C−H Bond Activation in Aqueous Solution: Kinetics and Mechanism of H/D Exchange in Alcohols Catalyzed by Molybdocenes

The mechanism of the catalytic H/D exchange in alcohol substrates promoted by molybdocenes in
D2O was shown to occur by C−H bond activation. Primary alcohols selectively exchanged α-hydrogens in
aqueous solutions containing the catalyst precursor [Cp‘2Mo(μ-OH)2MoCp‘2](OTs)2, while some additional
β-hydrogen exchange was observed in secondary alcohols. Tertiary alcohols did not undergo H/D exchange.
Formation of chelate complexes such as the independently synthesized and crystallographically characterized
glycolate complex [Cp‘2MoOCH2CH2OH](OTs) inhibited the H/D exchange in multidentate alcohols. The
exchange reaction was shown to proceed by formation of ketone hydride molybdocene intermediate [Cp‘2Mo(OCR1R2)H]+, which can reversibly dissociate the ketone ligand. The molybdocene hydride complex resulting
from ketone dissociation was identified by independent synthesis and crystallographic characterization of the
hydride complex Cp‘2MoH(OTf). The H/D exchange reaction proceeds stepwise, with the active catalyst being
derived from the monomeric complex [Cp‘2Mo(OH)(OH2)]+. At T = 90 °C, the exchange of the first methylene
hydrogen of PhCH2OH occurs with a rate constant k = 1.16 × 10-4 (±9.88 × 10-7) s-1. The activation
parameters were determined as ΔH⧧ = 19.4 (±0.2) kcal mol-1 and ΔS⧧ = −22.7 (±0.7) cal mol-1 K-1. A
primary kinetic isotope effect of kH,pD6.4/kD,pH6.5 = 2.2 was found.