Mechanisms Of The Acceptorless Primary Alcohol Dehydrogenation Catalyzed By Water-Soluble Iridium Complexes

With the increasing consumption of fossil fuels and CO₂ emission,energy shortage and environmental pollution have gradually become a serious global challenge.It is extremely urgent to find alternatives to non-renewable energy.Hydrogen energy is considered as a promising alternative source.The catalytic reaction system of alcohol dehydrogenation is a synthesis important method for the development of hydrogen econmy.The development of homogeneous catalysts with high conversion and good selectivity has become a research hotspot.On the one hand,the dehydrogenation of acceptorless alcohols can release two molecules of hydrogen and produce corresponding carboxylic acid.The advantages of this kind of reaction is green and environment friendly because the reaction does not need any oxidant and will not produce harmful by-products.On the other hand,carboxylic acid is an important raw material for organic industry,which can be used in practical production.In recent years,the research of catalysis system by iridium complex has made great progress.Compared with the traditional methods of hydrogen storage and hydrogen production,the dehydrogenation of organic liquid alcohol catalyzed by the homogeneous iridium complexes bearing pyridine ligands has the characteristics of mild reaction conditions,high selectivity and wide range of substrate.Water is the most ideal reaction medium.Using water as solvent to complete organic reactions has become a green,safe and sustainable development direction.The mechanisms of the acceptorless porimary alcohols dehydrogenation to generate carboxylic acid in water solution catalyzed by two novel Cp*Ir complexes have been investigated using density functional theory.According to the calculation result,the short-term intermediates and transition states have been located,the energy changes of the reaction paths are analysed and compared,and the optimal reaction path is obtained.Our work provides theoretical guidance for the development of metal-organic complexes compounds in the field of alcohol dehydrogenation.The main contents and conclusions of this paper are as follows:(1)The detailed mechanisms of the acceptorless ethanol-water dehydrogenation to generate acetic acid catalyzed by a novel iridium complex Cp*Ir[bpy O-N(Me)₂](H₂O)bearing an electron-rich functional bipyridonate ligand with N,N-dimethylamino substituents,have been investigated by using the B3PW91 and MN15 methods of density functional theory(DFT).The catalytic reaction occurs through three major steps,including the dehydrogenation of CH₃CH₂OH to CH₃CHO,the formation of ethanediol,the ethanediol dehydrogenation to CH₃COOH.The computational results show that the neutral concerted pathway has the lower free energy barrier than the anionic Ir-N bond dissociated stepwise path by 14.1 kcal/mol for ethanol dehydrogenation to CH₃CHO.It indicates that the neutral concerted pathway is more feasible than the anionic Ir-N bond dissociated stepwise path under the reflux temperature.The Na OH assisted reaction path for the dehydrogenation of ethanol to acetaldehyde is found to be without any barrier,and all the intermediate species have low relative energies.It is consistent with the excess sodium hydroxide added in the ethanol solution experimentally.Therefore,the Na OH assisted C-H activation of ethanol is the most faborable.Then,we considered and explored four possible pathways for the H-H bond formation,such as ethanol-bridged,water-bridged,Na⁺-coordinated,and direct dehydrogenation.The calculations show that the barrier of Na⁺promoting dehydrogenation is the lowest among the four paths.The analysis results of NBO calculations show that ligands of catalyst can store and release electrons,which make intermediates and transition states stable.Ethanediol dehydrogenation is a spontaneous exothermic process.Iridium complexes can catalyze the oxidation of ethanediol to acetic acid and release hydrogen in a similar manner to the mechanism of ethanol dehydrogenation.(2)The detailed mechanisms of the acceptorless benzyl alcohol dehydrogenation to generate benzoic acid catalyzed by a water-soluble dicationic iridium complex bearing a functional N-heterocyclic carbine ligand and pyridonate ligand have been investigated using the B3LYP and MN15 methods of density functional theory(DFT).The calculation results show that the C-H activation reaction energy barrier of the outer-sphere concerted path using complex Ir^a as the catalyst is only 4.8 kcal/mol lower than that of the inner-sphere stepwise path;whereas,the energy barrier of the inner-sphere stepwise dehydrogenation of benzyl alcohol triggered by complex Ir^b bearing a pyridonate ligand is very high,which is not conducive to the reaction.Therefore,the benzyl alcohol dehydrogenation catalyzed by complex Ir^a may proceed along the outer-sphere concerted path,and as the temperature increases the inner-sphere stepwise path may occur.However,using complex Ir^b as catalyst,the title reaction may only proceed along the outer-sphere concerted path.The calculated results show that the catalytic activity of complex Ir^a is higher than that of Ir^b,and the strong electron donor N-heterocyclic carbene ligand is more favorable for the acceptorless alcohol dehydrogenation which are in good agreement with the experimental observations.