Chalcogenide-bridged ditungsten (M–M) complexes: an experimental and theoretical study of the electronic structure and bonding in W2(μ-E)(μ-OCH2tBu)2(OCH2tBu)6, where E = O, S, Se or Te

Abstract

The series of compounds W₂(μ-E)(μ-OCH₂^tBu)₂(OCH₂^tBu)₆, where E = O, S, Se, or Te, has been isolated from the reactions between [W₂(OCH₂^tBu)₈]_n and pyridine N-oxide, elemental sulfur, elemental selenium and tri-n-butylphosphine telluride. The reactions were carried out in hexane or toluene solutions, and the new compounds were isolated as crystalline products by recrystallization from hexanes at low temperatures (E = O, S, Se) or from hot toluene (E = Te). Each compound displays temperature-dependent ¹H NMR spectra indicative of bridge ⇌ terminal alkoxide group exchange which is frozen out at ca. −65 °C. The low-temperature spectra are consistent with the observed molecular structures in the solid state as determined by single-crystal X-ray studies. There is a central O₃M(μ-O)₂(μ-E)MO₃ skeleton having virtual C_2ν symmetry. The W–W internuclear distances range from 2.55 to 2.66 Å and gradually increase along the series O → S → Se → Te. The compounds are colored (E = O: violet; E = S: blue; E = Se: blue-green; and E = Te: dark yellow/amber) as a result of absorptions in the visible region of the spectrum. The electronic structures of these compounds have been investigated using density functional theory (DFT) by examining the model compounds W₂(μ-E)(μ-OCH₃)₂(OCH₃)₆ (E = O, S, Se, or Te). These B3LYP(SDD,6-31G*) calculations reveal that the HOMO is a M–M σ-type molecular orbital, and with increasing mass of E, the orbital energies of the Ep_π lone-pair orbitals approach this level. A tentative assignment of the electronic absorption spectra is made on the basis of time-dependent DFT (TDDFT) calculations.