Reaction Mechanism Studies And Molecular Design Based On The Acid-Base Properties Of Materials

In this work, by using the quantum chemical methods, we theoretically performreaction mechanism stuedies and molecular design based on the acid-base properties ofmaterials. The main results of our scientific research are as follows:1. Very recently, zeolites have been used to effectively catalyze the hydration/amination ofethylene oxide. In this paper, we for the first time investigate the reaction mechanism ofthe ring-opening of ethylene oxide with ammonia catalyzed by Br nsted acid over Hβzeolite using theoretical calculation. Two stepwise reaction pathways and one concertedreaction pathways are disclosed. Based on the calculated intrinsic energy barriers andapparent energy barriers, among the stepwise pathways, the first adsorption of the ethyleneoxide leads to the Br nsted proton-initiated ring-opening, which is much morecompetitive than the first adsorption of the ammonia that results in the ammoniumproton-catalyzed ring-opening. For the concerted pathway, the ring-opening of ethyleneoxide occurs simultaneously with the addition of the ammonia, which is also initiated bythe ammonium proton. Thus, the stepwise pathway initiated by Br nsted proton isenergetically much more favored than the concerted one initiated by ammonium proton.Clearly, upon the zeolite catalysis, the ethylene oxide amination is mechanistically quitedifferent from the previously reported ethylene oxide hydration on H-ZSM-5, whichproceeds via a concerted reaction pathway. The dramatic discrepancy between theamination and hydration processes can be understood by the significantly variedprotonation ability between water and ammonia over ethylene oxide.2. Lewis acids of Hβ zeolite have been proposed to exhibit high activity in catalyticreactions. Yet, the catalytic role of these acids in the reactions is rather unclear. In thispaper, at the B3LYP/6-31G(d,p) level, we for the first time investigate the catalyticreactivity of various Lewis acid sites within Hβ zeolite (i.e., facile lattice Al, AlO+,Al(OH)2+and Al(OH)3EFAL sites) as well as the Br nsted acid site towards thering-opening reaction of ethylene oxide. The calculated apparent energy barriers for thering-opening of ethylene oxide at the at the Br nsted acid site, facile lattice Al site, AlO+ EFAL site, Al(OH)2+EFAL site and Al(OH)3EFAL site are14.5,-7.2,-15.9,19.2, and20.2kcal/mol (the intrinsic energy barriers are34.8,31.7,16.0,33.4and27.4kcal/mol),respectively. The facile lattice Al site and the AlO+EFAL site, in which the active centerAl is threefold coordinated, are more favorable than the Al(OH)2+EFAL site and theAl(OH)3EFAL site, in which the active center Al is fourfold coordinated, and theBr nsted acid site. Although the AlO+EFAL site has the highest catalytic reactivity, theresulted product may be too stable to undergo further reaction. Thus, we suggest that thefacile lattice Al site could be the most effective acid site for the ring-opening of ethyleneoxide in Hβ zeolite.3. The researches on the basic or acid-base bifunctional materials are becoming the hottopic due to the need of solid catalysts to replace liquid basic catalysts for efficient,noncorrosive and reproducible industrial applications. A new strategy for the aim of easilyprepare the–NH₂specials in zeolite is reported by Regli and co-workes using theboron-substituted B-SSZ-13zeolite interacted by NH3. In this work, the basicity ofboron-substituted zeolite interacted by NH3is studied by theoretical calculations usingDFT method at the B3LYP/6-311++G(d,p) level. The following basic groups areconsidered: Cage Si-O-Si, Cage Si-OH, Cage Si-NH-Si, Cage Si-NH₂,Cage B-NH₂(without Si-OH),Cage (B-NH₂+OH-Si),Cage B-OH (without Si-NH₂), Cage (B-OH+NH₂-Si).We also calculate the adsorption of CH3+on Cage (B-NH₂+OH-Si) and Cage(B-OH+NH₂-Si). We find that the B-NH₂group itself does not have high basicity.However, when the proton or Lewis acid is adsorbed to it, the coexisting Si-OH groupcontributes to the basicity by forming a hydrogen bond with B-NH₂. Besides, the Si-NH₂group and the coexisting B-OH group cooperatively have rather high basicity. Thus, theboron-substituted zeolite interacted by NH3, which contains the B-NH₂together withB-OH groups and the Si-NH₂together with B-OH groups, could be a kind of promisingbasic materials.4. The chemistry of the low-valent Group13elements (E=B, Al, Ga, In, Tl) has formedthe recent hot topic. Recently, a series of low-valent Group13-based compounds havebeen synthesized, i.e.,[E-Cp*-E]+cations, which have been termed as the interesting"inverse sandwich" complexes. To enrich the