can be observed in 1-Ti at 0.8 K. The absolute values of the Curie-Weiss temperatures of both 1-Ti (-139.5(7) K) and 2-Ti (-83.5(7) K) are larger than the SGL transition temperatures, which is indicative of geometrically frustrated spin glass (GFSG) states. All the SGL transitions are quenched with an applied field >0.1 T, which indicates novel magnetic phases emerge under small applied magnetic fields. The well-defined structure and the lack of structural disorder in 2-Ti suggest that 2-Ti is an ideal model compound for studying GFSG states and the potential transitions between spin liquid and GFSG states.

High-Frequency and -Field Electron Paramagnetic Resonance Spectroscopic Analysis of Metal-Ligand Covalency in a 4f7 Valence Series (Eu2+, Gd3+, and Tb4+).

The recent isolation of molecular tetravalent lanthanide complexes has enabled renewed exploration of the effect of oxidation state on the single-ion properties of the lanthanide ions. Despite the isotropic nature of the 8S ground state in a tetravalent terbium complex, [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4], preliminary X-band electron paramagnetic resonance (EPR) measurements on tetravalent terbium complexes show rich spectra with broad resonances. The complexity of these spectra highlights the limits of conventional X-band EPR for even qualitative determination of zero-field splitting (ZFS) in these complexes. Therefore, we report the synthesis and characterization of a novel valence series of 4f7 molecular complexes spanning three oxidation states (Eu2+, Gd3+, and Tb4+) featuring a weak-field imidophosphorane ligand system, and employ high-frequency and -field electron paramagnetic resonance (HFEPR) to obtain quantitative values for ZFS across this valence series. The series was designed to minimize deviation in the first coordination sphere from the pseudotetrahedral geometry in order to directly interrogate the role of metal identity and charge on the complexes' electronic structures. These HFEPR studies are supported by crystallographic analysis and quantum-chemical calculations to assess the relative covalent interactions in each member of this valence series and the effect of the oxidation state on the splitting of the ground state and first excited state.

Frustrated Magnetism in a 2-D Ytterbium Fluoride.

Geometric frustration of magnetic ions combines with spin-orbit coupling and structural disorder to give rise to potentially, highly entangled magnetic states such as quantum spin liquids. While fluoride-based frustrated magnets are common in the d-block, in the f-block, fluoride-based frustrated magnets are extremely rare. Herein, we report the synthesis of KYb2F5SO4, a fluoride-based distorted triangular lattice antiferromagnet with no structural disorder but significant geometric distortion from an ideal triangular lattice. In KYb2F5SO4, no long-range ordering can be observed down to 0.10 K, and the low-temperature Curie-Weiss temperature θcw = -0.46(2) K yields a frustration parameter of greater than 4.6. The magnetic entropy released at low temperature indicates an effective spin-1/2 Kramer's doublet ground state. However, the low saturation field and incomplete recovery of magnetic entropy down to 0.10 K under zero-field imply weak quantum entanglement.

Synthesis and Magneto-Structural Characterization of Yb3(OH)7SO4·H2O: a Frustrated Quantum Magnet with Tunable Stacking Disorder.

The quenched structural disorder in frustrated magnets can lead to apparent quantum spin liquid (QSL) behavior or to a valence bond glass state: the transition between these thermodynamic states has not been demonstrated experimentally. Herein, we report the synthesis of a novel layered rare earth hydroxide Yb3(OH)7SO4·H2O as single crystals. The interplay between the strong distortion of the triangular lattice and low point group symmetry of the three distinct Yb3+ sites leads to quenched disorder. The variable stacking disorder in Yb3(OH)7SO4·H2O is elucidated by comparison to the lutetium analogue, Lu3(OH)7SO4·H2O. The degree of disorder in Yb3(OH)7SO4·H2O is controlled by the chemical form of the starting material and solution pH. In a low magnetic field, Yb3(OH)7SO4·H2O displays QSL behavior, while, under a high field, a valence bond glass state is observed. The degree of stacking fault disorder in Yb3(OH)7SO4·H2O modulates the observed magnetic properties and the transition between QSL and valence bond glass states.

