Theoretical Design Of Chalcogen-stabilized Clusters With A Planar Hypercoordinate Center

Planar hypercoordination is among the most attractive nonclassical chemical bonding patterns in chemistry.Since Hoffmann sponsored a project over 50 years ago to stabilize the planar tetracoordinate carbon systems,a variety of clusters and/or molecules with planar hypercoordinate center（s）have been designed computationally.Electronic structure analyses indicate that the existence of delocalized multicenter?and/or?bonds plays the crucial role in maintaining the planar hypercoordinate configurations.In order to form such delocalized multicenter bonds,the electron-deficient elements,such as B,Al,and Be,are often employed as the key ligand atoms so that the planar hypercoordinate structures can be achieved.However,these electron-deficient elements are commonly exposed in the clusters,thus leading to high chemical reactivity of these computationally predicted clusters.This situation hinders their synthesis in the condensed phase and blocks the studies on their applications.In our initial computational studies in this dissertation on the stepwise oxidation of a milestone CAl₅⁺cluster,we find that O atoms can compensate for the electron-deficient Al atoms and provide steric protection to the CAl₅⁺core.Therefore,if CAl₅⁺is effectively surrounded by O atoms,the resultant CAl₅O₅⁺cluster possesses much higher chemical stability than the original CAl₅⁺cluster.Inspired by such interesting findings,this dissertation shall employ the state-of-the-art quantum chemistry methods to rationally design a series of clusters with a planar hypercoordinate center.Our primary strategy is to introduce the necessary chalcogens to the outer shell of these clusters.These theoretical studies aim to provide the potential candidate clusters through computational design,which may serve as promising targets for future experimental syntheses in the condensed phase,or in the gas phase.The main contents of the dissertation works are as follows:1.Oxygen-Stabilized Clusters with a Planar Pentacoordinate CarbonComputational design plays an important role in planar hypercoordinate carbon（ph C）chemistry.However,in numerous computationally predicted ph C species,none have been subsequently synthesized in the condensed phase.Oxidation and high reactivity around the periphery of the ph C clusters should be one of the key factors that lead to such a frustrated issue.In the present work,we shall study the influence of stepwise oxidation on the structures,stability,and physicochemical properties of the ph C species using a milestone planar pentacoordinate carbon（pp C）cluster CAl₅⁺as an example.Our theoretical results suggest that the pp C structure of CAl₅⁺would be destroyed directly in the presence of one,two,or six O atom（s）,and indirectly for the cases of three or four O atoms.In contrast,the pp C CAl₅⁺motif is maintained in the case of five O atoms,because the pp C structure of CAl₅O₅⁺cluster is kinetically stable as the global minimum（GM）of the system,possessingσandπdouble aromaticity.The first to fifth vertical oxygen affinities（VOAs）for CAl₅⁺are very high（–85.5 to–116.3 kcal/mol）,which are probably due to the existence of diffused peripheral Al–Al bond（s）.However,the sixth VOA is reduced significantly,down to–50.2 kcal/mol.The latter observation is consistent with the absence of diffused Al–Al bond in the CAl₅O₅⁺species,so that it becomes insensitive to oxidation.The pp C species,D_5hCAl₅O₅⁺,is likely to survive under the delicate control of oxidation level（five O atoms per molecule）.2.Sulfur-Stabilized Boron-Based Clusters with Planar Pentacoordinate CarbonAlthough boron is widely employed in designing nonclassical chemical species with planar hypercoordinate carbon（ph C）,it prefers multicenter bonding more than carbon.As a consequence,almost all boron-containing ph C species are less stable in energy than their corresponding isomers with a boron at the central position.In this dissertation,the GM structure of CB₅S₅⁺cluster is reported,which possesses a planar pentacoordinate carbon（pp C）inside a boron wheel and features doubleπandσaromaticity.The carbon atom is shown to stay favorably inside the B₅ring,because the electron deficiency of B atoms is significantly weakened by dative S→Bπbonding.Its isomer with a planar