The low-temperature reverse water-gas shift (LT-RWGS) is a critical and energy effective technology for syngas production and the mitigation of anthropogenic carbon emissions. Developing efficient and well-defined catalysts for the LT-RWGS, from which structure-activity relationships can be drawn, is a significant challenge. Herein we describe how the identification of the grafting properties of tetramesityldiiron (Fe2Mes4) helps with designing tailored and highly efficient catalysts of PtFe@SiO2 composition. To that end, a molecular analogue, Fe2Mes3OSi(OtBu)3, was synthesized and characterized by X-ray diffraction, 57Fe-Mössbauer and 1H-NMR spectroscopy. The results confirmed that tetramesityldiiron grafts onto silica via selective displacement of a single mesityl ligand, forming Fe2Mes3@SiO2, while steric hindrance likely prevents secondary interactions with surface siloxide bridges. This work highlights the potential of tetramesityldiiron as a versatile precursor for synthesizing bimetallic MFe@SiO2 systems, enabling the rational development of highly efficient LT-RWGS and CO2 hydrogenation catalysts.

[{Cp'Fe(μ-I)}2] (I; Cp' = η5-1,2,4-(Me3C)3C5H2) reacts with [Na(OCP)(1,4-dioxane)x] and K(OCAs) to yield [(Cp'Fe)2(μ-η2:η2-P2)(μ-CO)] (1) and [(Cp'Fe)2(μ-η2:η2-As2)(μ-CO)] (3), respectively. While complex 1 was previously accessed by Scherer and co-workers using rather harsh reaction conditions, the new synthetic method already proceeds at ambient temperature. UV-light irradiation of 1 and 3 induces CO release forming complexes [(Cp'Fe)2(μ-η2:η2-E2)] (E = P (2) and As (4)), respectively. However, both reactions also yielded several byproducts which were spectroscopically identified. Furthermore, upon thermally triggered CO elimination from 3 the Fe3As6-cluster [(Cp'Fe)3(As3)2] (5) is isolated in low yield. In addition, zero-field 57Fe Mössbauer spectra were recorded on complexes 1-4 and computational studies complement the experimental findings and provide additional insights into the bonding in these complexes and the reaction pathways resulting in the formation of complexes 1-4.

The reduction of the iron(II) half-sandwich precursors [Cp'FeN(R)(SiMe3)] (R = SiMe3, 2,6-di-iso-propylphenyl (dipp)) with KC8 yields the one-dimensional polymeric iron(I) complexes {[Cp'Fe(N(SiMe3)3)]K}n (4) and {[Cp'Fe(N(dipp)(SiMe3))]K}n (5). Addition of 18-crown-6 breaks up these chains and affords the monomeric salts [Cp'Fe(N(SiMe3)2)][K(18-crown-6)(thf)2] (6) and [Cp'Fe(N(dipp)(SiMe3))][K(18-crown-6)(thf)2] (7). All four compounds were structurally characterized by single-crystal X-ray diffraction analysis and adopt an S = 3/2 spin ground state. Zero-field 57Fe Mössbauer spectra uncover slow magnetic relaxation at low temperatures in each complex. Complementary CASSCF/NEVPT2 calculations reveal an S = 3/2 ground-state Kramers doublet that is ca. 100 cm-1 below the first excited state. These findings are confirmed by results from DC susceptibility and magnetization experiments. The slow relaxation of magnetization is further characterized via AC susceptibility measurements, unveiling detectable slow relaxation of magnetization at zero external field for 4-6. Complex 4 exhibits an unusual field dependence of the quantum tunneling of magnetization, akin to exchange-bias effects, which are commonly observed in lanthanide dimers.

