Cyclooctatetraene ditungsten alkoxides: W2(μ-η5,η5-COT)(OR)4, where R = CH2tBu, iPr, and tBu

Abstract

Four equivalents of neopentanol, isopropanol, or t-butanol react with the cyclooctatetraene-bridged dimethylamide, W₂(μ-η⁵,η⁵-COT)(NMe₂)₄, to give the corresponding tetraalkoxides, W₂(μ-η⁵,η⁵-COT)(OR)₄, in greater than 70% yields. The alkoxide W₂(COT)(OCH₂^tBu)₄ can be made alternatively by reacting W₂(COT)(O^tBu)₄ with 4 equivalents of neopentanol. X-ray diffraction data for the t-butoxide compound indicate a μ-η⁵,η⁵-COT ligand spans a W–W bond of distance 2.3887(1) Å. The t-butoxide is very similar in most structural parameters to the known W₂(COT)(NMe₂)₄ precursor as well as the bis-allyl compound W₂(allyl)₂(NMe₂)₄. The COT ring is fluxional about the metal–metal fragment, undergoing rapid 1,2-shifts on the NMR timescale as determined by ¹H EXSY NMR. The purple alkoxides exhibit maximum absorptions around 580 nm (ε ∼ 1700 M⁻¹ cm⁻¹) in the visible region of the electronic absorption spectrum. This transition agrees well with DFT calculations on the model compound W₂(μ-η⁵,η⁵-COT)(OH)₄ that reveal the calculated HOMO–LUMO gap to be a 1.94 eV transition involving principally the metal d orbitals. The calculations also reveal the structure with an antifacial (η⁵,η⁵)-COT to be 13.7 kcal mol⁻¹ more stable than the corresponding synfacial (η⁴,η⁴)-COT structure. In related studies, COT did not form adducts with W₂(OCH₂^tBu)₆(py)₂, W₂(O^tBu)₆, or W₂(OCH₂^tBu)₈ even upon heating to 80 °C for several hours.