Variable-temperature Magnetic Studies Research Articles

Synthesis, general physicochemical behaviour and magnetic studies of the macrocyclic bis-pyridine derivatives [TTDPzM(py)2]·2H2O (M = MnII, CoII, NiII)

In our extensive work on phthalocyanine-like macrocycles carrying peripherally attached strongly electron-withdrawing 1,2,5-thia/selenodiazol rings we recently reported on the FeII species of formula [TTDPzFe(py)2].2H2O (TTDPz = tetrakis(thiadiazole)porphyrazinato dianion). The present work deals with the synthesis and extended physicochemical studies of the series of related macrocycles [TTDPzM(py)2].2H2O (M = MnII, CoII, NiII). The diffractograms of three solid samples of the NiII complex [TTDPzNi(py)2]·2H2O, obtained from different synthetic procedures, indicate that the species is reproducibly obtained. All three samples are isomorphous with each other as shown by their X-ray powder diffractograms. Based on the comparison of the IR spectra of [TTDPzNi(py)2].2H2O and the related desolvated [TTDPzNi], the IR absorption peaks belonging to the pyridine molecules could be identified. The systematic presence of the two water molecules in all three species suggests that forms of contact do exist in the form of hydrogen bonds between them and the S and N atoms present peripherally in the thiadiazole groups of the macrocyclic units. As regard to the elimination of water, the thermograms (in inert atmosphere in the range 25–600 °C) indicate in all cases that loss of water occurs below 100 °C but loss of pyridine occurs over a broad range 100–300 °C for the MnII and CoII species whereas for the NiII complex it takes place in the narrow range 200–220 °C. In relation to the parallel series of metallophthalocyanines, i.e. [PcM] (M = MnII, FeII, CoII), it is noted that the NiII analog has not been reported in the literature and in a direct experiment it has been proved that [PcNi] heated in pyridine is obtained unchanged, the different behavior for the NiII TTDPz derivative due to the electron withdrawing effect of the external thiadiazole substituents, which makes favorable axial pyridine coordination in the complex. Completely insoluble in water, the triad of TTDPz derivatives is extremely low-soluble in non-donor or low donor solvents. This precluded any attempt to isolate single crystals for single-crystal X-ray work. Their UV–visible spectra show essential similarities in whatever organic solvent is used with clearly positioned Soret (300–400 nm) and Q-band absorptions (630–650 nm). Variable temperature magnetic susceptibility and magnetization studies reveal that the [TTDPzM(py)2].2H2O complexes all show paramagnetism with ground state spins of 3/2 (MnII), ½ (CoII) and 1 (NiII), supported by fitting the data to spin Hamiltonian formalism, vide infra. Comparisons are made with the magnetism of [PcM] analogues with key differences noted for M = NiII.

Conversion of Metal Pyrazolate/(Hydr)oxide Clusters into Nanojars: Solution vs Solid-State Structure and Magnetism.

Nanojars are a class of anion binding and extraction agents composed of a series of [Cu(μ-OH)(μ-pz)]n (pz = pyrazolate; n = 26-36) supramolecular metal-organic complexes. In contrast to other anion binding agents amenable to liquid-liquid extraction, nanojars only form by self-assembly around the target anion, and guest-free nanojar hosts cannot be isolated. An extraordinary binding strength toward highly hydrophilic anions such as carbonate and sulfate was demonstrated by the inability of Ba2+ ions to precipitate the corresponding insoluble barium salts from nanojars. Herein, we provide an additional proof for the superior robustness of the nanojar framework based on competition experiments with other transition metal pyrazolate/(hydr)oxide complexes. In addition to the mass spectrometric characterization, we present variable-temperature nuclear magnetic resonance studies with an emphasis on the influence of the paramagnetic Cu2+ centers on 1H hyperfine shifts, along with X-ray crystallographic analysis of two polymorphs of (MePh3P)2[CO3⊂{Cu(OH)(pz)}27], including the highest (cubic) symmetry nanojar crystal lattice obtained to date as well as magnetism studies for the first time. Furthermore, we provide evidence for the first molybdate-incarcerating nanojars, [MoO4⊂{Cu(μ-OH)(μ-pz)}n]2- (n = 28, 31-33), formed by rearrangement from [MoVI8O12(μ-O)9(μ-pz)6(pzH)6·3pzH] in the presence of Cu2+ ions.

