Nature of cyanoargentate bridges defining spin crossover in new 2D Hofmann clathrate analogues.

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)2}- are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)2{Ag(CN)2}2] (1) and {Fe(etpz)2[Ag2(CN)3][Ag(CN)2]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T1/2 ↓ = 233 K and T1/2 ↑ = 243 K for 1 and T1/2 ↓ = 188 K and T1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)2]- units and FeII cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of FeII cations bridged by both binuclear [Ag2(CN)3]- and mononuclear [Ag(CN)2]- units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯Netpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes.

Four-Step Spin Crossover in a New Cyano-Bridged Iron-Silver Coordination Polymer.

Spin-crossover complexes with multistep transitions attract much attention due to their potential applications as multi-switches and for data storage. A four-step spin crossover is observed in the new iron(II)-based cyanometallic guest-free framework compound Fe(2-ethoxypyrazine)2 {Ag(CN)2 }2 during the transition from the low-spin to the high-spin state. A reverse process occurs in three steps. Crystallographic studies reveal an associated stepwise evolution of the crystal structures. Multiple transitions in the reported complex originate from distinct FeII sites which exist due to the packing of the ligand with a bulky substituent.

1D iron(II)-1,2,4-triazolic chains with spin crossover assembled from discrete trinuclear complexes.

We report on a molecular cationic iron(II) complex with a 4-amino-1,2,4-triazole ligand and a tetraiodomercurate anion exhibiting an incomplete spin crossover (SCO). The complex exhibits an unusual disordered structure with a linear arrangement of ligand and water molecules that can potentially accommodate up to four iron atoms, but both terminal metal positions have half chemical occupancies, while occupancies of all ligands are full. This corresponds to the crystallisation of disordered trinuclear complexes arranged into 1D supramolecular chains. Iron cations have different N6 or N3O3 coordination environments, leading to the thermally induced SCO in two thirds of the metal centres. This SCO behaviour was characterised by magnetic susceptibility measurements and Mössbauer spectroscopy.

Hofmann-Like Frameworks Fe(2-methylpyrazine)n[M(CN)2]2 (M = Au, Ag): Spin-Crossover Defined by the Precious Metal.

Hofmann-like cyanometalates constitute a large class of spin-crossover iron(II) complexes with variable switching properties. However, it is not yet clearly understood how the temperature and cooperativity of a spin transition are influenced by their structure. In this paper, we report the synthesis and crystal structures of the metal-organic coordination polymers {FeII(Mepz)[AuI(CN)2]2} ([Au]) and {FeII(Mepz)2[AgI(CN)2]2} ([Ag]), where Mepz = 2-methylpyrazine, along with characterization of their spin-state behavior by variable-temperature SQUID magnetometry and Mössbauer spectroscopy. The compounds are built of cyanoheterometallic layers, which are pillared by the bridging Mepz ligands in [Au] but separated in [Ag]. The complex [Au] exhibits an incomplete stepped spin transition as a function of the temperature with TSCO1 = 170 K and TSCO2 = 308 K for the two subsequent steps. In contrast, the complex [Ag] attains the high-spin state over the whole temperature range. In the crystal structure of [Ag], weak interlayer contacts (Ag-π, Me-π, and Ag-N) are found that may be responsible for an unusual axial elongation of the FeN6 polyhedra. We propose that this structural distortion contributes to the trapping of iron in its high-spin state.

Phase Trapping in Multistep Spin Crossover Compound.

The dimeric motif is the smallest unit for two interacting spin centers allowing for systematic investigations of cooperative interactions. As spin transition compounds, dinuclear complexes are of particular interest, since they potentially reveal a two-step spin crossover (SCO), switching between the high spin-high spin [HS-HS], the high spin-low spin [HS-LS], and the low spin-low spin [LS-LS] states. Herein, we report the synthesis and characterization of six dinuclear iron(II) complexes [FeII2(µ2-L1)2](BF4)4 (C1), [FeII2(µ2-L1)2](ClO4)4 (C2), [FeII2(µ2-L1)2](F3CSO3)4 (C3), [FeII2(µ2-L2)2](BF4)4 (C4), [FeII2(µ2-L2)2](BF4)4 (C5), and [FeII2(µ2-L2)2](BF4)4 (C6), based on the 1,3,4-thiadiazole bridging motif. The two novel bis-tridentate ligands (L1 = 2,5-bis{[(1H-imidazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole and L2 = 2,5-bis{[(thiazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole) were employed in the presence of iron(II) salts with the different counterions. Upon varying ligands and counterions, we were able to change the magnetic properties of the complexes from a temperature-independent [HS-HS] spin state over a one-step spin transition toward a two-step SCO. When cooled slowly from room temperature, the two-step SCO goes along with two distinct phase transitions, and in the intermediate mixed [HS-LS] state distinct HS/LS pairs can be identified unambiguously. In contrast, rapid cooling precludes a crystallographically observable phase transition. For the mixed [HS-LS] state Mössbauer spectroscopy confirms a statistical (random) orientation of adjacent [HS-LS]·[HS-LS]·[HS-LS] chains.

