Reactivity of 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, structures and magnetic properties of polynuclear and polymeric Mn(ii), Cu(ii) and Cd(ii) complexes

Abstract

Five new complexes were obtained from solution or hydrothermal reactions of M(OAc)₂ (M = Mn, Cu and Cd) or CuCl₂ with 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole (abpt) and NaN₃ or 1,3,5-benzenetricarboxylic acid (btcH₃) in different molar ratios. Structural analysis reveals that Cd(abpt) units in [Cd(abpt)(μ_1,1-N₃)₂]_n (1) are bridged by double μ_1,1 end-on (EO) azides into 1D zigzag coordination chains. Similar structural motifs, i.e. the chelation of abpt to the metal center and the double bridges of EO azides, are found in [Mn₄(abpt)₄(μ_1,1-N₃)₈(H₂O)₂] (2). The terminal aqua molecules and the monodentate N₃⁻ groups lead to the formation of a tetranuclear complex rather than a polymeric compound. The abpt underwent deamination in the presence of copper ions during the process of coordination and became 3,5-bis(pyridin-2-yl)-1,2,4-triazolate (bpt-H) in 3–5. [Cu₄(bpt-H)₄(N₃)₄]·4.5H₂O (3) is a neutral tetranuclear grid-like complex, in which the azides act as monodentate ligands. A similar [Cu₄(bpt-H)₄]⁴⁺ grid-like unit was found in [Cu₄(bpt-H)₄(μ-btcH)Cl₂]·2H₂O (4) and a pair of symmetry-related copper atoms are bridged by the μ-btcH₂⁻ coligand in a butterfly-shaped structure. In [Cu₂(bpt-H)(μ₆-btc)(H₂O)]_n (5), the tetranuclear {Cu₄(μ-bpt-H)₂(μ₃-carboxylate)₂}⁴⁺ units are bridged by μ₆-btc³⁻ ligands in a 2D step-like layer structure. Temperature-dependent magnetic susceptibility measurements reveal that the double μ_1,1-N₃⁻ bridges in 2 transmit the ferromagnetic interactions between Mn²⁺ centers (J₁ = J₂ = +3.09(4) cm⁻¹, g_Mn(II) = 2.02(1)), and the μ-(bpt-H)⁻ bridges transmit moderate antiferromagnetic interactions in both 3 (J = −12.78(13) cm⁻¹) and 4 (J₁ = −14.96(11) cm⁻¹). In 4 the antiferromagnetic coupling via the μ-btcH²⁻ bridge was found as the second coupling pathway (J₂ = −9.48(7) cm⁻¹). The coexistence of ferromagnetic and antiferromagnetic coupling between four Cu²⁺ centers occurs in 5 (J₁ = −0.88(3) cm⁻¹ and J₂ = +5.01(2) cm⁻¹). The magneto–structural relationship for tetranuclear copper pyrazolate/triazolate compounds has been discussed.