Two manganese complexes, (py2(NMe)2)MnIICl2 (1) and [(py2(NMe)2)MnIIIF2]+ (2), are here described with the macrocyclic ligand py2(NMe)2 (py2(NMe)2 = N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane). For both, the crystal structure is reported. The UV-visible spectrum of 2 exhibits a very broad near-infrared (NIR) band corresponding to the transition between the two e(g)-type orbitals split by the Jahn-Teller effect. A negative D value of ca. -4 cm(-1) was estimated by high-field and high-frequency electron paramagnetic resonance (HF-EPR) spectroscopy, which was consistent with symmetry considerations. Density functional theory (DFT) calculations on 2 support the 5B1 electronic ground state predicted from the X-ray structure. Moreover, to explain the large value of the D parameter, a spin triplet first excited spin state was postulated to occur at low energy. This was confirmed by the DFT calculations.

We use the strategy of diamagnetic substitution for obtaining information on the crystal field effects in paramagnetic rare earth ions using the homologous series of compounds with the diamagnetic tropolonato ligand, Ln(Trp)(HBPz(3))(2), and the paramagnetic semiquinone ligand, Ln(DTBSQ)(HBPz(3))(2), (DTBSQ = 3,5-di-tert-butylsemiquinonato, Trp = tropolonate, HBPz(3)= hydrotrispyrazolylborate) for Ln = Sm(iii), Eu(iii), Gd(iii), Tb(iii), Dy(iii), Ho(iii), Er(iii) or Yb(iii). The X-ray crystal structure of a new form of tropolonate derivative is presented, which shows, as expected, a marked similarity with the structure of the semiquinonate derivative. The Ln(Trp)(HBPz(3))(2) derivatives were then used as a reference for the qualitative determination of crystal field effects in the exchange coupled semiquinone derivatives. Through magnetisation and susceptibility measurements this empirical diamagnetic substitution method evidenced for Er(iii), Tb(iii), Dy(iii) and Yb(iii) derivatives a dominating antiferromagnetic coupling. The increased antiferromagnetic contribution compared to other radical-rare earth metal complexes formed by nitronyl nitroxide ligands may be related to the increased donor strength of the semiquinone ligand.

The magnetic anisotropy of two Gd(III) derivatives has been investigated by multifrequency HF-EPR spectroscopy. The reliability of the method of diamagnetic substitution for the estimation of crystal field effects in rare earth compounds is discussed on the basis of the experimental results and of the estimation of the dipolar contribution to the anisotropy.

We describe here the synthesis, the magnetic properties, and a comparison of the X-band EPR spectra of complexes belonging to the homologous families Ln(III)(HBPz(3))(2)(3,5-DTBSQ) and Ln(III)(HBPz(3))(2) (tropolonate) (where HBPz(3) = hydrotrispyrazolborare, 3,5-DTUSQ = 3,4-di-tert-butylsemiquinone and Ln = Yb and Ho). This study shows new examples of violations to the rule that ferromagnetic coupling should hold for ions of the second half of the lanthanide series and an unpaired spin. (C) 2001 Academic des sciences / Editions scientifiques el medicales Elsevier SAS.

The compound [L-1(H2O)Fe(mu -O)Fe(OH2)L-1](ClO4)(4).2H(2)O [1(ClO4)(4).2H(2)O] [L-1 = N,N'-bis(1-methylimidazolyl-2-methyl)-N,N'-bismethyl-1,2-ethanediamine] was synthesized. It is characterized by a linear Fe-(mu -O)-Fe motif with an Fe Fe distance of 3.584(1) Angstrom. The measurement of the magnetic susceptibility as a function of temperature indicated a strong antiferromagnetic coupling between the two high-spin Fe-III ions (J = -223 cm(-1) with H = -J.S-1.S-2). In the solid state, the symmetric stretching vibration mode of the Fe-(mu -O)-Fe core unit was detected at 347 cm(-1), in agree-ment with a straight Fe-(mu -O)-Fe angle. Upon addition of up to one equivalent of triethylamine to an acetonitrile solution of 1, conversion to the [L-1(H2O)Fe(mu -O)Fe(OH)L-1](3+) complex 2 was observed. This last species was isolated as a perchlorate salt [2(ClO4)(3).H2O]. Susceptibility measurements and Raman and UV/Vis investigations on the powder and/or an acetonitrile solution confirmed the presence of an intramolecular hydrogen bond between the coordinated water molecule and the hydroxide group leading to a bent [Fe(mu -O)Fe](4+) core structure.

Two decanuclear cyclic Cr(iii) complexes have been synthesised in high yield by solvothermal techniques: magnetic susceptibility studies reveal ferromagnetic Cr . . . Cr exchange in one, and antiferromagnetic Cr . . . Cr exchange in the other.

The synthesis and characterisation of an asymmetric dinuclear gadolinium(III) semiquinonato complex, namely [Gd2(HBPz3)2(dtbsq)4] CHCl3 (1; HBPz3 = hydrotris(pyrazolyl)borate, dtbsq = 3,5-di-tert-butyl-O-semiquinone), is reported. The crystal structure of 1 was determined at room temperature. It crystallises in the triclinic system P1, with a = 16.735(5) A, b = 17.705(5) A, c = 19.553(5) A, alpha = 99.680(5) degrees, beta = 109.960(5), gamma = 107.350(5) degrees, Z = 2 and R = 9.96. The structure of 1 consists of a dinuclear asymmetric unit in which the two gadolinium(III) ions have coordination numbers of eight and nine. Three of the dioxolene molecules act as asymmetric bridging ligands, while the fourth molecule behaves as a bidentate ligand towards a single metal ion. The magnetic properties of 1 were investigated by means of susceptibility measurements and high-field electron paramagnetic resonance (HF-EPR) spectroscopy. They revealed an S = 0 ground spin state with excited states of higher spin very close in energy and a small negative zero-field splitting with a transverse anisotropy term for a S = 7 state.

