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)(OCH2tBu)4 can be made alternatively by reacting W2(COT)(OtBu)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) A. 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 (e ∼ 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(OCH2tBu)6(py)2, W2(OtBu)6, or W2(OCH2tBu)8 even upon heating to 80 °C for several hours.