ISOTROPIC AND ANISOTROPIC MAGNETIC EXCHANGE INTERACTIONS THROUGH MU-N-1,N-2 1,2,4-TRIAZOLE AND MU-SULFATO BRIDGES - X-RAY CRYSTAL-STRUCTURE, MAGNETIC-PROPERTIES, AND SINGLE-CRYSTAL EPR STUDY OF (MU-4-AMINO-3,5-BIS(PYRIDIN-2-YL)-1,2,4-TRIAZOLE-N',N-1,N-2,N '')(MU-SULFATO-O,O')[(SULFATO-O)AQUACOPPER(II)]TRIAQUACOPPER(II) HYDRATE

The crystal and molecular structure of (mu-4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole-N'N-1,N-2,N'') (mu-sulfato-O,O')-[(sulfato-O)aquacopper(II)]triaquacopper hydrate, [Cu-2(abpt)(SO4)(2)(H2O)(4)]. H2O (1) (C12H20Cu2N6O13S2), in which abpt 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, was determined by X-ray diffraction methods.; Crystal data for 1: T = 298 K, monoclinic, space group = P2(1), with a = 9.9704(3) Angstrom, b 14.7093(3) Angstrom, c = 7.4034(6) Angstrom, beta = 96.8641(6)degrees, Z = 2 (dinuclear molecules), V = 1078 Angstrom(3). The least-squares refinement based on 2717 significant reflections [I > 2 sigma(I)] converged to R = 0.0410 and R(w) = 0.0500. The structure of 1 consists of asymmetric dinuclear units, in which the copper(II) ions are linked in the equatorial plane by an N-1,N-2 bridging chelating abpt ligand and a didentate bridging sulfate anion. The Cu-O(sulfate) distances are very short (1.937(4) and 1.908(5) Angstrom). The Cu(1)-Cu(2) distance is 4.415(1) Angstrom. The compound represents the first example of a dinuclear copper(II) compound having a single N-1,N-2 1,2,4-triazole bridge. Both Cu(II) ions have four short equatorial distances to the bridging sulfate, the bridging 1,2,4-triazole, the pyridyl group, and a monodentate sulfate (Cu(1)) or a water molecule (Cu(2)). Cu(1) has one additional apical water molecule, whereas Cu(2) has two additional axial water molecules. Hydrogen bonding appears to play an important role in the stabilization of the asymmetric dinuclear cluster. The magnetic susceptibility data (295-6 K) are interpreted on the basis of the spin Hamiltonian (H) over cap - 2J[(S) over cap(Cu(1)).((S) over cap(Cu(2))], yielding J = -34.5 cm(-1), g = 2.15, and p = 0.66%. The isotropic exchange constant is compared to the ones of doubly N-1,N-2 1,2,4-triazole bridged dinuclear copper(Il) compounds. The single-crystal X-band EPR spectra recorded at room temperature are typical of a triplet spin state with g(xx) = 2.09(3), g(yy) = 2.10(3), g(zz) = 2.32(2), D-x'x' = 0.018(2) cm(-1), D-y'y' = 0.04(3) cm(-1), and D-z'z' = -0.058(3) cm(-1) The g directions in the xy plane are affected by a large experimental error, due to the very small anisotropy of g in this plane. The g(zz) and D-z'z' directions are almost parallel; the angle between these vectors is only 2 degrees. These vectors make an angle of about 12 degrees with the normal vector to the Cu(1)-Cu(2) coordination plane. The zero-field-splitting parameter D shows evidence of dominant exchange contributions, since its largest component is observed orthogonal to the Cu(1)-Cu(2) coordination plane. The magnetic and EPR data are discussed in comparison with those obtained for the doubly N-1,N-2 1,2,4-triazole bridged dinuclear copper(II) derivatives.