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1.
Dalton Trans ; 2020 Mar 18.
Artigo em Inglês | MEDLINE | ID: mdl-32186313

RESUMO

A hexacoordinated complex [Co(pydm)2](mdnbz)2 from the family of pincer complexes was prepared and structurally characterized. The complex behaves as an S = 3/2 spin system with a considerable zero-field splitting parameter D/hc ∼ +50 cm-1. The AC susceptibility measurements show a slow magnetic relaxation with three relaxation channels: at the low-frequency (LF), intermediate-frequency (IF) and high-frequency (HF) domains. At T = 2.0 K and an external field BDC = 0.25 T, the relaxation times of the individual modes are τ(LF) = 282 ms, τ(IF) = 3.1 ms, and τ(HF) = 0.16 ms, and the mole fractions of the slowly relaxing species are x(LF) = 0.19, x(IF) = 0.45, and x(HF) = 0.37. A comparison with the analogous complex [Co(pydm)2](dnbz)2 possessing a demethylated counter anion and identical metal cation shows that even small modifications in the composition of SIMs are no longer underestimated for the slow magnetic relaxation.

2.
Chem Commun (Camb) ; 55(92): 13868-13871, 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31674626

RESUMO

A mononuclear pentacoordinate iron(iii) complex shows slow magnetic relaxation with three relaxation channels. The high-frequency relaxation time of the order of microseconds is prolonged on cooling in accordance with the direct and Raman processes. The low frequency relaxation time is little dependent on temperature and varies in the range τ(LF) = 0.52-0.79 s.

3.
Dalton Trans ; 48(37): 13943-13952, 2019 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-31441924

RESUMO

Three new isomorphous complexes [Ni(o-van-en)LnCl3(H2O)] [H2(o-van-en) = N,N'-ethylene-bis(3-methoxysalicylaldiminate; Ln = Ce (1), Gd (2), Dy (3)] were prepared by a stepwise reaction using mild conditions and were structurally characterised as dinuclear molecules in which Ni and Ln are coordinated by the compartmental Schiff base ligand (o-van-en)= and doubly bridged by O atoms. While the nickel(ii) centre is diamagnetic within the N2O2 square-planar coordination of the Schiff base ligand, the lanthanide atoms are octa-coordinated to give an {LnCl3O5} chromophore with a fac-arrangement of the chlorido ligands. AC magnetic measurements revealed that all three complexes, including the nominally isotropic Gd(iii) system, show field induced slow magnetic relaxation with two or three relaxation channels: at T = 1.9 K the low-frequency relaxation time is τLF(1) = 0.060 s at BDC = 0.5 T, τLF(2) = 0.37 s at BDC = 0.3 T, and τLF(3) = 1.29 s at BDC = 0.15 T.

4.
Dalton Trans ; 48(31): 11647-11650, 2019 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-31318366

RESUMO

A hexacoordinate mononuclear [Ni(pydm)2](dnbz)2 complex shows field-induced slow magnetic relaxation with two or three relaxation channels that are strongly supported by an external magnetic field. At BDC = 0.8 T and T = 1.9 K, the low-frequency (LF) relaxation time is as slow as τ(LF) = 1.3 s with the mole fraction of x(LF) = 0.47.

5.
Dalton Trans ; 48(30): 11278-11284, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-31268102

RESUMO

The novel organic-inorganic hybrid salt [L]2[CoCl4] (1) and molecular complex [CoLCl3] (2), where L+ is 2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridinium cation, feature simple {CoCl4} and {CoCl3N} tetrahedral environments of negligible (1) and a slightly higher distortion (2) that are responsible for rather low positive magnetic anisotropy of CoII ion with D/hc = 12.1(6) (1) and 19.4(15) cm-1 (2). Both compounds exhibit field-induced slow magnetic relaxation with three relaxation channels [low- (LF), intermediate- and high-frequency (HF) modes] that is frequency and field dependent. With the increased DC field, the peaks referring to the LF relaxation path are moved to lower frequencies so that the applied DC field causes prolongation of the relaxation time. The opposite is true for the HF relaxation branch: the peak is moved to higher frequencies. Considering the simplicity of the coordination environment and moderate magnetic anisotropy of the metal ion in 1 and 2, the compounds are unique with respect to the remarkably long relaxation time for a given applied DC field and temperature: τLF = 0.54(4) s at BDC = 1.0 T and T = 2.0 K for 1, and τLF = 1.8(2) s at BDC = 1.2 T and T = 1.9 K for 2.