Determining the Effects of Zero-Field Splitting and Magnetic Exchange in Dimeric Europium(II) Complexes.

Related BAP [BAP = bis(acyl)phosphide] and Acac (Acac = β-diketonate) molecules perform as robust supports for both lanthanide and actinide metals. Here, a molecular bimetallic Eu2+ complex was successfully targeted and isolated by employing sodium bis(mesitoyl)phosphide [Na(mesBAP)] in a salt metathesis with EuI2, producing [Eu(mesBAP)2(et2o)]2 (et2o = metal-coordinated diethyl ether). The corresponding Acac-Eu2+ complex was targeted using mesAcac- (1,3-dimesityl-1,3-propanedione), generating [Eu(mesAcac)2(et2o)]2. Both complexes were characterized by single-crystal X-ray diffraction, UV-vis, IR, and NMR spectroscopies, and variable-temperature magnetic susceptibility. [Eu(mesBAP)2(et2o)]2 was persistent under anaerobic, anhydrous conditions, whereas the analogous [Eu(mesAcac)2(et2o)]2 showed evidence of decomposition under identical conditions. Variable-temperature magnetic susceptibility and magnetization studies of [Eu(mesBAP)2(et2o)]2 and [Eu(mesAcac)2(et2o)]2 were performed, resulting in similar magnetic exchange coupling values of Jex = -0.018 and -0.023 cm-1 and axial zero-field-splitting D values of -0.38 and -0.51 cm-1, respectively.

Heterometallic Co–Sr −2,3-pyridinedicarboxylate complex and its related perovskite oxide: Synthesis, structural characterization and magnetic properties

A new heterometallic complex {[CoSr(2,3-pdc)2(H2O)5]3H2O}n (1), where 2,3-H2pdc stands for 2,3-pyridinedicarboxylic acid, has been synthesized and investigated by single crystal X-Ray diffractometry, IR spectroscopy and thermal analysis. The compound is self-organized in a polymeric structure forming a 2D framework. The 2,3-pyridinedicarboxylic acid is fully deprotonated performing several types of coordination: monodentate, bridging chelating, bidentate bridging. Thermal decomposition of the heterometallic 2,3-pyridinedicarboxylate 1 leads to the formation of a perovskite type oxide of brownmillerite series, 2, with 20.8 % yield. Magnetic studies of 1 show the paramagnetic behavior of Co(II) ions with a significant influence of a ground state zero-field splitting at low temperatures. Variable field and temperature magnetic studies of the oxide product 2 (obtained by the controlled heating of 1) indicate the presence of the Sr6Co5O15, detected by the diffraction method. Also, trace amounts of SrCoO2.75 and SrCoO2.5 were revealed by magnetic and DSC methods, respectively.

Ligand Modifications Produce Two‐Step Magnetic Switching in a Cobalt(dioxolene) Complex

AbstractMononuclear monodioxolene valence tautomeric (VT) cobalt complexes typically exist in their low spin (l.s.) CoIII(cat2−) and high spin (h.s.) CoII(sq⋅−) forms (cat2−=catecholato, and sq⋅−=seminquinonato forms of 3,5−di−tBu‐1,2‐dioxolene), which reversibly interconvert via temperature‐dependent intramolecular electron transfer. Typically, the remaining four coordination sites on cobalt are supported by a tetradentate ligand whose properties influence the temperature at which VT occurs. We report that replacing one chelating pyridyl arm of tris(2‐pyridylmethyl)amine (tpa) with a weaker field ortho‐anisole moiety facilitates access to a third magnetic state, and examine a series of related complexes. Variable temperature crystallographic, magnetic, calorimetric, and spectroscopic studies support that this third state is consistent with l.s. CoII(sq⋅−). Thus, our ligand modifications not only provide access to the VT transition from l.s. CoIII(cat2−) to l.s. CoII(sq⋅−), but at higher temperatures, the complex undergoes spin crossover from l.s. CoII(sq⋅−) to h.s. CoII(sq⋅−), representing the first example of two‐step magnetic switching in a mononuclear monodioxolene cobalt complex. We hypothesize that ligand dynamicity may facilitate access to the rarely observed l.s. CoII(sq⋅−) intermediate state, suggesting a new design criterion in the development of stimulus‐responsive multi‐state molecular switches.

Ligand Modifications Produce Two-Step Magnetic Switching in a Cobalt(dioxolene) Complex.

