Synthesis, structure, and exchange reactions of rhenium alkoxide and aryloxide complexes. Evidence for both proton and hydrogen atom transfer in the exchange transition state

Article abstract of DOI:10.1021/om00027a031

A series of rhenium(I) aryloxide and alkoxide complexes have been prepared. The aryloxide complexes may be prepared by treatment of the methyl complex (CO)₃(PPh₃)₂ReCH₃ (1) with p-cresol, affording (CO)₃(PPh₃)₂ReOC₆H ₄CH₃ (3a). The PPh₃ ligands may be displaced by a number of other ligands (PMe₃, 1,2-(dimethylarsino)benzene (diars), or t-BuNC), providing a route to substituted cresolate complexes. Alkoxide complexes of the type (CO)₃(L)₂ReOR (R = CH₃ (6), CH₂CH₃ (7), CH(CH₃)₃ (8); L, L₂ = PMe₃, diars, depe, or (S,S)-bdpp) may be prepared by treatment of the triflate complexes (CO)₃(L)₂ReOSO₂CF₃ with the sodium salt of the appropriate alcohol. These alkoxide complexes are quite stable and do not decompose by β-hydride elimination. The coordinated alkoxide ligands exchange rapidly with added alcohols in solution, while the exchange of aryloxide ligands with added phenols is much slower. The alkoxide complexes may be converted into the aryloxide complexes by addition of the desired phenol and heating to 44°C. This reaction proceeds via the intermediacy of a hydrogen-bonded adduct formed by coordination of the phenol O-H proton to the alkoxide oxygen atom. The hydrogen bonds are quite strong, and the adducts may be isolated if desired. One of the hydrogen-bonded adducts of 7e and p-cresol, (CO)₃(depe)ReOCH₂CH₃?HOC ₆H₄CH₃, has been structurally characterized. The O-O distance is 2.532(5) A?, a value commonly observed for strong hydrogen bonds. The rates of conversion of a series of these complexes (CO)₃(diars)ReOCH₃?HOC₆H₄X (6b?HOC₆H₄X) to (CO)₃(diars)ReOC₆H₄X (3b) and methanol were studied. The rate of this reaction was found to be cleanly first order in the hydrogen-bonded complex 6b?HOC₆H₄X and was unaffected by the addition of excess phenol and added bases. The rate was found to increase when the substituent X either lowers the O-H bond dissociation energy or increases the acidity of the phenol, indicating that the transition state for this reaction can have either proton or hydrogen atom transfer character.