6.
Dalton Trans ; 48(28): 10526-10536, 2019 Jul 16.
Artigo em Inglês | MEDLINE | ID: mdl-31231741

RESUMO

Reactions of bis[5-(2-pyridyl)-1,2,4-triazol-3-yl]alkanes (alkane spacers = -CH2- in L2, -C3H6- in L3, -C4H8- in L4) with M(ii)A2 salts (M = Ni, Co, Fe) resulted in the preparation of five series of mononuclear ([M(L2)(H2O)2]2+, 1a-c) or binuclear ([M2(L3)2(H2O)4]4+, 2a-c; ([M2(L4)2(H2O)4]4+, 3a-c, [M2(L3)2(µ-ox)]2+, 4a-c; [M2(L4)2(µ-ox)]2+, 5a-c) complexes. The crystal structures of ten complexes were determined by single-crystal X-ray crystallography. Magnetic properties of the compounds were characterized by SQUID magnetometry and were analyzed by fitting on a spin Hamiltonian model. It was revealed that Fe(ii) and Co(ii) compounds exhibit non-negligible anisotropy and in the case of 2a-c and 3a-c complexes weak ferromagnetic interactions between the metal centers were observed. In the case of complexes containing an {M2(µ-ox)}2+ core strong antiferromagnetic interactions were observed within the dimer. Remarkably, solid state luminescence of Co(ii) and Fe(ii) complexes (1b, 2b, 3b and 1c, 2c, 3c) was observed.

7.
Inorg Chem ; 58(2): 1134-1146, 2019 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-30601007

RESUMO

Two solvent-free mononuclear Fe(III) complexes [Fe(L)2]NO3 (1) and [Fe(L)2]ClO4 (2) have been synthesized by employing a new π-conjugated azo-phenyl substituted ligand, 2-(( E)-((2-(ethylamino)ethyl)imino)methyl)-4-(2-phenyldiazenyl)phenol (HL). The noncoordinated azo-phenyl part of the ligand adopts two different conformations which can exert a varied local distortion around the metal center affecting the spin crossover behavior. The magnetic data (2-450 K) reveal that complex 1 displays spin crossover above room temperature where the ligand is in linear form, while complex 2 shows an incomplete spin transition where the ligand adopts a skew form in the solid state. These complexes represent rare examples of high-temperature spin transition for mononuclear Fe(III) complexes with T1/2 > 350 K with very high thermal stability. Presence of strong intermolecular interactions and solvent-free nature of the complexes leads to exceptional thermal stability up to 485 K (for 1) and 496 K (for 2) as revealed by thermogravimetric analysis. The magnetic data for complex 1 have been analyzed by employing an Ising-like model with vibrations yielding the enthalpy change Δ H and entropy change Δ S of the spin transition along with the critical temperature T1/2 and the solid-state cooperativeness Γ. Spin crossover behavior of complex 1 has also been characterized by differential scanning calorimetry and electron paramagnetic resonance measurements. Ab initio calculations have been performed to analyze the difference in energies of the ground state and excited states of the complexes.

