Synthesis, Structure, and Magnetism of Hexanuclear Copper(II) Phosphonates

The reaction of Cu(ClO4)2·6H2O with cyclopentylphosphonic acid and 1,10-phenanthroline (phen) in the presence of triethylamine afforded a hexanuclear copper(II) complex [Cu6(C5H9PO3)4(1,10-phen)6(MeOH)4](ClO4)4 (1) in over 80% yield. The hexanuclear assembly is held together by the coordination action of four tridentate dianionic [RPO3]2− ligands giving rise to two Cu2P2O4 eight-membered rings in the top and the bottom that are connected to each other by a central Cu2O2 four-membered ring. Every copper atom in 1 is bound by a chelating phenanthroline ligand. Each of the two terminal pairs of copper atoms in the hexanuclear assembly contains methanol molecules of coordination. These labile methanol molecules can be replaced by 1,3-bis(4-pyridyl)propane (bpp) to afford [Cu6(C5H9PO3)4(1,10-phen)6(bpp)2](ClO4)4 (2) where the pincer-like bipyridine ligand acts as a stopper to close both ends of the open hexanuclear cage. Instead, treatment of the in situ generated [Cu6(C5H9PO3)4(2,2′-bpy)6(MeOH)4](ClO4)4 with 4,4′-bipyridine results in the formation of a rail-road-like one-dimensional polymer [{Cu6(C5H9PO3)4(2,2′-bpy)6(4,4′-bpy)2}(ClO4)4]n (3). In addition to structural studies, detailed magnetic studies have been carried out on 1−3 which reveal an S = 1 spin ground-state with low lying excited states.