Charged Molecular Alloy Clusters And All-Metal Sandwich Complexes: Structures, Chemical Bonding, And Aromaticity

Charged molecular alloys and all-metal sandwich compounds are among the research frontiers in synthetic chemistry,whereas recent remarkable developments on nanocluster science include the new concepts of superatom and all-metal aromaticity.In the past 20 years,a wide variety of endohedral Zintl ions,cage compounds,and intermetalloid clusters have been accumulated in the literature.However,effort to elucidate such novel synthetic compounds and understand their nature of chemical bonding using the state-of-the-art quantum chemical tools has been limited.Of particular interest is whether the synthetic crystal compounds can be described using the new concepts from cluster systems?such as superatom,multifold p/s aromaticity,all-metal aromaticity,and even d-orbital aromaticity?.Exploration along this direction should offer key insight into these synthetic compounds,as well as help reinforce the concepts of superatom,all-metal aromaticity,and d-orbital aromaticity in chemistry,which are anticipated to be of fundamental scientific importance.In this thesis,we have mined the vast literature of inorganic synthesis and selected a number of crystal compounds for pursuit of novel chemical bonding using quantum chemical calculations.The computations are performed primarily at the density-functional theory level.Specifically,we choose to investigate the structures and chemical bonding of a series of charged molecular alloy clusters: [P₇ZnP₇]^4-,[Pd₂As₁₄]^4-,[Ge₉Au₃Ge₉]^5-,and [Pd₃Sn₈Bi₆]^4-.On the basis of the computational data,we propose for the first time to introduce bonding concepts of superatoms,“fragmental aromaticity”,all-metal aromaticity,multifold p/s aromaticity,and especially d-orbital aromaticity to elucidate the structures,stability,and bonding of these synthetic compounds.The aforementioned new aromatic systems go beyond the traditional aromaticity,which should help appease debates regarding the nature of aromaticity.We have examined metal-metal interactions,aurophilicity,and effective charge states of metal centers in the compounds.As a second thematic topic in the thesis,we explore the chemical bonding in all-metal aromatic sandwich complex [Sb₃Au₃Sb₃]^3-.Our analyses offer quantitative understanding how electron donation and back-donation redistribute the charges within the system.We also shed light on the bonding nature of interlayer Sb–Au–Sb edges,which is key to stabilizing an all-metal sandwich.Our computational data lead to a thorough and in-depth understanding of the all-metal aromatic sandwich.Lastly,using [Al[Al?NH₂?]₆]^-cluster as a model system,we introduce for the first time a bonding concept of ligand-stabilized all-main-group-metal aromatic sandwich complex,which is new in the chemistry of sandwich compounds.The main results of the thesis work are briefly described as follow:1.Superatom-Like Charged Molecular Alloy [P₇ZnP₇]^4-Cluster.We present a density-functional theory study on the structures,electronic properties and chemical bonding of a synthetic dumbbell-shaped charged molecular alloy cluster C₂ [P₇ZnP₇]^4-and its relevant C3 v [P₇]^3-Zintl ion.For the cage-like [P₇]^3-ion,three isolated bridging P sites carry the majority of negative charges,which are largely P 3p2 lone-pairs in nature,but also participate in secondary P–P ? bonding along bridging sites.Chemical bonding analyses show that [P₇ZnP₇]^4-cluster can be approximately formulated as [P₇]^2-[Zn]⁰[P7]^2-,in which [P₇]^2-ligands inherit the structural and bonding of [P₇]^3-despite their difference in charge state.Two [P₇]^2-ligands collectively bind with Zn center via four bridging P sites,forming a quasi-tetrahedral Zn P₄ core with the magic eight-electron counting.The cluster can alternatively be considered as a superatom,which concept underlies its stability.The Zn–P bonding in the Zn P4 core involves both P 3p nonbonding and two-center Zn–P bonding components,collectively generating four two-center two-electron?2c-2e?Zn–P ? “single” bonds,albeit with an effective bond order of around 0.5.The current finding provides an example of using the superatom concept to interpret the stability of charged molecular alloy cluster.2.Charged Molecular Alloy [Pd₂As₁₄]^4-Cluster and Relevant [Au₂Sb₁₄]^4-Model Cluster: ?-Aromaticity in Synthetic Crystal Compounds.Based on quantum chemical calculations,we report that the rod-like charged molecular alloy cluster D2 h [Pd₂As₁₄]^4-and model cluster D_2h [Au₂Sb₁₄]^4-,the former being known as a crystal compound,can be viewed as Pd₂ or Au₂ centers inlaid on the surface of As₁₄ or Sb₁₄ polyanion framework.Four neighboring As or Sb atoms are interconnected via each Pd or Au center,thus featuring distorted square-planar Pd As₄ or Au Sb₄ fragments.No covalent Pd–Pd or Au–Au bond is present in the systems.Chemical bonding analyses show that [Pd₂As₁₄]^4-possesses two globally delocalized ? electrons,whereas [Au₂Sb₁₄]^4-has two ? sextets?each associated with an Au Sb₄ unit?.The electron counting conforms to the?4n + 2?Hückel rule,with n = 0 and 1 respectively,rendering these clusters ? aromaticity.The discovery of 6? aromaticity is especially important since the sextet of delocalized electrons is at the heart of the aromaticity concept.We believe ?