Mononuclear monodioxolene valence tautomeric (VT) cobalt complexes typically exist in their low spin (l.s.) CoIII (cat2- ) and high spin (h.s.) CoII (sq⋅- ) forms (cat2- =catecholato, and sq⋅- =seminquinonato forms of 3,5-di-t Bu-1,2-dioxolene), which reversibly interconvert via temperature-dependent intramolecular electron transfer. Typically, the remaining four coordination sites on cobalt are supported by a tetradentate ligand whose properties influence the temperature at which VT occurs. We report that replacing one chelating pyridyl arm of tris(2-pyridylmethyl)amine (tpa) with a weaker field ortho-anisole moiety facilitates access to a third magnetic state, and examine a series of related complexes. Variable temperature crystallographic, magnetic, calorimetric, and spectroscopic studies support that this third state is consistent with l.s. CoII (sq⋅- ). Thus, our ligand modifications not only provide access to the VT transition from l.s. CoIII (cat2- ) to l.s. CoII (sq⋅- ), but at higher temperatures, the complex undergoes spin crossover from l.s. CoII (sq⋅- ) to h.s. CoII (sq⋅- ), representing the first example of two-step magnetic switching in a mononuclear monodioxolene cobalt complex. We hypothesize that ligand dynamicity may facilitate access to the rarely observed l.s. CoII (sq⋅- ) intermediate state, suggesting a new design criterion in the development of stimulus-responsive multi-state molecular switches.

Magneto-structural studies on [CuII2{L3-(μ2-1,1-CO2–)2}2]•MeOH•H2O and [CuII4{L4-(μ2-1,3-CO2–)(μ2-1,3-CO2Me)}4](ClO4)4 using carboxylate- / carboxymethylester-appended (2-pyridyl)-alkylamine ligands differing in alkyl chain length

Two discrete complexes [CuII2{L3-(CO2–)2}2]•MeOH•H2O (1) and [CuII4{L4-(CO2–)(CO2Me)}4](ClO4)4•unknown solvent (2) have been synthesized from the reaction between copper(II) salts and L3-(CO2Me)2 / L4-(CO2Me)2 [3,3′-[(pyridin-2-ylmethyl)imino]dipropionate dimethylester / 3,3′-[(pyridin-2-ylethyl)imino]dipropionate dimethylester]. Structural analysis reveals that 1 is a monatomic bridging carboxylate dinuclear complex with CuII2(μ2-1,1-OCO–)22+ structural unit. Each Cu(II) centre has distorted square-pyramidal CuN2O3 coordination, provided by two N (pyridyl and amine) and two carboxylate O from L3-(CO2–)2 and the fifth coordination is provided by an O atom belonging to the carboxylate group of an adjacent molecule. Complex 2 is a syn-anti carboxylate-bridged tetranuclear coordination cluster with a {CuII4{(μ2-1,3-OCO–)(μ2-1,3-OCOMe)}44+ structural motif. Here also, each Cu(II) centre has distorted square-pyramidal CuN2O3 coordination, and an O atom belonging to the carboxylate group of an adjacent molecule completes the fifth coordination. Variable temperature magnetic studies reveal antiferromagnetic coupling in 1 (J = –8.37(6) cm−1), while 2 exhibits ferromagnetic behaviour (J = +13.1(1) cm−1).

Synthesis, Crystal Structure Determination and Magnetic Study of a New [2 × 2] Grid Tetranuclear Fe(II) and Ni(II) Complexes Derived from the Ligand 1, 5-Bis(1-(Pyridin-2-Yl) Ethylidene) Carbonohydrazide)

Complexes of nickel (II) and iron (III) are easily synthetized using the symmetrical ligand 1,5-bis(1-(pyridin-2-yl)ethylidene)carbonohydrazide) (H2L) and metal nitrate salts. Square [2x2] grids structure of one tetranuclear iron and one tetranuclear nickel complex were isolated. X ray diffraction analysis reveals that the crystal structures of the two complexes are similar. The asymmetric unit of each complex consists of four cationic ligand molecules and four metal ions. Each ligand acts in its monodeprotonated form through five coordination sites such as two pyridine nitrogen atoms, two azomethine nitrogen atoms and one oxygen atom. Each of the four ligand molecules acts as a bridge between two metal ions yielding a square 2 x 2 grid structure. Each of the metal ion is hexacoordinated and is situated in a N4O2 core and the environment is best described as a severely distorted square bipyramidal geometry. Electrochemical studies show two electrons process for complex 1 and one electron process for complex 2. Variable temperature magnetic study shows that antiferromagnetic coupling is stronger in the nickel(II) than in the iron(II) complex. Perfect correlation between the magnetic properties and the crystallographic data are observed in both complexes.

