Slow magnetic relaxations in manganese(III) tetra(meta-fluorophenyl)porphyrin-tetracyanoethenide. Comparison with the relative single chain magnet ortho compound.

Mn(III) tetra(meta-fluorophenyl)porphyrin-tetracyanoethenide coordination polymer (abbreviated meta-F) was synthesized and crystallographically and magnetically characterized. The compound crystallizes in the space group C2/c with four equivalent molecules in the unit cell arranged along two symmetry related nonparallel linear chain directions. Magnetic properties were studied by SQUID dc magnetization and ac susceptibility techniques and high field-high frequency electron spin resonance (HF-ESR). Glassy transition to a ferromagnetic-like state is observed at 10 K accompanied by slow magnetic relaxations. The glassiness is interpreted as due to 3D domain wall pinning. In a bias dc magnetic field the width of the relaxation time distribution decreases and the relaxations become similar to the relaxations of the single chain magnet Mn(III) tetra(ortho-fluorophenyl)porphyrin-tetracyanoethenide (abbreviated ortho-F), for which comparative HF-ESR studies were also conducted in this work. Magnetic properties of these two compounds are compared, and the nature of magnetic relaxations in meta-F is discussed.

Magnetooptical and structural investigations of five dimeric cobalt(II) complexes mimicking metalloenzyme active sites.

Four novel cobalt(II) complexes mimicking metalloenzyme active sites, novel C(14)H(22)Cl(12)Co(2)O(13)·2C(3)H(8)O (1), C(28)H(36)Cl(24)Co(4)O(28)·4C(4)H(8)O(2) (2), C(16)H(22)Cl(12)Co(2)O(13)·C(2)HCl(3)O(2) (3), C(16)H(22)Cl(12)Co(2)O(13) (4), and one known C(40)H(78)Cl(8)Co(2)O(17) (5) are composed of the same core of two high-spin cobalt(II) centers triply bridged by water and two trichloroacetato (1-4) or two pivalate (5) groups but differ in terminal ligands. The crystal structures of new compounds 1-4 belong to the space groups P ̅1, P2(1)/c, P ̅1, and Pbcn, respectively. All five investigated complexes contain Co atoms in distorted octahedral coordination. The complexes were characterized by magnetic susceptibility and magnetization measurements and by variable-temperature variable-field magnetic circular dichroism spectroscopy. Experimental data were analyzed in the frame of the theoretical model, which includes an unquenched orbital moment of the Co(II) ions. All investigated compounds are antiferromagnetically coupled with exchange constants in the range -1.5 to -2.1 cm(-1). However, there is a significant difference between the crystal-field-splitting parameters.

Exchange-coupled cobalt(II) dimer with unusual magnetic circular dichroism saturation behavior.

The exchange-coupled complex [Co(2)(mu-H(2)O)(mu-OAc)(2)(OAc)(2)(tmen)(2)] (OAc = CH(3)COO(-) acetato; tmen = N,N,N',N'-tetramethylenediamine) has been studied by magnetic circular dichroism (MCD) spectroscopy and magnetization measurements. A peculiar behavior of the MCD spectra was observed with a change in the magnetic field. The intensity of particular lines initially increases, then decreases with an increase in the magnetic field strength, disappears with a further field increase, and appears again with the opposite sign. This behavior was explained by the overlap of the electronic transitions at different but near wavelengths. The obtained MCD magnetization curves, together with magnetization obtained by a SQUID magnetometer, are well reproduced using the Hamiltonian, which takes into account an orbital magnetic moment.

Extended triple-bridged Ni(II)- and Co(II)-hydroxamate trinuclear complexes: synthesis, crystal structures, and magnetic properties.

Crystal structures of new trinuclear complexes [Ni 3(mu-OAc F) 4(mu-AA) 2(tmen) 2], [Ni 3(mu-OAc F) 4(mu-BA) 2(tmen) 2], and [Co 3(mu-OAc F) 4(mu-BA) 2(tmen) 2] have been determined (OAc F = CF 3COO (-), AA = acetohydroxamate anion, BA = benzohydroxamate anion, tmen = N, N, N', N'-tetramethylethylenediamine). In each structure, the metal ions have distorted octahedral coordination and are triply bridged by one hydroxamate and two trifluoroacetate bridges. Magnetic properties of these compounds and of relative [Co 3(mu-OAc F) 4(mu-AA) 2(tmen) 2] were studied by susceptibility and magnetization measurements. It was shown that for nickel trimers the intramolecular magnetic coupling is weak ferromagnetic in the case of the complex with the AA group, and there is nearly no coupling in the case with BA group. Rather large zero field splitting was obtained for the distorted octahedral environments of the terminal nickel ions. The cobalt trimers were additionally studied by magnetic circular dichroism (MCD) measurements. The exchange interaction of the cobalt complexes is antiferromagnetic.

Orbital angular momentum contribution to the magneto-optical behavior of a binuclear cobalt(II) complex.

We report magnetic and magnetic circular dichroism investigations of a binuclear Co(II) compound. The Hamiltonian of the system involves an isotropic exchange interaction dealing with the real spins of cobalt(II) ions, spin-orbit coupling, and a low-symmetry crystal field acting within the (4)T(1g) ground manifold of each cobalt ion. It is shown that spin-orbit coupling between this ground term and the low-lying excited ones can be taken into consideration as an effective g factor in the Zeeman part of the Hamiltonian. The value of this g factor is estimated for the averaged experimental values of Racah and cubic ligand field parameters for high-spin cobalt(II). The treatment of the Hamiltonian is performed with the use of a irreducible tensor operator technique. The results of the calculation are in good agreement with experimental observations. Both a large effective g factor for the ground state and a large temperature-independent part of the magnetic susceptibility arise because of a strong orbital contribution to the magnetic behavior of the Co(II) dimer.