Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991, Russia.
Dalton Trans. 2013 Sep 28;42(36):12794-818. doi: 10.1039/c3dt51004b. Epub 2013 Jul 19.
This brief essay consists of a few "exciting stories" devoted to relations within a metal-complex catalyst between a metal ion and a coordinated ligand. When, as in the case of a human couple, the rapport of the partners is cordial and a love cements these relations, a chemist finds an ideal married couple, in other words he obtains a catalyst of choice which allows him to functionalize C-H bonds very efficiently and selectively. Examples of such lucky marriages in the catalytic world of ions and ligands are discussed here. Activity of the catalyst is characterized by turnover number (TON) or turnover frequency (TOF) as well as by yield of a target product. Introducing a chelating N,N- or N,O-ligand to the catalyst molecule (this can be an iron or manganese derivative) sharply enhances its activity. However, the activity of vanadium derivatives (with additionally added to the solution pyrazinecarboxylic acid, PCA) as well as of various osmium complexes does not dramatically depend on the nature of ligands surrounding metal ions. Complexes of these metals are very efficient catalysts in oxidations with H2O2. Osmium derivatives are record-holders exhibiting extremely high TONs whereas vanadium complexes are on the second position. Finally, elegant examples of alkane functionalization on the ions of non-transition metals (aluminium, gallium etc.) are described when one ligand within the metal complex (namely, hydroperoxyl ligand HOO(-)) helps other ligand of this complex (H2O2 molecule coordinated to the metal) to disintegrate into two species, generating very reactive hydroxyl radical. Hydrogen peroxide molecule, even ligated to the metal ion, is perfectly stable without the assistance of the neighboring HOO(-) ligand. This ligand can be easily oxidized donating an electron to its partner ligand (H2O2). In an analogous case, when the central ion in the catalyst is a transition metal, this ion changing its oxidation state can donate an electron to the coordinated H2O2 fragment. This provokes the O-O bond rupture in the hydrogen peroxide molecule as is assumed for the role of Fe(2+) ions in the Fenton system.
这篇短文包含了几个“激动人心的故事”，致力于研究金属配合物催化剂中金属离子与配位配体之间的关系。当合作伙伴之间的关系融洽，并且有一种爱将这些关系固定下来时，就像人类夫妻一样，化学家就会发现一个理想的夫妻关系，换句话说，他得到了一个理想的催化剂，这种催化剂能够非常高效和选择性地实现 C-H 键的功能化。这里讨论了离子和配体催化世界中这种幸运婚姻的例子。催化剂的活性由转化数（TON）或转化频率（TOF）以及目标产物的产率来衡量。将螯合 N,N-或 N,O-配体引入催化剂分子（可以是铁或锰衍生物）会显著提高其活性。然而，钒衍生物（外加溶液中的吡嗪羧酸，PCA）以及各种锇配合物的活性并不强烈依赖于包围金属离子的配体的性质。这些金属的配合物是非常有效的过氧化氢氧化催化剂。锇衍生物是记录保持者，表现出极高的 TON，而钒配合物则位居第二。最后，当金属配合物中的一个配体（即过氧氢配体 HOO(-)）有助于配合物中的另一个配体（与金属配位的 H2O2 分子）分解成两种物质时，描述了非过渡金属（铝、镓等）的烷烃功能化的优雅例子，生成非常活泼的羟基自由基。没有邻近 HOO(-)配体的帮助，即使配位到金属离子上的过氧化氢分子也是完全稳定的。这种配体可以很容易地被氧化，将电子捐给它的配体（配位到金属上的 H2O2 分子）。在类似的情况下，当催化剂中的中心离子是过渡金属时，该离子可以改变其氧化态，并将电子捐给配位的 H2O2 片段。这会导致过氧化氢分子中的 O-O 键断裂，就像 Fenton 体系中 Fe(2+)离子的作用一样。
