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The development and technological applications of molecular spin systems require versatile experimental techniques to characterize and control their static and dynamic magnetic properties. In the latter case, bulk spectroscopic and magnetometric techniques, such as AC magnetometry and pulsed electron paramagnetic resonance, are usually employed, showing high sensitivity, wide dynamic range, and flexibility. They are based on creating a nonequilibrium state either by changing the magnetic field or by applying resonant microwave radiation. Another possible source of perturbation is a laser pulse that rapidly heats the sample. This approach has proven to be one of the most useful techniques for studying the kinetics and mechanism of chemical and biochemical reactions. Inspired by these works, we propose an inductive detection of temperature-induced magnetization dynamics as applied to the study of molecular spin systems and describe the general design and construction of a particular induction probehead, taking into account the constraints imposed by the cryostat and electromagnet. To evaluate the performance, several coordination compounds of VO2+, Co2+, and Dy3+ were investigated using low-energy pulses of a terahertz free electron laser of the Novosibirsk free electron laser facility as a heat source. All measured magnetization dynamics were qualitatively or quantitatively described using a proposed basic theoretical model and compared with the data obtained by alternating current magnetometry. Based on the results of the research, the possible scope of applications of inductive detection and its advantages and disadvantages in comparison with standard methods are discussed.
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In the present work, the study of the unusual interaction between copper hexafluoroacetylacetonate and the diacetyliminoxyl radical resulted in two discoveries from different fields: the determination of the oxime radical spatial structure and the introduction of an oxime radical into the field of molecular magnetic material design. Oxime radicals are key plausible intermediates in the processes of oxidative CH-functionalization and in the synthesis of functionalized isoxazolines from oximes. Due to the lack of X-ray diffraction data for oxime radicals, the knowledge about their structure is based mainly on indirect approaches, spectroscopic methods (electron paramagnetic resonance and IR), and quantum chemical calculations. The structure of the oxime radical was determined for the first time by stabilizing the diacetyliminoxyl radical in the form of its complex with copper (II) hexafluoroacetylacetonate (Cu(hfac)2), followed by single-crystal X-ray diffraction analysis. Although oxime radicals are known to undergo oxidative coupling with acetylacetonate ligands in transition-metal complexes, a complex is formed with intact hfac ligands. X-ray diffraction studies have shown that the oxime radical is coordinated with copper ions through the oxygen atoms of the carbonyl groups without the direct involvement of the CâN-O⢠radical moiety. The structure of the coordinated diacetyliminoxyl is in good agreement with the density functional theory (DFT) prediction for free diacetyliminoxyl due to the very weak interaction of the radical molecule with copper ions. Remarkably, both weak ferromagnetic and antiferromagnetic interactions between Cu (II) and oxime radicals have been revealed by modeling the temperature dependence of magnetic susceptibility and confirmed by DFT calculations, rendering diacetyliminoxyl a promising building block for the design of molecular magnets.
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The chemical immobilization of cobalt(II) ions in a silica aerogel matrix enabled the synthesis of the first representative example of aerogel-based single-ion magnets. For the synthesis of the lyogels, methyl-trimethoxysilane and N-3-(trimethoxysilyl)propyl ethylenediamine were co-hydrolyzed, then the ethylenediamine groups that were immobilized on the silica matrix enabled the subsequent binding of cobalt(II) ions. Lyogels with various amounts of ethylenediamine moieties (0.1-15 mol %) were soaked in isopropanol solutions of cobalt(II) nitrate and further supercritically dried in carbon dioxide to obtain aerogels with a specific surface area of 210-596 m2·g-1, an apparent density of 0.403-0.740 cm3·g-1 and a porosity of 60-78%. The actual cobalt content in the aerogels was 0.01-1.50 mmol per 1 g of SiO2, which could easily be tuned by the concentration of ethylenediamine moieties in the silica matrix. The introduction of cobalt(II) ions into the ethylenediamine-modified silica aerogel promoted the stability of the diamine moieties at the supercritical drying stage. The molecular prototype of the immobilized cobalt(II) complex, bearing one ethylenediamine ligand [Co(en)(MeCN)(NO3)2], was synthesized and structurally characterized. Using magnetometry in the DC mode, it was shown that cobalt(II)-modified silica aerogels exhibited slow magnetic relaxation in a nonzero field. A decrease in cobalt(II) concentration in aerogels from 1.5 mmol to 0.14 mmol per 1 g of SiO2 resulted in a weakening of inter-ion interactions; the magnetization reversal energy barrier likewise increased from 4 to 18 K.
