Orbital symmetry control of electronic coupling in a symmetrical, all-carbon-bridged "mixed valence" compound: Synthesis, spectroscopy, and electronic structure of [{Mo(dppe)(ν-C7H7)} 2(μ-C4)]n+ (n = 0, 1, or 2)

Article abstract of DOI:10.1021/om200712m

The cycloheptatrienyl molybdenum alkynyl complex [Mo(C≡CH)(dppe) (ν-C₇H₇)], 1 (dppe = Ph₂PCH ₂CH₂PPh₂), undergoes oxidative dimerization on reaction with [FeCp₂]PF₆ in thf at -78 °C to give the bis(vinylidene) [{Mo(dppe)(ν-C₇H₇)}₂(μ-C= CHCH=C)][PF₆]₂, [2][PF₆]₂. Deprotonation of [2][PF₆]₂ with KOBu^t yields butadiyndiylbridged [{Mo(dppe)(ν-C₇H₇)} ₂(μ-C≡C-C≡C)], 3, which undergoes in situ aerial oxidation to give [{Mo(dppe)(ν-C₇H₇)} ₂(μ-C₄)][PF₆], [3]PF₆, as the major product. The cyclic voltammogram of [3]PF₆ exhibits a series of four redox processes indicative of sequential formation of [{Mo(dppe)(ν- C₇H₇)}₂(μ-C₄)]ⁿ⁺ (n = 0, 1, 2, 3, 4) with the comproportionation constant, K_C, for [3]PF₆ of 1.9 × 10₇. Spectroscopic investigations on [3]PF₆ by IR, Raman, NIR, and EPR spectroscopy reveal properties characteristic of a d⁵/d⁶ mixed valence complex with a localized electronic structure and an estimated intramolecular electron transfer rate in the range 10⁸-10¹⁰ s^-1. The experimental NIR spectrum of [3]PF₆ is consistent with the predicted spectral characteristics of a three-state model for bridgemediated, electron transfer in a weakly coupled, symmetrical mixed valence system. The dication [3][PF₆]₂ was isolated by chemical oxidation and structurally characterized; magnetic susceptibility measurements on [3][PF ₆]₂ in the temperature range 2-300 K reveal strong antiferromagnetic coupling with the exchange coupling constant J_ab (defined according to the Hamiltonian H_spin = -J _abS_aS_b) determined as -406 (±3) cm^-1.