pentacoordinate boron is located 61.6 kcal/mol higher in energy,which can be attributed to the loss ofσaromaticity.The CB₅S₅⁺cluster is also dynamically viable.As the GM structure,cluster CB₅S₅⁺has a wide HOMO–LUMO gap（7.47 e V）,a low vertical electron affinity（4.31 e V）,as well as a high vertical detachment energy（13.22 e V）,hinting that it may be a promising target for experimental realization.3.Chalcogen-Stabilized Boron-Based Clusters with a Planar Tetracoordinate CarbonAccording to the initial motivation of chemists to design planar hypercoordinate carbon（ph C）structures,they prefer to use boron as key ligand atom（s）around the carbon center,because the C–B bond is more covalent than that between carbon and amphoteric metals.Nevertheless,the electron-deficiency of boron is stronger than amphoteric metals,like beryllium and aluminum.Consequently,boron generally prefers the planar hypercoordinate center positions as compared to carbon.This fact leads to the absence of thermodynamically stable boron-based clusters with a ph C,especially those with a planar tetracoordinate carbon（pt C）.In this dissertation,by taking a CB₄unit as the parent structure,we computationally design two boron-based clusters with a pt C,namely CB₄Se₅and CB₄S₆,by the introduction of bridging chalcogen atoms（Xs）,S or Se,at the B–B edge.The two pt C clusters possessσandπdouble aromaticity.Electronic structure analysis shows that the peripheral Se or S atoms（X）can weaken the electron deficiency of ligand boron atoms through strong X→B dativeπbonds,so that carbon can better occupy the central position of the ring formed by B atoms.As the dynamically stable GM structures,CB₄Se₅and CB₄S₆clusters are expected to be synthesized in the gas phase and characterized using spectroscopic techniques in the future.4.Chalcogen-Stabilized Clusters with a Planar Tetracoordinate NitrogenThe design of clusters featuring nonclassical planar hypercoordinate atoms（ph As）often depends on the delocalized multicenter bonds associated to reactive electron-deficient elements.This situation both destabilizes the clusters and leads to difficulty in achieving the ph A arrangement for electronegative elements,like nitrogen,due to their preference for localized bonds.In this dissertation,we computationally design a series of aluminum chalcogenide NAl₄X₄⁺（X=S,Se,Te）clusters,by elaborating a chemically unstable NAl₄⁺cluster with planar tricoordinate nitrogen using chalcogen.These predicted clusters have the desired planar tetracoordinate nitrogen center,and their chemical stability is meaningfully improved.The latter is evidenced by their wide HOMO–LUMO gaps（6.51–7.23 e V）,high molecular rigidity（dynamically stable up to1500 K）,and exclusively low energy as the GM structures.The alternative isomeric structures are located at least 51.2 kcal/mol higher above the GM structures.Remarkably,these clusters are stabilized by peripheral chalcogens atoms,which not only sterically protect the NAl₄core moiety,but also electronically compensate for the electron-deficient aluminum atoms via X→Al dativeπbonds.The NAl₄X₄⁺（X=S,Se,Te）clusters meet the description of our recently proposed“electron-compensation”strategy.5.Beryllium Oxide Cluster with a Planar Pentacoordinate OxygenAs mentioned above,planar hypercoordination is hard to achieve for elements with high electronegativity.Based on the successful design of chalcogen-stabilized clusters with a pt N,we further explore the computational design of a beryllium oxide cluster O₆Be₅^2–with a planar pentacoordinate oxygen（pp O）.The pp O cluster benefits from the stabilization effect provided by chalcogens,as well as the high beryllium-oxygen affinity.Detailed electronic structure analysis indicates that the central pp O is a dianion in the zeroth-order approximation.In the interaction with five peripheral Be atoms,such a pp O serves as the electron donor to form four delocalized six-center two electron（6c-2e）dative bonds,including threeσbonds and oneπbond.They not only contribute to maintain the planar structure,but also render the 6σ+2πdouble aromaticity,which further stabilize the pp O structure.As a dynamically stable GM structure of the system,the pp O O₆Be₅^2–cluster is promising for gas phase generation and characterization in experiments,probably as a salt complex due to its dianion nature.