Variable-temperature Magnetic Measurements Research Articles

Ga2.52V2·48O7·33(OH)0.67, a synthetic member of the nolanite/akdalaite-type family of oxyhydroxides containing trivalent vanadium

The oxyhydroxide Ga2.52V2·48O7·33(OH)0.67 is prepared by reaction between Ga metal and Na3VO4 in a 1:1 monoethanolamine:water mixture at 240 ​°C. Powder neutron diffraction shows the material to be isostructural with the minerals nolanite and akdalaite, with cations occupying tetrahedral and octahedral interstitial sites in a hexagonal close-packed array of oxide/hydroxide (P63mc, a ​= ​5.7906(2) Å, c ​= ​9.2550(5) Å). Rietveld refinement against the data shows that Ga preferentially occupies tetrahedral sites, as well as some octahedral sites, and hence all V is octahedrally coordinated. The oxidation state of vanadium is confirmed as close to V3+ using V K-edge X-ray absorption near-edge structure spectroscopy, consistent with the refined chemical composition. The material is metastable, dehydrating around 300 ​°C and then decomposing above 500 ​°C, as shown by thermogravimetric analysis and thermodiffraction. The oxide Ga2.52V2.48O8 produced after dehydration at 300 ​°C is shown to contain a larger proportion of V4+ than the parent oxyhydroxide, to ensure charge balance, but the essential hexagonal structure is maintained. Variable temperature magnetisation measurements show that although both materials appear to obey the Curie-Weiss law at high temperatures, at low temperatures the inverse susceptibility curves are non-linear. There is, however, no evidence for strong magnetic exchange and the extracted effective moments are consistent with the presence of more V3+ in the oxyhydroxide compared to the oxide.

Metal-directed supramolecular architectures based on the bifunctional ligand 2,5-bis(1H-1,2,4-triazol-1-yl)terephthalic acid.

By the solvothermal reactions of 2,5-bis(1H-1,2,4-triazol-1-yl)terephthalic acid (H2L) with transition-metal ions, two novel polymeric complexes, namely, poly[diaqua[μ4-2,5-bis(1H-1,2,4-triazol-1-yl)terephthalato]cobalt(II)], [Co(C12H6N6O4)(H2O)2]n, (1), and poly[[diaqua[μ4-2,5-bis(1H-1,2,4-triazol-1-yl)terephthalato]nickel(II)] dihydrate], {[Ni(C12H6N6O4)(H2O)2]·2H2O}n, (2), were isolated. Both polymers have been characterized by FT-IR spectroscopy, elemental analysis and single-crystal X-ray diffraction analysis. The complexes have similar two-dimensional layered structures and coordination modes. Furthermore, the two-dimensional layered structures bear distinct intermolecular hydrogen-bonding interactions and π-π stacking interactions to form two different three-dimensional supramolecular networks based on 44-subnets. The structural variation depends on the nature of the metal cations. The results of variable-temperature magnetization measurements (χMT-T and χM-1-T) show that complexes (1) and (2) display antiferromagnetic behaviour.

2,6-/1,5-Naphthoquinodimethane bridged porphyrin dimer diradicaloids

2,6-Naphthoquinodimethane (2,6-NQDM)- and 1,5-naphthoquinodimethane (1,5-NQDM)-bridged porphyrin dimers, 2,6-P2 and 1,5-P2, were synthesized as relatively stable compounds. Both exhibit open-shell singlet ground state according to variable-temperature (VT) NMR and magnetic measurements, as well as restricted active space spin-flip (RAS-SF) calculations. The 1,5-P2 isomer has a larger diradical character ([Formula: see text], based on the RAS-SF calculations) and smaller singlet-triplet energy gap ([Formula: see text] kcal/mol, based on SQUID measurements) compared to the 2,6-P2 isomer ([Formula: see text], [Formula: see text] kcal/mol). In addition, 2,6-P2 shows intense one-photon absorption (OPA) ([Formula: see text] nm, [Formula: see text] M[Formula: see text] cm[Formula: see text] and a large two-photon absorption (TPA) cross-section ([Formula: see text] GM at 1400 nm) in the near-infrared region, while 1,5-P2 with larger diradical character displays red-shifted but weaker OPA ([Formula: see text] nm, [Formula: see text] M[Formula: see text] cm[Formula: see text] and a smaller TPA cross-section ([Formula: see text] GM at 1600 nm). Both compounds show four reversible redox waves and 1,5-P2 has a smaller electrochemical energy gap (1.06 eV vs.1.16 eV for 2,6-P2). Therefore, the bridge structure has a significant impact on the diradical character, electronic properties, and magnetic behaviors of the obtained porphyrin-based diradicaloids.

