Electronic Communication in Dinuclear C₄-Bridged Tungsten Complexes

The dinuclear tungsten carbyne [X(CO)₂(dppe)WC₄W(dppe)(CO)₂X] (dppe = 1,2-bis(diphenylphosphino)ethane; X = I (3), Cl (7)) complexes were prepared from the bisacetylide precursor Li₂[(CO)₃(dppe)WC₄W(CO)₃(dppe)] (2) via oxidative replacement of one CO group at each tungsten center with a halide substituent. The iodide ligand in 3 could be substituted with isothiocyanate or triflate resulting in [X(CO)₂(dppe)WC₄W(dppe)(CO)₂X] complexes (X = NCS (8), OTf (9)). Substitution of two and all four CO ligands in 3 was achieved via subsequent photolytic or thermal activation with dppe. The “half-substituted” complex [I(CO)₂(dppe)WC₄W(dppe)₂I] (11) allows reversible one-electron oxidation which results in the monocationic species [I(CO)₂(dppe)WC₄W(dppe)₂I][PF₆] (11[PF₆]). The “all-dppe substituted” complex [I(dppe)₂WC₄W(dppe)₂I] (10) possesses two reversible redox states leading to the stable monocationic [I(dppe)₂WC₄W(dppe)₂I][PF₆] (10[PF₆]) and the dicationic [I(dppe)₂WC₄W(dppe)₂I][PF₆]₂ (10[PF₆]₂) compounds. The complexes 2, 3, [W(CO)₃(dppe)(CCPh)(I)] (4), [X(CO)₂(dppe)WC−C(Me)C(Me)−CW(dppe)(CO)₂X] (X = I (5), Cl (6)), 7, 8, 10, 11 and 11[PF₆] were characterized by single crystal X-ray diffraction. The electronic properties of complexes 10, 10[PF₆], 10[PF₆]₂, as well as of compounds 11 and 11[PF₆], were investigated using cyclic voltammetry (CV), EPR, IR, near-IR spectroscopy, and magnetization measurements. These studies showed that the [W]C−CC−C[W] canonical form of the bridged system with strong tungsten−carbon interaction contributes significantly to the electronic coupling in the mixed-valent species 10[PF₆] (comproportionation constant K_c = 7.5 × 10⁴) and to the strong antiferromagnetic coupling in the dicationic complex 10[PF₆]₂ (exchange integral J = −167 cm⁻¹). In addition, the rate for electron transfer between the tungsten centers in 10[PF₆] was evaluated by near-IR and IR studies.