Synthesis, crystal structure and magnetic properties of bis[μ-bis(3-pyridin-2- yl)-1,2,4-triazole-N',N1,N2] bis[triaqua nickel(II)] tetranitrate

A new dinuclear nickel(II) coordination compound of formula Ni₂(Hpt)₂(H₂O)₆(NO3)4 (Hpt=3-(pyridin-2-yl)-1,2,4-triazole) was synthesized. The structure was solved at 298 K by single-crystal X-ray analysis. Bisμ-bis(3-(pyridin-2-yl)-1,2,4-triazole-N′,N1,N2) bistriaqua nickel(II) tetranitrate (C₁₄H₂₄ N₁₂O₁₈Ni₂) crystallizes in the triclinic space group P-1, a=7.2266(1), b=9.9144(1), c=11.2647(1) Å, α=66.854(1)°, β=74.000(1)°, γ=86.400(2)°, V=712.42(1) ų, Z=1 (dinuclear units), 2823 reflections (1 > 2σ(I)), R1=0.0442, wR2=0.1197. The Ni(II) ions are bridged by two neutral Hpt ligands coordinating via N1 and N2 in the equatorial plane, Ni(1)-Ni(1)′=4.098(1) Å. The Hpt bridging mode is asymmetric (Ni(1)-N(1)′=2.074(2) Å, Ni(1)-N(2)=2.052(2) Å, N(1)-N(2)-Ni(1)=138.8(1)°, N(2)-N(1)-Ni(1)′=124.3(1)°). The octahedral environment around nickel(II) is completed by the coordinating pyridyl group (Ni(1)-N(31)=2.123(2) Å) and three coordinating water molecules (Ni(1)-O(1)=2.083(2) Å, Ni(1)-O(2)=2.053(2) Å, Ni(1)-O(3)=2.049(2) Å). The Ni(II) pairs are antiferromagnetically coupled, with J = −4.65 cm−1 (based on the spin Hamiltonian: H = −2JS_Ni1 · S_Ni2). The J value was rationalized on the basis of the geometry around Ni(II). The superexchange pathway involves the σ orbitals of the N1,N2-diazine moiety of the bridging 1,2,4-triazole network. The nature and the magnitude of the antiferromagnetic exchange is discussed on the basis of the structural features.