Effects of deuteration on the structure and magnetic properties of bis-quinolinium tetrabromidocuprate(II) dihydrate

The partially [d6, 2] and fully [d20, 3] deuterated analogues of (QuinH)2CuBr4·2H2O (d0, 1) were prepared and their crystal structures were determined [Quin = quinoline]. In both compounds, there is a clear disorder in the positions of the bromide ions which was resolved. This led to a reexamination of the structure of the parent, fully protonated compound (1) where a small percentage of previously unrecognized disorder was also observed and the structure rerefined. Variable temperature magnetization measurements over the range 1.8–310 K indicate that all three materials behave as magnetically well-isolated layers that can be evaluated using the 2D-quantum Heisenberg antiferromagnetic model. Final fitting results for the partially (J = −5.96(5) K) and fully (J = −5.77(2) K) deuterated compounds indicate slightly weaker exchange compared to the protonated compound (J = −6.17(3) K), likely as a result of the increased disorder in the deuterated phases.

Unusual coordination behavior by a hydroxypyridine/pyridone ligand: Synthesis and structure of [(2-bromo-4-hydroxypyridine)2(2-bromo-1(H)-4-pyridone)2copper(II)] perchlorate • 2(2-bromo-4-hydroxypyridine) • 2(2-bromo-1(H)-4-pyridone)

The synthesis and structure of [Cu(2-Br-4-HOpy)2(2-Br-4-pyone)2](ClO4)2 • 2(2-Br-4-HOpy) • 2(2-Br-4-pyone) is reported [2-Br-4-HOpy = 2-bromo-4-hydroxypyridine; 2-Br-4-pyone = 2-bromo-(1H)4-pyridone]. The compound forms as a co-crystal in the triclinic P-1 space group with four-coordinate Cu(II) ions and perchlorate geganions interacting with both the hydroxypyridine and pyridone forms of the organic molecule through hydrogen and halogen bonding. The coordination complex exhibits the unusual coordination motif of having both the hydroxypyridine and pyridone tautomers of the ligand coordinated to the same metal ion. The isolation of the Cu complex by the organic groups suggests an absence of magnetic exchange within the sample which is confirmed by variable temperature magnetization measurements and EPR data.

Formation of a carbonato bridged Ni4-complex by atmospheric CO2 fixation: Crystal structure and magnetic properties

Reaction of NiCl2·6H2O with a simple N2O donor tridentate reduced Schiff base ligand, 2-[(3-methylamino-propylamino)-methyl]-4-nitro-phenol (HL) yielded a tetranuclear complex, [Ni4L4Cl2(µ4-CO3)(CH3CN)2]·0.5CH3CN·H2O (1) which was structurally characterized. Complex 1 is an example of atmospheric CO2 fixation model with uniqueness in a new µ4-κ2:κ2:ƞ1:ƞ1-carbonato coordination mode to Ni(II). Involvement of this type of NNO donor ligands for the synthesis of carbonato bridged polynuclear compounds by spontaneous atmospheric CO2 fixation is very rare. All Ni(II) ions are in octahedral environment in 1 with phenoxido and hexadentate µ4-carbonato bridging. Variable temperature magnetic measurements showed overall antiferromagnetic coupling between Ni(II) centers with J1 = −60.32 cm−1, J2 = −19.52 cm−1, J3 = 2.6 cm−1 and J4 = 3.3 cm−1. The magnetic behavior of the complex has been explained considering the carbonato and phenoxido bridging angles.