Comparative investigation on a hexane-degrading strain with different cell surface hydrophobicities mediated by starch and chitosan.

Bioremediation usually exhibits low removal efficiency toward hexane because of poor water solubility, which limits the mass transfer rate between the substrate and microorganism. This work aimed to enhance the hexane degradation rate by increasing cell surface hydrophobicity (CSH) of the degrader, Pseudomonas mendocina NX-1. The CSH of P. mendocina NX-1 was manipulated by treatment with starch and chitosan solution of varied concentrations, reaching a maximum hydrophobicity of 52%. The biodegradation of hexane conformed to the Haldane inhibition model, and the maximum degradation rate (ν max) of the cells with 52% CSH was 0.72 mg (mg cell)-1·h-1 in comparison with 0.47 mg (mg cell)-1·h-1 for cells with 15% CSH. The production of CO2 by high CSH cells was threefold higher than that by cells at 15% CSH within 30 h, and the cumulative rates of O2 consumption were 0.16 and 0.05 mL/h, respectively. High CSH was related to low negative charge carried by the cell surface and probably reduced the repulsive electrostatic interactions between hexane and microorganisms. The FT-IR spectra of cell envelopes demonstrated that the methyl chain was inversely proportional to increasing CSH values, but proteins exhibited a positive effect to CSH enhancement. The ratio of extracellular proteins and polysaccharides increased from 0.87 to 3.78 when the cells were treated with starch and chitosan, indicating their possible roles in increased CSH.

A cytosine analogue 5-azacitidine improves the accumulation of licochalcone A in licorice Glycyrrhiza inflata.

Licochalcone A (LCA) is a characteristic compound of Glycyrrhiza inflata with anti-inflammatory, antioxidant and antitumor activities. However, G. inflata produces LCA in low quantities that does not meet the market demand. In this study, we found that DNA methylation inhibitor 5-azacitidine (5-azaC) successfully improved the LCA contents in G. inflata seedlings. Transcriptome analysis revealed a series of differentially expressed genes (DEGs), including transcription factors such as MYB, ERF, WRKY, and some structural genes related to flavonoid biosynthesis. However, whole genome bisulfite sequencing (BS-seq) results showed little effect of the 5-azaC treatment on the alteration of DNA methylation on these genes, indicating the possibility that 5-azaC acts as a stimulus, but not an epigenetic modulation factor to improve the LCA content in G. inflata. Additionally, we applied the 5-azaC treatment to field plants and hairy roots and successfully increased the LCA contents in both cases. This research demonstrates the feasibility of 5-azaC treatments in future applications to improve plant production of LCA.

Tetrathiafulvalene-2,3,6,7-tetrathiolate linker redox-state elucidation via S K-edge X-ray absorption spectroscopy.

Sulfur K-edge XAS data provide a unique tool to examine oxidation states and covalency in electronically complex S-based ligands. We present sulfur K-edge X-ray absorption spectroscopy on a discrete redox-series of Ni-based tetrathiafulvalene tetrathiolate (TTFtt) complexes as well as on a 1D coordination polymer (CP), NiTTFtt. Experiment and theory suggest that Ni-S covalency decreases with oxidation which has implications for charge transport pathways.

Tuning symmetry and magnetic blocking of an exchange-coupled lanthanide ion in isomeric, tetrametallic complexes: [LnCl6(TiCp2)3].

