Theoretical Study Of Transition-metal Iron-Catalyzed Carbon-Hydrogen/halogen Bonds Activation Reactions

Since, C–H bond activation is a research field of people’s attention. The catalytic activationof the C-halogen bond is an efficient tool for selectively converting simple educts, via C-C bondformation, into more complex compounds. How to achieve efficient, C–H/X bond activation isan important challenge in the field of chemistry, with low or non-toxic, environmentally friendlynature of the cheap iron as catalyst, as researchers preferred. However, relative to the Pt, Pd andother precious metals, theoretical research on iron catalyzed by C–H activation reaction, is stillquite scarce.This dissertation makes systematic theoretical study on the reaction mechanism ofiron catalyzed by C–H/X bond activation based on the quantum chemistry. It providestheoretical clues for future design and activity of C–H/X bond activation of iron-based catayst.This dissertation is composed of dynamic studies on:1) Alkane C-H bond activation by various catalysts and enzymes has attracted considerableattention recently and but many issues are still unanswered. The conversion of ethane toethanol and ethene by bare [FeIII=O]+has been explored using density functional theory andcoupled-cluster method comprehensively. Two possible reaction mechanisms are availablefor the entire reaction, the direct H abstract mechanism and the concerted mechanism. First,in the direct H abstract mechanism, a direct H abstraction is encountered in the initial step,going through a collinear transition state C H O-Fe and then leads to the generation of anintermediate Fe–OH bounded to alkyl radical weakly. The final product of the direct Habstract mechanism is ethanol which is produced by the hydroxyl group back transfer to thecarbon radical. Second, in the concerted reaction mechanism, the H abstraction process ischaracterized via overcoming a four/five-centered transition states6/4TSH_c5or4TSH_c4.The second step of the concerted mechanism can lead either to product ethanol or ethene.Moreover, the major product ethene can be obtained through two different pathways, the onestep pathway and the stepwise pathway. It is the first report that the former pathway starting from^6/4IM_c to product can be better described as a proton coupled electron transfer （PCET）.It plays important role in the product ethene generation according to the CCSD（T） results.The spin-orbital coupling （SOC） calculations demonstrate that the title reaction shouldproceed via two-state reactivity （TSR） pattern that the spin-forbidden transition couldslightly lower the rate-determining energy barrier height. This thorough theoretical study,especially the explicit electronic structure analysis, may provide important clues forunderstanding and studying the C–H bond activation promoted by iron-based artificialcatalysts.2) We study the iron（IV）–oxo catalyzed methane C H activation reactions for complexes inwhich the FeIV=O core is surrounded by five negatively charged ligands. We analyze thereaction pathways in the simple [FeIV（O）（CN）5]3–model and then gradually substitute theCN–ligands with NC–or F–ligands. We study whether these strongly negatively chargedcompounds have a similar reactivity as previously studied models:1) Can the same fourreactions pathways be found for these reactions?2) Do the transition states of the σ and πmechanisms have the same structures?3) How do the equatorial ligands modulate thereactivity of these iron（IV）-oxo species? The calculations have allowed us to gain atomisticinsight into catalytic reactivity of these enzymes and shed light on how new reagents maymodify C H bonds with high efficiency and specificity.3) The reactions of Fe²⁺with CH₃X （X=H, Cl） have been studied by density functional theorymethod detailedly. The results demonstrated that the H abstraction in eq.4can proceed viathe lowest activation barrier （Ga=0.23kcal/mol） in all feasible pathways. However, themechanisms of oxidative insertion and the SN2substitution are not competitive because ofthermodynamic factors. The electronic structure analysis suggests that the overlap betweenmetal3d orbital and substrate σ*C–Xresults in the preference of Fe²⁺front side attack on theC–X bond. This study is expected to shed light on the nature of the title reactions andprovide theoretical clues and foundation for future research.4) The C–X bond functionalization catalyzed by iron and their complexes has attracted muchattention. Using density functional theory （DFT）, we herein studied the reactivity andmechanism of iron cation （Fe⁺） towards C–X bond cleavage of CH₃X （X=Cl, Br, I） atB3LYP/def2-SVP level of theory. The results show that there are two possible pathways available for the title reactions, i.e. the insertion mechanism and the SN2mechanism,respectively. Mechanistically, in the insertion mechanism, the reactions stem from Fe⁺attacking on the side of CH₃X and results in the generation the products FeX+and CH₃;whereas in the SN2mechanism, the Fe⁺initially attacks the substrate from the back of C–Xyielding FeCH₃+and X. The results show that the sextet and the quartet states of Fe⁺demonstrate quite distinct reactivity towards the cleavage of C–X bonds in the mostpotential pathways, and the quartet pathways are dominant in all the pathways. The relativehigher barriers in the SN2pathways results in their lower competitiveness in the titlereactions. In addition, our results show that, for all the three reaction systems, the insertionmechanism is exothermic; whereas for the SN2mechanism, only X=I is exothermic, however.Furthermore, the calculations also show that these reactions demonstrate two-state reactivity（TSR） scenario. There are minimum energy crossing points （MECPs） between the sextet andquartet state on potential energy surfaces （PESs） both at the entrance and export sides for thetwo reaction mechanisms. On the other hand, the electron transfer evolution analysisindicates that the spin polarization plays important role in the stabilization of the potentialenergy surfaces and as a result, it controls the pathways by which it takes place and thebranch ratio of the major products and byproducts. This thorough theoretical study,especially the detailed electronic structure analysis, may provide important clues forunderstanding the C X bond activation and theoretical fundamental evidences foriron-based catalysts design.