family of inverse sandwiches, we report ourtheoretical design of a new type of inverse sandwiches E-C4H4-E (E=Al, Ga, In, Tl) forstabilizing the low-valent Group13elements. The calculated dissociation energies indicatethat unlike [E-Cp-E]+that dissociates via loss of the charged atom E+, E-C4H4-E dissociates via loss of the neutral atom E with the bond strengths of Al> Ga> In> Tl.Moreover, E-C₄H₄-E are more stable in dissociation than [E-Cp-E]+cations. By comparingwith other various isomers, we found that the inverted E-C₄H₄-E should be kineticallyquite stable with the least conversion barriers of33.5,33.5,35.2and36.9kcal/mol forE=Al, Ga, In and Tl, respectively. Furthermore, replacement of cyclobutadiene-H atomsby the highly electron-positive groups such as SiH3and Si(CH₃)₃could significantlystabilize the inverted form in thermodynamics. Possible synthetic routes are proposed forE-C4H4-E. With no need of counterions, the newly-designed neutral complexes E-C4H4-Ewelcome future synthesis.5. As the electronic feather the η⁵-pentadienyl anion (η⁵-C-5H7) is analogous to that ofcyclopentadienyl anion (η⁵-C₅H₅), whether pentadienylboron is a kind of borylene likeCpB becomes interesting. In this paper, the potential energy surface of pentadienylboron(C₅H₇B) as well as the conformers of [(OC)₄Fe-BC₅H₇] are calculated using densityfunctional theory (DFT). C₅H₇B adopts14conformers that can be divided into two kinds:Lewis acidic conformers with high valent boron (+3), and Lewis basic conformers withlow valent boron (+1)(borylene). We find that the most stable isomers are the cyclicLewis acidic conformer3and conformer4, meanwhile, the cyclic Lewis basic conformer6that has high relative energy is stable in dynamics. The chain-shaped conformers, nomatter they are Lewis acid or Lewis base, are all instable and can easily interconvert intoeach others due to the small energy barriers, and furthermore are easy to form the cyclicLewis acidic conformer3. Thus, the most stable conformer of free C₅H₇B is the cyclicLewis acidic conformer3. However, we find in the lowest energy conformer of[(OC)₄Fe-BC₅H₇], the C₅H₇B acts as Lewis base (borylene), of which the structure iscorresponding to the cyclic conformer6of free C₅H₇B. By studying the conformers ofC₅H₇BCl2as precursor for the synthesis of [(OC)₄Fe-BC₅H₇], we predict the stable C₅H₇Bborylene in [(OC)₄Fe-BC₅H₇] would be attainable in experiments.6. Since the pentadienyl anion is open system containing6Ï€ delocalized electrons, itscyclic reaction according to the Woodward-Hoffman rules becomes of interest. As far aswe know, the electrocyclization of acyclic and meanwhile all carbon pentadienyl anion isnot attainable yet. Aiming at the electrocyclization of acyclic and all carbon pentadienylanion, in this work, a systematic density functional study on the electrocyclization ofC₅H₅R₂(R=H, CH₃, NH₂, OH, F, SiH₃, PH₂, SH, Cl) with substituent groups R introducedat C2and C4positions, is performed. To predict whether the electrocyclization of the substituted pentadienyl anion occurs in practical application, the electrocyclization ofC-nH_2n-5(n=6,7,8,9) is investigated as well. We find that:(1) The energy barrier for thecyclization of U-C₅H₇^-is relative high. Meanwhile, the product O-C5H-7is less stable thanU-C-5H7in energy.(2) The bulky substitution groups R (SiH₃, PH₂, SH) with lowelectronegatives greatly reduce energy barrier for the cyclization of the U-C5H5R2ˉanions,and better help stabilize O-C5H5R-2in energy.(3) The energy barriers for the cyclization ofU-C₅H₅R₂(SiH3, PH₂, SH) are close to or lower than those of R-CnH-2n-5(n=8,9). Thus,the synthesis of O-C-5H₅R₂(SiH₃, PH₂, SH) from U-C-5H5R2anions would be allowedthermodynamically and kinetically in experiments.7. Early in1990s, Kirss and his coworks produced (η⁵-2,4-Me₂C₅H₃)₂M (Me=methyl;M=Ru, Os) from (η⁵-2,4-Me₂C₅H₃)₂M. This is the only example of producing metallocenefrom open metallocene. However, since then there is no further investigation on thereaction mechanism of the cyclization of open metallocene. In this article, we investigatethe reaction mechanism of producing (η⁵-C₅H₅)₂Ru from (η⁵-C₅H₇)₂Ru by theoreticalcalculations using DFT (Density Functional Theory) method at theM06-2X/def2-TZVPP//B3LYP/def2-TZVPP+ZPE level. The reaction mechanism is that:Firstly, a η⁵-C₅H₇of (η⁵-C₅H₇)2Ru undergoes the cyclization by the carbon-carbon bondformation and produces (η3-C₅H₇)Ru(η⁵-C₅H₇), then, after the two steps of hydrogenmigration, dehydrogenation occurs and produces (η⁵-C5H5)Ru(η⁵-C₅H₇). Subsequently, theother η⁵-C₅H₇repeats the cyclization, hydrogen migration and dehydrogenation. Finally,the (η⁵-C5H5)2Ru is obtained.