Theoretical Studies On Dehydrogenation Reactions Catalyzed By Water-soluble Transition Metal Complexes

Hydrogen as a clean, efficient and inexhaustible energy is crucial for any greenenergy policy. Dehydrogenation reaction catalyzed by water-soluble transition metalcomplex is currently the important study issue of the efficient green energy plan.Therefore, we performed the detailed mechanism study for several essentialdehydrogenation reactions catalyzed by water-soluble transition metal complexes withthe density functional theory (DFT). In this reactions, using water as the solvent withliberation of dihydrogen represents a safe and clean process for such oxidations.Fukuzumi et al reported the dehydrogenation of ethanol to dihydrogen catalyzed [C,N] cyclometalated Cp*Ir-complex [IrIII(Cp*)-(4-(1H-pyrazol-1-yl-κN2)benzoicacid-κC3)(H2O)]2SO4. The main features of the catalytic system are catalytic cyclefacilitated by the adjusting of pH and the transformation between two active catalystsunder photoirradiation condition. Our theoretical studies well explain the effect of pHon the catalytic cycle and the delicate role of the photoirradiation condition incatalytic system. The same [C, N] cyclometalated complex also can catalyze theinterconversion between formic acid and dihydrogen, which is an excellent hydrogenstorage/supply catalytic system based on pH changing. Our calculations give therational interpretations for the experimental observations of the catalytic system. Inaddition, we also explored the N-alkylation of alcohol with ammonia catalyzed byYamaguchi goup’s water-soluble catalyst [Cp*IrIII(NH3)3][I]2and alcohol (orα-hydroxy carboxylic acids) dehydrogenation as well as the N-alkylation of alcoholwith ammonia catalyzed by Yamaguchi goup’s[IrIII(Cp*)-(6,6′-dihydroxy-2,2′-bipyridine)(H2O)]2[OTf]2. Except for the investigation on the specific reaction steps and manners, we also explored the influences oftransition metal center, ligand, and surrounding solvent environment on the catalyticsystem on the basis of the both kinetic and thermodynamic laws reflected by profiles.Our study purpose is to optimize the catalytic process and design the novel transitionmetal catalyst.1. Developing efficient dehydrogenation is critical to understanding organichydride hydrogen storage. The catalytic mechanism of the pH-dependentacceptorless-alcohol-dehydrogenation in aqueous solution catalyzed by a novel [C, N]cyclometalated Cp*Ir-complex,[IrIII(Cp*)-(4-(1H-pyrazol-1-yl-κN2)benzoicacid-κC3)(H2O)]2SO4, has been investigated by density functional theory (DFT). Theoverall catalytic cycle has been fully characterized. The precatalyst firstly lose twoligand protons to give the intermediate present in basic solution by the increasing ofpH. The formed intermediate reacts with the ethanol in basic solution to generate anessential active metal hydride complex via an inner-sphere mechanism, involving thehemi-decoordination of [C, N] ligand followed by the β-H elimination. Subsequently,due to the decrease of pH, the metal hydride complex interacts with the protons inacid solution to generate H2molecule, which is a downhill process nearly withoutenergy barrier. The present free energy results have shown that both the hydroxyl inbasic solution and the proton in acidic solution effectively promot the whole catalyticcycle. Therefore, our results theoretically demonstrated a significant dependence ofthe reaction system studied on pH value. The present study also predicted that the newcatalyst (at the first triplet excited state, T1) formed from metal hydride under laserexcitation can catalyze the dehydrogenation of ethanol. Remarkably, the replacementof Ir by Ru may get efficient catalyst in the present system.2. A complete reaction mechanism for interconversion between hydrogen andformic acid catalyzed by [C,N] cyclometallated organoiridium complex,[IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoic acid-kC3)(H2O)]2·SO4, has been revealedby density functional theory (DFT) calculations. On the basis of the pH, two catalyticcycles occurring in differect solution enviremnet have the various active catalysts. Forboth the hydrogen storage catalytic cycle I and hydrogen evolution catalytic cycle II, the detailed reaction profiles with the key transition states and intermediates arerevealed. Catalytic cycle I show that the ligand dihydrogen heterolysis facilitated byOH-gives the considerable stable iridium hydride intermediate followed by anouter-sphere hydrogen transfer to afford a metal-formate complex. Upon theincreasing of pH, catalytic cycle II occurs via the generation of the metal-formatecomplex followed by the outer-sphere β-H elimination to form a metal-hydridecomplex, which is subsequently protonated by the hydrated proton H3O+to afforddihydrogen. The formation of free CO2from HCO3-and the β-hydride elimination offormate are found to be the rate-determining step for cycle I and II, respectively. Ourstudies are well in agreement with experimental results. The acid-base equilibriumbetween the hydroxy and oxyanion form on the catalyst [C, N] ligand has aconsiderable influence on the catalytic hydrogen transfer. Remarkablely, newcomputational designed low-cost cobalt(III) and iron (II) complexes as promisingcatalysts exhibit the highly catalytic activity for the reverse reaction betweenhydrogen and formic acid.3. The catalytic mechanism for the multialkylation of benzyl alcohol with ammoniacatalyzed by the novel water-soluble catalyst,[Cp*IrIII(NH3)3][I]2, is computationallyinvestigated by density functional theory (DFT). On the basis of the assistance ofcounter iodine anion I-, the precatalyst Cp*IrIII(NH3)3firstly was activated to thecatalytic active species Cp*IrIII(NH3)(PhCH2O), which catalyzes the three interrelatedand succession catalytic cycles. Each catalytic cycle is consisted of three stages:(stage I) the dehydrogenation of active catalyst to free benzaldehyde and the keyiridium hydride reductant Cp*IrIII(NH3)(H);(stage II) the dehydration coupling ofbenzaldehyde with amine-reactant to form intermediate imine;(stage IIII) thereduction of imine by Ir-H reductant to afford amine-product. In three catalytic cycles,the hydride transfer steps including both the β-H elimination and imine reductions arefound to be essential for the smoothly proceeding of catalytic system. Both electroniceffect and steric effect caused by diverse amine-reactant in every catalytic cycletogether affect the catalytic activity of the corresponding catalytic cycle. The iminereduction process in cycle III is believed to be the rate-determining step in this catalytic reaction. Remarkablely, counter anion I-can more effectively facilitate theimine reduction by the outer-sphere hydrogen-bonding interaction than halogen anionboth Cl-and Br-.4. DFT calculations were performed to elucidate the mechanism of thedehydrogenative oxidation of various primary alcohols (or α-hydroxy carboxylic acids)and the dehydrogenative coupling of alcohols with ammonia catalyzed by the samewater-soluble Cp*Ir complex bearing a2-pyridonate-based ligand A-Ir,[IrIII(Cp*)-(6,6′-dihydroxy-2,2′-bipyridine)(H2O)]2[OTf]2. Another two Rh and Osnew catalysts are computationally designed for the dehydrogenative oxidation ofalcohols. The plausible pathway for alcohol dehydrogenation includes three steps:alcohol oxidation to aldehyde (step I); the generation of dihydrogen in the metalcoordination sphere (step II); the liberation of dihydrogen accompanied with theregeneration of active catalyst (step III). Among them, the step I follows bifunctionalconcerted double hydrogen transfer mechanism rather than the β-H elimination. Forstep II, the energy barriers involving the addition of one or two water molecules arehigher than in absence of water. Our results also confirm that A-Ir can be applied inthe dehydrogenation of various α-hydroxy carboxylic acids by the similar mechanism.Remarkably, A-Ir is also found to be efficient for the coupling reactions of variousprimary benzyl alcohols with ammonia to afford amides.