Electronic structure and reactivity of three-coordinate iron complexes.
Electronic structure and reactivity of three-coordinate iron complexes.
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三坐标铁复合物的电子结构和反应性。
DOI：
10.1021/ar700267b
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发表时间：
2008-08
影响因子：
18.3
通讯作者：
Holland, Patrick L.
中科院分区：
文献类型：
--
作者：
Holland, Patrick L.
配位化合物中金属的身份和氧化态通常被认为是其反应性的最重要决定因素。然而，配位数（与金属的键的数目）可以同样有影响。这个帐户描述了铁配合物的配位数只有三个，这大大不同于铁配合物的八面体（六配位）的几何形状，就其磁性，电子结构，配体的偏好，和反应性。具有三角平面几何形状的三配位络合物可使用庞大的阴离子双齿配体（β-二酮亚胺酸盐）获得，所述配体将单齿配体引导到其两个氮供体的平面中。这种策略导致了各种三配位铁络合物，其中铁处于+1，+2和+3氧化态。对这些配合物的电子结构的系统研究有助于解释它们的性质。铁离子通常是高自旋的，具有可用于与配体的π相互作用的单占据轨道。σ-键的趋势表明，铁（II）配合物有利于电负性配体（O，N供体）比正电性配体（氢化物）。静电σ-键合和π-相互作用的可用性的组合使氟化铁（II）和氧代络合物稳定。同样的因素使氢化铁络合物不稳定，氢化铁络合物的反应性足以将氢原子添加到不饱和有机分子中并参与自由基反应。铁（I）络合物使用强π-背键将电荷从铁转移到配位的炔和N2中，而铁（III）接受来自π-供体亚氨基配体的电荷。虽然亚胺铁（III）络合物通过三角平面几何中的π键稳定，但添加吡啶作为第四给体削弱了π键，这使得能够从烃中提取H原子。三配位铁化合物的不寻常的键合和反应模式可能会导致新的催化剂的氧化和还原反应，并可能被自然用于固氮酶的瞬时中间体。
The identity and oxidation state of the metal in a coordination compound are typically thought to be the most important determinants of its reactivity. However, the coordination number (the number of bonds to the metal) can be equally influential. This Account describes iron complexes with a coordination number of only three, which differ greatly from iron complexes with octahedral (six-coordinate) geometries with respect to their magnetism, electronic structure, preference for ligands, and reactivity. Three-coordinate complexes with a trigonal-planar geometry are accessible using bulky, anionic, bidentate ligands (β-diketiminates) that steer a monodentate ligand into the plane of their two nitrogen donors. This strategy has led to a variety of three-coordinate iron complexes in which iron is in the +1, +2, and +3 oxidation states. Systematic studies on the electronic structures of these complexes have been useful in interpreting their properties. The iron ions are generally high spin, with singly occupied orbitals available for π interactions with ligands. Trends in σ-bonding show that iron(II) complexes favor electronegative ligands (O, N donors) over electropositive ligands (hydride). The combination of electrostatic σ-bonding and the availability of π-interactions stabilizes iron(II) fluoride and oxo complexes. The same factors destabilize iron(II) hydride complexes, which are reactive enough to add the hydrogen atom to unsaturated organic molecules and to take part in radical reactions. Iron(I) complexes use strong π-backbonding to transfer charge from iron into coordinated alkynes and N2, whereas iron(III) accepts charge from a π-donating imido ligand. Though the imidoiron(III) complex is stabilized by π-bonding in the trigonal-planar geometry, addition of pyridine as a fourth donor weakens the π-bonding, which enables abstraction of H atoms from hydrocarbons. The unusual bonding and reactivity patterns of three-coordinate iron compounds may lead to new catalysts for oxidation and reduction reactions, and may be used by nature in transient intermediates of nitrogenase enzymes.
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