A methylenediphosphonate bridged copper(II) tetramer: Synthesis, structural, thermal, and magnetic characterization of [Cu4(H2O)2(phen)4(μ-P2O6CH2)2]·21H2O

Abstract

A novel tetracopper(II) complex namely, [Cu4(H2O)2(phen)4(μ-P2O6CH2)2]·21H2O (1) was synthesized by using copper(II) chloride, 1,10-phenanthroline (phen), and methylene diphosphonic acid (MDP). The structure of 1 was characterized by single crystal X-ray diffraction and spectroscopy. Compound 1 crystallizes in the triclinic system, P1¯ space group and its crystal structure consists of neutral centrosymmetric tetranuclear copper(II)-methylene diphosphonate units where each peripheral {Cu(phen)(P2O6CH2)}2− fragment adopts the bidentate(Oeq-P-Oeq)/monodentate (Oax) bridging mode towards each of the two inner {Cu(phen)}2+ entities resulting in a “clamshell”-like orientation. The two crystallographically independent copper(II) ions in 1 (Cu1/Cu2) are five-coordinate with two nitrogen atoms from a bidentate phen molecule and two phophonate-oxygens building the basal plane (Cu1/Cu2) and either a water molecule (Cu2) or a phosphonate-oxygen (Cu1) filling the apical position of somewhat distorted square pyramidal surroundings. The values of the intramolecular copper–copper separation are 3.3219(8) and 3.2062(8) Å for Cu1⋯Cu2 and Cu1⋯Cu1i, respectively. Cryomagnetic measurements on a polycrystalline sample of 1 reveal the occurrence of an overall antiferromagnetic behaviour leading to a diamagnetic low-lying spin state. The values of the intramolecular magnetic interactions are J1 = −22.60(4) cm−1 (across the bis-bidentate methylene diphosphonate) and J2 = +4.9(3) cm−1 (through the double-oxo(phosphonate) bridge), the spin Hamiltonian being defined as H = −J1(SCu1·SCu2 + SCu1i·SCu2i) − J2(SCu1·SCu1i). These values are analyzed through simple orbital symmetry considerations and compared with those for related systems in the literature.