Athanassios K. Boudalis

A Family of Enneanuclear Iron(II) Single‐Molecule Magnets

Complexes [Fe9(X)2-(O2CMe)8{(2-py)2CO2}4] (X(-)=OH(-) (1), N3(-) (2), and NCO(-) (3)) have been prepared by a route previously employed for the synthesis of analogous Co(9) and Ni(9) complexes, involving hydroxide substitution by pseudohalides (N3(-), NCO(-)). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3, leading to higher ground spin states compared to that of 1. Variable-field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out-of-phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single-molecule magnets, with energy barriers of 41 K for 2 (tau 0=3.4 x 10(-12) s) and 44 K for 3 (tau 0=2.0 x 10(-11) s). Slow magnetic relaxation has also been observed by zero-field 57Fe Mössbauer spectroscopy. Characteristic integer-spin electron paramagnetic resonance (EPR) signals have been observed at X-band for 1, whereas 2 and 3 were found to be EPR-silent at this frequency. 1H NMR spectrometry in CD3CN has shown that complexes 1-3 are stable in solution.

Slow magnetic relaxation of a ferromagnetic Ni(II)5 cluster with an S = 5 ground state.

Complex [Ni 5{pyCOpyC(O)(OMe)py} 2(O 2CMe) 4(N 3) 4(MeOH) 2].2MeOH.2.6H 2O ( 1.2MeOH.2.6H 2O) was synthesized by the reaction of Ni(O 2CMe) 2.4H 2O with pyCOpyCOpy and NaN 3 in refluxing MeOH. It crystallizes in the monoclinic C2/ c space group and consists of five Ni (II) atoms in a helical arrangement. Direct current magnetic susceptibility studies reveal ferromagnetic interactions between the Ni (II) ( S = 1) ions, stabilizing an S = 5 ground state. Alternating current susceptibility experiments revealed the existence of out-of-phase signals indicative of slow magnetic relaxation. Analysis of the signals showed that they are composite, suggesting more than one relaxation process, while analysis of their magnitudes suggests not all molecules undergo slow magnetic relaxation. Magnetization field-sweep experiments revealed hysteresis at 1.8 K, and magnetization decay experiments clearly verified the appearance of slow magnetic relaxation at that temperature.

Use of the di-2-pyridyl ketone/3,5-di-tert-butylcatechol “blend” in iron(III) chemistry: a cationic tetranuclear cluster and an anionic trinuclear complex

The use of di-2-pyridyl ketone ((py)2CO)/3,5-di-tert-butylcatechol (H2dbcat) “blend” in iron(III) chemistry has yielded a cationic tetranuclear cluster and an anionic trinuclear complex. Both complexes were prepared under anaerobic conditions. The Fe(ClO4)3·6H2O–(py)2CO–H2dbcat–NEt3 (1 ∶ 1 ∶ 1 ∶ 2) reaction system in MeOH gives [Fe4{(py)2C(OMe)O}2{(Hpy)(py)C(OMe)O}2(dbcat)4](ClO4)21, whereas reaction of Fe(ClO4)3·6H2O with (py)2CO, H2dbcat and NEt3 (1 ∶ 1 ∶ 1 ∶ 3) in MeCN gives (HNEt3)[Fe3{(py)2C(OH)O}2(dbcat)4]·MeCN 2·MeCN. The centrosymmetric tetranuclear cation of 1 contains a zigzag array of six-coordinate FeIII ions. The inner FeIII ions are bridged by two catecholate oxygen atoms from two η1:η2:μ2 dbcat2− groups, while one η1:η2:μ2 dbcat2− group and one η1:η2:η1:μ2 (py)2C(OMe)O− ligand bridge each inner FeIII to its outer FeIII neighbour. Each outer metal is chelated by a single bidentate (+Hpy)(py)C(OMe)O− zwitterion. The trinuclear anion of 2·MeCN consists of a triangular unit, in which the Fe2 edges are bridged by two η1:η2:μ2 and one η1:η2:μ3 dbcat2− groups, and one η1:η2:η1:μ2 (py)2C(OH)O− ligand. Two FeIII ions are six-coordinate, while the third is five-coordinate. One six-coordinate FeIII centre is chelated by a bidentate dbcat2− group and the other one by a bidentate (py)2C(OH)O− ligand. Variable-temperature magnetic susceptibility studies in the 2–300 K range reveal antiferromagnetic exchange interactions in both complexes. Variable-temperature Mossbauer spectra of 1 analyse as two quadrupole-split doublets which were assigned to the two different high-spin iron(III) sites in the complex, while those of 2 analyse as one (averaged) quadrupole-split doublet.

