Orbital symmetry control of electronic coupling in a symmetrical, all-carbon-bridged 'mixed valence' compound: Synthesis, spectroscopy, and electronic structure of [{Mo(dppe)(ν-C _7H _7)} _2(μ-C _4)] ~(n+) (n = 0, 1, or 2)

摘要

The cycloheptatrienyl molybdenum alkynyl complex [Mo(C≡CH)(dppe) (ν-C _7H _7)], 1 (dppe = Ph _2PCH _2CH _2PPh _2), undergoes oxidative dimerization on reaction with [FeCp _2]PF _6 in thf at -78 °C to give the bis(vinylidene) [{Mo(dppe)(ν-C _7H _7)} _2(μ-C= CHCH=C)][PF _6] _2, [2][PF _6] _2. Deprotonation of [2][PF _6] _2 with KOBu ~t yields butadiyndiylbridged [{Mo(dppe)(ν-C _7H _7)} _2(μ-C≡C-C≡C)], 3, which undergoes in situ aerial oxidation to give [{Mo(dppe)(ν-C _7H _7)} _2(μ-C _4)][PF _6], [3]PF _6, as the major product. The cyclic voltammogram of [3]PF 6 exhibits a series of four redox processes indicative of sequential formation of [{Mo(dppe)(ν- C _7H _7)} _2(μ-C _4)] ~(n+) (n = 0, 1, 2, 3, 4) with the comproportionation constant, K C, for [3]PF _6 of 1.9 × 10 _7. Spectroscopic investigations on [3]PF _6 by IR, Raman, NIR, and EPR spectroscopy reveal properties characteristic of a d ~5/d ~6 mixed valence complex with a localized electronic structure and an estimated intramolecular electron transfer rate in the range 10 ~8-10 ~(10) s ~(-1). The experimental NIR spectrum of [3]PF _6 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][PF _6] _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 ? _(spin) = -J _(ab)? _a? _b) determined as -406 (±3) cm ~(-1).