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Imãs , Dióxido de Silício , Dióxido de Silício/química , Cobalto/química , Magnetismo , EtilenodiaminasRESUMO
Heat-up and hot-injection methods were employed to synthesize Ni nanoparticles (NPs) with narrow size distribution in the presence of hyperbranched pyridylphenylene polymer (PPP) as a stabilizing agent. It was shown that depending on the synthetic method, Ni NPs were formed either in a cross-linked polymer network or stabilized by a soluble hyperbranched polymer. Ni NPs were characterized by a combination of transmission electron microscopy (TEM), scanning TEM, thermogravimetric analysis, powder X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray analysis, and magnetic measurements. The architecture of polymer support was found to significantly effect Ni NPs characteristics and behavior. The Ni NPs demonstrated a high catalytic activity in a model Suzuki-Miyaura cross-coupling reaction. No significant drop in activity was observed upon repeated use after magnetic separation in five consecutive catalytic cycles. We believe that hyperbranched PPP can serve as universal platform for the controllable synthesis of Ni NPs, acting as highly active and stable catalysts.
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Nanopartículas , Polímeros , Oxirredução , Catálise , Nanopartículas/química , Microscopia Eletrônica de Transmissão e VarreduraRESUMO
A new 1D-coordination polymer [Co(Piv)2(NH2(CH2)6NH2)]n (1, Piv is Me3CCO2- anion) was obtained, the mononuclear fragments {Co(O2CR)2} within which are linked by µ-bridged molecules of hexamethylenediamine (NH2(CH2)6NH2). For this compound, two different monoclinic C2/c (α-1) and P2/n (ß-1) phases were found at room temperature by single-crystal X-ray diffraction analysis, with a similar structure of chains and their packages in unit cells. The low-temperature phase (γ-1) of crystal 1 at 150 K corresponds to the triclinic space group P-1. As the temperature decreases, the structural phase transition (SPT) in the α-1 and ß-1 crystals is accompanied by an increase in the crystal packing density caused by the rearrangements of both H-bonds and the nearest ligand environment of the cobalt atom ("octahedral CoN2O4 around the metal center at room temperature" â "pseudo-tetrahedral CoN2O2 at 150 K"). The SPT was confirmed by DSC in the temperature range 210-150 K; when heated above 220 K, anomalies in the behavior of the heat flow are observed, which may be associated with the reversibility of SPT; endo effects are observed up to 300 K. The SPT starts below 200 K. At 100 K, a mixture of phases was found in sample 1: 27% α-1 phase, 61% γ-1 phase. In addition, at 100 K, 12% of the new δ-1 phase was detected, which was identified from the diffraction pattern at 260 K upon subsequent heating: the a,b,c-parameters and unit cell volume are close to the structure parameters of γ-1, and the values of the α,ß,γ-angles are significantly different. Further heating leads to a phase transition from δ-1 to α-1, which both coexist at room temperature. According to the DC magnetometry data, during cooling and heating, the χMT(T) curves for 1 form a hysteresis loop with ~110 K, in which the difference in the χMT values reaches 9%. Ab initio calculations of the electronic structure of cobalt(II) in α-1 and γ-1 have been performed. Based on the EPR data at 10 K and the ab initio calculations, the behavior of the χMT(T) curve for 1 was simulated in the temperature range of 2-150 K. It was found that 1 exhibits slow magnetic relaxation in a field of 1000 Oe.
