Phenoxyl radical complexes of gallium, scandium, iron and manganese

The hexadentate macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1 ,4,7-triazacyclononane ((LH3)-H-CH3), 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane ((LH3)-H-Bu) and 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1 ,4,7-triazacyclononane (L-OCH3- H,) form very stable octahedral neutral complexes LMIII with trivalent (or tetravalent) metal ions (Ga-III, Sc-III, Fe-III, Mn-III, Mn-IV). The following complexes have been synthesized: [(LM)-M-Bu], where M = Ga (1), Sc (2), Fe (3); [(LMnIV)-Mn-Bu]PF6 (4'); [(LM)-M-OCH3], where M = Ga (1a), Sc (2a), Fe (3a); [(LMnIV)-Mn-OCH3]PF6 (4a'); [(LM)-M-CH3], where M = Sc (2b), Fe (3b), Mn-III (4b); [(LMnIV)-Mn-CH3](2)(ClO4)(3)(H3O)(H2O)(3) (4b'). 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 salts [(LFeIII)-Fe-OCH3]ClO4 and [(LGaIII)-Ga-OCH3]ClO4 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 3 a and [3 a](+) 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) (S-1 = 5/2) and a phenoxyl radical (S-2 = 1/2) (H = - 2JS(1)S(2); J = - 80 cm(-1)). The manganese complexes undergo metal- and ligand-centred redox processes, which were elucidated by spectroelectrochemistry; a phenoxyl radical Mn-IV complex [(MnLOCH3)-L-IV](2+) is accessible.