Isomorphous replacement of MII ions in MII–GdIII dimers (MII = CuII, MnII, NiII, CoII, ZnII): magnetic studies of the products

Complexes [MIIGdIII{pyCO(OEt)pyC(OH)(OEt)py}3](ClO4)2·EtOH [MII = CuII (1), MnII (2), NiII (3), CoII (4) and ZnII (5)] crystallize in the monoclinic Cc space group and contain one hexacoordinate MII ion and one enneacoordinate GdIII ion, bridged by three {pyCO(OEt)pyC(OH)(OEt)py}− ligands. Magnetic susceptibility measurements indicate a ferromagnetic interaction for 1 and antiferromagnetic interactions for 2–4. Using the Ĥ = −JŜGdIIIŜMII spin Hamiltonian formalism, fits to the magnetic susceptibility data yielded J values of +0.32 cm−1 for 1, −1.7 cm−1 for 2, and −0.22 cm−1 for 3. In complex 4, the orbital contributions of CoII precluded the determination of the magnetic coupling. The complex follows the Curie–Weiss law with θ = −2.07 K (−1.44 cm−1).

Aromatic N-oxide bridged copper(II) coordination polymers: Synthesis, characterization and magnetic properties

Abstract The complexes [Cu2(o-NO2–C6H4COO)4(PNO)2] (1), [Cu2(C6H5COO)4(2,2′-BPNO)] n (2), [Cu2(C6H5COO)4(4,4′-BPNO)] n (3), [Cu(p-OH–C6H4COO)2(4,4′-BPNO)2·H2O] n (4), (where PNO=pyridine N-oxide, 2,2′-BPNO=2,2′-bipyridyl-N,N′-dioxide, 4,4′-BPNO=4,4′-bipyridyl-N,N′-dioxide) are prepared and characterized and their magnetic properties are studied as a function of temperature. Complex 1 is a discrete dinuclear complex while complexes 2–4 are polymeric of which 2 and 3 have paddle wheel repeating units. Magnetic susceptibility measurements from polycrystalline samples of 1–4 revealed strong antiferromagnetic interactions within the {Cu2}4+ paddle wheel units and no discernible interactions between the units. The complex 5, [Cu(NicoNO)2·2H2O] n ·4nH2O, in which the bridging ligand to the adjacent copper(II) ions is nicotinate N-oxide (NicoNO) the transmitted interaction is very weakly antiferromagnetic.

Expanding the 3d-4f heterometallic chemistry of the (py)2CO and pyCOpyCOpy ligands: structural, magnetic and Mössbauer spectroscopic studies of two FeII–GdIII co mplexes

Complex [Fe(II)Gd(III){pyCO(OEt)pyCOH(OEt)py}(3)](ClO(4))(2) (1) crystallizes in the Cc space group and contains one hexacoordinate ferrous ion and one enneacoordinate Gd(III) ion. Complex [Fe(2)(II)Gd(III){pyCO(OEt)py}(4)(NO(3))(H(2)O)][Gd(NO(3))(5)](0.5)(ClO(4)) (2) crystallizes in the C2/c space group and contains two hexacoordinate ferrous ions and one octacoordinate Gd(III) ion. Both complexes have been prepared by the metal-assisted ethanolysis of ligands di-2,6-(2-pyridylcarbonyl)pyridine (pyCOpyCOpy, dpcp) and di-2-pyridyl ketone ((py)(2)CO, dpk), which exhibit similar structures. Mössbauer spectroscopic studies of 2 revealed the presence of two quadrupole-split doublets of equal intensities, each assigned to a ferrous site. These doublets exhibit similar isomer shifts (δ(1) = 1.14 mm s(-1), δ(2) = 1.11 mm s(-1)) but quite different quadrupole splittings (ΔE(Q1) = 3.55 mm s(-1), ΔE(Q2) = 2.74 mm s(-1)). Magnetic studies revealed weak ferromagnetic Fe(II)-Gd(III) interactions for both complexes (J(FeGd) = +0.68 cm(-1), D(Fe) = 12.0 cm(-1) for 1 and J(FeGd) = +0.03 cm(-1), J(FeFe) = -1.73 cm(-1) for 2, according to the -JS(i)S(j) spin-Hamiltonian formalism).

