Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, USA.
Inorg Chem. 2011 May 16;50(10):4322-6. doi: 10.1021/ic102287j. Epub 2011 Apr 8.
The all-ferrous, carbene-capped Fe(4)S(4) cluster, synthesized by Deng and Holm (DH complex), has been studied with density functional theory (DFT). The geometry of the complex was optimized for several electronic configurations. The lowest energy was obtained for the broken-symmetry (BS) configuration derived from the ferromagnetic state by reversing the spin projection of one of the high spin (S(i) = 2) irons. The optimized geometry of the latter configuration contains one unique and three equivalent iron sites, which are both structurally and electronically clearly distinguishable. For example, a distinctive feature of the unique iron site is the diagonal Fe···S distance, which is 0.3 Å longer than for the equivalent irons. The calculated (57)Fe hyperfine parameters show the same 1:3 pattern as observed in the Mössbauer spectra and are in good agreement with experiment. BS analysis of the exchange interactions in the optimized geometry for the 1:3, M(S) = 4, BS configuration confirms the prediction of an earlier study that the unique site is coupled to the three equivalent ones by strong antiferromagnetic exchange (J > 0 in J Σ(j<4)Ŝ(4)·Ŝ(j)) and that the latter are mutually coupled by ferromagnetic exchange (J' < 0 in J' Σ(i<j<4)Ŝ(i)·Ŝ(j)). In combination, these exchange couplings stabilize an S = 4 ground state in which the composite spin of the three equivalent sites (S(123) = 6) is antiparallel to the spin (S(4) = 2) of the unique site. Thus, DFT analysis supports the idea that the unprecedented high value of the spin of the DH complex and, by analogy, of the all-ferrous cluster of the Fe-protein of nitrogenase, results from a remarkably strong dependence of the exchange interactions on cluster core geometry. The structure dependence of the exchange-coupling constants in the Fe(II)-(μ(3)-S)(2)-Fe(II) moieties of the all-ferrous clusters is compared with the magneto-structural correlations observed in the data for dinuclear copper complexes. Finally, we discuss two all-ferric clusters in the light of the results for the all-ferrous cluster.
通过密度泛函理论（DFT）研究了邓和霍尔姆（DH 复合物）合成的全铁、卡宾封端的 Fe(4)S(4)簇。该复合物的几何形状经过优化，适用于几种电子构型。通过反转一个高自旋（S(i) = 2）铁的自旋投影，从铁磁状态获得的非对称（BS）构型具有最低能量。后一种构型的优化几何形状包含一个独特的和三个等效的铁位，它们在结构和电子上都明显可区分。例如，独特铁位的一个显著特征是对角 Fe···S 距离，比等效铁位长 0.3 Å。计算得到的 (57)Fe 超精细参数显示出与穆斯堡尔谱中观察到的相同的 1:3 模式，与实验结果非常吻合。对优化的 1:3、M(S) = 4、BS 构型中的交换相互作用进行 BS 分析，证实了早期研究的预测，即独特的位点通过强反铁磁交换（J > 0 在 J Σ(j<4)Ŝ(4)·Ŝ(j)）与三个等效的位点耦合，而后者通过铁磁交换（J' < 0 在 J' Σ(i<j<4)Ŝ(i)·Ŝ(j)）相互耦合。组合起来，这些交换耦合稳定了一个 S = 4 的基态，其中三个等效位点的复合自旋（S(123) = 6）与独特位点的自旋（S(4) = 2）反平行。因此，DFT 分析支持这样的观点，即 DH 复合物以及氮酶的全铁簇前所未有的高自旋值是由于交换相互作用对簇核几何形状的显著依赖性。在全铁簇的 Fe(II)-(μ(3)-S)(2)-Fe(II) 部分中，比较了交换耦合常数的结构依赖性与双核铜配合物数据中观察到的磁结构相关性。最后，我们根据全铁簇的结果讨论了两个全铁簇。
