Kathleen Lonsdale Building, Department of Chemistry, University College London, Gower Street, London, United Kingdom WC1E 6BT.
Dalton Trans. 2011 Jan 14;40(2):402-12. doi: 10.1039/c0dt00820f. Epub 2010 Nov 23.
New insights into the structural, electronic and catalytic properties of Fe complexes are provided by a density functional theory study of model as well as real [Fe(II)(H)(2)(diphosphine)(diamine)] systems. Calculations conducted using several different functionals on the trans- and cis-isomers of [Fe(II)(H)(2)(S-xylbinap)(S,S-dpen)] complexes show that, as with the [Ru(II)(H)(2)(diphosphine)(diamine)] complexes, the trans-[Fe(II)(H)(2)(diphosphine)(diamine)] complex is the more stable isomer. Analysis of the spin states of the trans-[Fe(II)(H)(2)(diphosphine)(diamine)] complexes also shows that the singlet state is significantly more stable than the triplet and the quintet, as with the [Ru(II)(H)(2)(diphosphine)(diamine)] complexes. Calculations of the catalytic cycle for the hydrogenation of ketones using two model trans-[M(II)(H)(2)(PH(3))(2)(en)] catalysts, where M = Ru and Fe, show that the mechanism of reaction as well as the activation energies are very similar, in particular: (i) the ketone/alcohol hydrogen transfer reaction occurs through the metal-ligand bifunctional mechanism, with energy barriers of 3.4 and 3.2 kcal mol(-1) for the Ru- and Fe-catalysed reactions, respectively; (ii) the heterolytic splitting of H(2) across the M[partial double bond, bottom dashed]N bond for the regeneration of the Ru and Fe catalysts has an activation barrier of 13.8 and 12.8 kcal mol(-1), respectively, and is expected to be the rate determining step for both catalytic systems. The reduction of acetophenone by trans-[M(II)(H)(2)(S-xylbinap)(S,S-dpen)] complexes along two competitive reaction pathways, shows that the intermediates for the Fe catalytic system are similar to those responsible for the high enantioselectivity of (R)-alcohol in those proposed trans-[Ru(II)(H)(2)(S-xylbinap)(S,S-dpen)] catalysed acetophenone hydrogenation reaction. Thus the high enantiomeric excess in the hydrogenation of acetophenone could, in principle, be achieved using Fe catalysts.
通过对模型和真实的[Fe(II)(H)(2)(二膦)(二胺)]系统的密度泛函理论研究，提供了对 Fe 配合物结构、电子和催化性质的新见解。使用几种不同的函数对[Fe(II)(H)(2)(S-xylbinap)(S,S-dpen)]配合物的顺式和反式异构体进行的计算表明，与[Ru(II)(H)(2)(二膦)(二胺)]配合物一样，顺式-[Fe(II)(H)(2)(二膦)(二胺)]配合物是更稳定的异构体。对顺式-[Fe(II)(H)(2)(二膦)(二胺)]配合物的自旋态的分析还表明，单重态比三重态和五重态更稳定，与[Ru(II)(H)(2)(二膦)(二胺)]配合物一样。使用两个模型顺式-[M(II)(H)(2)(PH(3))(2)(en)]催化剂[其中 M = Ru 和 Fe]计算酮的加氢催化循环，表明反应机制和活化能非常相似，特别是：(i) 酮/醇氢转移反应通过金属-配体双功能机制进行，Ru 和 Fe 催化反应的能垒分别为 3.4 和 3.2 kcal mol(-1)；(ii) H(2)在 M[部分双键，底部虚线]N 键上的异裂用于 Ru 和 Fe 催化剂的再生，其活化能垒分别为 13.8 和 12.8 kcal mol(-1)，预计这两种催化体系的速率决定步骤都是如此。通过顺式-[M(II)(H)(2)(S-xylbinap)(S,S-dpen)]配合物沿两条竞争性反应途径还原苯乙酮，表明 Fe 催化体系的中间体与那些负责高对映选择性 (R)-醇的中间体相似在那些提出的顺式-[Ru(II)(H)(2)(S-xylbinap)(S,S-dpen)]催化的苯乙酮加氢反应中。因此，原则上可以使用 Fe 催化剂实现苯乙酮加氢的高对映过量。