High-spin bis[(S or R)-N-1-(Ar)ethyl-salicylaldiminato-κ2N,O]-Λ/Δ-iron(II): Combined studies of syntheses, spectroscopy, diastereoselection, electrochemistry, paramagnetic, thermal, PXRD and DFT/TDDFT

The enantiopure Schiff bases (S or R)-N-1-(Ar)ethyl-salicylaldimine (S or R-HL) react with the iron(II) chloride to give high-spin bis[(S or R)-N-1-(Ar)ethyl-salicylaldiminato-κ2N,O]-Λ/Δ-iron(II) {Ar = C6H5 (FeSL1 or FeRL1), p-OMeC6H4 (FeSL2 or FeRL2) and p-BrC6H4 (FeSL3 or FeRL3)}. ECD spectra show expected mirror-image relationship for the enantiomeric pairs and confirm the enantiopurity of the complexes in methanol. The electronic spectra calculated by DFT/TDDFT fit well to the experimental spectra. Comparisons of experimental ECD spectra for FeRL2 (or FeSL2) with those of the calculated spectra for Λ-FeRL2/Δ-FeRL2 (or Δ-FeSL2/Λ-FeSL2) exhibit diastereoselection and induced chirality (Λ vs. Δ) at-metal centre by R- or S-ligand and prefer formation of Δ-FeRL2 (or Λ-FeSL2) in solution. CV results demonstrate three redox peaks associated to the FeI/0, FeII/I and FeIII/II couples, respectively in acetonitrile. Thermal analyses by DSC suggest an irreversible phase transformation from solid to isotropic-liquid phase. Variable temperature magnetic measurements show relatively high magnetic moment values (i.e., μeff. = 4.6–5.0 μB), correspond to the tetrahedral high-spin iron(II)-Schiff bases complexes in methanol. Fair matching between experimental and simulated PXRD patterns indicates the ligands and complexes having well define crystalline nature with various degrees of crystallinity.

The Importance of Electronic Dimensionality in Multiorbital Radical Conductors.

The exceptional performance of oxobenzene-bridged bis-1,2,3-dithiazolyls 6 as single-component neutral radical conductors arises from the presence of a low-lying π-lowest unoccupied molecular orbital, which reduces the potential barrier to charge transport and increases the kinetic stabilization energy of the metallic state. As part of ongoing efforts to modify the solid-state structures and transport properties of these so-called multiorbital materials, we report the preparation and characterization of the acetoxy, methoxy, and thiomethyl derivatives 6 (R = OAc, OMe, SMe). The crystal structures are based on ribbonlike arrays of radicals laced together by S···N' and S···O' secondary bonding interactions. The steric and electronic effects of the exocyclic ligands varies, affording one-dimensional (1D) π-stacked radicals for R = OAc, 1D cofacial dimer π-stacks for R = SMe, and a pseudo two-dimensional (2D) brick-wall arrangement for R = OMe. Variable-temperature magnetic and conductivity measurements reveal strong antiferromagnetic interactions and Mott insulating behavior for the two radical-based structures (R = OAc, OMe), with lower room-temperature conductivities (σRT ≈ 1 × 10-4 and ∼1 × 10-3 S cm-1, respectively) and higher thermal activation energies ( Eact = 0.24 and 0.21 eV, respectively) than found for the ideal 2D brick-wall structure of 6 (R = F), where σRT ≈ 1 × 10-2 S cm-1 and Eact = 0.10 eV. The performance of R = OMe, OAc relative to that of R = F, is consistent with the results of density functional theory band electronic structure calculations, which indicate a lower kinetic stabilization energy of the putative metallic state arising from their reduced electronic dimensionality.