An Old Crystallization Technique as a Fast, Facile, and Adaptable Method for Obtaining Single Crystals of Unstable "Li2TCNQF4" and New Compounds of TCNQ or TCNQF4: Syntheses, Crystal Structures, and Magnetic Properties.

Detailed structural information is essential for understanding the properties of TCNQ and TCNQF4 compounds (TCNQ = 7,7,8,8-tetracyanoquinodimethane; TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). The ineludible requirement of obtaining crystals of a size and quality sufficient to yield a successful X-ray diffraction analysis has been challenging to satisfy because of the instability of many of these compounds in solution. Crystals of two new complexes of TCNQ, [trans-M(2ampy)2(TCNQ)2] [M = Ni (1), Zn (2); 2ampy = 2-aminomethylpyridine], as well as unstable [Li2(TCNQF4)(CH3CN)4]·CH3CN (3), can be prepared in minutes by a horizontal diffusion technique and can be harvested easily for X-ray structural studies. Compound 3, previously described as "Li2TCNQF4," forms a one-dimensional (1D) ribbon. Compounds 1 and 2 can also be obtained as microcrystalline solids from methanolic solutions of MCl2/LiTCNQ/2ampy. Their variable-temperature magnetic studies confirmed a contribution of strongly antiferromagnetically coupled pairs of TCNQ•- anion radicals at higher temperatures with exchange coupling J/kB = -1206 K and J/kB = -1369 K for 1 and 2, respectively, estimated using a spin dimer model. The presence of magnetically active anisotropic Ni(II) atoms with S = 1 in 1 was confirmed, and the magnetic behavior of 1, representing an infinite chain of alternating S = 1 sites and S = 1/2 dimers, was described by a spin-ring model suggesting ferromagnetic exchange coupling between Ni(II) sites and anion radicals.

Syntheses of two copper(II) coordination polymers from a morpholine-based tridentate Schiff base ligand: crystal structures and magnetic properties

Two new one-dimensional (1-D) end-to-end azido and thiocyanato bridged Cu(II) coordination polymers, [Cu(L)(µ-1,3-N3)] n (ClO4) n (1) and [Cu(L)(µ-1,3-NCS)] n (ClO4) n (2) with a morpholine-based tridentate N,N,N-donor Schiff base ligand (1-methyl-1H-imidazol-2-yl)-N-[2-(morpholin-4-yl)ethyl]methanimine, L) have been synthesized. Compounds 1 and 2 have been characterized by elemental analyses and spectroscopic techniques. The X-ray single crystal structures of 1 and 2 show the formation of regular zigzag chains with µ-1,3-azido and µ-1,3-thiocyanate bridges, respectively. Variable temperature (2–300 K) magnetic studies indicate that 1 shows a ferromagnetic intrachain exchange interaction with g = 2.135(2) and J = 2.13(2) cm−1, while in 2 this interaction is antiferromagnetic with g = 2.1412(3) and J = –0.277(2) cm−1. X-band EPR spectra of 1 and 2 in frozen (143 K) DMF solution corroborate our magnetic studies.

A 3D Coordination Polymer Based on Syn-Anti Bridged [Mn(RCOO)2]n Chains Showing Spin-Canting with High Coercivity and an Ordering Temperature of 14 K

A new 3D manganese(II) coordination polymer, formulated as [Mn3(HL)6] (1) (where H2L = 6-hydroxypicolinic acid), has been hydrothermally synthesized and characterized by single-crystal X-ray crystallographic analysis along with other spectroscopic and magnetic techniques. Structural analysis shows that the compound crystallizes in the monoclinic C2/c space group and is a non-porous 3D coordination polymer formed by three different Mn(II) centres connected by 6-hydroxypicolinic acid ligands in their keto form. Each Mn(II) centre shows a distorted octahedral coordination environment. Neighbouring Mn(II) centres are connected by two different syn-anti bridging carboxylate groups to form regular coordination chains. There are two different [Mn2(RCOO)2] units along the chain, formed by two crystallographically independent Mn centres (Mn1 and Mn2). These chains are further connected by HL− ligands to form a 3D coordination network. Interestingly, both the hydroxy and the carboxylate groups of the ligands are deprotonated and coordinated to the metal centres, whereas the pyridyl group is protonated and uncoordinated, although it participates in strong hydrogen bonding interactions with oxygen atoms of the HL− ligand, as shown by the Hirshfeld surface analysis. Both the absorption and emission spectra of the compound have also been measured. Variable temperature magnetic studies reveal the presence of a spin-canted antiferromagnetic behaviour with a high coercivity of 40 mT at 2 K and an ordering temperature of 14 K.

