Orbital Symmetry Control of Electronic Coupling in a Symmetrical, All-Carbon-Bridged “Mixed Valence” Compound: Synthesis, Spectroscopy, and Electronic Structure of [{Mo(dppe)(η-C7H7)}2(μ-C4)]n+ (n = 0, 1, or 2)
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The cycloheptatrienyl molybdenum alkynyl complex [Mo(CCH)(dppe)(η-C7H7)], 1 (dppe = Ph2PCH2CH2PPh2), undergoes oxidative dimerization on reaction with [FeCp2]PF6 in thf at −78 °C to give the bis(vinylidene) [{Mo(dppe)(η-C7H7)}2(μ-CCH-CHC)][PF6]2, [2][PF6]2. Deprotonation of [2][PF6]2 with KOBut yields butadiyndiyl-bridged [{Mo(dppe)(η-C7H7)}2(μ-CC-CC)], 3, which undergoes in situ aerial oxidation to give [{Mo(dppe)(η-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)(η-C7H7)}2(μ-C4)]n+ (n = 0, 1, 2, 3, 4) with the comproportionation constant, KC, 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 bridge-mediated, 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][PF6]2 in the temperature range 2–300 K reveal strong antiferromagnetic coupling with the exchange coupling constant Jab (defined according to the Hamiltonian Ĥspin = –Jab·Ŝa·Ŝb) determined as −406 (±3) cm–1.