A [Cu3] Cluster-Based Chain Featuring Linkages of Acylhydrazone N–N Single Bonds and Cl− Ions: Synthesis, Structure and Magnetic Properties

A Cu coordiantion polymer, namely, [Cu3(ovph)(Py)2Cl2] (1) [H4ovph = N,N′-bis(o-vanillidene)pyridine-2,6-dicarbohydrazide]; Py = Pyridine], was synthesized by solvothermal reaction of copper acetate and diacylhydrazone ligand H4ovhp. It was characterized by element analysis, FT-IR, thermogravimetric analysis, PXRD and single crystal X-ray diffraction. Structural analysis indicated that the N–N bonds of its ovph4− ligands bridge the Cu2+ ions to form quasi-linear [Cu3] cluster-based units, which were further linked together by in situ generated µ-Cl−, giving rise to a rare Cu chain structure containing two kinds of magnetic exchange pathways. Variable temperature magnetic measurements revealed that both N–N and Cl− bridges convey antiferromagnetic couplings, with the best fitting JN–N = − 99.95 cm−1, JCl = − 5.70 cm−1, zj = − 1.17 cm−1 and g = 2.00.

Synthesis, structural characterization and C[sbnd]H activation property of a tetra-iron(III) cluster

A non-heme tetra-iron cluster, [Fe4III(μ-O)2(μ-OAc)6(2,2′-bpy)2(H2O)2](NO3−)(OH−) (1), [OAc = acetate; 2,2′-bpy = 2,2′-bipyridine] containing oxido- and acetato-bridges was synthesized and structurally characterized by different spectroscopic methods including single crystal X-ray diffraction studies. X-ray crystal structure analysis of 1 revealed that tetra-iron complex was crystallized in monoclinic system with C2/c space group. Each of the Fe centres in 1 was found to exist in octahedral geometry and interconnected by oxido- and acetato-bridges. Bond valence sum (BVS) calculation recommended the existence of iron centres in +3 oxidation state. Variable temperature magnetic measurement authenticated the dominating antiferromagnetic ordering among the iron centres in the solid state of 1. This tetra-iron cluster was also evaluated as an efficient catalytic system towards the oxidation of both linear & cyclic alkanes without production of primary CH bond oxidation products. Oxidation of secondary CH bonds attested the formation of both the corresponding alcohols & ketones in 27–900 TONs. The tetra-iron catalytic system with Alcohol/Ketone values 0.2–1.7 indicated the involvement of freely diffusing carbon-centered radicals rather than metal based oxidant.

Trapping of a Pseudotetrahedral CoIIO4 Core in Mixed-Valence Mixed-Geometry [Co5] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism.

A systematic one-step one-pot multicomponent reaction of Co(ClO4)2·6H2O, H3L (2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol), and readily available carboxylate salts (RCO2Na; R = CH3, C2H5) resulted in the two structurally novel coordination aggregates [CoIICoIII4L2(μ1,3-O2CCH3)2(μ-OH)2](ClO4)4·4H2O (1) and [CoIICoIII4L2(μ1,3-O2CC2H5)2(μ-OH)(μ-OMe)](ClO4)4·5H2O (2). At room temperature, reactions of H3L in MeOH with cobalt(II) perchlorate salts led to coassembly of initially formed ligand-bound {CoII2} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L(μ1,3-O2CCH3)(μ-OH/OMe)Co2}+ fragments were utilized to trap a central CoII ion during the formation of [Co5] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo- T d CoII ion present. The structural distortion leads to an easy-axis magnetic anisotropy ( D = -31.31 cm-1 for 1 and -21.88 cm-1 for 2) and a small but non-negligible transverse component ( E/ D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O-Co-O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co2(μ-L)} hosts controls the distortion of the central {CoO4} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.

Characterization of Oxo-Bridged Iron Amino-bis(phenolate) Complexes Formed Intentionally or in Situ: Mechanistic Insight into Epoxide Deoxygenation during the Coupling of CO2 and Epoxides.

