Combined Experimental and Computational Study of the Mechanism of Acceptorless Alcohol Dehydrogenation by POCOP Iridium Pincer Complexes

Iridium pincer complexes
of the type (POCOP)Ir (POCOP = 2,6-(tBu2PO)2C6H3) are very productive
catalysts for dehydrogenation
of secondary alcohols. To our surprise, we found that turnover frequencies
demonstrated by (POCOP)­IrH2 (IrH2) are higher in more dilute solutions of the catalyst,
which triggered a mechanistic study of alcohol dehydrogenation by IrH2. Here, we provide strong evidence
that acceleration by dilution is related to the rate-limiting mass
transfer of hydrogen, which, so far, has not received much attention
in the literature. Using experimental and computational methods, we
show that dehydrogenation has two high-barrier steps, namely the reaction
of IrH2 with alcohol to give
(POCOP)­IrH­(OR) (IrH­(OR)) and subsequent β-elimination
in the latter. Depending on the alcohol and reaction conditions, IrH­(OR) can be formed via an associative pathway that includes
proton transfer to the hydride or a dissociative mechanism that involves
hydrogen elimination from IrH2 to give a 14e (POCOP)Ir species. Rapid re-hydrogenation of IrH­(OR) or the 14e (POCOP)Ir by dissolved hydrogen is responsible
for the rate retardation in more concentrated solutions of the catalyst.
The suggested mechanism gives a satisfactory quantitative description
of the catalytic cycle, such that kinetic curves and reaction orders
in the catalyst can be reproduced.