A tetracobalt(II) cluster with a two vertex truncated dicubane topology endogenously supported by carboxylate-based (2-pyridyl)methylamine ligands: magneto-structural and DFT studies.

A reaction between CoCl2 and L3-(CO2-)2 (2 : 1 stoichiometry) in CH3OH affords a discrete complex [CoII4-{L3-(CO2-)2}2(μ3-OCH3)2(CH3OH)2(H2O)2Cl2] (1) [L3-(CO2-)2 = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear CoII{L3-(CO2-)2}Cl units connected by a dimeric CoII2(μ3-OCH3)2(CH3OH)2(H2O) unit present in the centre through two methoxo (μ3-OCH3)- and two carboxylate (μ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral CoIIN2O3Cl and the central Co2 has CoIIO6 coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J1 = +3.3 cm-1 (Co⋯Co 3.176 Å; μ-1,1-OCO- and μ3-OCH3 bridges), J2 = -2.5 cm-1 (Co⋯Co 3.228 Å; μ-1-OCO- and μ3-OCH3 bridges) and J3 = +10.6 cm-1 (Co⋯Co 3.084 Å; two μ3-OCH3 bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin-orbit coupling (λ = -158 cm-1) and axial distortion (energy gap Δ = -975 cm-1 between singlet and doublet levels), rationalized by density functional theory (DFT) calculations.

Impact of Counteranion on Reversible Spin-State Switching in a Series of Cobalt(II) Complexes Containing a Redox-Active Ethylenedioxythiophene-Based Terpyridine Ligand.

The self-assembly of a redox-active ethylenedioxythiophene (EDOT)-terpyridine-based tridentate ligand and cobalt(II) unit with different counteranions has led to a series of new cobalt(II) complexes [Co(L)2](X)2 (X = BF4 (1), ClO4 (2), and BPh4 (3)) (L = 4'-(3,4-ethylenedioxythiophene)-2,2':6',2″-terpyridine). The impact of various counteranions on stabilization and spin-state switching of the cobalt(II) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, electrochemical, and spectroelectrochemical studies. All three complexes 1-3 consisted of an isostructural dicationic distorted octahedral CoN6 coordination environment offered by the two L ligands in a bis-meridional fashion and BF4-, ClO4-, and BPh4- as a counteranion, respectively. Complex 2 with ClO4- counteranion showed a reversible, gradual, and nearly complete spin-state switching between low-spin (LS) (S = 1/2) and high-spin (HS) (S = 3/2) states, while an incomplete spin-state switching behavior was observed for complexes 1 (BF4-) and 3 (BPh4-) in the measured temperature range of 350-2 K. The non-covalent cation-anion interactions played a significant role in stabilizing the spin-state in 1-3. Additionally, complexes 1-3 also exhibited interesting redox-stimuli-based reversible paramagnetic HS cobalt(II) (S = 3/2) to diamagnetic LS cobalt(III) (S = 0) conversion, offering an alternate way to switch the magnetic properties.

Near-Infrared-Absorbing B-N Lewis Pair-Functionalized Anthracenes: Electronic Structure Tuning, Conformational Isomerism, and Applications in Photothermal Cancer Therapy.

B-N-fused dianthracenylpyrazine derivatives are synthesized to generate new low gap chromophores. Photophysical and electrochemical, crystal packing, and theoretical studies have been performed. Two energetically similar conformers are identified by density functional theory calculations, showing that the core unit adopts a curved saddle-like shape (x-isomer) or a zig-zag conformation (z-isomer). In the solid state, the z-isomer is prevalent according to an X-ray crystal structure of a C6F5-substituted derivative (4-Pf), but variable-temperature nuclear magnetic resonance studies suggest a dynamic behavior in solution. B-N fusion results in a large decrease of the HOMO-LUMO gap and dramatically lowers the LUMO energy compared to the all-carbon analogues. 4-Pf in particular shows significant absorbance at greater than 700 nm while being almost transparent throughout the visible region. After encapsulation in the biodegradable polymer DSPE-mPEG2000, 4-Pf nanoparticles (4-Pf-NPs) exhibit good water solubility, high photostability, and an excellent photothermal conversion efficiency of ∼41.8%. 4-Pf-NPs are evaluated both in vitro and in vivo as photothermal therapeutic agents. These results uncover B-N Lewis pair functionalization of PAHs as a promising strategy toward new NIR-absorbing materials for photothermal applications.