8.
Inorg Chem ; 57(22): 14314-14321, 2018 Nov 19.
Artigo em Inglês | MEDLINE | ID: mdl-30406658

RESUMO

Instead of assembling complex clusters and/or expensive lanthanide-based systems as single ion magnets, we are focusing on mononuclear cobalt(II) systems among which the complex under study, [Co( pydca)( dmpy)]2·H2O (1), shows a field supported slow magnetic relaxation on the order of seconds at low temperature ( pydca = pyridine-2,6-dicarboxylato, dmpy = 2,6-dimethanolpyridine). The low-frequency relaxation time is as slow as τ(LF) = 1.35(6) s at T = 1.9 K and BDC = 0.4 T. The properties of 1 are compared to the previously reported nickel and copper analogues which were the first examples of single ion magnets in the family of Ni(II) and Cu(II) complexes.

9.
Dalton Trans ; 47(44): 15745-15750, 2018 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-30346456

RESUMO

A [Co(N3)2(mqu)] complex with a 1D chain architecture (mqu - 4-methylquinoline) displays the paramagnetic behaviour with a slight exchange interaction of antiferromagnetic nature among Co(ii) centres. It shows slow magnetic relaxation with three distinct modes. The relaxation time strongly depends upon the applied external field. The slowest relaxation time refers to the low-frequency mode τLF = 1.6(2) s at BDC = 0.8 T and T = 1.9 K.

10.
Inorg Chem ; 57(20): 12740-12755, 2018 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-30277381

RESUMO

Three pentacoordinate complexes of the type [Co( pypz)X2], where pypz is a tridentate ligand 2,6-bis(pyrazol-1-yl)pyridine and X = Cl- (1), NCS- (2), and NCO- (3), have been synthesized, and their structures have been determined by X-ray analysis. The DC magnetic data show a sizable magnetic anisotropy, which was confirmed by high-field high-frequency electron paramagnetic resonance (HF EPR) measurements. Well-resolved HF EPR spectra of high spin cobalt (II) were observed over the microwave frequency range 100-650 GHz. The experimental spectra of both complexes were simulated with axial g tensor components, a very large positive D value, and different E/ D ratios. To determine the exact D value for 2 (38.4 cm-1) and 3 (40.92 cm-1), the far-infrared magnetic spectroscopy method was used. Knowledge of the zero field splitting parameters and their signs is crucial in interpreting the single-molecule magnet or single chain magnet behavior. The AC susceptibility data confirm that these complexes exhibit a slow magnetic relaxation under small applied DC field with two (1 and 3) or three (2) relaxation modes.

11.
Dalton Trans ; 47(43): 15523-15529, 2018 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-30338326

RESUMO

Cobalt(ii) benzoate trihydrate prepared by the reaction of CoCO3 with benzoic acid (HBz) in boiling water followed by crystallization has been structurally characterized as a chain-like system with the formula unit [Co(Bz)(H2O)2]Bz·H2O where the Co(ii) atoms are triply linked by one bridging syn-syn benzoato (Bz) and two aqua ligands; additional benzoate counter ions and solvate water molecules are present in the crystal structure. DC magnetic measurements reveal a sizable exchange coupling of a ferromagnetic nature between the Co(ii) atoms. At TN = 5.5 K the paramagnetic phase switches to the antiferromagnetic phase. Though the remnant magnetization is zero, the magnetization curve shows two lobes of a hysteresis loop and the DC relaxation experiments confirm a long relaxation time at T = 2.0 K. AC susceptibility data confirm a slow relaxation of magnetization even in the antiferromagnetic phase. In the absence of the magnetic field, two relaxation channels exist. The relaxation time for the low frequency channel is as slow as τLF > 1.6 s and data fitting yields τLF (2.1 K) = 14 s. The high-frequency relaxation time obeys the Orbach process at a higher temperature whereas the Raman process dominates the low-temperature region. Three slow relaxation channels are evidenced at the applied magnetic field BDC = 0.1 T.

