Coordination chemistry of 2-(8-aminoquinolino)-4,6-di-tert-butylphenol with manganese(iv), iron(iii), and cobalt(ii/iii):N,O-coordinated o-iminobenzosemiquinonate(1−) π radical monoanions vs.o-iminophenolate(2−) dianions

Abstract

The potentially tridentate ligand 2-(8-aminoquinolino)-4,6-di-tert-butylphenol, H[¹L^AP], has been synthesized and its coordination chemistry with Mn^IV, Fe^III, and Co^III has been investigated by X-ray crystallography, electro- and magnetochemistry, electronic, Mössbauer and EPR spectroscopies. The following complexes have been prepared [Mn^IV(¹L^AP − H)₂] (1), S_t = ³/₂; [Mn^IV(¹L^AP − H)(¹L^ISQ)](PF₆)·CH₂Cl₂ (2), S_t = 1; [Fe^III(¹L^ISQ)₂](ClO₄)·0.5H₂O (3), S_t = ¹/₂; [Fe^III(¹L^ISQ)(3,5-dtcat)]₂ (4), S_t = 0; K[Co^II(¹L^ISQ)(¹L^IBQ)](NCS)₂ (5), S_t = 1; [Co^III₂(NCS)₂(¹L^AP − H)₂(AQ)] (6), S_t = 0, where (¹L^AP − H)²⁻ corresponds to the o-iminophenolate(2−) dianion, (¹L^ISQ)⁻ is the o-iminobenzosemiquinonate(1−) π radical derivative of the ligand H[¹L^AP], (¹L^IBQ)⁰ is the neutral iminobenzoquinone ligand, and 3,5-dtcat is 3,5-di-tert-butylcatecholate(2−) and N,N-coordinated (AQ) is 8-aminoquinoline. It is shown that the π radical anions in 2, 3, 4 couple antiferromagnetically to the respective paramagnetic metal ion. Complex 4 is a dinuclear neutral complex with weak antiferromagnetic coupling between two [Fe^III(¹L^ISQ)(3,5-dtcat)] halves. The asymmetrically ligated complex 6 contains two bridging (¹L^AP − H)²⁻ ligands and two diamagnetic Co^III ions. In contrast, 5 is correctly described as [Co^II(¹L^ISQ)(¹L^IBQ)]⁺ rather than [Co^III(¹L^ISQ)₂]⁺ since it possesses a temperature-independent magnetic moment of μ_eff (10–298 K) = 3.0 μ_B indicating an S_t = 1 ground state which is attained via strong antiferromagnetic coupling (|J| ≥ 200 cm⁻¹) between a high spin cobalt(II) ion (S_Co = ³/₂) and a single π radical anion (S_rad = ¹/₂).