Mononuclear chromium(III), manganese(II) and iron(III) complexes of the pentadentate ligand 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane

Abstract

Chromium(III), manganese(II) and iron(III) complexes of dmptacn [1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane] with general formula [M(dmptacn)Cl] ⁿ⁺ have been prepared by reaction of the macrocycle with the corresponding metal chloride salts and their physicochemical properties examined. Magnetic susceptibility, Mössbauer and ESR measurements have shown that the iron(III) complex [Fe(dmptacn)Cl][PF ₆ ] ₂ ·MeOH exhibits spin-crossover behaviour, existing in the low-spin form below 50 K and as a mixture of low- and high-spin forms at 298 K. The structure of [Cr(dmptacn)Cl][ClO ₄ ] ₂ confirmed that the isomer formed has the secondary nitrogen adjacent to the chloro ligand and that there is significant distortion from octahedral geometry in the complex, both common features of complexes of dmptacn. Two enantiomeric forms of the complex are found in the unit cell, vis-à-vis in one enantiomer the δδδ conformation of the tacn ring is coupled with a λλ conformation of the pyridyl arms while the opposite conformations are observed in the other.