Syntheses and characterization of iron(II) and iron(III) complexes of a tripodal ligand derived from tris(2-aminoethyl)methane

The trihydrochloride salt of tris(2-aminoethyl)methane (tram·3HCl) was deprotonated in methanolic potassium hydroxide and reacted with three molar equivalents of imidazole-2-carboxaldehyde to give a new Schiff base ligand, HC(CH₂CH₂N{double bond, long}CH-2ImH)₃. The ligand, H₃(1), was reacted in situ with iron(II)chloride tetrahydrate. Addition of excess sodium perchlorate resulted in the isolation of the dark red [FeH₃(1)](ClO₄)₂·KClO₄. The neutral emerald green iron(III) tripodal complex, Fe(1), was prepared by the aerial oxidation of the iron (II) complex on addition of three equivalents of potassium hydroxide. The complexes are characterized by EA, IR, ESI-MS, Mössbauer, magnetic susceptibility and single crystal XRD. The spectroscopic and structural data support a low spin assignment for both the iron(II) and iron(III) complexes at 295 K. The overall conformation of the tram backbone in these complexes has the apical carbon atom, C_ap, pointed away from the iron atom with an average non-bonded distance of 3.83 Å. However, C_ap is distorted from tetrahedral geometry toward trigonal monopyramidal. This is indicated by a narrowing of the H-C_ap-C angles, an expansion of the C-C_ap-C angles and a compression along the C-H axis so that C_ap approaches the plane defined by its three carbon substituents. Two unusual supramolecular features are exhibited in [FeH₃(1)](ClO₄)₂·KClO₄. These are a polymeric [K(ClO₄)₃^2-]_n anion and a bidentate hydrogen bonding donor, N_imine{double bond, long}CH-C_imidazole-N_imidazoleH, on each arm of the tripodal ligand. Density Functional Theory (DFT) calculations using the B3LYP functional were performed on the low spin and high spin states of both complexes. B3LYP correctly predicts that the low spin state is favored in both systems and closely matches the important metrical parameters that are indicative of spin state. B3LYP shows that the C_ap-out conformation of the tram backbone would be nearly identical in the low and high spin forms.