Divalent first-row transition metal complexes of the rigid pendant-arm ligand1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane

Abstract

The rigid pendant-arm macrocyclic ligand 1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane (L¹) has been synthesized. It reacts with divalent first row transition metal perchlorate salts to yield complexes of general formula [M^II(L¹)][ClO₄]₂ (M = Fe^II, Ni^II, Cu^II or Zn^II). Single crystal X-ray crystallography indicated that [Fe^II(L¹)][ClO₄]₂ and [Ni^II(L¹)][ClO₄]₂ are isostructural with both complex cations consisting of distorted pseudo-octahedral six-co-ordinate metal centres with three macrocyclic N-donors and three anilino donors. The mean Fe–N bond length in [Fe^II(L¹)][ClO₄]₂ was found to be 2.10 Å, which is intermediate between the normal ranges of high and low spin Fe^IIN₆ species. Magnetometry and EPR measurements indicated that single crystals of [Fe^II(L¹)][ClO₄]₂ were predominately low spin, with approximately 5% of a low spin iron(III) species present in the lattice. It is postulated that this latter species is the mono anilido complex [Fe^III(L^1−H)][ClO₄]₂. The crystal structure of [Ni^II(L¹)][ClO₄]₂ indicated a short mean Ni–N bond length of 2.09 Å. The ligand field strength for this material was found to have a very high value of 12,330 cm⁻¹ (B = 850 cm⁻¹) and is compared to that of some related trigonal Ni^IIN₆ species. The crystal structure of [Cu^II(L¹)][ClO₄]₂ was also determined and indicated a distorted six-co-ordinate metal centre. The X-band EPR spectrum of this complex in magnetically dilute solid solution indicated a predominantly axial species, although a slight rhombic distortion may be present. The variable temperature ¹H NMR of [Zn^II(L¹)][ClO₄]₂ in CD₃CN solution revealed a non-fluxional six-co-ordinate complex. Electrochemical analyses of the complexes indicated only irreversible oxidation and reduction processes for [Ni^II(L¹)][ClO₄]₂ and [Cu^II(L¹)][ClO₄]₂. [Fe^II(L¹)][ClO₄]₂ displays at least two reversible oxidation processes with the first at +0.181 V (vs. Fc–Fc⁺) being assigned as a metal centred Fe^II → Fe^III process and the second at +0.475 V as a ligand centred process.