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

• Published in: Polyhedron 2019-09, Vol.169, p.162-168
• Main Authors: Greenfield, Tiffany J., Takemoto, Tamra, Cano, Joan, Lloret, Francesc, Julve, Miguel, Zubieta, Jon, Doyle, Robert P.
• Format: Article
• Language: English
• Subjects: Copper(II); Crystal structure; Magnetic properties; Methylene diphosphonate complexes; Polynuclear compounds
• ISSN: 0277-5387
• DOI: 10.1016/j.poly.2019.05.022

We present here a novel methylene diphosphonate-containing tetracopper(II) complex of formula [Cu4(H2O)2(phen)4(O3PCH2PO3)2]·21H2O, which has been magneto-structurally characterized. The bidentate/monodentate coordination mode of the methylene diphosphonate ligand connecting the peripheral and inner copper(II) ions within the zig zag tetracopper(II) entity provides the two intramolecular exchange pathways that mediate intermediate antiferro- and weak ferro-magnetic interactions leading to a low-lying diamagnetic spin state. [Display omitted] 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.