12.
Dalton Trans ; 47(32): 10941-10952, 2018 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-30019733

RESUMO

The novel hexanuclear complexes [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4(tBuDea)4Cl] (2) were prepared through one-pot self-assembly reactions of copper powder and iron(ii) or manganese(ii) chloride with N-tert-butyldiethanolamine (H2tBuDea) and pivalic acid (HPiv) in acetonitrile. Crystallographic studies revealed the uncommon molecular core type M6(µ-X)7(µ3-X)2 in 1 and 2, which can be viewed as a combination of two trimetallic M3(µ-X)2(µ3-X) fragments joined by three bridging atoms. The analysis and classification of the hexanuclear complexes having a M3(µ-X)2(µ3-X) moiety as a core forming fragment using data from the Cambridge Structural Database (CSD) were performed. Variable-temperature (1.8-300 K) magnetic susceptibility measurements of 1 showed a decrease of the effective magnetic moment value at low temperature, indicative of antiferromagnetic coupling between the magnetic centres (JFe-Cu/hc = -6.9 cm-1, JCu-Cu/hc = -4.1 cm-1, JFe-Fe/hc = -24.2 cm-1). Complex 1 acts as a catalyst in the reaction of mild oxidation of cyclohexane with H2O2, showing the yields of products, cyclohexanol and cyclohexanone, up to 17% using pyrazinecarboxylic acid as a promoter. In the oxidation of cis-1,2-dimethylcyclohexane with m-chloroperoxybenzoic acid (m-CPBA), 70% of retention of stereoconfiguration was observed for tertiary alcohols. Compound 1 also catalyses the amidation of cyclohexane with benzamide. In all three catalytic reactions the by-products were investigated in detail and discussed.

13.
Dalton Trans ; 47(24): 7879-7882, 2018 Jun 19.
Artigo em Inglês | MEDLINE | ID: mdl-29850759

RESUMO

A mononuclear Ni(ii) complex, [Ni(pydca)(dmpy)]·H2O, pyridine-2,6-dicarboxylato-2,6-dimethanolpyridine nickel(ii) monohydrate, exhibits a slow magnetic relaxation under an applied magnetic field with two and/or three relaxation channels. While at BDC = 0.2 T, the low-frequency relaxation mode is weakly developed, its intensity grows rapidly with the field so that at BDC = 0.6 T, this is the dominating relaxation path giving rise to a relaxation time of τLF = 322 ms at T = 2.0 K. At BDC = 1.2 T the relaxation time is as slow as τLF = 876 ms.

14.
Inorg Chem ; 57(8): 4352-4358, 2018 Apr 16.
Artigo em Inglês | MEDLINE | ID: mdl-29600851

RESUMO

Three compounds with octahedral-tetrahedral Co(II) moieties of [Co( dppm O, O)3][CoX4] type, where X = SCN (1), Cl (2), or I (4) have been synthesized and characterized by the X-ray structure analysis (1 and 4), and spectroscopic methods. The dc magnetic measurements show high magnetic anisotropy for octahedral centers whereas tetrahedral sites possess moderate D values. These results are confirmed by the ab initio calculations. The ac susceptibility data reveals a slow magnetic relaxation for 2 and 4, similar to that of the X = Br analogue (3), whereas 1 displays no ac-absorption signal. There are two relaxation channels; the slower for 2 (4) possesses a relaxation time as long as τLF= 178 (588) ms at T = 1.9 K and Bdc = 0.7 T. Also, the half-Zn analogue, [Co( dppm O, O)3][ZnI4], shows slow magnetic relaxation with two relaxation channels conditioned by the cationic unit [Co( dppm O, O)3]2+.

