Three novel dimolybdenum dimers [Mo2(DAniF)3]2(μ-OSCC6H4CSO), [Mo2(DAniF)3]2(μ-O2CC6H4CS2), and [Mo2(DAniF)3]2(μ-S2CC6H4CS2) (DAniF = N,N'-di(p-anisyl)formamidinate) have been synthesized and characterized by single-crystal X-ray diffractions. Together with the terephthalate analogue, the four compounds, denoted as [O2-O2], [OS-OS], [S2-S2], and [O2-S2], have similar molecular skeletons and Mo2···Mo2 separations (∼12 Å), but varying sulfur contents or symmetry. The singly oxidized complexes [O2-O2](+), [OS-OS](+), [S2-S2](+), and [O2-S2](+) display characteristic intervalence transition absorption bands in the near- and mid-IR regions, with differing band energy, intensity, and shape. Applying the geometrical length of the bridging group "-CC6H4C-" (5.8 Å) as the effective electron transfer distance, calculations from the Mulliken-Hush equation yield electronic coupling matrix elements (H(ab)) in the range 600-900 cm(-1). Significantly, this series presents a transition from electron localization to "almost-delocalization" as the carboxylate groups of the bridging ligand are successively thiolated. In terms of Robin-Day's scheme, [S2-S2](+) is best described as an intermediate between Class II and III, while [O2-O2](+) and [OS-OS](+) belong to Class II. It is unusual that the Class II-III transition occurs in such a weakly coupled system (H(ab) < 1000 cm(-1)). This is attributed to the d(δ)-p(π) conjugation between the Mo2 center and bridging ligand. By electrochemical and spectroscopic methods, the internal energy difference for [O2-S2](+) is determined to be 2250 ± 80 cm(-1), which controls the charge distribution of the cation radical. The experimental results and theoretical analyses illustrate that the unsymmetrical geometry leads to unbalanced electronic configurations and asymmetrical redox and optical behaviors.
Reactions of Mo(2)(O(2)CCH(3))(DAniF)(3), DAniF = N,N'-di-p-anisylformamidinate, with oxamidate dianions [ArNC(O)C(O)NAr](2-), Ar = C(6)H(5) and p-anisyl, give pairs of isomeric compounds where the [Mo(2)] units are bridged by the oxamidate anions. For the alpha isomers, the C-C unit of the dianion is nearly perpendicular to the Mo-Mo bonds, and these are essentially perpendicular to each other. For the beta isomers, the corresponding C-C unit and the Mo-Mo bonds are essentially parallel to each other. Each type of isomer is stable in solution. The electronic communication as measured by the DeltaE(1/2) for the oxidation of each of the Mo(2) units is significantly better for the beta isomers. This is supported also by the appearance of what is conventionally called an intervalence charge-transfer band in the near infrared region upon oxidation of the beta isomers but not the alpha isomers. Molecular mechanics and DFT calculations help explain the relative conformations in the alpha isomers and the relative energy differences between the alpha and beta isomers.
Three symmetrical and one unsymmetrical dimolybdenum dimers, namely, [Mo2(DAniF)3]2(E2CC6H4CE2) (DAniF = N,N′-di(p-anisyl)formamidinate and E = O or S), are structurally and electronically closely related. The mixed-valence cation radicals display well-defined metal to ligand (ML), ligand to metal (LM), and metal to metal (MM) charge transfer absorption bands. Successive thiolations of the complexes result in steady increases of the electronic coupling between the two [Mo2] units. The electronic coupling matrix elements (H ab) calculated from the Hush model fall in the range of 600–900 cm–1, which are remarkably consistent with the results from the CNS superexchange formalism. Spectroscopic analyses suggest that the intramolecular electron transfer occurs by electron-hopping and hole-hopping in concert. The rate constants (k et) are estimated in the range of 1011–1012 s–1 for the symmetrical analogues and 107 s–1 for the unsymmetrical species. The ultrafast electron transfer in such a weakly coupled system (H ab < 1000 cm–1) is attributed to the d(δ)–p(π) conjugation between the dimetal centers and the bridge.
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