Cyclic Trinuclear and Chain of Cyclic Trinuclear Copper(II) Complexes Containing a Pyramidal Cu3O(H) Core. Crystal Structures and Magnetic Properties of [Cu3(μ3-OH)(aaat)3(H2O)3](NO3)2·H2O [aaat = 3-Acetylamino-5-amino-1,2,4-triazolate] and {[Cu3(μ3-OH)(aat)3(μ3-SO4)]·6H2O}n [aat = 3-Acetylamino-1,2,4-triazolate]: New Cases of Spin-Frustrated Systems

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Abstract
New copper(II) complexes of the cyclic trinuclear type with 1,2,4-triazole ligands, [Cu3(μ3-OH)(aaat)3(H2O)3](NO3)2·H2O [Haaat = 3-acetylamino-5-amino-1,2,4-triazole] (1) and {[Cu3(μ3-OH)(aat)3(μ3-SO4)]·6H2O}n [Haat = 3-acetylamino-1,2,4-triazole] (2), have been prepared and characterized by X-ray crystallography and magnetic measurements. Compound 1, the first reported with the ligand (H)aaat, consists of discrete trinuclear cations, associated NO3- anions and lattice water molecules. Compound 2 consists of unusual chains of trinuclear units with a tridentate sulfato group linking the trimeric units and water molecules stabilizing the crystal lattice. In both complexes, 1 and 2, the trinuclear [Cu3(OH)L3] unit contains a pyramidal Cu3-μ3OH core, and an almost flat Cu3N6 ring formed by the N,N-bridging triazolato groups. The Cu···Cu‘ intratrimeric distances are 3.35−3.37−3.39 Å in 1 and 3.34−3.34−3.36 Å in 2. The copper atoms are five-coordinated with a distorted square-pyramidal geometry. Magnetic measurements have been performed in the 1.9−300 K temperature range. In the high-temperature region (T > 90 K), experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = −J(S1S2 + S2S3 + S1S3), with J = −194.6 cm-1 and g = 2.08 for 1, and J = −185.1 cm-1 and g = 2.10 for 2. The magnitude of the antiferromagnetic exchange in both complexes is discussed on the basis of their structural features by comparison with reported N,N-pheripherically bridged trinuclear systems. In order to fit the experimental magnetic data at low temperature, an antisymmetric exchange term, HAS = G(S1xS2 + S2xS3 + S1xS3), had to be introduced, with G = 27.8 (1) and 31.0 (2) cm-1. Crystal data: C12H27Cu3N17O14 (1) (MW = 824.13) crystallizes in the triclinic space group, P1̄, Z = 2, with the cell dimensions a = 8.852(2) Å, b = 11.491(3) Å, c = 15.404(3) Å, α = 70.43(3)°, β = 75.11(2)°, γ = 88.43(2)°, and V = 1423.8(5) Å3, Dcalcd = 1.922 g cm-3; the final agreement values were R1 = 0.0822 and wR2 = 0.2300 for 4989 unique reflections. C12H28Cu3N12O14S (2) (MW = 787.14) crystallizes in the triclinic space group, P1̄, Z = 2, with the cell dimensions a = 7.146(6) Å, b = 14.26(1) Å, c = 15.35(2) Å, α = 109.0(9)°, β = 93.6(9)°, γ = 99.5(7)°, and V = 1448(2) Å3, Dcalcd = 1.806 g cm-3; the final agreement values were R1 = 0.0628 and wR2 = 0.1571 for 3997 “observed” reflections.

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