Azide-bridged manganese(III) one-dimensional chain: synthesis, structure, and magnetic study

A new end-to-end azide-bridged one-dimensional coordination polymer, [MnIII(L)(μ1,3–N3)] n (1) (where L represents the dianionic form of the neutral tetradentate Schiff base H2L obtained by 1 : 2 condensation of ethane-1,2-diamine, and 5-chloro-2-hydroxyacetophenone in methanol), has been synthesized and characterized by elemental analyses, IR spectroscopy, single-crystal X-ray diffraction, and variable temperature magnetic study. X-ray crystal structure determination of 1 reveals that a chain system with the repeating unit [MnIII(L)(μ 1,3–N3)]n bridged by μ 1,3 azide. Each manganese(III) is a distorted octahedral geometry. Variable-temperature magnetic study (between 2 and 300 K) suggests a moderate antiferromagnetic interaction in this complex, similar to other studies reported.

Dynamic versus Static Character of the Magnetic Jahn–Teller Effect: Magnetostructural Studies of [Fe3O(O2CPh)6(py)3]ClO4·py

Complex [Fe3O(O2CPh)6(py)3]ClO4·py (1) crystallizes in the hexagonal P63/m space group, and its cation exhibits a crystallographically imposed D3h symmetry due to a C3 axis passing through the oxide of its {Fe3O}7+ core. Single-crystal unit-cell studies carried out with synchrotron radiation confirmed that this symmetry is retained down to 4.5 K; a full crystal structure determination carried out at 90 K resolved the previously reported disorder of the perchlorate anion. Magnetic susceptibility and electron paramagnetic resonance (EPR) data for complex 1 were interpreted with a model considering the retention of the threefold crystallographic symmetry while predicting a lowering of the magnetic symmetry. This model considered the effects of atomic vibrations of the central oxide on the magnetic properties of the complex by incorporating these movements into the spin Hamiltonian through angular overlap considerations of the atomic orbitals; no ad hoc magnetic Jahn-Teller effect was considered. The derived magnetostructural correlations achieved an improvement in the interpretation of the magnetic susceptibility data using the same number of free variables. They also improved the simulations of the EPR data, which exhibit a complicated set of at least five axial resonances; improved simulations were achieved using only two spectral components. Due to the thermal effects on the oxide vibrations, the model predicts a temperature dependence of the magnetic coupling J, which should not be viewed as a constant but as a variable.

The [Fe{(SePPh2)2N}2] Complex Revisited: X-ray Crystallography, Magnetometry, High-Frequency EPR, and Mössbauer Studies Reveal Its Tetrahedral FeIISe4 Coordination Sphere: The [Fe{(SePPh2)2N}2] Complex Revisited: X-ray Crystallography, Magnetometry, High-Frequency EPR, and Mössbauer Studies Reveal Its Tetrahedral FeIISe4

The synthesis and characterization of a preparation of the [Fe{(SePPh 2) 2 N} 2 ] complex (1 Td) is described. X-ray crystallography shows that this system contains a tetrahedral FeSe 4 coordination sphere. The structural features of 1 Td are compared with those of similar first row transition element complexes, including a recently reported preparation of [Fe{(SePPh 2) 2 N} 2 ] (1 SP) exhibiting a highly unusual square planar FeSe 4 coordination sphere. The electronic structure of 1 Td was elucidated by magnetometry, high-frequency EPR, and Mossbauer spectroscopic studies, which reveal zero-field splitting (ZFS) parameters typical of high spin S = 2 tetrahedral Fe II sites. Accurate ZFS parameters (D = +8.22 cm −1 , E/D = 0.104) were obtained by analysis of the EPR spectra and compared with those of the analogous [Fe{(SPPh 2) 2 N} 2 ] complex. The findings of this work call for a thorough structural and physicochemical characterization of the literature 1 SP system.

The [Fe{(SePPh : The [Fe{(SePPh2)2N}2] Complex Revisited: X-ray Crystallography, Magnetometry, High-Frequency EPR, and Mössbauer Studies Reveal Its Tetrahedral FeIISe4

The synthesis and characterization of a preparation of the [Fe{(SePPh 2) 2 N} 2 ] complex (1 Td) is described. X-ray crystallography shows that this system contains a tetrahedral FeSe 4 coordination sphere. The structural features of 1 Td are compared with those of similar first row transition element complexes, including a recently reported preparation of [Fe{(SePPh 2) 2 N} 2 ] (1 SP) exhibiting a highly unusual square planar FeSe 4 coordination sphere. The electronic structure of 1 Td was elucidated by magnetometry, high-frequency EPR, and Mossbauer spectroscopic studies, which reveal zero-field splitting (ZFS) parameters typical of high spin S = 2 tetrahedral Fe II sites. Accurate ZFS parameters (D = +8.22 cm −1 , E/D = 0.104) were obtained by analysis of the EPR spectra and compared with those of the analogous [Fe{(SPPh 2) 2 N} 2 ] complex. The findings of this work call for a thorough structural and physicochemical characterization of the literature 1 SP system.