Abstract Cooperativity among spin centres has long been the royal road in spin crossover (SCO) research to impose magnetic bistability in terms of thermal hysteresis. In this work we access magnetic multi-inert states of the iron(III) compound {FeL2[B(Ph)4]} ≡ FeB at low temperature, in addition to thermal bistability. The packing of the low-spin and high-spin forms of crystalline FeB differs only marginally what ultimately leads to structural conservatism. This indicates that the SCO-immanent breathing of the complex cation is almost fully compensated by the anion matrix. The unique cooling rate dependence of the residual low-temperature magnetisation in FeB unveils continuous switching between the trapped high-spin (ON) and the relaxed low-spin state (OFF). The macroscopic ratio of the spin states (ON:OFF) can be adjusted as a simple function of the cooling rate. That is, cooperative spin crossover can be the source of bistable and multi-inert system states in the very same material.

The unsymmetrical amino-imidazolin-2-imine ligand [HAmIm, 1,2-(DippNH)-C6H4-N=C(NiPrCMe)(2)] is employed in the synthesis of the iron(I) arene complex [(AmIm)Fe(eta(6)-C6H6)] and the iron(II) neosilyl complex [(AmIm)Fe(CH2SiMe3)]. These compounds are highly efficient precatalysts in H/D exchange reactions with deuterium (D-2) in hydrosilanes. The scope comprises primary to tertiary silanes at a catalyst loading of 1 mol % at ambient temperature. In-depth mechanistic studies including various control experiments and the syntheses of isolated iron-hydride and iron-silyl compounds are performed. These studies reveal that the activation of both Fe(I) and Fe(II) complexes generates Fe-H/D species as key catalytic intermediates. An alternative catalytic pathway involving an iron-silyl intermediate, although shown to be less feasible by DFT calculations, may also be operative.

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre-two-electron Yb-γ-Me-β–Si secondary metal-ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb-carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.

Co-precipitated 14 nm ?-Fe2O3 and 35 nm MgO nanoparticles (NPs) decorated on zeolite 5 A (Z5A) templates were systematically studied.Fe- 57 Mossbauer and transmission electron microscopy data indicated a broad particle size distribution. The spinel structure of ?-Fe2O3 is strongly affected by size effects (particles smaller than 10 nm in size have an octahedral/tetrahedral iron site ratio of 7/3 instead of 5/3 found in bulk). Al-27 nuclear magnetic resonance (NMR) showed the formation of octahedral Al species after boric acid reaction with MgO NPs.B- 11 NMR spectra indicated adsorption of B on the MgO/Z5A surface, whereas B adsorption in the pure Z5A framework is absent. Adsorption isotherms and kinetic studies of the nanohybrids showed that the B-rich species adsorption is described by Freundlich and fractal-like models, respectively. B adsorption occurs dominantly by the MgO fraction. The nanohybrids presented a good B removal efficiency when compared with other materials reported in literature.