A Mixed-Valent Iron (II/III) Diamond Chain with Single-Ion Anisotropy.

The geometrically frustrated diamond spin chain system has yielded materials with a diversity of interesting magnetic properties but is predominantly limited to compounds with single-spin components. Here, we report the compound [(CH3)2NH2]6[FeIII4FeII2(µ3-O)2(µ3-OH)2(µ3-SO4)8] (1), which features the mixed-valent iron(II/III) diamond chain: ∞[FeIII-(FeIII)2-FeIII-(FeII)2]. 57Fe Mössbauer spectroscopy shows that two-thirds of the total spins in the ∞[FeIII4FeII2] diamond chain are spin-5/2 (high-spin FeIII), while the remaining one-third are spin-2 (high-spin FeII). To date, 1 is the only diamond-chain compound composed of more than one type of dimer, namely, (FeIII)2 and (FeII)2. On the basis of temperature-dependent 57Fe Mössbauer spectroscopy data, an alternating noncollinear 90° magnetic structure is proposed. Both the (FeIII)2 and (FeII)2 dimers are antiferromagnetically coupled and align in the direction along the chain axis ≈ [010], whereas the moments of the bridging FeIII monomers are oriented orthogonally. The spin canting, arising from the anisotropy of the FeII ions, leads to ferrimagnetic ordering at low temperatures.

Gold(ii) in redox-switchable gold(i) catalysis.

Gold(ii) species catalyse the cyclisation of N(2-propyn-1-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline without halide abstraction while the saturated gold(i) complex is inactive. Redox-switching between gold(ii) and gold(i) turns catalytic turnover on and off.

Room temperature hysteretic spin crossover in a new cyanoheterometallic framework.

A new iron(ii)-based spin-crossover compound with thermal hysteresis operating under ambient conditions is reported. This complex exhibits a high reproducibility of the spin transition in many successive thermal cycles, stability of both spin states at room temperature and an attractive operational temperature range.

Screw- Type Motion and Its Impact on Cooperativity in BaNa2Fe[VO4]2.

BaNa2Fe[VO4]2 contains a Jahn-Teller active ion (FeII, 3d6, high-spin) in an octahedral coordination. On the basis of a combination of temperature-dependent X-ray diffraction and Mössbauer and Raman spectroscopies, we demonstrate the coupling of lattice dynamics with the electronic ground state of FeII. We identify three lattice modes combined to an effective canted screw- type motion that drives the structural transition around room temperature from the high-temperature ( P3̅) via intermediate phases to the low-temperature phase ( C2/ c). The dynamics of the electronic ground state of Fe(II) are evident from Mössbauer data with signatures of a motion-narrowed doublet above 320 K, a gradual evolution of the 5Eg electronic state below 293 K, and finally the signature of the thermodynamically preferred orbitally nondegenerate ground state (5Ag) of Fe(II) below 100 K. The continuous nature of the transition is associated with the temperature-dependent phonon parameters derived from Raman spectroscopy, which point out the presence of strong electron-phonon coupling in this compound. We present a microscopic mechanism and evaluate the collective component leading to the structural phase transition.

Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO.

From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary [Formula: see text] can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [Formula: see text] layers. The two polymorphs have similar unit cells but different crystal symmetries (Cmc21 and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d6) and high-spin, as proven by X-ray spectroscopy, Mössbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (TN = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth.

Indefinitely stable iron(IV) cage complexes formed in water by air oxidation.

In nature, iron, the fourth most abundant element of the Earth's crust, occurs in its stable forms either as the native metal or in its compounds in the +2 or +3 (low-valent) oxidation states. High-valent iron (+4, +5, +6) compounds are not formed spontaneously at ambient conditions, and the ones obtained synthetically appear to be unstable in polar organic solvents, especially aqueous solutions, and this is what limits their studies and use. Here we describe unprecedented iron(IV) hexahydrazide clathrochelate complexes that are assembled in alkaline aqueous media from iron(III) salts, oxalodihydrazide and formaldehyde in the course of a metal-templated reaction accompanied by air oxidation. The complexes can exist indefinitely at ambient conditions without any sign of decomposition in water, nonaqueous solutions and in the solid state. We anticipate that our findings may open a way to aqueous solution and polynuclear high-valent iron chemistry that remains underexplored and presents an important challenge.

