Chang Christopher H, Boone Amy J, Bartlett Rodney J, Richards Nigel G J
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, USA.
Inorg Chem. 2004 Jan 26;43(2):458-72. doi: 10.1021/ic0350032.
The metal coordination and spin state of the Fe(III) center in nitrile hydratase (NHase) has stimulated the synthesis of model complexes in efforts to understand the reactivity and spectroscopic properties of the enzyme. We report density functional theory (DFT) calculations on a number of Fe(III) complexes that have been prepared as models of the NHase metal center, together with others having similar ligands but different ground state spin multiplicities. Our results suggest that a DFT description of specific spin configurations in these systems does not suffer from significant amounts of spin contamination. In particular, B3LYP calculations not only reproduce the observed spin state preferences of these Fe(III) complexes but also predict spin-dependent structural properties consistent with those expected on the basis of ligand field models. An analysis of the natural bond orbital (NBO) transformation of the Kohn-Sham wave functions has enabled quantitation of the overall contribution to covalency of ligand-to-metal sigma-donation and pi-donation, and metal-to-ligand pi-back-bonding in these Fe(III) complexes at their BLYP-optimized geometries. Although sulfur ligands are the primary source of covalency in the Fe(III) complexes, our quantitative analysis suggests that hyperbonding between metal-bound nitrogens and an Fe-S bond represents a mechanism by which Fe-N covalency may arise. These studies establish the computational methodology for future theoretical investigations of the NHase Fe(III) center.
腈水合酶(NHase)中Fe(III)中心的金属配位和自旋态激发了模型配合物的合成,以努力理解该酶的反应活性和光谱性质。我们报告了对一些已制备作为NHase金属中心模型的Fe(III)配合物以及其他具有相似配体但基态自旋多重性不同的配合物的密度泛函理论(DFT)计算。我们的结果表明,这些体系中特定自旋构型的DFT描述不存在大量的自旋污染。特别是,B3LYP计算不仅再现了这些Fe(III)配合物观察到的自旋态偏好,还预测了与基于配体场模型预期一致的自旋相关结构性质。对Kohn-Sham波函数的自然键轨道(NBO)变换的分析能够定量配体到金属的σ-给予和π-给予以及金属到配体的π-反馈键合对这些Fe(III)配合物在其BLYP优化几何构型下共价性的总体贡献。尽管硫配体是Fe(III)配合物中共价性的主要来源,但我们的定量分析表明,金属结合的氮与Fe-S键之间的超键合代表了一种可能产生Fe-N共价性的机制。这些研究为未来对NHase Fe(III)中心的理论研究建立了计算方法。