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

摘要

Three new transition metal complexes using 2-pyrimidineamidoxime (pmadH_2) as multidentate chelating and/or bridging ligand have been synthesized and characterized. The ligand pmadH_2 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(pmadH_2)_2(NO_3)](NO_3) (1) was obtained upon treatment of Cu(NO_3)_2·3H_2O with pmadH_2 at room temperature in the absence of base, a high temperature reaction in the presence of base yielded a tetranuclear Cu ~(II)-complex [Cu_4(pmad)_2(pmadH) _2(NO_3)](NO_3)(H_2O) (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(NO_3)_2·3H_2O and NiCl_2·6H_2O yielded a tetranuclear heterometallic Cu~(II)_2Ni~(II)_2 complex [Cu_2Ni_2(pmad)_2(pmadH)_2Cl _2]·H_2O (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 (J_1 = +10.7 cm~(-1) and J_2 = -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 J _1 = -3.4 cm~(-1) and g = 2.08. DFT calculations have been used to corroborate the magnetic results.