Oxotris(oxalate)niobate(V): An oxalate delivery agent in the design of building blocks

Abstract

<p>This work concerns the oxalate delivery process that occurs when using (NH₄)₃[NbO(C₂O₄)₃]·6H₂O as a suitable oxalate source in the synthesis of two compounds, [Cu(dmphen)(C₂O₄)(H₂O)] (<b>1</b>) and [{Cu(dmphen)(CH₃OH)}₂(μ-C₂O₄)](ClO₄)₂ (<b>2</b>) (dmphen = 2,9-dimethyl-1,10-phenanthroline). {[Fe{HB(pz)₃}(CN)₂(μ-CN)]₂[{Cu(dmphen)}₂(μ-C₂O₄)]}∙<i>x</i>CH₃OH (<b>3</b>) (2.0 ≤ <i>x</i> ≤ 2.4) was obtained by reacting <b>2</b> and PPh₄[Fe{HB(pz)₃}(CN)₃]∙H₂O [ = tetraphenylphosphonium and  = tris(pyrazolyl)borate]. Crystal structures of <b>1</b>–<b>3</b> have been determined by single-crystal X-ray diffraction experiments: <b>1</b> is a mononuclear trigonal bipyramidal copper(II) species, <b>2</b> is a centrosymmetric oxalato-bridged dicopper(II) complex, and <b>3</b> consists of centrosymmetric tetranuclear units with intramolecular iron–copper and copper–copper distances around 5.010(1) and 5.1833(9) Å, respectively. Variable-temperature magnetic measurements of <b>2</b> and <b>3</b> were carried out from 50 to 350 (<b>1</b>) and 1.9 to 300 K (<b>3</b>). A strong antiferromagnetic interaction between copper(II) ions occurs in <b>2</b> (<i>J</i> = −340 cm⁻¹, the spin Hamiltonian being defined as ). Analysis of the magnetic data of <b>3</b> shows magnetic interactions across the oxalate (<i>J</i>₁ = −341 cm⁻¹) and single cyanide (<i>J</i>₂ = +12.9 cm⁻¹) … (<i>J</i>₂ = +12.9 cm⁻¹) bridges . Simple symmetry considerations of the interacting magnetic orbitals in <b>2</b> and <b>3</b> provide a clear picture of the exchange pathways involved in these complexes.</p