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Cobalto , Temperatura Baixa , Cobalto/química , Cristalografia por Raios X , Transição de FaseRESUMO
Novel mononuclear cymantrenecarboxylate complexes of transition metals, [Co(H2O)6](CymCO2)2·4H2O (Cym = (η5-C5H4)Mn(CO)3) (1), [Ni(H2O)6](CymCO2)2·4H2O (2), [Zn(H2O)6](CymCO2)2·4H2O (3), [Co(CymCO2)2(imz)2] (imz = imidazole, 4), [Co(CymCO2)2(bpy)2]·2PhMe (bpy = 2,2'-bipyridyl, 5), [Ni(CymCO2)(bpy)2(H2O)][CymCO2]·0.5MePh·2H2O (6), [Cu(CymCO2)2(imz)2] (7), and [Cu(CymCO2)2(bpy)(H2O)] (8), were obtained and characterized by single-crystal X-ray analysis. Complexes 1-3 are isostructural. Magnetism of the Co complexes 1, 4, and 5 was studied; it was shown that they exhibit the properties of field-induced single-molecule magnets with magnetization reversal barriers (ΔE/kB) of 44, 13, and 10 K, respectively. Thermal decomposition of complexes 1-8 was studied by means of DSC and TGA methods. The final products of thermolysis of 1-6 in air, according to powder XRD data, are the pure spinel phases MMn2O4; for the cases of copper complexes, the mixtures of CuMn2O4 and CuO were found in the products.
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New CoII substituted malonate field-induced molecular magnets {[Rb6Co3(cpdc)6(H2O)12]â6H2O}n (1) and [Cs2Co(cpdc)2(H2O)6]n (2) (where cpdc2- stands for cyclopropane-1,1-dicarboxylic acid dianions) were synthesized. Both compounds contain mononuclear bischelate fragments {CoII(cpdc)2(H2O)2}2- where the quasi-octahedral cobalt environment (CoO6) is complemented by water molecules in apical positions. The alkali metal atoms play the role of connectors between the bischelate fragments to form 3D and 2D polymeric structures for 1 and 2, respectively. Analysis of dc magnetic data using the parametric Griffith Hamiltonian for high-spin CoII supported by ab initio calculations revealed that both compounds have an easy axis of magnetic anisotropy. Compounds 1 and 2 exhibit slow magnetic relaxation under an external magnetic field (HDC = 1000 and 1500 Oe, respectively).
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Reversible nucleophilic addition to a phthalocyanine core was observed for the first time for the electron-deficient cationic phosphorus(V) complex [PcP(OMe)2]+, whose reaction with KOH afforded a highly distorted nonaromatic adduct bearing an OH group at one of the α-pyrrolic carbon atoms. This adduct was characterized by single-crystal X-ray diffraction, ESI HRMS, and NMR, and UV-vis spectroscopy, together with quantum-chemical modeling. The acidic treatment of this adduct restored aromaticity and recovered the starting cationic complex. The reversible aromaticity breakage resulted in dramatic changes in the photophysical properties of the studied complex, which could pave the way to novel switchable Pc-based compounds and materials.
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Reaction of 2,2'-bipyridine (2,2'-bipy) or 1,10-phenantroline (phen) with [Mn(Piv)2(EtOH)]n led to the formation of binuclear complexes [Mn2(Piv)4L2] (L = 2,2'-bipy (1), phen (2); Piv- is the anion of pivalic acid). Oxidation of 1 or 2 by air oxygen resulted in the formation of tetranuclear MnII/III complexes [Mn4O2(Piv)6L2] (L = 2,2'-bipy (3), phen (4)). The hexanuclear complex [Mn6(OH)2(Piv)10(pym)4] (5) was formed in the reaction of [Mn(Piv)2(EtOH)]n with pyrimidine (pym), while oxidation of 5 produced the coordination polymer [Mn6O2(Piv)10(pym)2]n (6). Use of pyrazine (pz) instead of pyrimidine led to the 2D-coordination polymer [Mn4(OH)(Piv)7(µ2-pz)2]n (7). Interaction of [Mn(Piv)2(EtOH)]n with FeCl3 resulted in the formation of the hexanuclear complex [MnII4FeIII2O2(Piv)10(MeCN)2(HPiv)2] (8). The reactions of [MnFe2O(OAc)6(H2O)3] with 4,4'-bipyridine (4,4'-bipy) or trans-1,2-(4-pyridyl)ethylene (bpe) led to the formation of 1D-polymers [MnFe2O(OAc)6L2]n·2nDMF, where L = 4,4'-bipy (9·2DMF), bpe (10·2DMF) and [MnFe2O(OAc)6(bpe)(DMF)]n·3.5nDMF (11·3.5DMF). All complexes were characterized by single-crystal X-ray diffraction. Desolvation of 11·3.5DMF led to a collapse of the porous crystal lattice that was confirmed by PXRD and N2 sorption measurements, while alcohol adsorption led to porous structure restoration. Weak antiferromagnetic exchange was found in the case of binuclear MnII complexes (JMn-Mn = -1.03 cm-1 for 1 and 2). According to magnetic data analysis (JMn-Mn = -(2.69 ÷ 0.42) cm-1) and DFT calculations (JMn-Mn = -(6.9 ÷ 0.9) cm-1) weak antiferromagnetic coupling between MnII ions also occurred in the tetranuclear {Mn4(OH)(Piv)7} unit of the 2D polymer 7. In contrast, strong antiferromagnetic coupling was found in oxo-bridged trinuclear fragment {MnFe2O(OAc)6} in 11·3.5DMF (JFe-Fe = -57.8 cm-1, JFe-Mn = -20.12 cm-1).
