Phenoxyl Radical Complexes of Zinc(II)

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.

一系列锌(II)苯氧自由基配合物已在溶液中制备得到，其中一例以固体物质(5)的形式被分离，旨在通过电子顺磁共振(Electron Paramagnetic Resonance, EPR)、共振拉曼(resonance Raman, RR)以及紫外-可见(UV−vis)光谱学研究其光谱特性。本系列配合物可作为含铜真菌酶半乳糖氧化酶活性形式的模型配合物。所合成的配合物包括[Zn(L1H2)]BF4·H2O(1)、[Zn(L2H2)]BF4·H2O(2)、[Zn(L2H)](2a)、[Zn(L3)(Ph2acac)](3)、[Zn(L4)(Ph2acac)](4)以及[Zn(L4)(Me-acac)](6)，均由Zn(BF4)2·4H2O与对应配体的溶液反应制得：其中L1H3 = 1,4,7-三(3,5-二叔丁基-2-羟基苄基)-1,4,7-三氮杂环壬烷；L2H3 = 1,4,7-三(3-叔丁基-5-甲氧基-2-羟基苄基)-1,4,7-三氮杂环壬烷；L3H = 1,4-二甲基-7-(3,5-二叔丁基-2-羟基苄基)-1,4,7-三氮杂环壬烷；L4H = 1,4-二甲基-7-(3-叔丁基-5-甲氧基-2-羟基苄基)-1,4,7-三氮杂环壬烷；Ph2acac- = 1,3-二苯基-1,3-丙二酮基；Me-acac- = 3-甲基-2,4-戊二酮基。配合物2、3·0.5甲苯·1正己烷以及4通过单晶X射线衍射法完成了结构表征。对该系列配合物在乙腈(CH3CN)和/或二氯甲烷(CH2Cl2)溶液中的电化学研究显示，配位酚盐配体可发生可逆单电子氧化，生成配位苯氧自由基。合成层面，通过1当量六氟磷酸二茂铁在无水二氯甲烷中对4进行单电子氧化，成功制备出微晶态、顺磁性（有效磁矩μeff = 1.7 μB）的[Zn(L4)(Ph2acac)]PF6(5)固体。配合物1、2及2a中配位酚悬垂臂的氧化则需要显著更高的电位，且为不可逆过程。目前已针对该类自由基在溶液中的紫外-可见(UV−vis)、电子顺磁共振(EPR)以及共振拉曼(RR)光谱展开研究，借此可阐明这类配位苯氧自由基配合物的电子结构。