A Magnetocaloric Glass from an Ionic-Liquid Gadolinium Complex.

[Gd5 (L)16 (H2 O)8 ](Tf2 N)15 was obtained from reaction of Gd2 O3 with 1-carboxymethyl-3-ethylimidazolium chloride (LHCl). The material was found to be an ionic liquid that freezes to glassy state on cooling to -30 °C. Variable-temperature magnetic studies reveal the presence of weak magnetic intramolecular interactions in the glass. Isothermal variable-field magnetization demonstrates a magnetocaloric effect (MCE), which is the first finding of such an effect in a molecular glass. This MCE is explainable by an uncoupled representation, with a magnetic entropy change of -11.36 J K-1 kg-1 at 1.8 K for a 0-7 T magnetic field change, and with a refrigerant capacity of 125.9 J kg-1 , in the 1.8-50 K interval.

Long-Range Intramolecular Spin Coupling through a Redox-Active Bridge upon Stepwise Oxidations: Control and Effect of Metal Ions.

Dinickel(II) and dicopper(II) porphyrin dimers have been constructed in which two metalloporphyrin units are widely separated by a long unconjugated dipyrrole bridge. Two macrocycles are aligned somewhat orthogonally to each other, while oxidation of the bridge generates a fully π-conjugated butterfly-like structure, which, in turn, upon stepwise oxidations by stronger oxidants result in the formation of the corresponding one- and two-electron-oxidized species exhibiting unusual long-range charge/radical delocalization to produce intense absorptions in the near-infrared (NIR) region and electron paramagnetic resonance (EPR) signals of a triplet state due to interaction between the unpaired spins on the Cu(II) ions. Although the two metal centers have a large physical separation through the bridge (more than 16 Å), they share electrons efficiently between them, behaving as a single unit rather than two independent centers. Detailed UV-vis-NIR, electrospray ionization mass spectrometry, IR, variable-temperature magnetic study, and EPR spectroscopic investigations along with X-ray structure determination of unconjugated, conjugated, and one electron-oxidized complexes have been exploited to demonstrate the long-range electronic communication through the bridge. The experimental observations are also supported by density functional theory (DFT) and time-dependent DFT calculations. The present study highlights the crucial roles played by a redox-active bridge and metal in controlling the long-range electronic communication.

Two Polynuclear Manganese(II) Complexes Based on Multidentate N-Heterocyclic Aromatic Ligand and V-Shaped Polycarboxylate Ligand: Synthesis, Crystal Structure Analysis and Magnetic Properties

The reaction of MnCl2, 2-(5-{6-[5-(Pyrazin-2-yl)-1H-1,2,4-triazol-3-yl]pyridin-2-yl}-1H-1,2,4-triazol-3-yl)pyrazine (H2ptptp), 4,4′-sulfonyldibenzoic acid (H2sdba) or 4-(4-carboxyphenoxy)phthalate acid (H3cpop) and [BMI]Br ionic liquids (BMI = 1-butyl-3-methylimidazolium) gave rise to two complexes, {[Mn4(ptptp)2(sdba)2(H2O)2]·2H2O}n (1) and {[Mn3(ptptp)(cpop)Br(H2O)2]·2H2O}n (2). The compounds have been well characterized by elemental analysis, IR spectra, thermogravimetric analysis, as well as single-crystal and powder X-ray diffraction. The structure feature of 1 is that Mn(II) ions in the [Mn6(sdba)2] loops of V-shaped sdba2− ligands are ptptp2− ligands, respectively, and a 2D layer is constructed from sdba2− and ptptp2− ligands. The adjacent 2D layers are connected by O–H⋯N hydrogen bonds to form a 3D supramolecular network. The neighbouring trinuclear Mn(II) clusters in 2 are linked by V-shaped cpop3− ligands to give a 2D layer, which is penetrated by ptptp2− ligands to form self-threading structure. The results of variable-temperature magnetic studies have shown that the magnetic interactions between the Mn(II) ions in 1 and 2 are mainly due to antiferromagnetic coupling.