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Acetatos/química , Complexos de Coordenação/química , Compostos Heterocíclicos/química , Manganês/química , Valeratos/química , Adsorção , Complexos de Coordenação/síntese química , Cristalografia por Raios X , Espectroscopia de Ressonância de Spin Eletrônica , Fenômenos Magnéticos , Conformação Molecular , Temperatura , Termogravimetria , Valeratos/síntese química , Difração de Raios XRESUMO
The reaction of the redox active 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-BIAN) and iron(II) iodide in acetonitrile led to a new complex [(dpp-BIAN)FeIII2] (1). Molecular structure of 1 was determined by the single crystal X-ray diffraction analysis. The spin state of the iron cation in complex 1 at room temperature and the magnetic behavior of 1 in the temperature range of 2-300 K were studied using Mossbauer spectroscopy and magnetic susceptibility measurements, respectively. The neutral character of dpp-BIAN in 1 was confirmed by IR and UV spectroscopy. The electrochemistry of 1 was studied in solution and solid state using cyclic voltammetry. The generation of the radical anion form of the dpp-BIAN ligand upon reduction of 1 in a CH2Cl2 solution was monitored by EPR spectroscopy.
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A series of heterometallic carboxylate 1D polymers of the general formula [LnIIICd2(piv)7(H2O)2]n·nMeCN (LnIII = Sm (1), Eu (2), Tb (3), Dy (4), Ho (5), Er (6), Yb (7); piv = anion of trimethylacetic acid) was synthesized and structurally characterized. The use of CdII instead of ZnII under similar synthetic conditions resulted in the formation of 1D polymers, in contrast to molecular trinuclear complexes with LnIIIZn2 cores. All complexes 1-7 are isostructural. The luminescent emission and excitation spectra for 2-4 have been studied, the luminescence decay kinetics for 2 and 3 was measured. Magnetic properties of the complexes 3-5 and 7 have been studied; 4 and 7 exhibited the properties of field-induced single-molecule magnets in an applied external magnetic field. Magnetic properties of 4 and 7 were modelled using results of SA-CASSCF/SO-RASSI calculations and SINGLE_ANISO procedure. Based on the analysis of the magnetization relaxation and the results of ab initio calculations, it was found that relaxation in 4 predominantly occurred by the sum of the Raman and QTM mechanisms, and by the sum of the direct and Raman mechanisms in the case of 7.
