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)

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 Ĥ spin = -J abŜ aŜ b) determined as -406 (±3) cm -1. © 2011 American Chemical Society.