The tetradentate ligand N,N‘-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-phenylenediamine, H4L1, has been prepared, and its square planar complexes [CuII(L3)] and [ZnII(L3)] have been synthesized from the reaction of H4L1 with [CuI(NCCH3)4](ClO4) or Zn(BF4)2·2H2O in methanol in the presence of air. The dianion (L3)2- represents the two-electron oxidized form of (L1)4-, namely N,N‘-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-diiminoquinone. Complexes [CuII(L3)]·CH3CN and [Zn(L3)]·CH3CN have been characterized by X-ray crystallography, EPR spectroscopy, and magnetochemistry; [CuII(L3)] has an S = 1/2 ground state, and [Zn(L3)] is diamagnetic. Cyclic voltammetry established that both complexes undergo two successive reversible one-electron oxidations and two successive reversible one-electron reductions. Thus, the coordinated ligand exists in five oxidation levels. The species [MII(L4)]PF6 (M = CuII, ZnII) and [MII(L5)](ClO4)2 (M = CuII, ZnII) have been isolated and characterized by UV/vis, EPR, and 1H NMR spectroscopy and by magnetic susceptibility measurements, where (L4)- represents the monoanion N-(3,5-di-tert-butyl-2-hydroxyphenyl)-N‘-(3,5-di-tert-butyl-2-phenoxyl)-1,2-diiminoquinone and (L5) is the neutral ligand N,N‘-bis(3,5-di-tert-butyl-2-phenoxyl)-1,2-diiminoquinone. Similarly, two complexes of the type [MII(L1H2)] (M = CuII, ZnII) have been isolated from the reaction of L1H4 with CuII(ClO4)2·6H2O or Zn(ClO4)2·6H2O under anaerobic conditions in the presence of NEt3. Complexes [CuII(L4)]PF6 and [Zn(L4)]PF6 selectively oxidize primary alcohols (including methanol and ethanol) in a stoichiometric fashion under anaerobic conditions, yielding the corresponding aldehydes and [MII(L2H2)]+ (M = CuII, ZnII), where (L2)3- is the trianionic form of N,N‘-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-diiminosemiquinone. Since the latter reduced forms react rapidly with dioxygen with formation of [MII(L4)]+ (M = Cu, Zn) and 1 equiv of H2O2, these oxidized species are catalysts for the air oxidation of primary alcohols, including ethanol and methanol, with concomitant formation of H2O2 and aldehydes. The kinetics of the stoichiometric reactions and of the catalyses (initial rate method) have been measured. Large kinetic isotope effects show that H-abstraction from the α-carbon atom of a coordinated alcoholato ligand is the rate-determining step in all cases.

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.

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...

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The pendent arm macrocyclic ligand 1-ethyl-4,7-bis(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, H2L, forms stable complexes in methanol with zinc(II) and copper(II) ions:  [ZnII(L)]·H2O (1); [CuII(L)]·0.5 CH2Cl2 (2); [CuII(LH)](ClO4) (3). The crystal structures of 1 and 2 have been determined by X-ray crystallography:  1 crystallizes in the orthorhombic space group Pbca with a = 21.100(4) Å, b = 10.267(2) Å, c = 28.896(6) Å, V = 6260(2) Å3, Z = 8; 2 crystallizes in the monoclinic space group C2/c with a = 14.447(2) Å, b = 25.522(4) Å, c = 17.296(3) Å, V = 6300(2) Å3, Z = 8. In CH2Cl2 solution complexes 1 and 2 can electrochemically be reversibly oxidized by two successive one-electron processes generating the stable phenoxyl mono- ([1]•+; [2]•+) and diradicals ([1]2•2+, [2]2•2+). In contrast, 3 containing a coordinated phenol and one phenolate can only be oxidized to the monoradical [3]•2+. The electronic structure of these mono- and diradicals have been established by UV/vis and EPR spectroscopy in fluid and/or frozen solution. All oxidations are ligand-centered generating coordinated phenoxyl radicals. In [1]2•2+ the two unpaired electrons interact with each other via exchange and weak dipolar couplings of the order of −3 and 10-2 cm-1, respectively. The monoradicals [2]•+ and [3]•2+ are nearly EPR-silent; an S t = 1 excited state for both species is barely observable due to large zero-field splitting. In [1]•+ the phenoxyl radical electron is localized on one phenyl ring whereas for [2]•+ some degree of delocalization over both phenyl rings may be present. The diradical [1]2•2+ possesses a diamagnetic whereas [2]2•2+ has an S t = 3/2 ground state.