A theoretical analysis of chemical bonding, vibronic coupling, and magnetic anisotropy in linear iron(II) complexes with single-molecule magnet behavior
A theoretical analysis of chemical bonding, vibronic coupling, and magnetic anisotropy in linear iron(II) complexes with single-molecule magnet behavior
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DOI：
10.1039/c2sc21394j
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发表时间：
2013-01-01
期刊：
影响因子：
8.4
通讯作者：
Neese, Frank
中科院分区：
文献类型：
--
作者：
Atanasov, Mihail;Zadrozny, Joseph M.;Neese, Frank
六种具有线性 FeX2 (X = C, N, O) 核的高自旋 Fe-II 配合物、Fe[N(SiMe3)(Dipp)](2) (1)、Fe[C(SiMe3)(3)](2) (2)、Fe[N(H)Ar'](2) (3)、Fe[N(H)Ar*](2) (4) 的电子结构和磁各向异性， Fe[O(Ar')](2) (5) 和 Fe[N(t-Bu)(2)](2) (7) [Dipp = C6H3-2,6-Pr-2(i)； Ar' = C6H3-2,6-(C6H3-2,6-Pr-2(i))(2)； Ar* = C6H3-2,6-(C6H2-2,4,6-Pr-2(i))(2)； Ar-# = C6H3-2,6-(C6H2-2,4,6-Me-3)(2)] 和一种弯曲 (FeN2) 配合物 Fe[N(H)Ar-#](2) (6) 已使用完全活性空间自洽场 (CASSCF) 波函数结合 N 电子价态微扰理论 (NEVPT2) 和准副生成微扰理论进行了理论研究（QDPT）用于处理磁场和自旋相关的相对论效应。穆斯堡尔对化合物 2 的研究表明，Fe-II 核处存在前所未有的内部磁场（151.7 T）。这被解释为由 Fe-II 中心的 (5)Delta 基态的一阶角动量引起 (J. Am. Chem. Soc. 2004, 126, 10206)。使用来自X射线结构数据的几何形状，使用角度重叠模型到多参考波函数的1:1映射来提取Fe-配体键的配体场参数。结果表明，这些配合物中的金属-配体键合具有以下特征：(i) 强 3d(z2)-4s 混合（在所有配合物中），(ii) 涉及强 pi 供体酰胺配体（1、3-4、6 和 7 中）的 pi 键合各向异性，以及 (iii) Fe-O 键的 sigma-pi 型轨道混合（5 中的错误价态）。所有三种效应的相互作用导致 (5)Delta (3d(xy), 3d(x2-y2)) 基态的对称性明显降低和分裂。效应强度按 1 < 5 < 7 的顺序增加，类似于 6。然而，微分键合效应在很大程度上被一阶自旋轨道耦合所推翻，这导致 L - 1 的轨道贡献几乎不减少，从而产生约 6 mu(B) 的净磁矩。这种独特的自旋轨道驱动磁性受到几何畸变效应的显着调制：弯曲复合体6的静态畸变和系列1-5的增强强度的Renner-Teller类型的动态振动耦合效应。基于 1 和 2 的 X 射线数据的几何形状的从头计算可以很好地再现磁数据。采用动态 Renner-Teller 振动耦合模型计算磁子能级和波函数，其中振动耦合参数根据小型 Fe(CH3)(2) 截断模型复合体的从头计算结果进行调整。该模型再现了观察到的轨道矩的减小，并在引入振动耦合强度 (f) 作为单个可调参数后定量再现了 3-5 的磁化率数据。该值在整个系列中的变化范围很窄 (f = 0.142 +/- 0.015)。结果表明，该系统接近从静态到动态 Renner-Teller 效应过渡的边界。Renner-Teller 电子振动活性用于解释自旋反转势垒 U-eff 沿系列 1 到 5 的大幅减小。基于理论分析，制定了生成具有增强磁各向异性和更长弛豫时间的新型单分子磁体的指南。
The electronic structure and magnetic anisotropy of six complexes of high-spin Fe-II with linear FeX2 (X = C, N, O) cores, Fe[N(SiMe3)(Dipp)](2) (1), Fe[C(SiMe3)(3)](2) (2), Fe[N(H)Ar'](2) (3), Fe[N(H)Ar*](2) (4), Fe[O(Ar')](2) (5), and Fe[N(t-Bu)(2)](2) (7) [Dipp = C6H3-2,6-Pr-2(i); Ar' = C6H3-2,6-(C6H3-2,6-Pr-2(i))(2); Ar* = C6H3-2,6-(C6H2-2,4,6-Pr-2(i))(2); Ar-# = C6H3-2,6-(C6H2-2,4,6-Me-3)(2)], and one bent (FeN2) complex, Fe[N(H)Ar-#](2) (6), have been studied theoretically using complete active space self-consistent field (CASSCF) wavefunctions in conjunction with N-Electron Valence Perturbation Theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for the treatment of magnetic field and spin-dependent relativistic effects. Mossbauer studies on compound 2 indicate an internal magnetic field of unprecedented magnitude (151.7 T) at the Fe-II nucleus. This has been interpreted as arising from first order angular momentum of the (5)Delta ground state of Fe-II center (J. Am. Chem. Soc. 2004, 126, 10206). Using geometries from X-ray structural data, ligand field parameters for the Fe-ligand bonds were extracted using a 1 : 1 mapping of the angular overlap model onto multireference wavefunctions. The results demonstrate that the metal-ligand bonding in these complexes is characterized by: (i) strong 3d(z2)-4s mixing (in all complexes), (ii) pi-bonding anisotropy involving the strong pi-donor amide ligands (in 1, 3-4, 6, and 7) and (iii) orbital mixings of the sigma-pi type for Fe-O bonds (misdirected valence in 5). The interplay of all three effects leads to an appreciable symmetry lowering and splitting of the (5)Delta (3d(xy), 3d(x2-y2)) ground state. The strengths of the effects increase in the order 1 < 5 < 7 similar to 6. However, the differential bonding effects are largely overruled by first-order spin-orbit coupling, which leads to a nearly non-reduced orbital contribution of L - 1 to yield a net magnetic moment of about 6 mu(B). This unique spin-orbital driven magnetism is significantly modulated by geometric distortion effects: static distortions for the bent complex 6 and dynamic vibronic coupling effects of the Renner-Teller type of increasing strength for the series 1-5. Ab initio calculations based on geometries from X-ray data for 1 and 2 reproduce the magnetic data exceptionally well. Magnetic sublevels and wavefunctions were calculated employing a dynamic Renner-Teller vibronic coupling model with vibronic coupling parameters adjusted from the ab initio results on a small Fe(CH3)(2) truncated model complex. The model reproduces the observed reduction of the orbital moments and quantitatively reproduces the magnetic susceptibility data of 3-5 after introduction of the vibronic coupling strength (f) as a single adjustable parameter. Its value varies in a narrow range (f = 0.142 +/- 0.015) across the series. The results indicate that the systems are near the borderline of the transition from a static to a dynamic Renner-Teller effect.Renner-Teller vibronic activity is used to explain the large reduction of the spin-reversal barrier U-eff along the series from 1 to 5. Based upon the theoretical analysis, guidelines for generating new single-molecule magnets with enhanced magnetic anisotropies and longer relaxation times are formulated.