Abstract 4-(2,6-Di(2H-indazol-2-yl)pyridin-4-yl)benzoic acid (1) and 10-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)anthracene-9-carboxylic acid (2) were required for adsorption studies on Ag(111), with a view to subsequent iron(II) complexation and formation of well-ordered spin-responsive self-assembled monolayers. While the generation of these compounds has remained elusive, several intermediates and by-products were obtained, potentially useful as dipyrazolylpyridine-related derivatives and for metal ion coordination. 3,5-Dichloro-2,6-diindazolylpyridine-4-amine, which forms as a mixture of regioisomers, was synthesised, the mixture separated, and the components characterised (3,5-dichloro-2,6-di(2H-indazol-2-yl)pyridin-4-amine; 3,5-dichloro-2-(1H-indazol-1-yl)-6-(2H-indazol-2-yl)pyridin-4-amine; 3,5-dichloro-2,6-di(1H-indazol-1-yl)pyridin-4-amine). Their iron(II) complexes have been prepared and fully characterised, including single crystal X-ray structure determination. The complexes are instructive examples of the influence of ligand design (“steric jamming”) on the spin-crossover (SCO) activity of FeII centres. Bulky substitution, which entails twisted ligand conformation, increases intramolecular crowding. This prevents contraction of the metal coordination sphere, which would be a prerequisite for thermally inducible SCO. Mössbauer spectroscopy has revealed that the complexes remain predominantly high-spin (HS) between 20 and 200 K, and that a mixture of conformational HS isomers is present in the microcrystalline solid.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Multiferroic BiFeO3 (BFO) ceramics are synthesized by non-thermodynamically stable methods, such as fast firing, cryomilling and Spark Plasma Sintering (SPS) with applied pressure of 60 MPa and high heating rate of 150 K/min. We obtained BFO ceramics and powders that present enhanced magnetic and electrical properties. The observed enhancements are caused by the chosen routes, such as cryomilling, which causes a significant reduction of crystallite size, reaching 23 nm, and a high micro-strain level, reaching 1.2%. This is reflected in the magnetic curve which is changed from antiferromagnetic-like to weak-ferromagnetic-like, with an increased maximum magnetization from 0.085 emu/g to 0.369 emu/g. The dielectric constant reached a high value, close to epsilon'= 102 coupled with low dielectric loss (tan delta similar to 0.019) over a wide range of frequencies (1 kHz to 2 MHz), reaching values close to 0.005 near 2 MHz. Such dielectric constant (dielectric loss) values are generally present for highly resistive ceramics. By equivalent circuit fitting we managed to separate the grain and grain boundary contributions. These results and those from XRD diffraction and Mossbauer spectroscopy indicate a migration of defects to the surface of the grains. In summary, we managed to consolidate this sintering method as a powerful method to obtain BFO ceramics with enhanced electrical and magnetic properties. Such improvements, especially the high resistivity and low tangent losses, can open new avenues for future technological applications of BiFeO3 nanostructured ceramics, such as ferroelectric memories and the magnetic enhancements for spintronic based devices.

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f -shell. Here, we showcase the use of state-of-the-art magnetic resonance spec- troscopy (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the structure of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, that features a notable structure according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex; taken together, they provide a detailed understanding of this paramagnetic compound. Namely, this complex displays a three-centre-two-electron Yb-γ-Me-β–Si secondary metal-ligand interaction, whose NMR spectroscopic signature was acquired for the first time for a lanthanide paramagnetic species. The electronic configuration of Yb(III)[CH(SiMe3)2]3 is demonstrated to be close to the one of the free Yb(III) ion, with the partially filled f -shell of the Yb atom having little influence on its bonding properties and with minimal delocalization of f -electron density from Yb to the directly bonded carbons.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The combination of polymers with nanoparticles offers the possibility to obtain customizable composite materials with additional properties such as sensing or bistability provided by a switchable spin crossover (SCO) core. For all applications, a precise control over size and shape of the nanomaterial is highly important as it will significantly influence its final properties. By confined synthesis of iron(II) SCO coordination polymers within the P4VP cores of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) micelles in THF we are able to control the size and also the shape of the resulting SCO nanocomposite particles by the composition of the PS-b-P4VP diblock copolymers (dBCPs) and the amount of complex employed. For the nanocomposite samples with the highest P4VP content, a morphological transition from spherical nanoparticles to worm-like structures was observed with increasing coordination polymer content, which can be explained with the impact of complex coordination on the self-assembly of the dBCP. Furthermore, the SCO nanocomposites showed transition temperatures of T1/2 = 217 K, up to 27 K wide hysteresis loops and a decrease of the residual high-spin fraction down to γHS = 14% in the worm-like structures, as determined by magnetic susceptibility measurements and Mössbauer spectroscopy. Thus, SCO properties close or even better (hysteresis) to those of the bulk material can be obtained and furthermore tuned through size and shape control realized by tailoring the block length ratio of the PS-b-P4VP dBCPs.