A series of iron(III) chloride and iron(III) μ-oxo compounds supported by tetradentate amino-bis(phenolate) ligands containing a homopiperazinyl backbone were prepared and characterized by electronic absorption spectroscopy, magnetic moment measurement, and MALDI-TOF mass spectrometry. The solid-state structures of three iron(III) μ-oxo compounds were determined by single crystal X-ray diffraction and revealed oxo-bridged bimetallic species with Fe-O-Fe angles between 171.7 and 180°, with the iron centers in distorted square pyramidal environments. Variable temperature magnetic measurements show the oxo complexes exhibit strong antiferromagnetic coupling between two high-spin S = 5/2 iron(III) centers. The oxo complexes exhibit poor activity for the reaction of carbon dioxide and epoxides in the presence of a cocatalyst, under solvent free conditions to yield cyclic carbonates. The least active iron oxo compound bears tert-butyl groups on the phenolate donors, and we propose that steric congestion around the iron center reduces catalytic activity in this case. We provide evidence that an epoxide deoxygenation step occurs when employing monometallic iron(III) chlorido species as catalysts. This affords the corresponding μ-oxo compounds which can then enter their own catalytic cycle. Deoxygenation of epoxides during their catalytic reactions with carbon dioxide is frequently overlooked and should be considered as an additional mechanistic pathway when investigating catalysts.

Comparative Studies on Optical and Electronic Behavior of Lanthanide-based Coordination Polymers: Synthesis, Structure, Absorption-Emission and Magnetic Properties

A series of lanthanide(III) based coordination polymers have been synthesized using chelidamic acid (CA) as a ligand, $$[\hbox {Ln}(\hbox {C}_{7}\hbox {H}_{2}\hbox {NO}_{5})(\hbox {H}_{2}\hbox {O})_{3}].\hbox {H}_{2}\hbox {O}$$ (Ln = Eu, Gd, Tb, and Dy). All the compounds were prepared by solvothermal technique using $$\hbox {H}_{2}\hbox {O-DMF}$$ as solvents (4:1, volume ratio). Compounds were characterized through various instrumental techniques such as single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, IR spectroscopy, etc. In all the cases lanthanide ion adopts distorted dodecahedron geometry. And these dodecahedrons are connected through chelidamic acid ligand to form the two-dimensional structure. These layers are H-bonded to form the three-dimensional supramolecular structure. The optical band gap energy measurements exhibit that the variation of the band gap energy is independent of the f-electrons. This is due to the weak bonding connectivity between the metal ion and ligands, which could not perturb the density of states significantly. The variable-temperature magnetic measurements exhibit the paramagnetic behavior of all the compounds though the effective magnetic moments rise with increasing number of f-electrons. The present study illustrates the usefulness of the lanthanides for the structure building as well as the role of f-electrons for opto-electronic behaviors. SYNOPSIS A series of two-dimensional isostructural Lanthanum coordination polymers have been synthesized and comparisons of various properties have been made. Figure shows the reaction scheme for the observed coordination polymer.

Hydrothermal synthesis, crystal structure, luminescent and magnetic properties of a new mononuclear GdIII coordination complex

A new GdIII coordination complex, {[Gd(2-stp)2(H2O)6].2(4,4'-bipy).4(H2O)}, complex 1, (2-stp = 2-sulfoterephthalate anion and 4,4'-bipy = 4,4'-bipyridine), has been synthesized by hydrothermal method and characterized by elemental analysis, solid state UV–Vis and FT-IR spectroscopy, single-crystal X-ray diffraction, solid state photoluminescence and variable-temperature magnetic measurements. The crystal structure determination shows that GdIII ions are eight coordinated and adopt a distorted square-antiprismatic geometry. Molecules interacting through intra- and intermolecular (O-H⋯O, O-H⋯N) hydrogen bonds in complex 1, give rise to 3D hydrogen bonded structure and the discrete lattice 4,4'-bipy molecules occupy the channel of the 3D structure. π-π stacking interactions also exist 4,4'-bipy-4,4'-bipy and 4,4'-bipy-2-stp molecule rings in 3D structures. Additionally, solid state photoluminescence properties of complex 1 at room temperature have been investigated. Under the excitation of UV light (at 349 nm), the complex 1 exhibited green emissions (at 505 nm) of GdIII ion in the visible region. Furthermore, Variable-temperature magnetic susceptibility and isothermal magnetization as function of external magnetic field studies reveal that complex 1 displays possible antiferromagnetic interaction.