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Boron-cross-linked cobalt(II) pseudoclathrochelate was obtained by the template reaction of 2-acetylpyrazoloxime, phenylboronic acid, and a new DMF cobalt(II) solvato complex with a decachloro-closo-decaborate dianion. As confirmed by single-crystal X-ray diffraction, this complex crystallizes with two symmetry-independent cobalt(II) pseudoclathrochelate cations, one decachloro-closo-decaborate dianion, one benzene, one dichloromethane solvent molecule, and two molecules of DMF. The latter act as pseudocapping fragments to the monocapped tris-pyrazoloximate ligands by forming N-H···O hydrogen bonds with their pyrazole groups. The CoIIN6-coordination polyhedra adopt a nearly ideal TP geometry with distortion angles φ equal to 1.22(16) and 2.58(17)° for two symmetry-independent pseudoclathrochelate cations, both containing the encapsulated cobalt(II) ion in its high-spin state (Co-N 2.115(4)-2.198(3) Å). Magnetic properties of this complex were studied both by dc-magnetometry and by solution-state NMR spectroscopy to reveal a high magnetic anisotropy, thus suggesting a large magnetic susceptibility tensor anisotropy (25.8 × 10-32 m3 at 298 K) and a large negative zero-field splitting energy (-85 cm-1). The results of magnetometry studies in the ac magnetic field suggest a single molecule magnet behavior of this TP complex with an effective magnetization reversal barrier of approximately 130 cm-1. Its pseudocapping DMF molecules that form H-bonds with tris-pyrazoloximate fragments are easy to substitute by strong H-bond acceptors, such as chloride ions and di- and tetramethylureas, thus affecting the magnetic properties of a whole pseudomacrobicyclic paramagnetic system.
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Complexes [(dpp-BIAN)0CoIII2]·MeCN (I) and [(Py)2CoI2] (II) were synthesized by the reaction between cobalt(II) iodide and 1,2-bis(2,6-diisopropylphenylimino)acenaphthene (dpp-BIAN) or pyridine (Py), respectively. The molecular structures of the complexes were determined by X-ray diffraction. The Co(II) ions in both compounds are in a distorted tetrahedral environment (CoN2I2). The electrochemical behavior of complex I was studied by cyclic voltammetry. Magnetochemical measurements revealed that when an external magnetic field is applied, both compounds exhibit the properties of field-induced single ion magnets.
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Acenaftenos/química , Cobalto/química , Técnicas Eletroquímicas , Iodetos/química , Íons , Magnetismo , Piridinas/química , Anisotropia , Ligantes , Conformação Molecular , Estrutura Molecular , Oxirredução , Temperatura , Difração de Raios XRESUMO
Herein, we report a new trigonal prismatic cobalt(II) complex that behaves as a single molecule magnet. The obtained zero-field splitting, which is also directly accessed by THz-EPR spectroscopy (-102.5â cm-1 ), results in a large magnetization reversal barrier U of 205â cm-1 . Its effective value, however, is much lower (101â cm-1 ), even though there is practically no contribution from quantum tunneling to magnetization relaxation.
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Vanadium(IV) complexes are actively studied as potential candidates for molecular spin qubits operating at room temperatures. They have longer electron spin decoherence times than many other transition ions, being the key property for applications in quantum information processing. In most cases reported to date, the molecular complexes were optimized through the design for this purpose. In this work, we investigate the relaxation properties of vanadium(IV) ions incorporated in complexes with lanthanides using electron paramagnetic resonance (EPR). In all cases, the VO6 moieties with no nuclear spins in the first coordination sphere are addressed. We develop and implement the approaches for facile diagnostics of relaxation characteristics in individual VO6 moieties of such compounds. Remarkably, the estimated relaxation times are found to be close to those of other vanadium-based qubits obtained previously. In the future, a synergistic combination of qubit-friendly properties of vanadium ions with single-molecule magnetism and luminescence of lanthanides can be pursued to realize new functionalities of such materials.
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Complexos de Coordenação/química , Espectroscopia de Ressonância de Spin Eletrônica , Elementos da Série dos Lantanídeos/química , Fenômenos Magnéticos , Vanádio/química , Algoritmos , Complexos de Coordenação/síntese química , Hidrogênio , Modelos Teóricos , Conformação Molecular , Transição de FaseRESUMO
High magnetic anisotropy is a key property of paramagnetic shift tags, which are mostly studied by NMR spectroscopy, and of single molecule magnets, for which magnetometry is usually used. We successfully employed both these methods in analyzing magnetic properties of a series of transition metal complexes, the so-called clathrochelates. A cobalt complex was found to be both a promising paramagnetic shift tag and a single molecule magnet because of it having large axial magnetic susceptibility tensor anisotropy at room temperature (22.5 × 10-32 m3 mol-1) and a high effective barrier to magnetization reversal (up to 70.5 cm-1). The origin of this large magnetic anisotropy is a negative value of zero-field splitting energy that reaches -86 cm-1 according to magnetometry and NMR measurements.