The half-sandwich complex [Cp'Fe{N(dipp)(SiMe3)}] (Fe-dipp; Cp' = 1,2,4-tri-tert-butylcyclopentadienyl and dipp = 2,6-diisopropylphenyl) and the mixed metallocene [Cp'Fe{(η5-C6H3iPr2)═N(SiMe3)}] (Fe-chd) formed in the reaction between [{Cp'Fe(μ-I)}2] and [Li{N(dipp)(SiMe3)}]2 were characterized by NMR spectroscopy and X-ray diffraction analysis. Fe-dipp complements the series of low-coordinate, quasi-linear iron amido half-sandwich complexes [Cp'Fe{N(tBu)(SiMe3)}] (Fe-tBu) and [Cp'Fe{N(SiMe3)2}] (Fe-tms) reported earlier, and all three compounds were characterized in the solid state by zero-field 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements, confirming their S = 2 electronic ground state. Moreover, the Mössbauer absorption spectra reveal slow paramagnetic relaxation at low temperatures with large internal magnetic hyperfine fields of Bhf = 96.4 T (Fe-dipp, 20 K), Bhf = 101.3 T (Fe-tBu, 15 K), and Bhf = 96.9 T (Fe-tms, 20 K). The magnetic measurements further confirm that the presence of significant axial zero-field splitting and slow relaxation of magnetization is detected, which is revealed even in the absence of a static magnetic field in the case of Fe-tBu. Supplementary ab initio and density functional theory calculations were performed and support the experimental data.

We report on the application of cryomilling for the synthesis of oxide ceramics nanoparticles with controlled dislocation densities using the rhombohedral BiFeO3 compound as a model system. Multiferroic BiFeO3 nano -particles were prepared with controlled crystallite size, micro-strain and dislocation density. After milling for 150 min the average crystallite size is reduced to 23 nm, the levels of micro-strain increased to 1.2 % and the dislocation density reaches 1.7 x 1013cm 2. This shows that the cryomilling promotes the nanostructuration of the samples and the introduction of atomic defects such as dislocations. In addition we found an enhancement of the magnetic properties resulting from a combination of the presence of small nanoparticles and uncompensated spins along the dislocation lines. The isothermal magnetization measurements showed a weak-ferromagnetic behavior with 0.58 emu/g at 15 kOe at room temperature. Mo center dot ssbauer spectroscopy ruled out the formation of Fe2+ ions and also revealed the presence of a superparamagnetic contribution. The control of crystallite size along with dislocation density in BiFeO3 is proposed as a powerful tool for tuning its magnetic properties.

AbstractCooperativity among spin centres has long been the royal road to impose magnetic bistability in terms of thermal hysteresis. In this work we access magnetic multi-metastability of the iron(III) complex [Fe(L)2][BPh4] (1) at low temperature, in addition to thermal bistability. The packing of the low-spin and high-spin forms of crystalline1differs only marginally what ultimately leads to very minor thermal variation in the lattice constants. This indicates that the SCO-immanent breathing of the complex cation is almost fully compensated by the anion matrix. We believe that this structural conservatism is the origin of the unique cooling-rate dependence of the residual low-temperature magnetisation in1. The system state of1can be continuously tuned between the trapped high-spin (ON) and the relaxed low-spin state (OFF), as a simple function of the cooling rate. That is, cooperative spin crossover can be the source of bistable and multi-metastable system states in the very same material.

(allyl-Me = C3H4-2-Me (eta 3-methallyl); BArF4 = B(3,5-(F3C)2C6H3)4; 1) was treated with the phosphine ligands tBu3P, Ad3P (Ad = tri-1adamantylphosphine), and TMPP (TMPP = tris(2,4,6trimethoxyphenyl)phosphine). Although these phosphines display similar steric bulk and differ only moderately in their electronic properties, very different reaction products are isolated. For tBu3P, only the phosphonium salt [tBu3P-C3H4-2-Me][BArF4] (2) is formed; this contrasts with the reactivity with TMPP, in which the mesitylene ligand is readily displaced to yield [(eta 3-allyl-Me)Ni(tmpp)][BArF4] (3). With Ad3P, we observe the formation of an unusual C-H bond activation product [(toluene)Ni(kappa C:P-C10H14-PAd2][BArF4] (4). Nevertheless, Ad3P successfully stabilizes the two-coordinate Ni(I) species [(Ad3P)2Ni] [BArF4] (5). Complexes 3-5 have been evaluated in the homopolymerization of ethylene and norbornene after addition of the Lewis acid B(C6F5)3.