Oxotris(oxalate)niobate(V): An oxalate delivery agent in the design of building blocks

This work concerns the oxalate delivery process that occurs when using (NH4)3[NbO(C2O4)3]·6H2O as a suitable oxalate source in the synthesis of two compounds, [Cu(dmphen)(C2O4)(H2O)] (1) and [{Cu(dmphen)(CH3OH)}2(μ-C2O4)](ClO4)2 (2) (dmphen = 2,9-dimethyl-1,10-phenanthroline). {[Fe{HB(pz)3}(CN)2(μ-CN)]2[{Cu(dmphen)}2(μ-C2O4)]}∙xCH3OH (3) (2.0 ≤ x ≤ 2.4) was obtained by reacting 2 and PPh4[Fe{HB(pz)3}(CN)3]∙H2O [ = tetraphenylphosphonium and = tris(pyrazolyl)borate]. Crystal structures of 1–3 have been determined by single-crystal X-ray diffraction experiments: 1 is a mononuclear trigonal bipyramidal copper(II) species, 2 is a centrosymmetric oxalato-bridged dicopper(II) complex, and 3 consists of centrosymmetric tetranuclear units with intramolecular iron–copper and copper–copper distances around 5.010(1) and 5.1833(9) Å, respectively. Variable-temperature magnetic measurements of 2 and 3 were carried out from 50 to 350 (1) and 1.9 to 300 K (3). A strong antiferromagnetic interaction between copper(II) ions occurs in 2 (J = −340 cm−1, the spin Hamiltonian being defined as ). Analysis of the magnetic data of 3 shows magnetic interactions across the oxalate (J1 = −341 cm−1) and single cyanide (J2 = +12.9 cm−1) … (J2 = +12.9 cm−1) bridges . Simple symmetry considerations of the interacting magnetic orbitals in 2 and 3 provide a clear picture of the exchange pathways involved in these complexes.

Synthesis, structural and magnetic characterizations of a dinuclear copper(II) complex with an (N,S,O) donor ligand: Catecholase and phenoxazinone synthase activities

Abstract A new dinuclear Cu(II) complex (1) was synthesized and crystallographically characterized. Each of the Cu(II) centres has penta coordination and been found to adopt square pyramidal geometry. Variable temperature magnetic measurements showed that there is weak ferromagnetic interaction between the Cu(II) centres in 1. 1 shows catecholase as well as phenoxazinone synthase activities in different solvents. The turn over numbers for the catecholase activity were 4.02 × 103 h−1 (MeOH) and 9.57 × 103 h−1 (MeCN), and that of phenoxazinone synthase activity were 1.065 × 103 h−1 (MeOH), 2.13 × 102 h−1 (MeCN) and 2.844 × 103 h−1 (DCM).

The diversity of five metal–organic complexes based on an unsymmetrical biphenyl tetracarboxylate: Synthesis, structures, magnetism and luminescence

