Monomeric, Tetrameric, and Polymeric Copper Di-tert-butyl Phosphate Complexes Containing Pyridine Ancillary Ligands

Abstract

The reaction of di-tert-butyl phosphate ((tBuO) 2P(O)(OH), dtbp-H) with copper acetate in the presence of pyridine (py) and 2,4,6-trimethylpyridine (collidine) has been investigated. Copper acetate reacts with dtbp-H in a reaction medium containing pyridine, DMSO, THF, and CH3OH to yield a one-dimensional polymeric complex [Cu(dtbp) 2-(py)2(μ-OH2)]n (1) as blue hollow crystalline tubes. The copper atoms in 1 are octahedral and are surrounded by two terminal phosphate ligands, two pyridine molecules, and two bridging water molecules. The μ-OH2 ligands that are present along the elongated Jahn-Teller axis are responsible for the formation of the one-dimensional polymeric structure. Recrystallization of 1 in a DMSO/THF/CH3OH mixture results in the reorganization of the polymer and its conversion to a more stable tetranuclear copper cluster [Cu 4(μ3-OH)2(dtbp)6(py) 2] (2) in about 60% yield. The molecular structure of 2 is made up of a tetranuclear core [Cu4(μ3-OH)2] which is surrounded by six bidentate bridging dtbp ligands. While two of the copper atoms are pentacoordinate with a tbp geometry, the other two copper atoms exhibit a pseudooctahedral geometry with five normal Cu-O bonds and an elongated Cu-O linkage. The pentacoordinate copper centers bear an axial pyridine ligand. The short Cu⋯Cu nonbonded distances in the tetranuclear core of 2 lead to magnetic ordering at low temperature with an antiferromagnetic coupling at ∼20 K (JP = -44 cm-1, Jc = -66 cm-1, g = 2.25, and ρ = 0.8%). When the reaction between di-tert-butyl phosphate (dtbp-H) and copper acetate was carried out in the presence of collidine, large dark-blue crystals of monomeric copper complex [Cu(dtbp)2(collidine)2] (3) formed as the only product. A single-crystal X-ray diffraction study of 3 reveals a slightly distorted square-planar geometry around the copper atom. Thermogravimetric analysis of 1-3 revealed a facile decomposition of the coordinated ligands and dtbp to produce a copper phosphate material around 500 °C. An independent solid-state thermolysis of all the three complexes in bulk at 500-510 °C for 2 days produced copper pyrophosphate Cu2P2O 7 along with small quantities of Cu(PO3)2 as revealed by DR-UV spectroscopic and PXRD studies.