Computational Studies On C-H Bond Activation Using Nonheme Lron(iv)-oxo Complexes And Transition Metal Complexes With Bidentate Directing Groups

Selective and efficient functionalization of unactivated C H bonds under mildconditions poses a great and pressing challenge for chemists. In bioinorganicchemistry, the bio-inspired catalysts based on nonheme iron oxygenases can catalyzethe oxidation of abundant substrates including alkanes, alkenes, phenols via “green”catalysis like oxygen molecules and H2O2. On the other hand, bidentate directinggroups can today be considered as an interesting alternative to mondentate system forthe development of new catalytic strategies for the functionalization of sp2and sp3C H bonds in the field of organic chemistry. On the basis of systematic DensityFunctional Theory (DFT) computations, the C H bond activation reactions mediatedby the nonheme Iron(IV) oxo complexes in bioinorganic chemistry and transitionmetal complexes with bidentate directing groups in organic chemistry aretheoretically investigated in this thesis, respectively. We have elaborated theequatorial and axial ligand effects on the C H bond activation catalyzed by thenonheme Iron(IV) oxo complexes, designed a series of nonheme Iron(IV) oxomodel complexes, focusing on comparing the relevant geometric structures, electronicproperties and reaction mechanisms, and further predicting potential catalysts basingon the energies. Moreover, with comparing the substrate Gibbs free energy (ΔG) andC H bond activation barrier (ΔG), the origins of experimentally-observed results ofselective sp2and sp3C H bond activations using transition metal (including Ni, Pd,Ru, and Cu) complexes with bidentate directing groups have been deciphered,providing beneficial referencing values for the experiments.1. Comprehensive density functional theory computations on substratehydroxylation by a range of nonheme iron(IV) oxo model systems [FeIV(O)(NH3)4L]+ (where L=CF3CO2, F, Cl, N3, NCS, NC, OH) have been performed toestablish the effects of axial ligands with different degrees of electron donor ability onthe reactivity of the distinct reaction channels. The results show that theelectron-pushing capability of the axial ligand can exert a considerable influence onthe different reaction channels, where the σ-pathway reactivity decreases as theelectron-donating ability of the axial ligand strengthens, while the π-pathwayreactivity follows an opposite trend. Moreover, an apparently antielectrophilic trendobserved for the energy gap between the triplet π-and quintet σ-channel (ΔG(T Q))stems from the fact that the reaction reactivity can be fine-controlled by the interplaybetween the exchange-stabilization benefiting from the5TSHrelative to the3TSHbymost nonheme enzymes and the destabilization effect of the σ*z2orbital by theanionic axial ligand. When the former counteracts the latter, the quintet σ-pathwaywill be more effective than the other alternatives. Nevertheless, when the dramaticdestabilization effect of the σ*z2orbital by a strong binding axial σ-donor ligand likeOH counteracts but does not override the exchange-stabilization, the barrier in thequintet σ-pathway will remain identical to the triplet π-pathway barrier. As such, thetuning of the reactivity of the different reaction channels can be realized byincreasing/decreasing the electron pushing ability.2. The effect of the whole ligand environment on the C H bond activation hasalso captured our interest. A general comparison of fundamental distinctions betweenthe FeO2+and FeS2+complexes in an identical cyanide or isocyanide ligandenvironment for methane hydroxylation has been probed computationally in this workwith a series of hypothetical [FeIV(X)(CN)5]3,[FeIV(X)(NC)5]3,(X=O, S)complexes. We have detailed an analysis of the geometric and electronic structuresusing density functional theory calculations. In addition, their σ-and π-mechanisms inC H bond activation process have been described with the aid of the schematicmolecular orbital diagram. From our theoretical results, it is shown that (a) theiron(IV) sulfido complex apparently is able to hydroxylate C H bond of methane asgood as the iron(IV) oxo species,(b) the O CN, S CN complexes have an inherentpreference for the low-spin state, while for the cases of O NC and S NC in which S =1and S=2states are relatively close in energy,(c) each of the d block electronorbital plays an important role, not just spectator electron, and (d) in comparison tothe cyanide and isocyanide ligand environment, we can see that the FeS2+speciesprefer the cyanide ligand environment, while the FeO2+species favor the isocyanideligand environment. In addition, a remarkably good correlation of the σ-/π-mechanismfor hydrogen abstraction from methane with the gap between the Fe-dz2(a) and C H(α)pair as well as the Fe-dxz/yz(β) and C H(β) pair has been found.3. Based on the experimentally achieved nonheme octahedral iron(IV) oxomodel complex [FeIV=O(TMC)(CF3CO2)]+with the TMC ligand (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) in the equatorial plane andCF3CO2in the axial plane, we have boldly replaced O2with S2and N3in[FeIV=O(TMC)(CF3CO2)]+, and a systematic comparative study on their geometries,electronic properties, and reactivities in hydrogen atom abstraction reactionsregarding