Electronic Coupling in [Mo₂]–Bridge–[Mo₂] Systems with Twisted Bridges

In order to evaluate the impact of bridge conformation on electronic coupling in donor–bridge–acceptor triad systems, two Mo2 dimers, [Mo2(DAniF)3]2[μ-1,4-{C­(O)­NH}2-Naph] (1, DAniF = N,N′-di­(p-anisyl)­formamidinate and Naph = naphthalenyl) and [Mo2(DAniF)3]2[μ-1,4-(CS2)2-2,5-Me2C6H2] (2), have been synthesized and structurally characterized. These two compounds feature a large dihedral angle (>60°) between the central aromatic ring and the plane defined by the Mo–Mo bond vectors, which is distinct from the previously reported phenylene bridged analogues [Mo2(DAniF)3]2[μ-1,4-{C­(O)­NH}2-ph] (I) and [Mo2(DAniF)3]2[μ-1,4-(CS2)2-C6H4] (II), respectively. Unusual optical behaviors are observed for the mixed-valence (MV) species (1+ and 2+), generated by single-electron oxidation. While 2+ exhibits a weak intervalence charge transfer (IVCT) absorption band in the near-IR region, the IVCT band is absent in the spectrum of 1+, which is quite different from what observed for I+ and II+. Optical analyses, based on superexchange formalism and Hush model, indicate that, in terms of Robin–Day classification, mixed-valence species 1+ belongs to the electronically uncoupled Class I and complex 2+, with Hab = 220 cm–1, is assigned to the weakly coupled Class II. Together with I+ and II+, the four MV complexes complete a transition from Class I to Class II–III borderline as a result of manipulating the geometric topology of the bridge. Given the structural and electronic features for the molecular systems, the impacts of electrostatic interaction (through-space) and electron resonance (through-bond) on electronic coupling are discussed.为评估桥联构象对给体-桥体-受体（donor–bridge–acceptor）三单元体系中电子耦合效应的影响，本研究合成并结构表征了两种Mo₂二聚体：[Mo₂(DAniF)₃]₂[μ-1,4-{C(O)NH}₂-Naph]（1，其中DAniF = N,N′-二(对茴香基)甲脒基，Naph = 萘基）与[Mo₂(DAniF)₃]₂[μ-1,4-(CS₂)₂-2,5-Me₂C₆H₂]（2）。这两种化合物的中心芳环与Mo-Mo键矢量所定义的平面间存在大于60°的大二面角，这与此前报道的亚苯基桥联类似物[Mo₂(DAniF)₃]₂[μ-1,4-{C(O)NH}₂-ph]（I）和[Mo₂(DAniF)₃]₂[μ-1,4-(CS₂)₂-C₆H₄]（II）存在显著差异。通过单电子氧化得到的混合价态（mixed-valence, MV）物种1⁺和2⁺展现出异常的光学行为。其中2⁺在近红外区域呈现出弱的价间电荷转移（intervalence charge transfer, IVCT）吸收带，而1⁺的光谱中未观测到IVCT带，这与I⁺和II⁺的实验结果差异显著。基于超交换机制（superexchange formalism）与Hush模型的光学分析表明，依据Robin–Day分类法，混合价态物种1⁺属于电子解耦合的I类（Class I），而2⁺的Hab=220 cm⁻¹，被归为弱耦合II类（Class II）。连同I⁺与II⁺，这四种MV配合物通过调控桥体的几何拓扑结构，完整呈现了从I类到II-III类边界的过渡。鉴于该分子体系的结构与电子特征，本文讨论了静电相互作用（空间途径）与电子共振（键合途径）对电子耦合效应的影响。