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

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)3PO. 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.