Two new mononuclear iron complexes, [(LBzl2)Fe(II)Cl2]·H2O and [(LBzl2) Fe(III)Cl2]·PF6, (LBzl2=N,N′-bisbenzyl-N,N′-bis(2-pyridylmethyl)-ethane-1,2-diamine) have been synthesised in view of generating complexes to mimic the active site of methane monooxygenase. Their structures have been determined by X-ray analysis. In both species, the iron atom shows a pseudo-octahedral coordination with two pyridine nitrogen atoms in axial positions and two amine nitrogen atoms in the equatorial plane. Two other equatorial positions are occupied by chloride ions. The coordination bond lengths clearly indicate the sensitivity of the ligand to the oxidation state of the iron. Thus, the FeN and FeCl bond distances in Fe(II) complex are more elongated than corresponding distances in Fe(III). A statistical examination of the bond distances of hexacoordinated Fe(II) and Fe(III) complexes using the Cambridge Structural Database provides evidence to relate both complexes to their spin state. The redox potential of the Fe(III)/Fe(II) couple was determined by cyclic voltammetry. The UV–Vis spectra are dominated by charge transfer transitions. The X-band EPR spectrum of [(LBzl2) FeCl2]·PF6 is characteristic of an S=5/2 species with an unusual zero-field splitting.

The new bimetallic nickel(II) compound (PPh4)(4)[Ni-2(2)]. 6H(2)O (3), where H-8[2] stands for N,N',N",N'''-1,2,4,5-benzenetetrayltetrakis(oxamic acid), has been synthesized and its crystal structure determined by single-crystal X-ray diffraction. The structure of 3 consists of [Ni-2(eta(4):eta(4)-2)](4-) anions, tetraphenylphosphonium cations, and water molecules. Facile one-electron oxidation of the square-planar diamagnetic dinickel(II) complex [Ni-2(eta(4):eta(4)-2)](4-) generates the metallo-radical species [Ni-2(eta(4):eta(4)-2(.+))](3-) with characteristic intra-ligand pi-cation radical transitions in the visible region (475-550 nm) as well as a typical quasi-isotropic EPR signal at g approximate to 2.0.

A new series of monomeric copper(II) complexes of the related substituted oxamate ligands N,N'-naphthalene-1,8-diylbis(oxamate) (L-2) and N,N'-trimethylenebis(oxamate) (L-3) have been synthesized. The molecular structures of [NBu4](2)[CuL2] and [PPh4](2)[CuL3]. 2H(2)O have been determined by single-crystal X-ray analysis. The structure of the previously reported complex [PPh4](2)[CuL1], where L-1 is the parent o-phenylenebis(oxamate), has been also determined. These are mononuclear four-co-ordinate copper(II) complexes with the metal center in a more or less distorted square-planar environment formed by the two amido nitrogen and two carboxylate oxygen atoms from the two oxamato groups of each tetradentate chelating ligand. The bond lengths at the metal atom are similar for all three complexes, the Cu-N bond distances (1.89-1.93 Angstrom) being shorter than the Cu-O ones (1.93-1.97 Angstrom). The bond angles around the metal are different from one complex to the other. They are closer to 90 degrees, corresponding to the ideal square-planar geometry, for the copper(II)-L-2 and -L-3 complexes as a result of the alternating 5-6-5-membered chelate ring system afforded by L-2 and L-3, respectively. The values of the Cu-III-Cu-II redox potential in acetonitrile for this family of complexes range from 0.41 to 0.27 V (vs. saturated calomel electrode, 25 degrees C and 0.1 mol dm(-3) NEt4ClO4 as supporting electrolyte), the redox process being only reversible for the copper(II)-L-3 species. The stabilization of the trivalent oxidation state of copper in this complex is attributed to the stronger basicity of the aliphatic amido nitrogens with respect to that of the aromatic amido ones. The trend in formal potential along this series is mainly controlled by the size of the chelate rings around the metal ion.

The complex [L(H2O)Fe(mu-O)Fe(OH)L](ClO4)(3). H2O, where L=N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine, was synthesized. It crystallizes in the orthorhombic space group P2(1)2(1)2(1) With a = 13.283(5) Angstrom, b = 16.050(9) Angstrom, c = 20.050(9) Angstrom, V = 4476(6) Angstrom(3), and Z = 4. It presents the hydrogen-bonded [(H2O)Fe(mu-O)Fe(OH)](3+) core unit characterized by an Fe-O-Fe angle of 137.5(2)degrees and an Fe-Fe distance of 3.396(1) Angstrom. The measurement of the magnetic susceptibility as a function of the temperature indicated an antiferromagnetic coupling between the two high-spin Fe(III) ions J = -184 cm(-1) (H = -JS(1). S-2). In the solid state the symmetric stretching vibration was observed at 438 cm(-1). Upon dissolution in dry acetonitrile,this vibration was no longer detected and an intense band was observed at 600 cm(-1). This frequency can be correlated with an Fe-O-Fe angle of 111 degrees. This value suggests that the species which exists in these conditions is the protonated diamond core [Fe(mu-O)(mu-OH)Fe](3+), analogous to that identified by Zang et al. (J. Am. Chem. Sec. 1994, 116, 3653) and by Hazell et al. (J. Chem. Sec.; Dalton Trans. 1995, 707). Upon addition of water, the original aquated species is observed in equilibrium with the protonated diamond unit. By:analysis of the changes in UV-vis spectra as a function of the amount of water added, the equilibrium constant of the formation of the aquated species was found to be 5.4 M-1.