the iron(IV) oxo, sulfido and nitrido counterparts has been performedusing density functional theory methods. Further, considering that theiron(IV) nitrido model complexes have been synthesized and characterized withfantastic reactivity in the latest bioinorganic report, the relative importance of theaxial ligands on the reactivity of the iron(IV) nitrido systems is probed by samplingthe reactions of CH4with [FeIV=N(TMC)(Lax)]n+,(Lax=none, CH3CN, CF3CO2, N3,Cl, NC, and SR). As we found, one hydrogen atom is abstracted from the methaneby the iron(IV) nitrido species, leading to an FeIII(N) H moiety together with acarbon radical, similar to the cases by the iron(IV) oxo and sulfido compounds.DFT calculations show that, unlike the well-known iron(IV) oxo species with the S=1ground state where two-state reactivity (TSR) was postulated to involve, theiron(IV) nitrido and sulfido complexes stabilize in a high-spin (S=2) quintetground state, and they appear to proceed on the single-state reactivity via adominantly and energetically favorable low-lying quintet spin surface in theH-abstraction reaction that enjoys the exchange-enhanced reactivity. It is furtherdemonstrated that the iron(IV) nitrido complexes are capable of hydroxylating C Hbond of methane, and potential reactivities as good as the iron(IV) oxo and sulfido species have been observed. Additionally, analysis of the axial ligand effect revealsthat the reactivity of iron(IV) nitrido oxidants in the quintet state toward C H bondactivation enhances as the electron-donating ability of the axial ligand weakens.4. Different acceptor orbital can render various reaction pathways for the C Hbond activation catalyzed by the nonheme Iron(IV) oxo complexes. The tripletδ-mechanism different from the previously reported ones, i.e., the π-channel with theunoccupied π*xz/yz(FeO) orbital and the σ-channel involving the unoccupied α-spinFe-σ*z2orbital, has been theoretically described for the methane hydroxylation by[FeIV=O(TMC)(SR)]+and its derivative [FeIV=O(TMC)(OH)]+complex for the firsttime, and we have undertaken a detailed DFT study on the nature of this state byprobing its geometry, electronic property and reactivity in comparison to all otherpossibilities. The results indicate that the electron transfer for the3δ-channel occursthrough a complex mechanism, which is initiated by the original α-spin electrontransfers from the π*orbital of the catalyst to its σ*x2y2orbital, accompanied by theα-spin electron from the σC Horbital of the substrate shifting to the just empty α-spinπ*orbital of the catalyst via the O-px/ybased π*xz/yz-orbital concomitantly. It is alsofound that the electron-donating ability of the axial ligand could affect the reactionchannels, evident by the distinction that the electron-deficient F and CF3CO2ligandsreact via the3σ-channel, whereas the electron-rich SR and OH ligands proceed bythe3δ-channel. With respect to reactivity, the3δ-pathway has a comparable barrier tothe3π and5π-pathways, which may offer a new approach for the specific control ofC H bond activation by the iron(IV) oxo species.5. The strategy using N,N-bidentate directing groups is a promising way toachieve selective C(sp2) H activation inaccessible by that of monodentate directinggroups, as reported by the organic experiment recently. Herein, we focus on exploringthe origin of the N,N-bidentate directing groups assisting transition metal catalyzedselective C(sp2) H activation reactions, and have presented a rationale behind thisstrategy through Density Functional Theory calculations by comparing the substratebinding energy and C H bond activation barrier, which deciphers its key roles inC(sp2) H activation promoted by Ni, Pd, Ru, and Cu. The calculations reveal two key points:(a) Between the two coordination sites of the N,N-bidentate directing group,the proximal one influences more the C H activation barrier ΔG, whereas the distalsite affects more the free energy change ΔG relevant to the substrate coordination.(b)Enlarging/shrinking the chelation ring can exert different effects on the reactivity,depending on the metal identity and the ring size. Importantly, our computationalresults are in full agreement with previous experimental findings concerning reactivity.Furthermore, the prediction about the unprecedented reactivity from our theory isconfirmed by our corresponding experiments, lending more credence to the rationaleand insights gained in this study.6. Further extensions of the reactivity theory to inert C(sp3) H bond activation aswell as to other types of bidentate directing groups except for2-pyridinylmethylamine and8-aminoquinoline have been performed by DFTcalculations, attempting to get a general rationale behind bidentate directinggroup-assisted transition-metal-catalyzed sp2and sp3C H bonds activation. Moreover,we have compared the chelating ability of the most versatile and powerful bidentateauxiliaries screened in sp2or sp3C H bond activations, and also supplemented theeffect of the electronegativity of the bidentate substrates on the reactivity for theC(sp2) H bond reaction. It is our belief that this further investigation can providecomprehensive and valuable details upon the application of bidentate-chelationstrategy in sp2and sp3C H bond activations catalyzed by transition metals.