The synthesis and magnetic properties of two pairs of isomeric, exchange-coupled complexes, [LnCl6(TiCp2)3] (Ln = Gd, Tb), are reported. In each isomeric pair, the central lanthanide ion adopts either a pseudo-octahedral (O-Ln) or trigonal prismatic geometry (TP-Ln) yielding complexes with C 1 or C 3h molecular symmetry, respectively. Ferromagnetic exchange coupling is observed in TP-Ln as indicated by the increases in χ m T below 30 K. For TP-Gd, a fit to the susceptibility reveals ferromagnetic coupling between the Gd3+ ion and the Ti3+ ions (J = 2.90(1) cm-1). In contrast to O-Tb, which shows no single-molecule magnetic behavior, the TP-Tb complex presents slow magnetic relaxation with a 100s-blocking temperature of 2.3 K and remanent magnetization at zero field up to 3 K. The calculated electronic structures of both compounds imply that trigonal prismatic geometry of TP-Tb is critical to the observed magnetic behavior.

Elemental chalcogen reactions of a tetravalent uranium imidophosphorane complex: cleavage of dioxygen.

Herein we report the first example of a mononuclear uranium complex, [U4+(NP(pip)3)4] (1-U), that selectively reduces dioxygen to produce a terminal oxo complex, [U6+O(NP(pip)3)4] (2-U; [NP(pip)3]1- is tris(piperidinyl)imidophosphorane). Reactions between 1-U and the heavier elemental chalcogens, S8 or Se0, result in six-coordinate U(VI) complexes, [U6+(κ2-E3)(NP(pip)3)4] (E = S (3-U) or Se (4-U)).

Chalcogen-atom abstraction reactions of a Di-iron imidophosphorane complex.

Reaction of the complexes [Fe2(µ2-NP(pip)3)2(NP(pip)3)2] (1-Fe) and [Co2(µ2-NP(pip)3)2(NP(pip)3)2] (1-Co), where [NP(pip)3]1- is tris(piperidinyl)imidophosphorane, with nitrous oxide, S8, or Se0 results in divergent reactivity. With nitrous oxide, 1-Fe forms [Fe2(µ2-O)(µ2-NP(pip)3)2(NP(pip)3)2] (2-Fe), with a very short Fe3+-Fe3+ distance. Reactions of 1-Fe with S8 or Se0 result in the bridging, side-on coordination (µ-κ1:κ1-E22-) of the heavy chalcogens in complexes [Fe2(µ-κ1:κ1-E2)(µ2-NP(pip)3)2(NP(pip)3)2] (E = S, 3-Fe, or Se, 4-Fe). In all cases, the complex 1-Co is inert.

Synthesis of a d1-titanium fluoride kagome lattice antiferromagnet.

The kagome lattice, composed of a planar array of corner-sharing triangles, is one of the most geometrically frustrated lattices. The realization of a spin S = 1/2 kagome lattice antiferromagnet is of particular interest because it may host an exotic form of matter, a quantum spin liquid state, which shows long-range entanglement and no magnetic ordering down to 0 K. A few S = 1/2 kagome lattice antiferromagnets exist, typically based on Cu2+, d9 compounds, though they feature structural imperfections. Herein, we present the synthesis of (CH3NH3)2NaTi3F12, which comprises an S = 1/2 kagome layer that exhibits only one crystallographically distinct Ti3+, d1 site, and one type of bridging fluoride. A static positional disorder is proposed for the interlayer CH3NH3+. No structural phase transitions were observed from 1.8 K to 523 K. Despite its spin-freezing behaviour, other features-including its negative Curie-Weiss temperature and a lack of long-range ordering-imply that this compound is a highly frustrated magnet with unusual magnetic phase behaviours.

Homoleptic cerium tris(dialkylamido)imidophosphorane guanidinate complexes.

We report the synthesis of a new potassium tris(piperdino)imidophosphorane N,N'-dicyclohexylguanidinate, K[CyGNP(pip)3], and describe the synthesis and characterization of the tris-homoleptic compounds, [Ce(CyGNP(pip)3)3], 1-Ce, and [Ce(CyGNP(pip)3)3][BPh4], 2-Ce. The latter is an unusual cationic tetravalent cerium complex. Cyclic voltammetry studies of 1-Ce and 2-Ce revealed Epc potentials of -1.56 V and -1.81 V, and Epa potentials of -0.78 V and -0.66 V (200 mV s-1; THF, vs. Fc0/+), respectively. Compounds 1-Ce and 2-Ce were studied by L3-edge X-ray absorption near-edge spectroscopy (XANES), and the fit of the spectrum of 2-Ce revealed a white-line multiplet with an nf value of 0.50(2).

