C. Zanchini

Exchange coupling in dinuclear copper(II) complexes with oxalato, oxamidato and oxamato ligands

We have synthesized a series of dinuclear copper-(II) complexes in which the metal ions are bridged by oxamidato, oxamato and oxalato ligands, with the purpose of studying the influence of the ligands on the extent of the antiferromagnetic coupling between the two metal ions. Magnetic susceptibility measurements between 77 and 300 K showed that the singlet-triplet splitting ranges from 330 cm−1 to 550 cm−1; J is found to increase in the order oxalato s oxamato s oxamidato.

Synthesis, structure and EPR studies of mixed hexafluoroacetylacetonatecopper (II) complexes with some diimine ligands

The synthesis, structure and magnetic properties of [CuL(hfacac) 2 ] and [Cu 2 TPP(hfacac) 4 ], where L is 2,3-bis(2-pyridyl)pyrazine (DPP), 2,3-bis(2-pyridyl)quinoxaline (DPQ), 6-methyl-2,3-bis(2-pyridyl)quinoxaline (MeDPQ), 6,7-dimethyl-2,3-bis(2-pyridyl)quinoxaline (DMeDPQ), 6-chloro-2,3-bis(2-pyridyl)quinoxaline (ClDPQ), 2,3-bis(2-pyridyl)-benzoquinoxaline (BDPQ) and TPP is 2,3,5,6-tetrakis-(2-pyridyl)pyrazine and hfacac is hexafluoroacetylacetonate ion, are described. The [Cu(DMeDPQ)-(hfacac) 2 ] complex crystallizes in the monoclinic space group P 2 1 / a with cell parameters a =23.643(5), b =15.245(3), c =9.044(2) A, β=94.16(2)° and Z =4. The two copper-nitrogen(pyridine) bond distances are equal, 1.998(7) A. Polycrystalline powder EPR spectra of all the complexes and singe crystal EPR spectrum of [Cu(DMeDPQ)(hfacac) 2 ] were recorded at room temperature. The spectra of [CuL(hfacac) 2 ] complexes show axial distortion with g ∥ = 2.31(1) and g ⊥ = 2.08(1); for [Cu 2 (TPP)(hfacac) 4 ] g ∥ = 2.30(1) and g ⊥=2.09(1). In some cases weak evidence of hyperfine splitting in the parallel transition is shown.

Complexes with diimine ligands. Part III. Synthesis, structure and magnetic studies of mixed acetylacetonatecobalt(II) derivatives

Abstract The synthesis and properties of the mononuclear [Co(L)(hfacac) 2 ], [Co(L)(tfacac) 2 ] and of the dinuclear complexes [Co2(L)(acac)4], where L is 2,3-bis( 2− pyridyl)pyrazine (DPP), 2,3-bis( 2− pyridyl)quinoxaline (DPQ), 6-methyl-2,3-bis( 2− pyridyl)quinoxaline (MeDPQ), 6,7-dimethyl-2,3-bis( 2− pyridyl)quinoxaline (DMeDPQ), 6-chloro-2,3-bis( 2− pyridyl)quinoxaline (ClDPQ), 2,3-bis( 2− pyridyl)benzoquinoxaline (BDPQ); hfacac, tfacac and acac being hexafluoroacetylacetonate, trifluoroacetylacetonate and acetylacetonate, respectively, are described. The structures of [Co(L)(hfacac) 2 ] where L=BDPQ and DMeDPQ were solved. The [Co(BDPQ)(hfacac) 2 ] complex crystallizes in the Pi space group with cell parameters a =14.382( 4 ), b =12.853(3), c =9.094(3) A, α=92.55( 3 ), β=100.80(3), γ=94.54(3)° and Z =2. The [Co(DMeDPQ)(hfacac) 2 ] complex crystallizses in the P 2 1 / a space group with cell parameters a =19.130(5), b =9.248( 3 ), c =18.198(4) A,β=95.18( 3 ) and Z =4. Magnetic properties of the dinuclear complexes and a study of the coordination possibilities of this kind of ligands are presented.

EPR spectra of triplet states with large zero-field splitting. [Cu(bipy)OH)]2SO4·4H2O

Abstract The polycrystalline powder and single-crystal EPR spectra of [Cu(bipy)(OH)] 2 SO 4 · 4H 2 O were recorded in the range of temperatures at both X- and Q-band frequencies. The polycrystalline powder spectra were used to estimate that the zero-field splitting in this complex must be fairly large, > 1 cm −1 . The single-crystal spectra recorded in four different planes were analyzed through a diagonalization of the full Hamiltonian matrix, allowing for nonparallel g and D tensors. The difficulties which arise for the interpretation of the spectra were discussed. The values of g and D were found to have coincident z axes, with the x and y axes rotated by approximately 45°. The data obtained from the analysis of the single-crystal spectra were used to calculate the polycrystalline powder spectra, with satisfactory agreement with experiment.