The presented work shows a synthesis route to obtain nanoparticles of the hexagonal α-NiS phase and core-shell particles where the same material is grown onto previously prepared Au seeds. In the bulk, this nickel sulfide phase is known to exhibit a metal-insulator type phase transition (MIT) at 265 K which drastically alters its electrical conductivity. Since the produced nanoparticles show a localized surface plasmon resonance (LSPR) in the visible range of the electromagnetic spectrum, the development of their optical properties depending on the temperature is investigated. This is the first time an LSPR of colloidal nanoparticles is monitored regarding such a transition. The results of UV-vis absorbance measurements show that the LSPR of the particles can be strongly and reversibly tuned by varying the temperature. It can be switched off by cooling the nanoparticles and switched on again by reheating them above the transition temperature. Additional to the phase transition, the temperature-dependent magnetic susceptibility of α-NiS and Au-NiS nanoparticles suggests the presence of different amounts of uncompensated magnetic moments in these compounds that possibly affect the optical properties and may cause the observed quantitative differences in the LSPR response of these materials.

Structural characterization of novel Fe(III) basic acetate with 1,3,5,7-tetraazaadamantane (hexamine ? HEX), [Fe3(?3-O)(OAc)6(H2O)3)](NO3)(HEX)2(H2O)5] (1), revealed a molecular structure typical for Fe(III) basic carboxylates, with a triangular array formed by three iron atoms being equilateral and with an uncoordinated hexamine molecule and nitrate anion present in the crystal structure. Despite the hexamine molecule being uncoordinated, it plays a crucial role in the formation of a supramolecular network. The magnetic properties of the compound were characterized by magnetic susceptibility, Mo?ssbauer and EPR spectroscopy. These studies indicate the presence of high-spin iron ions in +3 oxidation state with non-Curie-like behaviour. A small room temperature value of the effective magnetic moment and a decrease of the product ?T with decreasing temperature both speak in favour of antiferromagnetic interactions between the three individual Fe(III) ions of complex 1 with a total ground state spin of St = 1/2 per trinuclear cluster. EPR spectroscopy confirmed the formation of a St = 1/2 ground state, but also suggested the presence of weak intermolecular exchange interactions between two (or more) clusters of neighbouring molecules. The temperature-dependent Mo?ssbauer spectra show a slowing-down of the fast spin-lattice relaxation rates of the paramagnetic Fe(III) ion below about 60 K, however, yet no static spin regime is reached down to 3.6 K.

The methyltransferase FliB posttranslationally modifies surface-exposed ɛ-N-lysine residues of flagellin, the protomer of the flagellar filament inSalmonella enterica (S.enterica). Flagellin methylation, reported originally in 1959, was recently shown to enhance host cell adhesion and invasion by increasing the flagellar hydrophobicity. The role of FliB in this process, however, remained enigmatic. In this study, we investigated the properties and mechanisms of FliB fromS.enterica in vivoandin vitro. We show that FliB is an S-adenosylmethionine (SAM) dependent methyltransferase, forming a membrane associated oligomer that modifies flagellin in the bacterial cytosol. Using X-band electron paramagnetic resonance (EPR) spectroscopy, zero-field57Fe Mössbauer spectroscopy, methylation assays and chromatography coupled mass spectrometry (MS) analysis, we further found that FliB contains an oxygen sensitive [4Fe-4S] cluster that is essential for the methyl transfer reaction and might mediate a radical mechanism. Our data indicate that the [4Fe-4S] cluster is coordinated by a cysteine rich motif in FliB that is highly conserved among multiple genera of the Enterobacteriaceae family.