The quadruply bonded metal-metal complex Mo2[O2P(OC6H5)2]4 reduces 1,2-dichloroalkanes and 1,2-dichloroalkenes upon its irradiation with visible light (λexc ≥ 495 nm). The inorganic product of the photochemistry in all cases is the MoIIMoIII mixed-valence binuclear complex Mo2[O2P(OC6H5)2]4+, which has been structurally characterized. Green prismatic crystals of the tetrafluoroborate salt are triclinic with a Pi space group: a = 10.917(8), b = 11.793(4), c = 12.430(4) Å; a = 63.45(2), β = 70.38(4), γ = 70.64(5)°; V= 1316.8(12) Å3; Z = 1. The structure was refined to values of R = 0.036 and Rw = 0.043. The Mo-Mo bond distance of 2.191 Å is 0.05 Å greater than its quadruple bond parent complex, Mo2[O2P(OC6H5)2]4·2THF. The mixed-valence dimer shows a vibrationally-structured δ → δ (2B1u ← 2B2g) absorption band in the near-infrared spectral region (λmax = 1469 nm, t = 142 M−1 cm-1) with a 308-cm-1 energy spacing, which is consistent with a progression in the symmetric metal-metal stretching vibration. The mixed-valence complex is reversibly reduced and oxidized by one-electron (E1/2(Mo2(II,III/II,II) = +0.067 V and E1/2(Mo2(III,III/II,III) = +0.997 V vs SCE). The nature of the organic product resulting from Mo2 [Mo2P(OC6H5)2]4 photochemistry depends on the organic substrate. For 1,2-dichloroalkanes, photoreaction is facile and affords the olefin (ϕp(1,2-dichloroethane) = 0.029; ϕp(1,2-transdichlorocyclohexane) = 0.040), whereas photoreaction of 1,2-dichloroalkenes yields monohalogenated alkenes (ϕp-(1,2-cis-dichloroethylene) = 5.5 × 10−6; ϕp(o-dichlorobenzene) = 1.4 × 10−4).
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