Structures and spectra of bis-tripodal iron(II) chelates, [FeL₂]²⁺, where L = tris(pyrazol-1-yl)methane, tris(pyridin-2-yl)methane, bis(pyrazol-1-yl)(pyridin-2-yl)methane and tris(pyridin-2-yl)phosphine oxide. Magnetism and spin crossover in the (pz)₃CH case

Abstract

A range of six-co-ordinate bis-tripodal iron(II) chelates, [FeL₂]²⁺, has been synthesized from iron-(II) or -(III) precursors where L = tris(pyrazol-1-yl)methane (pz)₃CH, tris(pyridyl-2-yl)methane (py)₃CH, bis(pyrazol-1-yl)(pyridin-2-yl)methane (pz)₂(py)CH and tris(pyridin-2-yl)phosphine oxide (py)₃P=O. Crystal structures have been determined for three compounds and in the (pz)₃CH case for two polymorphs (α and β) which were obtained in two different laboratories from iron(III) salts using different crystallisation solvents. The electronic properties of the [FeL₂]²⁺ series have been investigated by optical spectroscopic measurements. Low-spin d⁶ behaviour is indicated. Magnetic and Mössbauer effect measurements on [Fe{(pz)₃CH}₂]²⁺ show that a gradual spin crossover to the high-spin state occurs above 270 K, though this is far from developed at 350 K. Comparisons are made with the well studied boron ligand analogue [Fe{(pz)₃BH}₂], which displays similar crossover behaviour, and with bis(tridentate) ligand complexes containing the ligand 1,4,7-triazacyclononane.