The hexadentate macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L CH 3H3 ), 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L(Bu) H3 ) and 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane (L OCH 3-H3 ) 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) ]PF6 (4'); [L OCH 3M], where M = Ga (1 a), Sc (2 a), Fe (3 a); [L OCH 3Mn(IV) ]PF6 (4 a'); [L CH 3M], where M = Sc (2 b), Fe (3 b), Mn(III) (4 b); [L CH 3Mn(IV) ]2 (ClO4 )3 (H3 O)(H2 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) ]ClO4 and [L OCH 3Ga(III) ]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 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) (S1 = 5/2) and a phenoxyl radical (S2 = 1/2) (H = - 2JS1 S2 ; 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.

The coordination chemistry of the pendent-arm macrocycles 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L(Me)H(3), 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L(Bu)H(3), 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, L(OCH)()3H(3), and Tolman's ligand 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L(Pr)H, with vanadium and cobalt(III) has been studied. The following complexes containing a fac-N(3)O(3) donor set have been synthesized: [L(Me)V(III)] (1), [L(Me)V(IV)]PF(6) (2), [(L(Me)H)V(V)(O)]PF(6) (3), [L(Bu)V(IV)]PF(6) (4), [L(OCH)()3V(IV)]PF(6) (5), [L(Me)Co(III)] (6), [L(Bu)Co(III)] (7), [L(OCH)()3Co(III)] (8). In addition, two complexes containing the L(Pr)Co(III) fragment have been prepared: [L(Pr)Co(III)(acac)](ClO(4)) (9) and [L(Pr)Co(III)(Cl(4)cat)].CH(3)CN (10), where acac(-) represents the ligand pentane-2,4-dionate and Cl(4)cat(2)(-) is tetrachlorocatecholate. Complexes 9 and 10 have been characterized by single-crystal X-ray crystallography: 9 crystallizes in the triclinic space group P&onemacr; with a = 9.493(1) Å, b = 9.760(1) Å, c = 18.979(2) Å, alpha = 88.57(1) degrees, beta = 78.60(1) degrees, gamma = 79.24(1) degrees, V = 1693.3(3) Å(3), and Z = 2; 10 crystallizes in the monoclinic space group P2(1)/n with a = 10.184(2) Å, b = 24.860(5) Å, c = 14.872(3) Å, beta = 97.95(3) degrees, V = 3729(1) Å(3), and Z = 4. Electrochemically, complexes 2, 4, and 5 can be reversibly oxidized by one electron, yielding vanadium(V), and one-electron-reduced, affording vanadium(III) species; 3 can be reduced to [L(Me)HV(IV)(O)]. These redox processes are shown to be metal-centered. In contrast, the cyclic voltammograms of 7 and 8 display three reversible one-electron oxidations. For the monocations [7](*)(+) and [8](*)(+), EPR and UV-vis spectroscopies reveal that these are phenoxyl-cobalt(III) species. Thus, the redox processes are ligand-centered. Similarly, [9](+) is reversibly oxidized to the phenoxyl-cobalt(III) complex [9](*)(2+) (S = (1)/(2)). For 10, two reversible one-electron oxidation steps have been identified generating [10](*)(+) (S = (1)/(2)) and [10](2)(*)(2+) (S = 1). It is unambiguously shown by EPR and resonance Raman spectroscopies that [10](*)(+) contains a (phenolato)(semiquinonato)cobalt(III) unit whereas in [10](2)(*)(2+) a phenoxyl(semiquinonato)cobalt(III) unit prevails.