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

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)·CH₂ Cl₂ (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)(¹ L^IBQ)](NCS)₂ (5), S_t = 1; [Co₂^III(NCS)₂(¹ L^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).