Phenoxyl radical complexes of gallium, scandium, iron and manganese

Article abstract of DOI:10.1002/chem.19970030221

The hexadentate macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxy-benzyl)-1,4,7-triazacyclononane (L(CH1)H₃), 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L(CH1)H₃) and 1,4, 7-tris(3-tert-butyl-5-methoxy-2-hydro-xybenzyl)- 1,4,7-triazacyclononane (L(BH)H₃) and 1,4,7-tris()3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4-7-triazacyclononane (L)(OCH1)-H₃) form very stable octahedral neutral complexes LM¹¹¹ with trivalent (or tetravalent) metal ions (Ga¹¹¹, Se¹¹¹, Fe¹¹¹, Mn¹¹¹, Mn(IV)). The following complexes have been synthesized: [L(Bu)M], where M = Ga (1), Se (2), Fe (3); [L(Bu)Mn(IV)]PF₆ (4); [L(OCH)¹M], where M = Ga (1 a), Se (2a), Fe (3a); [L(OCH)³Mn(IV)]PF₆ (4a'); [L(CH)¹M], where M = Se (2b), Fe (3b), Mn¹¹¹ (4b); [L(CH)¹Mn(IV)]₂(ClO₄)₃(H₃O)(H₂ O)₃(4 b').An electrochemical study has shown that complexes 1, 2, 3, 1a, 2a and 3a each display three reversible, ligand-centred, one-electron oxidation steps. The stilts [L(OCH)³Fe]CIO₄ and [L(OCH)¹]CIO₄ have been isolated as stable crystalline materials. Electronic and EPR spectra prove that these oxidations produce species containing one, two or three coordinated phenoxyl radicals. The Mossbauer spectra of 3a and [3a] ⁺ show conclusively that both compounds contain high-spin iron(III) central ions. Temperature-dependent magnetic susceptibility measurements reveal that 3a has an S= 5/2 and [3a]⁺ an S = 2 ground state. The latter is attained through intramolecular antiferromagnetic exchange coupling between a high-spin iron(III) (S₁ = 5/2) and a phenoxyl radical (S₂=1/2) (II = 2JS₁S₂; J = 80cm^-1). The manganese complexes undergo metal- and ligand-centred redox processes, which were elucidated by spectroelectrochemistry; a phenoxyl radical Mn(IV) complex [Mn(IV)L(OCH)₄]²⁺ is accessible.