Theoretical Studys On The Catalytic Activation Of H₂ And CO₂ By Boron Containing Compounds

The catalytic activation of hydrogen and carbon dioxide is one of the most important research themes in in modem chemistry,and significant for tackling the serious problems of energy,environment and climate change.The organoboron compounds and their metal derivatives show unique bonding and chemical reactivity,and have been widely used in the field of organic chemistry and pharmaceutical chemistry.In the thesis,a systematic theoretical research has been performed on the catalytic activation of H2 and CO2 by boron-containing compounds,which includes the following contents:1.The hydroboration has emerged as a promising strategy for the CO2 fixation,and attracted considerable attention in recent years.We carried out a theoretical study on the mechanism of CO2 hydroboration using a model complex[Ru(CO)H(L)(PMe3)2](Al),and discovered the favorable hydride-shuttle mechanism.The reaction consists of four stages:(1)CO2 insertion into the C-B bond of A1 to form A4,(2)the reduction of A4 by HBpin to afford HCOOBpin(P2),(3)the reduction of P2 by HBpin to HCHO(P5),and(4)the reduction of P5 to CH3OBpin(P6).The boryl ester A4,featuring both Lewis acidic boron atom and Lewis basic carbonyl oxygen atom,is the catalytic species in reaction cycles,and able to form Lewis adducts with HBpin and carbonyl-containing compounds.Oftentimes,the hydride and Bpin moieties can transfer within the Lewis adducts in a concerted manner in our proposed hydride-shuttle mechanism.Lewis adducts play a key role in forming more relaxed polycyclic transition states and making the hydride more hydridic.Therefore,the energy barriers for the hydride transfer in all stages are very low(15.7-22.6 kcal/mol).In contrast,the direct hydroboration mechanism proposed in the literature have extremely high energy barriers because of the highly strained four-membered rings in the transition states and the unactivated hydride in the-(H)Bpin rmoiety.2.The boron compounds are critical for the development of frustrated Lewis pair(FLP)chemistry.A series of intramolecular frustrated B/N-Lewis pairs are designed with the cyclooctatetraene(COT)as skelecton.The origin of stability and catalytic activity of FLP is studied.It is discovered that the increase of the Lewis acidity of boron atom or the Lewis basicity of nitrogen atom will result in the higher catalytic activity of FLP in the hydrogen pyrolysis,but the lower stability of FLP which will be more inclined to form Lewis adducts.In addition,the introduction of specific substituents into the ortho-position of-NR2 or-BR2 moiety is found to be able to stabilize the generated zwitterion,with an increase of the catalytic activity of FLP.This also reflects the advantage of COT skelecton.Through the clamping strategy,we design the molecule A9 in which the COT skelecton is fused with a pyridine and a fluorinated borane-containing five-member ring.The computational results suggested the energy barrier of hydrogen activation by A9 is 15 kcal/mol,and the Lewis adducts B9 is 15 kcal/mol higher than A9,which suggests A9 has good stability and catalytic activity.Besides,we take full advantages of COT skelecton and design the molecule A10 featuring a higher capability for activating hydrogen.3.Two-coordinate boron cation(borinium)compounds,characterized by exceptional Lewis acidity and coordinative unsaturation,have shown particular catalytic performance.Through the coupled cluster CCSD(T)method,we investigated the structure of borinium[(C2H3)2B+]with the dominant conformation occuring at a nearly 90° τ(C1-C2-C3-C4)dihedral angle,while the cis and trans planar geometries proved to be transition states(the torsional energy barriers are in 2.8～2.3 kcal/mol).The heavier boron group congeners(C2H3)2M+(Al,Ga,In,and Tl)have generally analogous dominant conformation and transition states,but even lower torsional energy barriers(0.3～0.7 kcal/mol).With the natural bond orbital(NBO)analysis,electronic(hyperconjugative)and steric effects are proved to be the origins of the structural and torsional properties for elementary vinylboranes and cationic divinylboriniums.The unique reaction of(C2H3)2B+ with CO2 are predicted.The activation of CO2 by(C2H3)2B+ will generate the stable product 4 with being exothermic by 23.9 kcal/mol.The insertion of CO2 into the B-C bond is the rate-determining step.The decomposition of 4 into C2H3C=O+and C2H3B＝0 will undergo an increase of energy by 12.4 kcal/mol.4.Developing homogeneous catalysts based on common metals has been the research hotspot and a great challenge.A significant breakthrough was succeeded by Peters et al.with developing a family of metal catalysts supported by borane ligands.A DFT study on the hydrogen activation and alkene hydrogenation catalyzed by iron metallaboratrane complex has been performed in order to unveil the characteristic features of these iron metallaboratrane complexes and the role of the borane ligand.A detailed spin state analysis revealed there exists two minimum energy crossing points in the hydrogenolysis of(TPB)Fe(N2)1triplet to provide(TPB)(μ-H)Fe(H)3triplet.In the catalytic cycle of alkene hydrogenation,3triplet plays a role of the active species,and the triplet-dissociative pathway is the most favorable,which is in accordance with the experiment.The dissociation of phosphine atom of TPB ligand from Fe center in 3triplet occurs easily,because the anti-bonding dca is singly occupied.However,non-dissociative pathway is not likely to occur due to a very high energy barrier.The product is formed via theσ-bond metathesis,rather than the direct reductive elimination between bridging hydride(μ-H)and styryl ligand,because the Fe-(μ-H)-B bridging bond is very stable.