Redox Site Confinement in Highly Unsymmetric Dimanganese Complexes

A set of highly preorganized pyrazolate-bridged dimanganese complexes LMnMnX have been prepared and structurally
characterized. They can be described as hybrid organometallic/Werner-type systems that consist of a low-spin
CpMnI(CO)2 subunit (Mn1) and a proximate tripodal tetradentate {N4} binding pocket accommodating a high-spin
MnII ion (Mn2), with Mn···Mn distances of ∼4.3 Å and different coligands bound to Mn2. Density functional theory
(DFT) calculations (both the hybrid B3LYP and the pure BP86 functionals and the all-electron basis sets 6-311G
and 6-311G*) confirm that the valence α and β Kohn−Sham molecular orbitals (MOs) of these mixed-valent MnIMnII
compounds have predominant Mn(3d) character and an almost perfectly localized nature:  all five unpaired electrons
are essentially localized at the Werner-type Mn2, whereas Mn1 possesses an effective closed-shell structure with
the MOs of highest energy centered there. One-electron oxidation occurs in a clean process at approximately E1/2
= −0.6 V (versus ferrocene/ferrocinium), giving the low-spin/high-spin MnIIMnII species. UV/vis and IR
spectroelectrochemistry as well as a detailed theoretical analysis reveal that the redox process takes place with
strict site control at the organometallic subunit, while it does not significantly influence the spin and charge distribution
on the Werner-type site. Positions and shifts of the ν(C⋮O) absorptions are largely reproduced by the DFT
calculations. These systems thus represent an exceptional example of the effect the unsymmetry of a dinucleating
ligand scaffold has on the spin and charge distribution in homobimetallic complexes and might offer interesting
prospects for the study of the cooperative effects of bimetallic arrays.

本研究合成了一系列高度预组织的吡唑酸盐桥联二锰配合物LMnMnX，并对其进行了结构表征。该类配合物可被归类为杂化有机金属/维尔纳型（Werner-type）体系，其结构包含一个低自旋CpMnI(CO)2亚单元（记为Mn1），以及一个紧邻的三足四齿{N4}配位口袋，该口袋配位有一个高自旋MnII离子（记为Mn2）；配合物中Mn···Mn间距约为4.3 Å，且Mn2上结合有不同的辅助配体。密度泛函理论（Density Functional Theory，DFT）计算（采用杂化B3LYP与纯BP86泛函，以及全电子基组6-311G和6-311G*）结果表明，这类混合价态MnIMnII化合物的价层α和β自旋科恩-沙姆（Kohn−Sham）分子轨道（MOs）主要具有Mn(3d)轨道特征，且几乎完全定域：全部5个未成对电子均定域于维尔纳型Mn2位点，而Mn1则呈现有效的闭壳层结构，其最高占据分子轨道集中于该位点。该配合物可在约E1/2 = -0.6 V（相对于二茂铁/二茂铁正离子）处发生干净的单电子氧化过程，生成低自旋/高自旋的MnIIMnII物种。紫外-可见光谱与红外光谱电化学实验结合详细的理论分析结果表明，该氧化还原过程严格定域于有机金属亚单元位点发生，且不会显著影响维尔纳型位点的自旋与电荷分布。DFT计算可较好地重现ν(C≡O)吸收峰的位置与位移变化。因此，该类体系是一个极佳的研究范例，可用于探究双核配体骨架的不对称性对同双核配合物自旋与电荷分布的影响，同时也为双金属阵列的协同效应研究提供了颇具潜力的研究方向。