Phenoxyl radicals: H-bonded and coordinated to Cu(II) and Zn(II)
Phenoxyl radicals: H-bonded and coordinated to Cu(II) and Zn(II)
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DOI：
10.1039/b513221p
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发表时间：
2006-01-01
影响因子：
4
通讯作者：
Wilson, C
中科院分区：
文献类型：
--
作者：
Benisvy, L;Bill, E;Wilson, C
两个前配体（（LH）-L-R）由共价键合到苯并咪唑（（LH）-L-Bz）或4，5-二-对-甲氧基苯基取代的咪唑（（LH）-L-PhOMe）上的邻，对-二叔丁基取代的苯酚组成，其结构已被表征。它们都具有分子内的O-H中心点N氢键在酚基O-H基团和咪唑氮原子之间，H-1 NMR研究表明这种键保留在溶液中。每个（LH）-L-R经历电化学可逆的单电子氧化以形成[（LH）-L-R]（中心点+）自由基阳离子，其被认为通过分子内O中心点中心点H-N氢键稳定。在Et 3 N存在下，（LH）-L-R前配体与M（BF_4）（2）中心点H_2O（M = Cu或Zn）反应生成相应的[M（L-R）（2）]化合物。用X射线单晶衍射法测定了[Cu（L-Bz）（2）]（1），[Cu（L-PhOMe）（2）]（2），[Zn（L-Bz）（2）]（3）和[Zn（L-PhOMe）（2）]（4）的晶体结构。在每个化合物中，金属具有N2 O2配位球：在1个中心点4 MeCN和2个中心点2 MeOH中，{CuN 2 O2}中心具有扭曲的正方形平面几何形状;在3个中心点2 MeCN和4个中心点2 MeCN中，{ZnN 2 O2}中心具有扭曲的四面体几何形状。1和2在77 K的CH 2Cl 2- DMF（9：1）溶液中的X-带EPR谱与Cu（II）配合物的存在一致，所述Cu（II）配合物具有通过X射线晶体学鉴定的结构。电化学研究表明，1、2、3和4各自经历两个单电子氧化;这些过程的电位以及产物的UV/维斯和EPR性质表明每个氧化是基于配体的。第一次氧化产生[M（II）（L-R）（L-R（中心点））]（+），包含与酚盐（L-R）和苯氧基自由基（L-R（中心点））配体结合的M（II）中心;这些阳离子已经通过电化学产生，并且对于R = PhOMe，通过与Ag[BF 4]的化学氧化产生。第二次氧化产生[M（II）（L-R（中心点））（2）]（2+）。从这些研究中获得的信息表明，合适的前配体设计允许产生相对惰性的苯氧基自由基，通过氢键稳定，如[（LH）-L-R]（中心点+）（R = Bz或PhOMe），或通过与金属配位，如[M（II）（L-R）（L-R中心点）]（+）（M = Cu或Zn; R = Bz或PhOMe）。在稳定苯氧基自由基方面，与金属的配位比氢键更有效，并且Cu（II）在这方面比Zn（II）稍微更有效。
Two pro-ligands ((LH)-L-R) comprised of an o, p- di-tert-butyl-substituted phenol covalently bonded to a benzimidazole ((LH)-L-Bz) or a 4,5-di-p-methoxyphenyl substituted imidazole ((LH)-L-PhOMe), have been structurally characterised. Each possesses an intramolecular O - H center dot center dot center dot N hydrogen bond between the phenolic O - H group and an imidazole nitrogen atom and H-1 NMR studies show that this bond is retained in solution. Each (LH)-L-R undergoes an electrochemically reversible, one-electron, oxidation to form the [(LH)-L-R](center dot+) radical cation that is considered to be stabilised by an intramolecular O center dot center dot center dot H-N hydrogen bond. The (LH)-L-R pro-ligands react with M(BF4)(2) center dot H2O (M = Cu or Zn) in the presence of Et3N to form the corresponding [M(L-R)(2)] compound. [Cu(L-Bz)(2)] (1), [Cu(L-PhOMe)(2)] ( 2), [Zn(L-Bz)(2)] ( 3) and [Zn(L-PhOMe)(2)] ( 4) have been isolated and the structures of 1 center dot 4MeCN, 2 center dot 2MeOH, 3 center dot 2MeCN and 4 center dot 2MeCN determined by X-ray crystallography. In each compound the metal possesses an N2O2-coordination sphere: in 1 center dot 4MeCN and 2 center dot 2MeOH the {CuN2O2} centre has a distorted square planar geometry; in 3 center dot 2MeCN and 4 center dot 2MeCN the {ZnN2O2} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both 1 and 2, in CH2Cl2 - DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(II) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that 1, 2, 3 and 4 each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)(L-R)(L-R(center dot))](+), comprising a M(II) centre bound to a phenoxide (L-R) and a phenoxyl radical (L-R(center dot)) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF4]. The second oxidation produces [M(II)(L-R(center dot))(2)](2+). The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [(LH)-L-R](center dot+) (R = Bz or PhOMe), or by coordination to a metal, as in [M( II)(L-R)(L-R center dot)](+) (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu( II) is slightly more effective than Zn( II) in this respect.