Theoretical Studies On Several Chemical Reactions Catalyzed By Transition Metals And Ionic Liquids

Transition metal complexes are formed by the assembly of transition metals and organic fragments, which exhibit high activity, selectivity and performance stability in catalytic reactions and have versatile and extensive applications in many fields, including organic synthesis, green chemistry, nanoscience and life science. On the other hand, ionic liquids (ILs), a new kind of liquid compounds in the room temperature or near that are entirely composed of anion and cation, which exhibit excellent physical and chemical properties, such as nonvolatility, non-flammability, good reusability and designability. So far, ILs have been widely used in electrochemisty, functional materials, clean energy, separation and extraction. In recent decades, experimental and application researches on transition metal complexes- and ILs-catalyzed transformations both have make great progresses, however, theoretical studies are relatively laggard. Moreover, the elementary mechanism involved are not very clear and experimental observations can not be rationalized, which limit the exploitation and utilization of new high-efficiency catalysts to a certain extent. Alternatively, it is significance to carry out the relevant theoretical researches and further to explore the microscopic nature behind the macroscopic experimental observations.In this dissertation, a series of theoretical researches have been performed for the catalytic conversion reactions in the fileds of organometallic and biomass promoted by several transition metal complexes and ILs with excellent performances. The main idea is to elucidate the mechanism details and calculate the properties of thermodynamics and dynamics, so as to reveal the micro mechanism of the transition metal complexes- and ILs-catalyzed reactions. Currently, we have achieved a series of creative achievements, and enhanced our understanding and awareness of the important chemical reactions and phenomenas. It is expected that presented results would provide theoretical guide to design more novel and high efficiency catalysts.The main contributions and innovations of this dissertation are summarized as follows:1. Focusing on three typical substrates,2,6-difluorobenzophenone imine (A), 2,6-difluorobenzophenone (B), and 2,4’-difluorobenzophenone (C), the present work theoretically studied their C-H and C-F cyclometalation reactions promoted by activator Co(PMe3)4 or CoMe(PMe3)4. It is found that reaction A+Co(PMe3)4 favors the C-F activation, reaction A+CoMe(PMe3)4 prefers the C-H activation, whereas both the C-H and C-F activation pathways may be viable for reactions B+ CoMe(PMe3)4 and C+CoMe(PMe3)4. The experimentally observed C-H and C-F cyclometalation products have been rationalized by analyzing the thermodynamic and kinetic properties of two activation pathways. From calculated results combined with the experimental observations, we believe that three factors, i.e. the oxidation state of metal center in the activators, the anchoring group of substrates, and substituted fluoroatom counts of aromatic ring in substrates, affect the selectivity of C-H and C-F activations of fluoroaromatic ketones and imines. Calculated results are enlightening to rational design of activators and substrates of fluoroaromatic imines and ketones to obtain exclusive C-H or C-F bond activation product.The corresponding results have been published in Org. Biomol. Chem (2014,12, 1897-1907).2. By performing density functional theory (DFT) calculations, the present work for the first time provides a detailed mechanism study of the entire HDS process of thiophene by a representative tungsten complex W(PMe3)4(η2-CH2PMe2)H. In detail, the HDS process consists of four sub-processes: i) binding of thiophene to the metal center to give the metallathiacycle species W(PMe3)4(η2-SC4H4), ii) formation of the tungsten butadiene-thiolate intermediate, (η5-C4H5S)W(PMe3)2(η2-CH2PMe2), iii) hydrogenation of the butadiene-thiolate intermediate, and iv) liberation of the desulfur product but-1-ene. The overall barriers of the four sub-processes were calculated to be 25.5,26.7,22.5, and 44.1 kcal/mol, respectively, which qualitatively rationalizes the experimental observations that the tungsten butadiene-thiolate intermediate was observed under the mild temperature (60℃), whereas the desulfur product was obtained upon thermolysis at elevated temperature (100℃). The regeneration of the tungsten complex is also discussed to evaluate its potential possibility serving as a HDS catalyst.The corresponding results have been published in Applied Catalysis A:General [2014,487,54-61).3. DFT calculations have been performed to gain insight into the mechanism of formic acid (HCOOH) decomposition to H2 and CO2, catalyzed by a well-defined bifunctional cyclometallated iridium(Ⅲ) complex based on 2-aryl imidazoline ligand with a remote NH functionality. It is shown that the reaction features the direct protonation of the Ir-H hydride by HCOOH with the hydrogen shuttling between the NH group and the carbonyl group of HCOOH. Importantly, the simultaneous presence of two HCOOH molecules is proposed to be essential for the dehydrogenation, where one works as a hydrogen source and the other acts as a hydrogen shuttle to assist the long-range intermolecular hydrogen migration. The unique dehydrogenation mechanism is referred to as the HCOOH self-assisted concerted hydrogen migration. With such a mechanism, the barrier of the rate-determining step in the catalytic cycle is calculated to be 14.8 kcal/mol, which