Synthesis, structure, and magnetism of hexanuclear copper(II) phosphonates

Abstract

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.