Use of 2-pyrimidineamidoxime to generate polynuclear homo-/heterometallic assemblies: synthesis, crystal structures and magnetic study with theoretical investigations on the exchange mechanism

Abstract

Three new transition metal complexes using 2-pyrimidineamidoxime (pmadH₂) as multidentate chelating and/or bridging ligand have been synthesized and characterized. The ligand pmadH₂ has two potential bridging functional groups [μ-O and μ-(N–O)] and consequently shows several coordination modes. While a polymeric 1D Cu^II complex [Cu(pmadH₂)₂(NO₃)](NO₃) (1) was obtained upon treatment of Cu(NO₃)₂·3H₂O with pmadH₂ at room temperature in the absence of base, a high temperature reaction in the presence of base yielded a tetranuclear Cu^II-complex [Cu₄(pmad)₂(pmadH)₂(NO₃)](NO₃)(H₂O) (2). One of the Cu^II centers is in a square pyramidal environment while the other three are in a square planar geometry. Reaction of the same ligand with an equimolar mixture of both Cu(NO₃)₂·3H₂O and NiCl₂·6H₂O yielded a tetranuclear heterometallic Cu^II₂Ni^II₂ complex [Cu₂Ni₂(pmad)₂(pmadH)₂Cl₂]·H₂O (3) containing both square planar (Ni^II) and square pyramidal (Cu^II) metal centers. Complexes 1–3 represent the first examples of polynuclear metal complexes of 2-pyrimidineamidoxime. The analysis of variable temperature magnetic susceptibility data of 2 reveals that both ferromagnetic and antiferromagnetic interactions exist in this complex (J₁ = +10.7 cm⁻¹ and J₂ = −2.7 cm⁻¹ with g = 2.1) leading to a resultant ferromagnetic behavior. Complex 3 shows expected antiferromagnetic interaction between two Cu^II centers through –N–O– bridging pathway with J₁ = −3.4 cm⁻¹ and g = 2.08. DFT calculations have been used to corroborate the magnetic results.