Monomeric and polymeric oxovanadium(IV) complexes containing 5-(4′-alkyl-phenylazo)-8-hydroxy-quinoline ligands

Abstract A number of bis-[5-(4′-R-phenylazo)-8-hydroxy-quinolinato] vanadyl(IV) compounds (R=H, CH 3 , C 2 H 5 , n-C 3 H 7 , n-C 4 H 9 ) have been synthesized and characterized by elemental analyses and IR, EPR and UV–Vis spectroscopies. Data from the above show that they exist in two isomeric solid forms (which were separated for R=C 2 H 5 , n-C 3 H 7 and n-C 4 H 9 ) whose IR and EPR spectra account for isolated molecules (IR: ν (VO)≅961 cm −1 ; EPR: g ≅1.980) or for polymers (IR: ν (VO) about 890 cm −1 ; EPR: g x = g y =1.975(5), g z =1.960(2), A x = A y =40(5), A z =160(3)×10 −4 cm −1 ). The polymeric structure results from intermolecular VO⋯VO interactions.

Crystal and molecular structure, magnetic properties and scattered-wave description of the electronic structure of the hexanuclear octahedral clusters [Fe6(µ3-S)8(PEt3)6][PF6]n(n= 1 or 2)

The crystal structure of [Fe6(µ3-S)8(PEt3)6][PF6]2 has been determined. It is isostructural with the parent tetraphenylborate complex. The electronic structures of [Fe6(µ3-S)8(PEt3)6][PF6] and of [Fe6(µ3-S)8(PEt3)6][PF6]2 have been investigated experimentally by measuring the temperature variation of the magnetic susceptibility between 300 and 4.2 K, the field dependence of the magnetisation at three different temperatures in the range 2–10 K and the polycrystalline powder Mossbauer spectra at variable temperature. The complex [Fe6(µ3-S)8(PEt3)6][PF6] possesses a S= 7/2 spin state well isolated from the excited states, while [Fe6(µ3-S)8(PEt3)6][PF6]2 shows a marked temperature dependence of the magnetic susceptibility. The magnetic structures of the complexes have been characterised empirically with the Heisenberg–Dirac–van Vleck exchange spin Hamiltonian. The nature of the magnetic states is rationalised in the framework of Xα-SW theory.

Single-crystal EPR study of the bimetallic ferrimagnetic chain MnCu(EDTA)·6H2O

A single-crystal EPR study of the bimetallic chain compound MnCu(EDTA)·6H2O is reported. The angular dependence of the linewidth is discussed in relation to the magnetic dipole-dipole interaction, manganese zerofield splitting (ZFS) and copper hyperfine coupling. The calculation of the second moments indicates that ZFS is comparable to the dipolar contribution. The EPR data support the one-dimensional character of the compound.

ESR spectra of low-symmetry high-spin cobalt(II) complexes: Part 5. Pseudotetrahedral tetra-n-butylammonium tribromo (quinoline) cobaltate(II) and the calculation of the a2 tensor

Abstract X-ray structure of [N(n-C4H94)][CoBr3(quinoline)] shows that the complex possesses a symmetry near to C3, the Co-N bond direction being the pseudo C3 axis. The single crystal spectra are largely anisotropic: g1=6.31, g2=2.33, g3=1.61. g3, makes an angle of 20° with Co-N. These results are easily interpreted using a symmetry i n dependent Ligand Field Model. The application of the model to the calculation of the cobalt hyperfine coupling constant is also shown.

Single-crystal EPR spectra of the first alternating bimetallic chain compound MnCu(obp)(H2O)3·H2O (obp=oxamido bis(n,n′-propionato))

Abstract Single-crystal EPR spectra of the heterobimetallic alternating double-chain compound MnCu(obp) (H2O)3·H2O (obp=oxamido bis (N,N′-propionato)) were recorded in the 300-20 K range. Analysis of the spectra indicate a substantially dipolar-determined linewidth with enhancement of the secular term of the second moment due to spin diffusion effects. The anisotropic shifts in the resonance field observed in low-temperature spectra revealed that interchain interactions are relevant in determining the preferred spin orientations.

Magnetism and EPR spectra of the two‐sublattice manganese chain Mn2(EDTA)⋅9H2O

We report on the magnetic behavior and single‐crystal EPR spectra of the chain complex Mn2(EDTA)⋅9H2O characterized by two different alternating sites for the manganese ions. Magnetic susceptibility data are indicative of weak antiferromagnetic exchange interactions between the manganese ions, showing a maximum of about 3 K. This behavior can be accurately described on the basis of a Heisenberg chain model that assumes classical spins, giving J/k=−0.72 K and g=2.0. The EPR spectra are typically low dimensional, with an angular dependence of the linewidth of the type (3 cos2 θ−1)n (n=4/3 or 2). Nevertheless, such a behavior cannot be reproduced from a one‐dimensional model with dipolar broadening. An alternative broadening mechanism arising from single‐ion zero‐field splitting is then suggested.