Theoretical Study On Non-classical Bonding Patterns Of Alkaline Earth Metals And Mechanism Of Nickel-catalyzed Carbonyl-heck Reaction

Currently,the studies on the bonding of alkaline earth metals mainly focused on the classical patterns,including ionic bonds,the two-center two-electron（2c-2e）covalent bonds,and the coordination bonds involving their s and p orbitals.Nevertheless,the studies on the non-classical bonding patterns are relatively rare,which deters the bonding properties of alkaline earth metals and corresponding applications from being integrally comprehended.Bearing this insufficiency in mind,we propose in this dissertation to use the quantum chemistry methods to explore the non-classical bonding patterns of alkaline earth metals,including planar hexacoordinate bonding and the coordination bonds involving the d-orbitals of the alkaline earth metals.The chemical and material properties brought by these non-classical bonding will be explored simultaneously.The main contents are listed below:1.Theoretical design of planar pentacoordinate nitrogen species based on berylliumThe NBe₅H_n^n-3（n=0–5）species with a novel planar pentacoordinate nitrogen（pp N）were designed by the isoelectronic substitution of the C atom in planar pentacoordinate carbon（pp C）species CBe₅H_n^n-4（n=0–5）with an N atom.The result showed that nitrogen is more suitable than carbon to fit the Be₅H_n rings both geometrically and electronically,because all NBe₅H_n^n-3（n=0–5）species are planar,but CBe₅H₄ and CBe₅H₅⁺adopt the non-planar structures,and the HOMO-LUMO gaps of pp N species are all higher than the pp C species.Moreover,the nitrogen substitution fixed the highly flexible H atoms found in pp C species CBe₅H₂^2–and CBe₅H₃^–.The electronic structure analyses revealed that the N atoms in NBe₅H_n^n-3 were involved in three six-center two-electron（6c-2e）σbonds and one 6c-2eπbond.Therefore,these pp N species not only obey the octet rule,but also possessσandπdouble aromaticity.Consequently,NBe₅H₂^–,NBe₅H₄⁺,and NBe₅H₅²⁺are kinetically stable global energy minima,which are suitable for the gas phase generation and spectroscopic characterization.2.Stabilization of[3]CPP using the transition metal properties of alkaline earth metalsInspired by the transition metal properties of alkaline earth metals in the compound M（Bz）₃（M=Ca,Sr,Ba）,we have designed the complex[M?[3]CPP]with alkaline earth metals as the central metals and cycloparaphenylene（C₆H₄）₃（[3]CPP）as the ligand.The calculation results indicated that[Ca?[3]CPP]complex is the energy minimum with D_3hsymmetry,but D_3h[Sr?[3]CPP]and D_3h[Ba?[3]CPP]complexes are transition states.The molecular orbitals revealed that there are donation-backdonation interactions between the valence atomic orbitals of Ca and the delocalizedπorbitals of[3]CPP and d orbitals of Ca involves in chemical bonding.Remarkably,the metal center Ca shows typical bonding properties of transition metals by fullfilling the 18-electron rule.These studies also provide useful ideas for stabilizing[3]CPP.we observed that free[3]CPP is extremely unstable because the embedded high strain can make[3]CPP isomerize to[3]CPCP with the localizedπelectrons.The desired[3]CPP skeleton can be restored by a proper metal atom,including Ca,Sc⁺and Y⁺,because theπelectron delocalization can be significantly enhanced by the favorable host-guest interaction.Remarkably,the binding energy（–205.7 kcal/mol）of Sc⁺and[3]CPP is high enough to well compensate the strain energy of 170.3 kcal/mol in free[3]CPP.Simultaneously,[Sc?[3]CPP]⁺complex is kinetically stable up to 1500 K,so it would be a promising compound with[3]CPP skeleton that could be synthesized in the experiment.Although Ca is not an ideal metal to stablize[3]CPP,it is a powerful addition for transition metal properties of alkaline earth metals.3.Transition metal chemistry in alkaline earth metals complexes K₂[M（COT）₂]and M（CNT）₂（M=Ca,Sr,Ba）.The electronic structure analyses had been performed on the previously synthesized heavy alkaline earth metals（HAE）complexes K₂[Ca（COT）₂]（COT denotes the cyclooctatetraene）and Ba（CNT）₂（CNT denotes the cyclononatetraenyl）.The results disclosed that both complexes had 10occupied molecular orbitals（MOs）,in which 8 of them were related to the atomic orbitals（AOs）of central HAE metals,including four d AOs(d_xy,d_x2-y2,d_xz,and d_yz),so the HAE metals in these two complexes also exhibited the typical bonding behaviors of transition metals.Based on these results,we further designed HAE complexes K₂[M（COT）₂]and M（CNT）₂（M=Ca,Sr,Ba）.Note that the metals in these complexes are in their common stable positive oxidation state and the ligands are the anions,so the Coulomb attraction makes the complexes combined by the positive and negative ions more stable.The other part of this dissertation illustrates the mechanism of nickel-catalyzed carbonyl-Heck reaction.Palladium is the traditional catalyst in the Heck reaction,whose mechanism has been well studied,since the density functional theory has been proposed,while the studies on the Heck reaction mechanism catalyzed by nickel is rare.What is the similarity and the difference between the Pd-and Ni-catalyzed Heck reaction?To provide more information for answering this question,we carried out a detailed DFT mechanistic study on the first Ni-catalyzed direct carbonyl-Heck coupling of aryl triflates Ph OTf and aldehydes to afford ketones.The precatalyst Ni（COD）₂ is activated with the phosphine ligand,the activation was followed by the coordination of the substrate Ph OTf for intramolecular oxidative addition.The triflate in the ensuing complex was substituted by benzaldehyde through an interchange mechanism,leaving the triflate anion in the second coordination sphere held by Coulomb attraction.Then the Ni（II）complex cation undergoes benzaldehyde C=O insertion into the Ni-Ph bond,and the followedβ-hydride elimination produces the Ni（II）-bound benzophenone,which is released by interchange with triflate.The resulting neutral Ni（II）hydride complex leads to regeneration of the active catalyst following base-mediated deprotonation/reduction.The benzaldehyde C=O insertion is the rate-determining step.The triflate anion,while remaining in the second sphere,engages in electrostatic interactions with the first sphere,thereby stabilizing the intermediate/transition state and enabling the desired reactivity.The new insights gained from this study can help better understand and improve Heck-type reactions.