Novel Cu(II)-Based Frameworks Built from BIMAM and Oxalate: Syntheses, Structures, and Magnetic Characterizations (BIMAM = Bis(imidazol-yl) methylaminomethane)

Complexes with the formula [Cu(HBIMAM)(C2O4)]2[Cu(C2O4)2(OH2)2] (1) and [{Cu(HBIMAM)(OH2)(OClO3)}2(μ-C2O4)] (ClO4)2 (2) have been synthesized from copper(II) perchlorate, bis(imidazol-yl) methylaminomethane (BIMAM), and oxalic acid by adjusting the ratio of the reagents. Compound 1 is an ion pair made up of binuclear cations [Cu2(HBIMAM)2(C2O4)2]2+ and mononuclear anions [Cu(C2O4)2(OH2)2]2−, in which the oxalate acts as bidentate-monodentate and bidentate ligand, respectively. Neighboring cations and anions are connected to each other by a weak Cu-oxalate interaction and a significant H-bond, leading to a chain structure with the mononuclear anions sandwiched as bridges between the dimeric cations. The coordination of copper in the cationic and anionic moieties is very elongated square pyramidal and tetragonally elongated octahedral, respectively, with HBIMAM acting as a bidentate ligand in both. The intramolecular copper−copper distance within the cationic unit is 4.853(2) Å, while the shortest copper−copper interunit (cation−anion) separation within the chains is Cu(1)−Cu(2) = 5.231(2) Å. Compound 2 contains centrosymmetric binuclear cations and two nonbonded perchlorate counteranions. The two copper atoms are linked through a bis-bidentate oxalate group leading to a metal−metal separation of 5.149(3) Å. The coordination stereochemistry of the copper ions can be described as a tetragonally elongated octahedron of the 4 + 1 + 1′ type, in which the BIMAM acts as a bidentate ligand through two imidazole N atoms. The molecular building blocks (MBBs) in 1 and 2 are packed together by multiple-path H-bonding and π-π stacking interactions, giving rise to the supramolecular three-dimensional networks. EPR spectra are indicative of an essentially d x 2 − y 2 ground-state for the copper(II) ions in both compounds. Magnetic susceptibility measurements show antiferromagnetic exchange between the copper(II) ions, very weak in the case of compound 1 (J = −0.68(2) cm−1) and strong in compound 2 (J = −330 cm−1). The magnitudes of the exchange interactions are discussed in relation to the structural features of each compound and compared with published magneto-structural data on similar oxalato-bridged copper(II) compounds.