Exploring The Mechanisms Of The Acceptorless Alcohol Dehydrogenation Catalyzed By N-heterocyclic Carbene Iridium Complex

Acceptorless alcohol dehydrogenation?AAD?reaction is an atom-economical and environmentally benign method for the synthesis of carbonyl compounds without stoichiometric oxidants.These reactions are researched for the potential applications in hydrogen energy storage,since the readily available alcohols can serve as important sources for liquid organic hydrogen carriers which can be easily implemented in liquid fuel infrastructure.In recent years,significant progresses has been made in AAD reaction.But the reaction is mostly catalyzed by the noble metal?Ru,Os,Rh,and Re?complexes in organic solvents.In contrast to these metal complexes,iridium complexes can catalyze AAD reaction in aqueous phase.At the same time,the new catalysts formed by iridium and excellent chelating ligand have attracted a wide range of attention due to their high chemical selectivity.Compared with the traditional method,nitrogen heterocyclic carbene-iridium complex has the characteristics of wide substrate range,mild conditions and high catalytic activity for the dehydrogenation of 1-phenylethyl alcohol to acetophenone.The mechanism of the dehydrogenation of 1-phenylethyl alcohol catalyzed by two functional N-heterocyclic carbene iridium complexes has been studied by using density functional theory?DFT?.The intermediates and transition states have been located.In order to obtain the favorable path of the reaction,the energy change of all possible reaction paths are analyzed and discussed.It will provide reasonable explanation for experimental observations and also give theoretical guidance for the design of bifunctional catalysts applied in the acceptorless alcohol dehydrogenation.The main contents and results of the study are as follows:???The mechanism study on the dehydrogenation of 1-phenylethanol catalyzed by the hydroxy-functionalized N-heterocyclic carbene iridium complexesThe catalytic cycle of dehydrogenation of 1-phenylethyl alcohol to acetophenone catalyzed by Cp*Ir?NHC?is studied in detail by DFT method under the promotion of proton co-solvent.The acceptorless 1-phenylethanol dehydrogenation reaction goes through two major steps.The first step is the 1-phenylethanol dehydrogenation,which is more favored via an outer-sphere over an inner-sphere mechanism.The second step is H₂ formation,which is the rate-determining step,and this is in agreement with the experimental results.During the whole catalytic cycle,the alcohol/alkoxide moiety on the nitrogen heterocyclic carbene ligand participates in each step by receiving or releasing protons to form hydrogen bonds with the reactants.In the second step,the proton tBuOH solvent plays a bridging role in the intramolecular H-H proton transfer reaction,and simultaneously provides the proton for the formation of hydrogen molecules.The assistance of tBuOH leads to the decease of the reaction energy barrier by 4.4 kcal/mol,thereby,the catalytic activity of carbene iridium complex was enhanced.Therefore,we propose that the reaction follows the outer-sphere mechanism of alcohol oxidation and the proton solvent assisted H-H formation.Based on these results,a new catalyst?HMB?Ru?NHC??HMB=hexamethylbenzene?is designed theoretically,where a relatively cheaper and abundant 4d ruthenium element is employed with the NHC and HMB ligands.Ruthenium complex?HMB?Ru?NHC?and iridium complex Cp*Ir?NHC?have the similar reaction mechanism,and the reaction energy barrier is 19.3 kcal/mol.Thus,?HMB?Ru?NHC?may effectively catalyze the acceptorless alcohol dehydrogenation reaction.???The mechanism study on the dehydrogenation of 1-phenylethanol by the water-soluble hydroxypyridine iridium complexBased on our computations,it is found that hydroxyl group on pyridine ligands of hydroxypyridine iridium catalyst 1-Ir^a serves as a proton bridge in the catalytic reaction,and participates in almost each step of the catalytic cycle by giving or receiving protons to form hydrogen bond with substrates and intermediates.Unlike the hydroxy-functionalized N-heterocyclic carbene iridium catalytic system,the rate-determing step of the catalytic cycle promoted by hydroxypyridine iridium complex is related to the oxidation of 1-phenylethanol.In comparison with other iridium complexes?pyrimidine iridium catalyst 1-Ir^b,double pyridine iridium catalyst 1-Ir^c?,the hydroxyl pyridine iridium complex 1-Ir^a has the best catalytic activity in aqueous solution.The reaction energy barrier is only 13.0 kcal/mol.Based on the above research results,we designed a new catalyst Cp*Rh?NHC?for the dehydrogenation of alcohols.The calculated results show that:?i?the catalytic dehydrogenation of alcohol catalyzed by designed rhodium catalyst has the same geometric configuration and energy change as that of iridium catalyst;?ii?the energy barrier of dehydrogenation of 1-phenylethyl alcohol catalyzed by rhodium catalyst is 15.0 kcal/mol,which indicates that this complex may be used as a catalyst for the dehydrogenation of alcohol.Therefore,nitrogen heterocyclic carbene with[C,N]chelating ring as the main ligand structure is very important for the design of highly efficient catalyst for AAD reaction.