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.