No abstract

The dinuclear MnIII complex [Mn2O(PhCOO)2(bpy)2(OH)(NO3)]·H2O was prepared by controlled oxidation of manganous nitrate with n-tetrabutylammonium permanganate in the presence of benzoic acid (PhCOOH) and 2,2‘-bipyridine (bpy). Its structure was determined in a single-crystal X-ray diffraction experiment, and consists of a triply-bridged [Mn2(μ-O)(μ-PhCOO)2]2+ dinuclear core. Each manganese(III) ion bears a chelating bpy and a terminal X anion (X = OH- or NO3 -) completing a distorded octahedral coordination geometry. Although the terminal anions are located in disordered positions, the analysis of bond lengths and steric considerations led us to assign to the complex an asymmetric structure [(bpy)(OH)MnIII(μ-O)(μ-PhCOO)2MnIII(bpy)(ONO2)]. The product, with a chemical formula C34H29Mn2N5O10, crystallizes in the monoclinic system, space group C2/c, with a = 16.607(4) Å, b = 25.619(6) Å, c = 9.796(3) Å, β = 100.15(3)°, and Z = 4. Magnetic studies performed on a series of related compounds have revealed a moderate ferro- or antiferromagnetic interaction and significant zero-field splittings in line with the strong Jahn−Teller distortion of the high spin d4 manganese(III) ions. In the present work we interpreted the magnetic properties of the complex using a spin Hamiltonian which includes the Heisenberg exchange, axial and rhombic ZFS, and an anisotropic Landé factor, under the assumption of a pseudo-C 2 symmetry of the dinuclear core. In order to determine the anisotropy parameters with enough accuracy, we resorted to variable-temperature variable-field magnetization measurements over the 2−300 K range at fields of 0.5, 1.0, 2.5, and 5 T. The whole set of data was fit with a single set of parameters through diagonalization of the complete spin Hamiltonian. The best fit values J = +1.0(4) cm-1, D = +4.5(5) cm-1, E = 0, g x = g y = 1.96, and g z = 2.00 showed that a ferromagnetic interaction occurs between manganese(III) ions in a compressed octahedral environment. A magnetostructural relationship linking the ferromagnetic behavior of the dinuclear complex to the compression of the Mn(III) coordination sphere (and conversely of the antiferromagnetism to the elongation) is then proposed. It is substantiated by theoretical molecular orbital calculations of the extended Hückel type.

The complexes [Cu2(dpt)2Cl2]X2 (dpt = dipropylenetriamine; X = Cl, 1; X2 = (Cl)(BPh4), 2) have been prepared and their magnetic properties studied. The crystal structure of complex 1 has been solved. The compound belongs to the C2/c space group with Z = 4, a = 17.201(2) Å, b = 10.873(2) Å, c = 11.835(2) Å, and β = 99.720(10)°. The geometry about each copper approximates that of a distorted square pyramid. The three N-atoms of the dpt ligand and one of the Cl bridging ligands form the base of the pyramid while the other chloro bridging ligand occupies the axial site. The two pyramids share a base-to-apex edge with the basal planes being perpendicular, an arrangement which had not been previously observed for this type of dimers. The chloro counterions are responsible for interdimer interactions through hydrogen bonding forming chains of dimers throughout the lattice. Magnetic susceptibility data show a ferromagnetic coupling between the two Cu(II) centers (J = 42.94 cm-1 for 1 and J = 13.89 cm-1 for 2). Dimer−dimer interaction and ZFS have been considered with the J ‘ parameter (1, J ‘ = −3.92 cm-1; 2, J ‘ = −2.9 cm-1). Extended Hückel calculations on complex 1 clearly reveal that the intradimer magnetic interaction only takes place through the basal chloro bridging ligand (with a CuCl(1)Cu angle of 91.4°) which in turn is consistent with the observed ferromagnetic coupling.

Three new polynuclear Mn II complexes with chloroacetate bridges were obtained and characterized by X-ray diffraction: [Mn 3 (µ-ClCH 2 COO) 6 (bpy) 2 ] (1), [Mn 2 (µ-ClCH 2 COO) 2 -(phen) 4 ](ClO 4 ) 2 (2), and [Mn(µ-ClCH 2 COO) 2 (phen)] n (3). The Mn···Mn distance for 1 (3.624 Å ) is smaller than for 2 (4.613 Å ) and 3 (4.530 Å ); this is in agreement with the number of carboxylate bridges. The three compounds show a weak antiferromagnetic coupling. The J values are −3.82 cm −1 for

Three different types of polynuclear MnII complexes with carboxylate bridges were obtained from the reaction of Mn(RCOO)2 with 2,2‘-bipyridine (bpy). Dinuclear complexes [Mn2(μ-RCOO)2(bpy)4](ClO4)2 with R = 2-ClPh, 3-ClPh, 4-ClPh, Ph (1−4); trinuclear complexes [Mn3(μ-RCOO)6(bpy)2] with R = 2-ClPh, 3-ClPh (5, 6), [Mn3(μ-RCOO)6(2,2‘-Me2-bpy)2] with R = 2-ClPh, 3-ClPh, 4-ClPh (7−9), and 1D complexes [Mn(μ-RCOO)2(bpy)] n with R = 3-ClPh, 4-ClPh (10, 11). [Mn2(μ-PhCOO)2(bpy)4](ClO4)2 (4) and the [Mn(μ-3-ClPhCOO)2(bpy)] n ·nH2O (10) have been characterized by X-ray diffraction. Complex 4 crystallizes in the triclinic system, space group P1̄ with a = 9.1886(10) Å, b = 11.6135(9) Å, c = 13.595(2) Å, α = 66.225(13)°, β = 84.073(12)°, γ = 88.593(10)°, Z = 1. Complex 10 crystallizes in the monoclinic system, space group C2/c with a = 26.376(5) Å, b = 12.404(3) Å, c = 7.095(1) Å, β = 96.10(3)°, Z = 4. The other complexes were characterized by XANES and EXAFS studies, by comparison with analogous complexes of known X-ray crystal structure. The Mn···Mn distances in the dinuclear complexes and in the infinite chains were similar (av 4.5 Å) and longer than for the trinuclear complexes (av 3.6 Å); this is in agreement with the number of carboxylate bridges:  two in the first case and three for the trinuclear complexes. All the polynuclear complexes shown very weak antiferromagnetic coupling (J between −0.7 and −3.22 cm-1). So, at low temperatures more than one spin state may be populated and many possible transitions may be expected in the EPR spectra; each series shows a similar spectrum, which is different from the others.