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 (L   CH   3H 3 ), 1,4,7‐tris(3,5‐di‐ tert ‐butyl‐2‐hydroxybenzyl)‐1,4,7‐triazacyclononane (L Bu H 3 ) and 1,4,7‐tris(3‐ tert ‐butyl‐5‐methoxy‐2‐hydroxybenzyl)‐1,4,7‐triazacyclononane (L   OCH   3‐H 3 ) form very stable octahedral neutral complexes LM III with trivalent (or tetravalent) metal ions (Ga III , Sc III , Fe III , Mn III , Mn IV ). The following complexes have been synthesized: [L Bu M], where M = Ga ( 1 ), Sc ( 2 ), Fe ( 3 ); [L Bu Mn IV ]PF 6 ( 4′ ); [L   OCH   3M], where M = Ga ( 1 a ), Sc ( 2 a ), Fe ( 3 a ); [L   OCH   3Mn IV ]PF 6 ( 4 a' ); [L   CH   3M], where M = Sc ( 2 b ), Fe ( 3 b ), Mn III ( 4 b ); [L   CH   3Mn IV ] 2 (ClO 4 ) 3 (H 3 O)(H 2 O) 3 ( 4 b' ). An electrochemical study has shown that complexes 1, 2, 3, 1 a, 2 a and 3 a each display three reversible, ligand‐centred, one‐electron oxidation steps. The salts [L   OCH   3Fe III ]ClO 4 and [L   OCH   3Ga III ]ClO 4 , 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 Mössbauer 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 3 a 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 = ‐ 2 JS 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 [Mn IV L   OCH   3] 2+ is accessible.