Synthesis of a d2 kagome lattice antiferromagnet, (CH3NH3)2NaV3F12.

The ground-state of S = 1 kagome lattice antiferromagnets (KLAFs), in the presence of strong geometric frustration and the smallest integer spin, has the potential to host a range of non-trivial magnetic phases including a quantum spin liquid. The effect of local geometry and metal-ion electronic structure on the formation of these predicted phases remain unknown due to, in part, the lack of an ideal analyte. Herein, a kagome lattice compound, (CH3NH3)2NaV3F12 (1-V), featuring a single distinct V3+ (d2) site in the R3̄m space group, was synthesized hydrothermally. In this S = 1, d2 system, the trivalent vanadium ions are tetragonally compressed due to Jahn-Teller distortion. The interlayer methylammonium cations show static positional disorder with three possible orientations. The negative Curie-Weiss temperature and dominant antiferromagnetic interactions make 1-V a candidate to study S = 1 KLAF physics. The frequency-dependence of ac magnetic susceptibility and the heat capacity results suggest that 1-V has a spin glass ground state. This freezing of the spin dynamics may be due to competing exchange interactions, structural imperfection arising from the static disorder of the interlayer methylammonium cations or the presence of 'defect'-like spins.

Coinage metal tris(dialkylamido)imidophosphorane complexes as transmetallation reagents for cerium complexes.

We report the synthesis of tetrameric Cu(i) and Ag(i) homoleptic complexes supported by the tris(piperidinyl)imidophosphorane [NP(pip)3]1- and [NP(1,2-bis-tBu-diamidoethane)(NEt2)]1- ligands. These complexes demonstrate the redox stability of the imidophosphorane ligands to oxidizing salts, and the silver complexes can, in turn, serve as oxidative transmetallation reagents for the isolation of tetravalent cerium complexes from cerium metal or trivalent cerium precursors.

Synthesis of homoleptic, divalent lanthanide (Sm, Eu) complexes via oxidative transmetallation.

The direct synthesis of neutral, divalent samarium and europium complexes supported by the bulky bis(tris-tert-butoxysilyl)amide (BTTSA) ligand via oxidative transmetallation is reported. Through the use of a copper(i) ligand complex, conventional lanthanide halide starting materials for complex formation are circumvented and the clean formation of divalent complexes is achieved directly from the bulk metal. The structures of the [Ln(BTTSA)2] (Ln = Sm, Eu) complexes are isotypic, presenting divalent lanthanide ions with distorted, six-coordinate geometries.

[Removal of Volatile Sulfur Odor by the Biotrickling Filter].

The biodegradation of gas-phase mixtrue of dimethyl sulfide (DMS) and 1-propanethiol (PT) was examined in a biotrickling filter (BTF), inoculated with a microbial consortium composed of activated sewage sludge, and pure strains of Alcaligenes sp. SY1 and Pseudomonas putida. S-1. BTF could be successfully started up within only 11 days when the inlet concentrations of DMS and PT were both 50 mg·m-3 and EBRT was 30 s, with 90% removal efficiency (RE) of DMS and 100% RE of PT. In the steady state, the maximum elimination capacities of DMS and PT were 8.7 g·(m3·h)-1 and 12.4 g·(m3·h)-1, respectively. The presence of PT with a concentration up to 51 mg·m-3 showed an antagonistic removal pattern for DMS, but the opposite did not occur. Meanwhile, the BTF showed high efficiency in the biodegradation of H2S. When the concentration of H2S was as high as 230 mg·m-3, the RE of H2S could reach 98%. However, H2S showed a declining effect on the removal of DMS when the concentration exceeded 115 mg·m-3.