15.
Dalton Trans ; 46(38): 13135-13144, 2017 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-28945256

RESUMO

Two µ-phenoxo-µ1,1-azide dinuclear CoIIICoII complexes [CoIII(N3)2L1(µ1,1-N3)CoII(N3)]·MeOH (1) and [CoIII(N3)2L2(µ1,1-N3)CoII(N3)]·MeOH (2) (HL1 and HL2 are two Schiff base ligands having N2O-N2O compartments) both possess one hexacoordinate Co(iii) and one pentacoordinate Co(ii) center. DC magnetic susceptibility and magnetization measurements show an appreciable amount of positive magnetic anisotropy (D/hc∼ 40 cm-1) that is also confirmed by ab initio CASSCF calculations. AC susceptibility measurements of 1 reveal that it exhibits a slow magnetic relaxation with two relaxation channels. The external magnetic field supports the low-frequency (LF) channel that escapes on heating more progressively than the high-frequency (HF) branch. The relaxation time is as slow as τ = 255 ms at T = 1.9 K and BDC = 0.6 T, where the LF mole fraction is 69%. The complex 2 also displays similar field-supported slow magnetic relaxation with two relaxation channels.

16.
Dalton Trans ; 46(33): 10950-10956, 2017 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-28766674

RESUMO

A mononuclear hexacoordinate complex [Co(pydm)2](dnbz)2 formed from 2,6-pyridinedimethanol in the coordination sphere of Co(ii) and dinitrobenzoato anions exhibits magnetic anisotropy of an easy axis type and a field induced slow magnetic relaxation with manifold relaxation channels. The low-frequency relaxation time is as slow as τLF = 0.13 s at BDC = 0.4 T and T = 1.9 K.

17.
Dalton Trans ; 46(23): 7480-7494, 2017 Jun 13.
Artigo em Inglês | MEDLINE | ID: mdl-28561848

RESUMO

The "direct synthesis" approach, namely one-pot reaction of metal powders and ammonium salt with a methanol solution of a polydentate Schiff base (H2L) formed in situ from salicylaldehyde and ethanolamine, has been successfully used for the preparation of the new heterometallic compounds [Cu3Mn(L)4(CH3OH)3]I3 (1), [Cu3Mn(L)4(CH3OH)3(H2O)]NCS·H2O (2), [Cu3Mn(L)4(CH3OH)(H2O)2.55]Br·0.45H2O (3) and [Cu3Mn(L)4(H2O)3.4]BF4·0.6H2O (4). Crystallographic analysis revealed that 1-4 are based on the tetranuclear core {CuMnIII(µ3-O)4} where the metal centres are joined by the oxygen bridges of Schiff base ligands forming a cube-like arrangement. The novel heterometallic compound [Cu3Mn(L)4(CH3OH)3]2[Mn(NCS)4]·2CH3OH (5) has been obtained by the "building block" approach using the reaction of [Cu(HL)2] with manganese acetate and NH4NCS in methanol. The crystal structure of 5 revealed the {CuMnIII(µ-O)2(µ3-O)2} metal core which can be viewed as a double open cube. In spite of a similar {Cu3MnO4} atom set in the cores of 1-5, the complexes show rather different molecular structures and significantly differ by the number and combinations of coordinated CH3OH/H2O solvent molecules. Variable-temperature (2-300 K) magnetic susceptibility along with variable-field magnetization measurements of 1-5 showed a decrease of the effective magnetic moment value at low temperature, indicative of the antiferromagnetic coupling of medium size (-55 to -22 cm-1). For these systems resembling a compressed prism the coupling constant in walls J4 correlates with the averaged bonding angles in walls α: J4vs. α develops approximately according to a straight line.