Haloperoxidase Mimicry by CeO2-x Nanorods Combats Biofouling.

CeO2-x nanorods are functional mimics of natural haloperoxidases. They catalyze the oxidative bromination of phenol red to bromophenol blue and of natural signaling molecules involved in bacterial quorum sensing. Laboratory and field tests with paint formulations containing 2 wt% of CeO2-x nanorods show a reduction in biofouling comparable to Cu2 O, the most typical biocidal pigment.

Spin Crossover in Fe(II)-M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2-Substituted Pyrazines.

Discovery of spin-crossover (SCO) behavior in the family of Fe(II)-based Hofmann clathrates has led to a "new rush" in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe(2+), square planar tetracyanometalates M(II)(CN)4(2-) (M(II) = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe(II) ion has a pseudo-octahedral coordination environment supported by four µ4-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M(II) = Ni (1), Pd (2), Pt (3); X = Me and M(II) = Ni (4), Pd (5)) or one 2X-pyrazine and one water molecule (X = I and M(II) = Ni (7), Pd (8), Pt (9)), or, alternatively, two distinct Fe(II) positions with either two pyrazines or two water molecules (X = Me and M(II) = Pt (6)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN6 units (1-6) display cooperative spin transition, while Fe(II) ions in N5O or N4O2 surrounding are high spin (HS). Structural changes in the nearest Fe(II) environment upon low-spin (LS) to HS transition, which include ca. 10% Fe-N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of 1-9 (see abstract figure). Effects of a pyrazine substituent and M(II) nature on the hyperfine parameters in both spin states are established.

Hierachical Ni@Fe2O3 superparticles through epitaxial growth of γ-Fe2O3 nanorods on in situ formed Ni nanoplates.

One endeavour of nanochemistry is the bottom-up synthesis of functional mesoscale structures from basic building blocks. We report a one-pot wet chemical synthesis of Ni@γ-Fe2O3 superparticles containing Ni cores densely covered with highly oriented γ-Fe2O3 (maghemite) nanorods (NRs) by controlled reduction/decomposition of nickel acetate (Ni(ac)2) and Fe(CO)5. Automated diffraction tomography (ADT) of the Ni-Fe2O3 interface in combination with Mössbauer spectroscopy showed that selective and oriented growth of the γ-Fe2O3 nanorods on the Ni core is facilitated through the formation of a Fe0.05Ni0.95 alloy and the appearance of superstructure features that may reduce strain at the Ni-Fe2O3 interface. The common orientation of the maghemite nanorods on the Ni core of the superparticles leads to a greatly enhanced magnetization. After functionalization with a catechol-functional polyethylene glycol (C-PEG) ligand the Ni@γ-Fe2O3 superparticles were dispersible in water.

Enantioselective Guest Effect on the Spin State of a Chiral Coordination Framework.

The diversity of spin crossover (SCO) complexes that, on the one hand, display variable temperature, abruptness and hysteresis of the spin transition, and on the other hand, are spin-sensitive to the various guest molecules, makes these materials unique for the detection of different organic and inorganic compounds. We have developed a homochiral SCO coordination polymer with a spin transition sensitive to the inclusion of the guest 2-butanol, and these solvates with (R)- and (S)-alcohols demonstrate different SCO behaviours depending on the chirality of the organic analyte. A stereoselective response to the guest inclusion is detected as a shift in the temperature of the transition both from dia- to para- and from para- to diamagnetic states in heating and cooling modes respectively. Furthermore, the Mössbauer spectroscopy directly visualizes how the metallic centres in a chiral coordination framework differently sense the interaction with guests of different chiralities.

Synthesis of Nanocrystals and Particle Size Effects Studies on the Thermally Induced Spin Transition of the Model Spin Crossover Compound [Fe(phen)2(NCS)2].

Surfactant-free nanocrystals of the model spin-crossover compound [Fe(phen)2(NCS)2] (phen: 1,10-phenanthroline) have been synthesized applying the reverse micelle technique. The morphology of the nanocrystals, characterized by scanning electronic microscopy, corresponds to rhombohedric platelets with dimensions ranging from 203 × 203 × 106 nm to 142 × 142 × 74 nm. Variation of the concentration of the Fe(BF4)2·6H2O salt in the synthesis has been found to have little influence on the crystallite size. In contrast, the solvent-surfactant ratio (ω) is critical for a good particle growth. The spin transition of the nanocrystals has been characterized by magnetic susceptibility measurements and Mössbauer spectroscopy. The nanocrystals undergo an abrupt and more cooperative spin transition in comparison with the bulk compound. The spin transition is centered in the interval of temperature of 175-185 K and is accompanied by 8 K of thermal hysteresis width. The crystallite quality more than the crystallite size is responsible for the higher cooperativity. The magnetic properties of the nanocrystals embedded in organic polymers such as polyethylene glycol, nujol, glycerol, and triton have been studied as well. The spin transition in the nanocrystals is affected by the polymer coating. The abrupt and first-order spin transition transforms into a more continuous spin transition as a result of the chemical pressure asserted by the organic polymers on the Fe(II) centers.

