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

Fourier transform ion cyclotron resonance mass spectrometry was used to study the gas-phase reactions of three zirconium(IV) hydroxide ions, Cp2ZrOH+ (Cp = eta (5)-cyclopentadienyl), Cp2ZrOD+, and (Cp2ZrOH+)-O-18. Product distributions were determined for reactions with alcohols, amines, ethers, esters, and amides. Reactions with alcohols lead to O-H bond activation with formation of alkoxide complexes Cp2ZrOR+ and elimination of water. Equilibrium constants for the reactions were used to determine the relative energetics 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 order as the proton affinities of RO. Adducts of the alkoxide complex ion with alcohols are formed at long reaction times. Reaction with acetic acid leads 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 unsolvated zirconium(IV) hydroxide species can coordinate substrates and activate O-H bonds, but it does not efficiently cleave esters and amides despite its reactive, bound hydro ride and coordinative unsaturation. The relationship of these results to solution reactions of metal-hydroxides is discussed.