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A heterometallic phenylsilsesquioxane [(PhSiO1,5)22(CoO)3(NaO0.5)6]â (EtOH)6â (H2O) 1 cage architecture of Co(II) ions in a triangular topology exhibits a slow dynamic behavior in its magnetization, induced by the freezing of the spins of individual molecules.
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A series of monocapped cobalt(II) tris-pyrazoloximates was obtained through the template condensation of the corresponding pyrazoloxime, phenylboronic acid and a suitable cobalt(II) halogenide. Comparing 3-acetylpyrazoloxime versus its methine-containing homolog, the former produced cobalt(II) clathrochelates in substantially higher yields due to the electron donating effect of the methyl substituent, increasing the N-donor ability of its oxime group. Their less N-donor analog with the electron acceptor trifluoromethyl group did not form cobalt(II) complexes of this type. In all their solvent-free and solvent-containing crystals, the encapsulated cobalt(II) ion adopted a high-spin state, as gauged by the Co-N bond lengths of 2.112(4)-2.188(9) Å, and was located almost in the center of its CoN6-coordination polyhedron. Their CoN6-polyhedra had an almost ideal trigonal-prismatic (TP) geometry with distortion angles φ below 4°. This TP-like geometry was assisted by hydrogen bonding between their NH groups and the apical counterion. The absence of methyl groups makes them close to an ideal TP. In contrast, stronger N-Hâ¯Cl hydrogen bonds occurred in the methyl-containing complex, while the Co-N bond lengths stayed the same at 2.144(2) Å on average. In its solvates with benzene, chloroform and acetone, there is a clear tendency for φ to decrease from 2.7(3)° to 0.47(13)°. The comparable effects of the ribbed methyl substituents, the cross-linking counterion and the lattice solvent on their molecular geometry were observed; the larger the distortions from an ideal TP geometry, the stronger the hydrogen bonds to the corresponding apical halogenide anion. The analysis of the experimental AC- and DC-magnetometry data for their fine-crystalline samples suggests that the passing from the derivative of the methyl-substituted synthon to that of its methine-containing homolog caused a substantial decrease in the magnetic susceptibility value χT and an increase in the QTM contribution to the magnetic relaxation. The effect of a cross-linking halogenide counteranion on the Orbach remagnetization barrier is greater than that of the solvatomorphism of their crystals.
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We report a first example of field-induced (HDC = 2500 Oe) slow magnetization relaxation in the homotrinuclear linear heterospin manganese coordination compound with S = 7/2 ground state, based on the bidentate 3,5-di-tert-butyl-1,2-benzoquinone-1-monooxime (HL) ligand with composition {[MnL3]Mn[MnL3]}.
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The first Yb complexes comprising a quinoline-2-carboxylate (quinaldinate, Q-) ligand, namely 1D-polymeric [Yb(acac)2(Q)]n (1, acac- is the acetylacetonate (pentane-2,4-dionate) anion) and mononuclear [Yb(acac)2(Q)(Phen)] (2, Phen is 1,10-phenanthroline), are reported. The bifunctionality of both complexes as field-induced single-molecule magnets (SMMs) and near IR luminophores has been revealed. The SMM properties of 1 and 2 have been discussed in terms of the geometry and composition of the coordination environment. Also, 1 is the first example of 1D-polymeric SMMs with the capped octahedral surrounding of Yb3+. The photoluminescence quantum yields (PLQYs) of 1 and 2 are 2 and 4%, respectively. The origins of this difference are discussed. Surprisingly, the PLQY value of 2 is high for compounds comprising a lot of C-H vibrational quenchers, being the highest one for reliably characterized Yb ß-diketonate complexes, and surpassing those for complexes with a broad range of anionic ligands. In this respect, the role of the Phen ligand is to tune the coordination mode of Q- thereby decreasing the energy of coordinating C-O oscillators rather than to act as a typical antenna ligand. These results can give rise to an alternative route to elaborate efficient Yb-based luminophores via the substitution of the ß-diketonate ligands controlled by the introduction of appropriate neutral ligands.