Syntheses directed by ionic liquids: structures and properties of six novel lanthanide 1,3,5-benzenetrisbenzoate frameworks

Abstract Reactions of lanthanide nitrate, 1,3,5-benzenetrisbnzoic acid (H3BTB) and [RMI]Br ionic liquids (RMI = 1-alkyl-3-methylimidazolium; R = ethyl, propyl or butyl), gave rise to six novel lanthanide–organic frameworks (LOFs) [Ln(BTB)H2O], where Ln = Nd 1, Sm 2, Gd 3, Dy 4, Ho 5 and Er 6. These compounds have been characterized by elemental analysis, IR spectra, thermogravimetric analysis, single-crystal and powder X-ray diffraction. Compounds 1–6 are isostructural and consist of infinite rod-shaped lanthanide-carboxylate building units, which are further bridged by trigonal-planar BTB ligands to give non-interpenetrated open 3D frameworks. The results of variable-temperature magnetic studies have shown that the magnetic interaction between the Ln(III) ions in 3, 5 and 6 is mainly due to antiferromagnetic coupling as well as the depopulation of the Stark levels.

Structure diversity and magnetic properties of manganese and cobalt coordination polymers with multiple carboxyl bridges

Based on asymmetric lithium salts of 1-(2-pyridinylmethyl)-pyrazole-3,5-dicarboxylate (2-Li2pmpd) or 1-(4-pyridinylmethyl)-pyrazole-3,5-dicarboxylate (4-Li2pmpd), four polymers [Mn(2-pmpd)(H2O)]n (1), {[Mn2(4-pmpd)(pd)(H2O)3]·H2O}n (2), [Co(4-pmpd)(H2O)]n (3) and [Co(2-pmpd)(H2O)2]n (4) [pd = pyrazole-3,5-dicarboxylate] have been hydrothermally prepared and characterized structurally. 1 displays three-dimensional (3D) network with (44.84) topology, 2 and 3 show 2D framework with (42·5·63)(42·52·64·72)(52·8) and (43)2(46·66·83) topologies, respectively. 4 is a 1D chain structure. Magnetization data of 1 with the 1D carboxylate bridges chain structure, fitted by the 1D model, suggests the antiferromagnetic coupling with J = −2.07 cm−1. The carboxylate bridges in 1 adopt common syn-syn and unidentate oxygen-bridge modes. 2 with the Mn4 unit, exhibits antiferromagnetic coupling with J = −0.68 cm−1 and zJ′ = −0.317 cm−1. The carboxylate bridges in 2 adopt common syn-anti and the unidentate oxygen-bridge modes. In the Co2 units, magnetization data for 3 shows the weak antiferromagnetic coupling. Variable temperature magnetic studies of 4 reveal ferromagnetic intrachain interactions and antiferromagnetic interchain magnetization interactions. The M(H) curves for 3 and 4 are consistent with their antiferromagnetic or ferromagnetic natures.

Tuning of spin crossover properties in a series of mononuclear cobalt(II) complexes based on a macrocyclic tetradentate ligand and pseudohalide coligands.

The three mononuclear cobalt(II) complexes, [Co(L)(NCX)2] (L = N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane, and X = S (1), Se (2), and [C(CN)2] (3)), have been synthesized and characterized using variable temperature single-crystal X-ray crystallography, and spectroscopic, magnetic, and electrochemical studies. All three complexes have a similar distorted octahedral CoN6 coordination geometry produced by the macrocyclic tetradentate ligand L and two NCX- co-ligands in the cis position. Complexes 1 and 2 crystallized in the monoclinic P21/n (Z = 4) space group, while complex 3 was isolated in the monoclinic P21/c (Z = 4) space group. Interestingly, complexes 1 and 2 exhibit a reversible and gradual spin-crossover behavior between the high-spin (S = 3/2) and low-spin (S = 1/2) states at ca. 168 K and 255 K, respectively. However, for complex 3, a low-spin configuration persists up to 260 K and it exhibits an incomplete and reversible spin crossover even at 400 K. In addition, complex 1 displays a reversible redox behavior indicating the paramagnetic cobalt(II) to diamagnetic cobalt(III) conversion.