Distinct microbial populations are tightly linked to the profile of dissolved iron in the methanic sediments of the Helgoland mud area, North Sea.

Iron reduction in subseafloor sulfate-depleted and methane-rich marine sediments is currently a subject of interest in subsurface geomicrobiology. While iron reduction and microorganisms involved have been well studied in marine surface sediments, little is known about microorganisms responsible for iron reduction in deep methanic sediments. Here, we used quantitative PCR-based 16S rRNA gene copy numbers and pyrosequencing-based relative abundances of bacteria and archaea to investigate covariance between distinct microbial populations and specific geochemical profiles in the top 5 m of sediment cores from the Helgoland mud area, North Sea. We found that gene copy numbers of bacteria and archaea were specifically higher around the peak of dissolved iron in the methanic zone (250-350 cm). The higher copy numbers at these depths were also reflected by the relative sequence abundances of members of the candidate division JS1, methanogenic and Methanohalobium/ANME-3 related archaea. The distribution of these populations was strongly correlated to the profile of pore-water Fe(2+) while that of Desulfobacteraceae corresponded to the pore-water sulfate profile. Furthermore, specific JS1 populations also strongly co-varied with the distribution of Methanosaetaceae in the methanic zone. Our data suggest that the interplay among JS1 bacteria, methanogenic archaea and Methanohalobium/ANME-3-related archaea may be important for iron reduction and methane cycling in deep methanic sediments of the Helgoland mud area and perhaps in other methane-rich depositional environments.

A heteroleptic push-pull substituted iron(II) bis(tridentate) complex with low-energy charge-transfer states.

A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)](2+) with a strongly electron-withdrawing ligand (dcpp, 2,6-bis(2-carboxypyridyl)pyridine) and a strongly electron-donating tridentate tripyridine ligand (ddpd, N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine) is reported. Both ligands form six-membered chelate rings with the iron center, inducing a strong ligand field. This results in a high-energy, high-spin state ((5) T2 , (t2g )(4) (eg *)(2) ) and a low-spin ground state ((1) A1 , (t2g )(6) (eg *)(0) ). The intermediate triplet spin state ((3) T1 , (t2g )(5) (eg *)(1) ) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low-energy π(*) orbitals of dcpp allow low-energy MLCT absorption plus additional low-energy LL'CT absorptions from ddpd to dcpp. The directional charge-transfer character is probed by electrochemical and optical analyses, Mößbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)](3+) and [Fe(dcpp)(ddpd)](+) , augmented by density functional calculations. The combined effect of push-pull substitution and the strong ligand field paves the way for long-lived charge-transfer states in iron(II) complexes.

Diferrocenyl tosyl hydrazone with an ultrastrong NHFe hydrogen bond as double click switch.

The ultrastrong and short intramolecular NHFe hydrogen bond in diferrocenyl hydrazone raises the barrier for intramolecular electron transfer in its mixed-valent cation and is only disrupted by double oxidation to .

Spin crossover star-shaped metallomesogens of iron(II).

Three new types of spin crossover (SCO) metallomesogens of Fe(II) based on symmetric tripod ligands and their magnetic and structural properties are reported here. These were obtained by condensation of tris(2-aminoethyl)amin (tren) with the aldehyde derived from 3-alkoxy-6-methylpyridine (mpyN, N (number of carbon atoms in n-alkyl chains) = 8, 18), 1-alkyl-1H-imidazole (imN, N = 4, 16, 18, 20, 22), or 1-alkyl-1H-benzimidazole (bimN, N = 6, 14, 16, 18, 20). A complex derived from 1-octadecyl-1H-naphtho[2,3-d]imidazole (nim18) retains the high spin state at any temperature. Single crystals of the short-chain complexes were investigated by a combination of X-ray crystallography, magnetic measurements and Mössbauer spectroscopy. Generally, in comparison with the short-chain complexes the long-chain complexes display more gradual SCO and undergo a phase transition crystal-liquid crystal that is reflected in their magnetic properties. Characterization by X-ray powder diffractometry and differential calorimetry reveal formation of a smectic mesophase upon melting.