Cu K-edge XAS study of the [Cu-2(mu-O)(2)] core: Direct experimental evidence for the presence of Cu(III)
Cu K-edge XAS study of the [Cu-2(mu-O)(2)] core: Direct experimental evidence for the presence of Cu(III)
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DOI：
10.1021/ja9717673
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发表时间：
1997-09-10
影响因子：
15
通讯作者：
Hodgson, KO
中科院分区：
文献类型：
--
作者：
DuBois, JL;Mukherjee, P;Hodgson, KO
铜在其不常观察到的Cu （III）氧化态最近引起了人们的注意，特别是作为一种被提议的o2切割双核和三核配合物的成分。1,2一些被普遍接受为含有Cu （III）的离散分子物种是已知的。这些化合物中Cu （III）氧化态的赋值是基于各种光谱、电化学和结构证据。特别是，相对于它们的Cu （II）对应物的键长，它们非常短的Cu配体键距离表明它们具有更高的氧化态。然而，对金属氧化态的直接和明确的探测显然是缺失的。金属k边x射线吸收光谱（XAS），由于其能够直接探测吸收金属的核心电子环境，提供了一种明确检查铜氧化状态的方法。考虑到在某些Cu （II）体系中发生配体中心氧化，金属氧化态的特定探针特别相关。更一般地说，定义原子组成相似但电子分布不同的化合物之间的差异，可能与理解其反应性的变化有关。特别是，（μ -η2: η2-过氧）二铜（II）、1b、6双- μ -氧二铜（II）和双- μ -氧二铜（III） 1岩心在形式上都是等效的，尽管后两者在大多数光谱方法中可能在很大程度上无法区分。在这里，我们报告了两个双核铜配合物[(LME) 2Cu2O2] 2+(1)和[(led) 2Cu2O2] 2+, 1,9(2)的Cu K-edge XAS研究（图1），表明这些配合物最好的形式描述为Cu (III) 2（µ-O） 2，并给出了Cu （III）稳定在Cu2O2菱形中的第一个直接光谱证据。配合物1的Cu （III）价态可以根据其非常短的晶体学上确定的金属配体键长度来提出。10 Cu K-edge EXAFS分析11证实了这些键距离以及显著短的Cu-Cu距离，为2.73 Å。13,14对2的EXAFS分析给出了相似的测量细节（Cu-O） 1.80 Å， Cu-N) 1.92 Å， Cu-Cu) 2.75 Å)。通过EXAFS对这些结构的验证提供了对这些热敏分子中可能分解的内部检查。类似的cu配体和Cu-Cu距离在2e-， 2H+还原的双-µ-羟基铜（II）形式的1和2（分别为3和4）中要长0.1-0.2 Å。14,15最近报道的Cu2O2配合物（如[(Bn3-TACN) 2Cu2O2] 2+）的Cu-Cu分离时间要长~ 0.05 Å。为了与1-4进行比较，我们选择了两种公认的Cu （III）物质，即n30三肽螯合物Cu (III)-(H-2Aib3) 3a和固态材料KCuO2 17
Copper in its infrequently observed Cu (III) oxidation state has gained attention recently, in particular as a proposed constituent of O2-cleaving binuclear and trinuclear complexes. 1, 2 A few discrete molecular species generally accepted as containing Cu (III) are known. 3 Assignment of the formal Cu (III) oxidation state in these compounds is based on a variety of spectroscopic, electrochemical, and structural evidence. In particular, their very short Cu-ligand bond distances relative to bond lengths in their Cu (II) counterparts suggest a higher oxidation state. However, a direct and unambiguous probe of the metal oxidation state is notably absent. 4 Metal K-edge X-ray absorption spectroscopy (XAS), because of its ability to probe directly the core-electronic environment of the absorbing metal, offers a means of explicitly examining the oxidation state of the copper. A specific probe of the metal oxidation state is particularly relevant, given the occurrence of ligand-centered oxidation in some Cu (II) systems. 5 More generally, defining differences between compounds with similar atomic compositions but differing electronic distributions, is potentially relevant to understanding variations in their reactivities. In particular, the (µ-η2: η2-peroxo) dicopper (II), 1b, 6 bis-µ-oxyl dicopper (II), and bis-µ-oxo dicopper (III) 1 cores are all formally equivalent, though the latter two may be largely indistinguishable by most spectroscopic methods. 7 Here we report on a Cu K-edge XAS study of two binuclear copper complexes [(LME) 2Cu2O2] 2+(1) and [(LTEED) 2Cu2O2] 2+, 1, 9 (2)(Figure 1) showing that these complexes are best formally described as Cu (III) 2 (µ-O) 2 and giving the first direct spectroscopic evidence of stabilized Cu (III) in a Cu2O2 rhomb.The Cu (III) valence state for complex 1 may be proposed on the basis of its remarkably short, crystallographically determined metal-ligand bond lengths. 10 Cu K-edge EXAFS analysis11 confirms these bond distances as well as the notably short Cu-Cu distance of 2.73 Å for 1. 13, 14 EXAFS analysis of 2 gives similar metrical details (Cu-O) 1.80 Å, Cu-N) 1.92 Å, Cu-Cu) 2.75 Å). 14 Verification of these structures by EXAFS provides an internal check against possible decomposition in these thermally sensitive molecules. Analogous Cu-ligand and Cu-Cu distances are 0.1-0.2 Å longer in the 2e-, 2H+ reduced bis-µ-hydroxo dicopper (II) forms of 1 and 2 (ie, 3 and 4, respectively). 14, 15 The Cu-Cu separation is∼ 0.05 Å longer in related, recently reported Cu2O2 complexes (eg,[(Bn3-TACN) 2Cu2O2] 2+). 1b, 16 For comparison with 1-4, two accepted Cu (III) speciessthe N3O tripeptide chelate complex Cu (III)-(H-2Aib3) 3a and the solid state material KCuO2 17 shave been