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.

A series of phenoxyl radical complexes of zinc(II) have been generated in solution and, in one instance, isolated as solid material (5) in order to study their spectroscopic features by EPR, resonance Raman, and UV−vis spectroscopy. They serve as model complexes for the active form of the copper containing fungal enzyme galactose oxidase. The complexes [Zn(L1H2)]BF4·H2O (1), [Zn(L2H2)]BF4·H2O (2), [Zn(L2H)] (2a), [Zn(L3)(Ph2acac)] (3), [Zn(L4)(Ph2acac)] (4), and [Zn(L4)(Me-acac)] (6) were synthesized from solutions of Zn(BF4)2·4H2O and the corresponding ligand (L1H3 = 1,4,7-tris(3,5-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane; L2H3 = 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane; L3H = 1,4-dimethyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane; L4H = 1,4-dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, Ph2acac- = 1,3-diphenyl-1,3-propanedionate, and Me-acac- = 3-methyl-2,4-pentanedionate). Complexes 2, 3·0.5 toluene·1n-hexane, and 4 were structurally characterized by single-crystal X-ray crystallography. An electrochemical investigation of these complexes in CH3CN and/or CH2Cl2 solution revealed that the coordinated phenolate ligands undergo reversible one-electron oxidations with formation of coordinated phenoxyl radicals. Synthetically, the microcrystalline, paramagnetic (μeff = 1.7 μB), solid material of [Zn(L4)(Ph2acac)]PF6 (5) was produced by one electron oxidation of 4 by 1 equiv of ferrocenium hexafluorophosphate in dry CH2Cl2. Oxidation of coordinated phenol pendent arms in 1, 2, and 2a occurs at significantly higher potentials and is irreversible. Electronic (UV−vis), electron paramagnetic resonance (EPR), and resonance Raman (RR) spectra of the radicals have been studied in solution and allow the description of the electronic structure of these coordinated phenoxyl radical complexes.

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Resonance Raman (RR) spectroscopy has been employed to study coordinated phenoxyl radicals (M = Ga, Sc, Fe) which were electrochemically generated in solution by using 1,4,7-triazacyclononane-based ligands containing one, two, or three p-methoxy or p-tert-butyl N-substituted phenolates, i.e., 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (3Lbut), 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane (3Lmet), 1,4-bis(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-7-ethyl-1,4,7-triazacyclononane (2Lmet), and 1-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-4,7-dimethyl-1,4,7-triazacyclononane (1Lmet). A selective enhancement of the vibrational modes of the phenoxyl chromophores is achieved upon excitation in resonance with the π → π* transition at ca. 410 nm. The interpretation of the spectra was supported by quantum chemical (density functional theory) calculations which facilitate the vibrational assignment for the coordinated phenoxyl radicals and provide the framework for correlations between the RR spectra and the structural and electronic properties of the radicals. For the uncoordinated phenoxyl radicals the geometry optimization yields a semiquinone character which increases from the unsubstituted to the p-methyl- and the p-methoxy-substituted radical. This tendency is indicated by a steady upshift of the ν8a mode which predominantly contains the Cortho−Cmeta stretching coordinate, thereby reflecting strengthening of this bond. The calculated normal-mode frequencies for these radicals are in a good agreement with the experimental data constituting a sound foundation for extending the vibrational analysis to the 2,6-di-tert-butyl-4-methoxyphenoxyl which is the building block of the macrocyclic ligands 3Lmet, 2Lmet, and 1Lmet. The metal-coordinated radical complexes reveal a similar band pattern as the free radicals with the modes ν8a and ν7a (CO stretching) dominating the RR spectra. These two modes are sensitive spectral indicators for the structural and electronic properties of the coordinated phenoxyl radicals. A systematic investigation of complexes containing different ligands and metal ions reveals that two parameters control the semiquinone character of the phenoxyls:  (i) an electron-donating substituent in the para position which can accept spin density from the ring and (ii) an electron-accepting metal ion capable of withdrawing excess electron density, introduced by additional electron-donating substituents in ortho positions. It appears that both effects, which are reflected by (i) the frequency of the mode ν8a and (ii) the frequency difference of the modes ν8a and ν7a, balance an optimum electron density distribution in the phenoxyl radical. Along similar lines, it has been possible to interpret the RR spectral changes between the Fe monoradical, [Fe(3Lmet)]+•, and diradical, [Fe(3Lmet)]2+••, complexes. Both the parent as well as the radical complexes of Fe exhibit a phenolate-to-iron charge transfer band >500 nm. Excitation in resonance with this transition yiel...

The hexadentate ligand 1,4,7-tris(3 -tert-butyl-5-methoxy -2-hydroxybenzy1)-1,4,7-triazacyclononane, H3L, forms a very stable neutral complex [Cr111L].2MeCN 2 with chromium(m) which undergoes electrochemically three one-electron oxidations to yield [CrIIIL]+, [CrI"L]2+ and [CrIIIL]3+, respectively; these contain one, two and three coordinated phenoxyl radical ligands; [CrIIlL]-2MeCN and [Cr111L]C104.3MeCN are characterized by X-ray crystallography.