Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
Inorg Chem. 2011 Sep 5;50(17):8238-51. doi: 10.1021/ic200767e. Epub 2011 Aug 11.
An analysis of the electronic structure of the Mn(II)Mn(III)(μ-OH)-(μ-piv)(2)(Me(3)tacn)(2)(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn(III). All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the (55)Mn-ENDOR constrains the (55)Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn(II) and Mn(III) ions. The large effective hyperfine tensor anisotropy of the Mn(II), a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn(III) ion strongly perturbs the zero-field energy levels of the Mn(II)Mn(III) complex. An estimate of the fine structure parameter (d) for the Mn(III) of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn(II) ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn(III) is consistent with the known coordination environment of the Mn(III) ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.
报道了 Mn(II)Mn(III)(μ-OH)-(μ-piv)(2)(Me(3)tacn)(2)(2) (PivOH) 配合物的电子结构分析。它具有以下特征：(i) 基态 1/2 自旋态；(ii) 两个 Mn 离子之间的小交换 (J) 耦合；(iii) 单核-μ-羟桥，双-μ-羧基；(iv) 与 Mn(III) 强耦合的末端配位 N 配体。所有这些特征都在氧进化复合物 (OEC) 的结构模型中观察到。对该配合物进行了多频电子顺磁共振 (EPR) 和电子核双共振 (ENDOR) 测量，并使用自旋哈密顿形式主义模拟了所得光谱。(55)Mn-ENDOR 的强场依赖性限制了(55)Mn 超精细张量，从而可以推断出电子结构的唯一解。需要大的超精细各向异性来重现 Mn(II)和 Mn(III)离子的 EPR/ENDOR 光谱。通常表现出小各向异性的 d(5)离子 Mn(II)的大有效超精细张量各向异性，在一种形式主义中进行了解释，其中 Mn(III)离子的精细结构张量强烈扰动 Mn(II)Mn(III)配合物的零场能级。假设 Mn(II)离子的固有各向异性较小，Mn(III)的精细结构参数 (d) 估计为-4 cm(-1)。Mn(III)的精细结构和固有（原位）超精细张量的大小与从其晶体结构中观察到的 Mn(III)离子的已知配位环境一致。对晶体结构几何形状进行了broken symmetry 密度泛函理论 (DFT) 计算。DFT 值对于原位（固有）超精细张量的各向同性和各向异性分量，与上述 EPR/ENDOR 模拟推断的值相匹配，误差在 5%以内。这项研究表明，即使在描述孤立良好的 S = 1/2 基态开始失效的情况下，DFT 计算也能为混合价 Mn 配合物的光谱观测提供可靠的估计。