Careful investigation of the effects of metal ions and pH value resulted in five multi-dimensional metal−organic complexes, [Co5(μ3-OH)2(α-bptc)2(H2O)6]n (1), {(H3O)2[Co4(μ3-O)(α-bptc)2(H2O)]·4H2O}n (2), {K2[Zn4(μ3-OH)2(α-bptc)2(H2O)3]}n (3), [Zn4(α-bptc)2(H2O)3]n (4), and {[Cd4(α-bptc)2(H2O)7]·2H2O}n (5) (α-H4bptc = 2,3,3',4'-biphenyl tetracarboxylic acid). The complexes consisted of different highly connected secondary building units (SBUs) assembled via hydrothermal in situ routes. The pH value as well as the kinds of metal ions influenced the coordination modes of α-H4bptc and also introduced the hydroxyl ligand at high pH value, resulting in the diversity of SBUs and dimensionality. Variable temperature magnetic measurements indicated the antiferromagnetic behavior in the Co5 core in 1, and the intra-chain ferrimagnetic behavior in complex 2. Moreover, the luminescent properties of the complexes 3−5 indicated the existence of intra-ligand charge transfer.

Synthesis, Characterization, and Properties of Iron(II) Spin-Crossover Molecular Photoswitches Functioning at Room Temperature.

Spin-crossover molecular switches [FeII(H2B(pz)2)2L] (L = novel phenanthroline-based ligands featuring photochromic diarylethene units; pz = 1-pyrazolyl) were synthesized and thoroughly characterized by variable-temperature X-ray crystallography, Mössbauer spectroscopy, and magnetic measurements. The effect of substituents introduced into the phenanthroline backbone (L2) and into the photochromic diarylethene unit (L3) on photophysical properties of metal-free ligands and spin-crossover iron(II) complexes 2 and 3, respectively, were investigated in detail. Both ligands and complexes could be switched with light in solution at room temperature. The photocyclization of 2 was accompanied by a high-spin to low-spin photoconversion determined at 19%. The closed-ring isomers of L3 and 3 reveal the lifetimes in the range of minutes, whereas those of L2 and 2 are thermally stable for days in solutions at room temperature. The reversibility of the photoswitching can be improved by avoiding the photostationary states. Prospective introduction of anchoring groups to the phenanthroline backbone might allow the construction of chemisorbed self-assembled monolayers of spin-crossover species switchable with light at room temperature.

Syntheses, crystal structures and magnetic properties of four tetrahydrofuran-2,3,4,5-tetracarboxylato bridged transition metal complexes with N-donor heteroaromatic co-ligands

Four tetrahydrofuran-2,3,4,5-tetracarboxylato complexes, Cu(bpy)(H2THFTC) (1), [Cu(phen)(H2THFTC)]·0.5H2O (2), [Mn(phen)(H2THFTC)]·0.5H2O (3) and [Co4(H2O)4(phen)4(THFTC)2]·4H2O (4) (H4THFTC = tetrahydrofuran-2,3,4,5-tetracarboxylic acid, phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine) were synthesized in aqueous solution or under hydrothermal conditions, and characterized. In compounds 1–3, the metal centers are bridged by tetrahydrofuran-2,3,4,5-tetracarboxylate anions to form 1D chains, which are further assembled into 2D supramolecular layers based on interchain π···π interactions. Compound 4 consists of tetranuclear [Co4(H2O)4(phen)4(THFTC)2] complex molecules and lattice water molecules. The complex molecules are interconnected into hydrogen-bonded chains along the [100] direction, and interdigitation of phen ligands with interchain π···π stacking interactions assembles the hydrogen-bonded chains into 2D supramolecular layers parallel to the (010) plane. Variable-temperature magnetic measurements show an overall weak ferromagnetic behavior for complexes 1 and 2 and antiferromagnetic behavior for complexes 3 and 4.

Structures and Magnetic Properties of Three Heterobimetallic 3d–4f Hexanuclear Complexes

Three butterfly heterhexanuclear 3d–4f clusters have been prepared under solvothermal conditions and structurally characterized by IR spectroscopy, elemental analyses, thermal analyses, and single-crystal X-ray diffraction. Each of hexanuclear clusters was built from butterfly Co 2 III Ln 2 III clusters and two LnIII ions by the bridging of pivalate. Variable-temperature magnetic susceptibilities measurements of the complexes in the temperature range 2–300 K indicate dominant antiferromagnetic exchange interactions.