is consistent with the observed rapid dehydrogenation of HCOOH at mild conditions (40℃). The effectiveness of the self-assisted catalytic system is attributed, on the one hand, to the d-pπ conjugation between Ir center and proximal nitrogen, which increases the electron density at the Ir center and hence aids Ir-H bond cleavage, and on the other hand, to the hydrogen-shared three-center-four-electron (3c-4e) bond between formate and formic acid, which stabilizes transition states and hence reduces the free energy barriers for the reaction. In addition, calculated results also emphasize the importance of the concerted catalysis of the bifunctional catalyst: when y-NH functional group does not participate in the reaction or is replaced by O atom, the reaction becomes remarkably less favorable with a rate-determining barrier of 25.4 kcal/mol for the former and 26.0 kcal/mol for the latter. The present work rationalizes the experimental findings and provides help for understanding the unique catalysis of the bifunctional cyclometallated iridium(III) complexes.The corresponding results have been accepted for ACS Catal.4. Detailed DFT calculations have been launched to deelply study the mechanism of functionalized ionic liquids (ILs)-catalyzed degradation of cellulose, and further to investigate the effect of different kinds of functionalized ILs on this reaction.(1) The present work presents the first attempt to address the fundamental reaction chemistry of the catalytic transformation of cellulose to glucose by functionalized ILs. We have proposed an enzyme-like catalytic mechanism of ILs, in which the-SO3H group in the cation functioned as a proton donor and the Cl- anion acted as a nucleophile, or the-SO3- anion group in the cation as a proton acceptor, to promote glycosidic bond cleavage through the retaining or inverting mechanisms. DFT calculations show that both two mechanisms involve moderate barriers (<30 kcal/mol), which is in consistent with the catalytic performance of the ILs under mild conditions (<100℃). The "biomimetic" mechanism model proposed in this work is expected to be viable for understanding the unique catalytic activity of ILs under mild conditions.The corresponding results have been published in ChemPhysChem (2015,16, 3044-3048).(2) Based on the mechanism of enzyme-catalyzed degradation of cellulose, the present work provides a clear theoretical model that elucidates how imidazolium-based acidic ILs promote the glycosidic bond hydrolysis. DFT calculations show the mechanism detail of the glycosidic bond hydrolysis catalyzed by several typical ILs, including [C4SO3Hmim]HSO4, [C4SO3Hmim]H2PO4, [C4SO3Hmim]Cl, [C4SO3Hmim]Br and [C4COOHmim]Cl, and the thermodynamics and dynamics properties of catalytic reactions. It is found that the catalytic activities of ILs are not only depended on acidity of acidic groups in the cations, but also the nucleophilicity of anions. [C4SO3Hmim]Br is expected to be the most efficient acid catalyst. The calculated results provide theoretical guide to screen out the efficient IL catalysts for cellulose degradation.The corresponding results have been published in Scientia Sinica Chimica (2015,45,1299-1303).5. The detailed mechanisms for the conversion from glucose into 5-Hydroxymethylfurfural (HMF), and further into levulinic acid (LA) catalyzed by SO3H-functioned ILs have been investigated, the roles of anion and cation in ILs played in the reaction have been made clear and the main factors that control the reaction have been mastered.(1) Choosing 1-butyl-3-methylimidazolium chloride [C4SO3HmimCl] as a representative of SO3H-functioned ILs, this work presents a DFT study on the catalytic mechanism for conversion of glucose into 5-HMF. It is found that the conversion may proceed via two potential pathways and that throughout most of elementary steps, the cation of IL plays a substantial role, functioning as a proton shuttle to promote the reaction. The chloride anion interacts with the substrate and the acidic proton in the imidazolium ring via H-bond, as well as provides a polar environment together with the imidazolium cation to stabilize intermediates and transition states. The calculated overall barriers of the catalytic conversion along two potential pathways are 32.9 and 31.0 kcal/mol, respectively, which are compatible with the observed catalytic performance of the IL under mild conditions (100℃). The present results provide help for rationalizing the effective conversion from glucose to HMF catalyzed by SO3H-functionalized ILs and for designing IL catalysts used in biomass conversion chemistry.The corresponding results have been published in J. Phys. Chem. B (2015,119, 42,13398-13406).(2) We have performed a DFT calculation study on the conversion of 5-HMF into LA catalyzed by a representative IL, l-methyl-3-(3-sulfopropyl) imidazolium hydrogen sulfate ([C3SO3Hmim]HSO4). The pathway proposed in the previous literature has been examined, which seems not to be completely rationalized the experimental findings. Alternativley, we put forward a novel catalytic mechanism, which involves dissociation of fomic acid and then ring-opening. The overall barrier is calculated to be 35.0 kcal/mol, which is significantly lower than the barrier (49.2 kcal/mol) invovled in the pathway proposed in the previous literature. Throughout most of the elementary steps, the-SO3H group of the cation and the HSO4- anion of [C3SO3Hmim]HSO4 act as acid/base catalysts. The theoretical results have not only rationalized the experimental observations, but also presented a new catalytic mechanism for the important reaction.