Intrinsic reactivity of metal-hydroxide complexes: O-H bond activation andadduct formation in gas-phase reactions of Cp2ZrOH+

Fourier transform ion cyclotron resonance mass spectrometry was used to stu dy the gas-phase reactions of three zirconium(IV) hydroxide ions, Cp2ZrOH(Cp = eta (5)-cyclopentadienyl), Cp2ZrOD+, and (Cp2ZrOH+)-O-18. Product dis tributions were determined for reactions with alcohols, amines, ethers, eat ers, and amides. Reactions with alcohols lead to O-H bond activation with f ormation of alkoxide complexes Cp2ZrOR+ and elimination of water. Equilibri um constants for the reactions were used to determine the relative energeti cs of Zr-OR bonds, and the affinities of hydroxide and alkoxides toward the zirconium(IV) center in Cp2Zr2+ decrease (OH- > MeO- > EtO- approximate to iso-PrO- approximate to sec-BuO- approximate to teri-BuO-) in the same ord er as the proton affinities of RO. Adducts of the alkoxide complex ion with alcohols are formed at long reaction times. Reaction with acetic acid lead s to formation of the carboxylate complex ion Cp2ZrO2CCH3+ and elimination of water. Amines, ethers, esters, and amides all form adducts with Cp2ZrOH, but no other reaction pathways, such as hydrolysis, are observed. The uns olvated zirconium(IV) hydroxide species can coordinate substrates and activ ate O-H bonds, but it does not efficiently cleave esters and amides despite its reactive, bound hydro ride and coordinative unsaturation. The relation ship of these results to solution reactions of metal-hydroxides is discusse d. (C) 2001 Elsevier Science B.V.