Structure of the seven-coordinate aqua-(N-hydroxyethyl)ethylenediaminetriacetato iron(III) complex and solution coexistence between six- and seven-coordinate species

The molecular structure of [Fe(heed3a)(H2O)] (1, heed3a=(N-hydroxyethyl)ethylenediaminetriacetate) has been determined: orthorhombic, Fdd2 (No. 43), Z=16, a=25.666(6), b=26.634(7), c=7.631(2) Å, V=5216(2) Å3, R1=0.0244, wR2=0.0633. The seven-coordinate polyhedron of complex 1 is best described as distorted pentagonal bipyramidal. The solid state Vis–NIR spectrum of 1 is compared with its spectrum after redissolution in water and with the solid state and solution spectra of [Fe(edta)(H2O)]− where prevalence of seven-coordination is known for aqueous solutions. In difference to Fe(III)–edta, considerable amounts of 1 rearrange upon dissolution and an equilibrium mixture of six- and seven-coordinate species is formed in aqueous solution.