18.
Inorg Chem ; 56(12): 6999-7009, 2017 Jun 19.
Artigo em Inglês | MEDLINE | ID: mdl-28556666

RESUMO

The reaction of Co(CH3COO)2·4H2O with the Schiff base ligand LH4 derived from o-vanillin and tris(hydroxymethyl)aminomethane produces the dinuclear mixed-valence complex [CoIICoIII(LH2)2(CH3COO)(H2O)](H2O)3 (1), which has been investigated using IR spectroscopy, X-ray crystallography, temperature-dependent magnetic susceptibility, magnetization, HFEPR spectroscopy, and ac susceptibility measurements at various frequencies, temperatures, and external magnetic fields. The structure of 1 consists of neutral molecules in which two cobalt ions with distorted octahedral geometries, CoIIO6 and CoIIIN2O4, are bridged by two deprotonated -CH2O- groups of the two LH22- ligands. 1 completes a series with Cl, Br, NO3, and NCS anions published before by different authors. Low-temperature HFEPR measurements reveal that the ground electronic state of the Co(II) center in 1 is a highly anisotropic Kramers doublet; the effective g values of 7.18, 2.97, and 1.96 are frequency-independent over the frequency ranges 200-630, 200-406, and 200-300 GHz for the highest, intermediate, and lowest geff values, respectively. The two lower values were not seen at higher frequencies because the magnetic field was not high enough. Temperature-dependent magnetic susceptibility and field-dependent magnetization data confirm high magnetic anisotropy of the easy axis type. Complex 1 behaves as a single-ion magnet under a small applied external field and demonstrates two relaxation modes that strongly depend on the applied static dc field. The observation of multiple relaxation pathways clearly distinguishes 1 from the Cl and Br analogues.

19.
Chemistry ; 23(33): 7990-7996, 2017 Jun 12.
Artigo em Inglês | MEDLINE | ID: mdl-28326629

RESUMO

An unprecedented reversible dynamic transformation is reported in a metal-organic framework involving bond formation, which is accompanied by two important structural changes; achiral to chiral and two- to three-dimensions. Using two bent organic ligands (diimpym=4,6-di(1H-imidazol-1-yl)pyrimidine; H2 npta=5-nitroisophthalic acid) and CoII (NO3 )2 ⋅6 H2 O the coordination polymer Co(diimpym)(npta)⋅CH3 OH, (1⋅CH3 OH), was obtained solvothermally. Its structure consists of knitted pairs of square layers (44 -sql net) of five-coordinated Co and disordered methanol, and it crystallized in the achiral Pbca space group at room temperature. It undergoes a single crystal to single crystal (SC-SC) transformation to a 3D interpenetrated framework (α-polonium-type net, pcu) of six-coordinated Co and ordered methanol in the chiral P21 21 21 space group below 220 K. Most unusual is the dynamic temperature-dependent shortening of a Co⋅⋅⋅O connection from a non-bonded 2.640 Š(298 K) to a bonded 2.347 Šdistance (100 K) transforming the square pyramidal cobalt polyhedron to a distorted octahedron. The desolvated crystals (1) obtained at 480 K retain the full crystallinity and crystallize in the achiral Pbca space group between 100 and 298 K but the dynamic shortening of the Co⋅⋅⋅O distance connecting the layers into the 3D pcu framework structure is observed. Following post-synthetic insertion of ethanol (1⋅CH3 CH2 OH) it does not exhibit the transformation and retains the knitted 2D achiral Pbca structure for all temperatures (100-298 K) and the ethanol is always disordered. The structural analyses thus conclude that the ordering of the methanol induces the chirality while the available space controls the dynamic motion of the knitted 2D networks into the 3D interpenetrated framework. Consequently, 1 selectively adsorbs CO2 to N2 and exhibits Type-III isotherms indicating dynamic motion of the 2D networks to accommodate the CO2 at 273 and 298 K in contrast to the rigidity of the 3D framework at 77 K preventing N2 from penetrating the solid. The magnetic properties are also reported.

20.
Inorg Chem ; 56(3): 1478-1482, 2017 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-28068067

RESUMO

A mononuclear hexacoordinate Cu(II) complex shows a field induced slow magnetic relaxation that is not facilitated by an energy barrier to spin reversal due to the zero-field splitting. Two relaxation channels were found: the magnetic field strongly supports the low-frequency relaxation path with a relaxation time as long as τ = 0.8 s at T = 1.9 K and B = 1.5 T. The mechanism of the relaxation at low temperature involves the dominant Raman process for this S = 1/2 spin system along with a temperature-independent term belonging to a quantum tunneling.

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