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)

The cycloheptatrienyl molybdenum alkynyl complex [Mo(C≡CH)(dppe)(η-C 7 H 7 )], 1 (dppe = Ph 2 PCH 2 CH 2 PPh 2 ), undergoes oxidative dimerization on reaction with [FeCp 2 ]PF 6 in thf at −78 °C to give the bis(vinylidene) [{Mo(dppe)(η-C 7 H 7 )} 2 (μ-C═CH-CH═C)][PF 6 ] 2, [ 2 ][PF 6 ] 2 . Deprotonation of [ 2 ][PF 6 ] 2 with KOBu t yields butadiyndiyl-bridged [{Mo(dppe)(η-C 7 H 7 )} 2 (μ-C≡C-C≡C)], 3, which undergoes in situ aerial oxidation to give [{Mo(dppe)(η-C 7 H 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)(η-C 7 H 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 bridge-mediated, 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 ab ·S ̂ a ·S ̂ b ) determined as −406 (±3) cm –1 .