-aromaticity is governing the structures and stability of the two clusters.3.Sandwich-Shaped Charged Molecular Alloy Cluster [Ge₉Au₃Ge₉]^5-with d-Orbital Aromaticity.The aforementioned charged molecular alloy clusters take homoatomic clusters of Group 15 element as ligands.Based on first-principles calculations,we present herein the structural,electronic,and bonding properties of Cs [Ge₉Au₃Ge₉]^5-charged molecular alloy,in which the ligands involve the Group 14 element.The compound consists of two polyhedral Ge₉ cages interconnecting an Au₃ triangle,which results in a prismatic Ge₃Au₃Ge₃ sandwich core.Computational data show that [Ge₉Au₃Ge₉]^5-cluster can be formulated as [Ge9]2.5-[Au₃]0[Ge₉]^2.5-,in which [Ge₉]^2.5-ligands inherit chemical integrity of C_4v and D_3h [Ge₉]^4-Zintl ions,respectively.We adopt Wade's rules to elucidate the skeletal bonding in naked [Ge₉]^4-Zintl ions and in sandwich [Ge₉Au₃Ge₉]^5-cluster.The edge Ge–Au–Ge bonding involves an Au 6s/5d and Ge 4p mixed bond,as well as secondary bonding via six interfacial Ge₄s₂ and three Au 5d “lone-pairs”.Canonical molecular orbitals?CMOs?and adaptive natural density partitioning?Ad NDP?analyses reveal a delocalized ? bond over the Au₃ ring,which is composed of virtually pure Au 5d atomic orbitals?AOs?,thus rendering d-orbital aromaticity for this synthetic crystal compound.The finding offers a compelling and clear-cut case for d-orbital aromaticity in a bottleable compound,which should help solidate the aromaticity concept in all-metal and transition-metal systems.4.UFO-Like Charged Molecular Alloy Cluster [Pd₃Sn₈Bi₆]^4-: Multifold ?/? Aromaticity and d-Orbital Aromaticity.Here we present a density-functional theory study on the structural and electronic properties and chemical bonding of a synthetic ternary charged molecular alloy cluster,D_3h [Pd₃Sn₈Bi₆]^4-.The cluster features a core-shell shape with a Pd₃ core being embedded inside a Sn₈Bi₆ shell,which is sort of like an unidentified flying object?UFO?.Detailed CMO and Ad NDP analyses reveal that the cluster possesses five-fold p/s aromaticity collectively,which are derived from trigonal pyramid Sn₄ caps,peripheral Bi₆ ring,Pd₃ core?with d-orbital ? aromaticity?,and roof-like Sn₂Bi₂ walls.Five-fold aromaticity is believed to hold the key to the unique core-shell cluster.Note that the surface fragments of the cluster are rather flat.They support multicenter p/s bonding,whose electron-counting conforms to the?4n + 2?Hückel rule,with n = 0.The current finding indicates that synthetic all-metal cluster compound can support multifold?? and ??aromaticity,including d-orbital aromaticity,which further extends the all-metal aromaticity field.5.All-Metal Aromatic Sandwich Complex [Sb₃Au₃Sb₃]^3-.We report a density-functional theory study on the structure,stability,and chemical bonding of an all-metal aromatic sandwich cluster,D_3h [Sb₃Au₃Sb₃]^3-,which was synthesized and characterized recently by our group in collaboration with the Chinese Academy of Sciences.The cluster has a triangular prismatic structure,in which an Au₃ metal sheet is jammed between two metallic Sb₃ ligands.No covalent Au–Au bond is presented in the system,and the Au₃ sheet appears to be dominated by aurophilicity.In a zeroth order bonding model,the cluster can be formulated as [Sb₃1.5+Au₃3-Sb₃1.5+]3-,which involves an intramolecular transfer of three electrons from Sb to Au,resulting in an Au_<sup>33-layer.The three extra electrons manage to compensate for electron loss of Sb₃,leading to the Sb₃0 ligands.However,natural atomic charges from natural bond orbital?NBO?analysis are Sb^0.493- and Au^0.014-,which hint that the system is far more complex and covalent.Ad NDP and orbital composition analyses show that,in effect,there are three 3c-2e Sb–Au–Sb ? bonds.Seventeen CMOs participate in the Sb?Au electron donation and Sb?Au back-donation,which are characteristic of covalent bonding and redistribute electrons from the Sb₃ and Au₃ layers to the interlayer edges.This represents an intriguing collective effect,as the saying goes “particles of sand accumulated will form a towering pile”,which holds the key to the interlayer bonding in this all-metal sandwich system.The NICS analysis confirms remarkable ? aromaticity for the cluster.6.Ligand-Stabilized All-Main-Group-Metal Aromatic Sandwich Complex [Al[Al?NH₂?]₆]^-.Based on literature search and density-functional theory calculations,we present the viability of ligand-stabilized all-main-group-metal sandwich complex.We model a simplified D_3d [Al[Al?NH₂?]₆]^-cluster on the basis of a synthetic Al[Al[N?Si Me₃?₂]]^6- complex,the latter possessing a sandwich-like [Al₃–Al–Al₃]-core.CMO analyses suggest that the model cluster can be formulated as [Al?NH₂?]₃^2-Al₃+[Al?NH₂?]₃^2-,while quantitative orbital composition analysis and NBO data give a more realistic model of [Al?NH₂?]₃^0.35-Al^0.30-[Al?NH₂?]^30.35-.Electron donation and back-donation occur in between two [Al?NH₂?]₃ layers and the Al core,collectively accumulating as many as 4.5 electrons along the interlayer Al–Al edges,which stabilize the all-metal sandwich.The CMO and Ad NDP analyses reveal two delocalized ?/? bonds,rendering the system ?/? double aromaticity.The finding allows us to propose a bonding concept: ligand-stabilized all-main-group-metal aromatic sandwich.