Theoretical Investigations On The Novel Non-classic Group14Compounds

Group14compounds play an important role in morden chemistry，it has beenapplied in organic chemistry, inorganic chemistry and material chemistry. Especially,anti-traditional non-classic group14compounds has been hotly pursued in recentyears. Due to the uniqune structure and electronic property, these compounds can beviewed as the precursors and intermediates in many important reactions. In addition,some compounds can be used as the building block to design the potential lowdimensional materials and solar cell materials. In this thesis, using the quantumchemistry, we studied three classes of non-classic group14compounds. Through theglobal structure search program, we obtained some novel non-classic group14compounds. These researches enriched group14chemistry, and provided impressivetheoretical information for further isolation and characterization in experiment. Themain results are summarized in the following:1. Aiming to design novel low-valent silylenes, in this work, we made the firstsystematic research on the SiAlR5system. Based on the "topology" isomeric searchstrategy, four classes of isomers, i.e., RSi(μ-R)2AlR21, R3Si-AlR22, R2Si(μ-R)AlR23and R2Si-AlR34were identified. For12groups with electron lone-pair, i.e., R=F, NH2,OH, SH, OMe, PMe2, SMe, OEt, PEt2, SEt, OPh, PPh2, the tri-coordinate silylene1isthe most stable structure, which contains two sets of inter-molecular donor-acceptorinteractions. In accordance with the “silylene” term, the Si-center can undergo boththe nucleophilic reaction and insertion reaction. Moreover, coupling AlR3(R=OR’ andPR’2)(R’=H, Me, Et, Ph) with the known cyclic di-coordinate silylene could lead tostable tri-coordinate silylenes. These novel tri-coordinate silylenes strongly welcomelaboratory studies in near future.2. Design of stable heavier carbene analogues M(II)(M=Ge,Sn,Pb) with structural motifs different from the traditional di-coordinate analogues is extremely ofinterest and challengeable. In this work, we computationally designed a novel class oftri-coordinate cyclic heavier analogues of carbenes covering a number of substituents,i.e., RM(μ-R)2AlR2(M=Ge, Sn, Pb)1, in which two sets of inter-moleculardonor-acceptor interactions were formed between the di-coordinate carbene MR2andthe AlR3moiety. The M-center was found to undergo both the nucleophilic andinsertion reactions, indicative of the “carbene” character. We also designed c-1starting from the known di-coordinate cyclic MR2and AlR3under16kinds ofsubstituents (NR’2, OR’, PR’2, SR’(R’=H, Me, iPr, Ph)), c-1also has favorablethermodynamic stability. After the only known cyclic tri-coordinate M(II)[M2,R4]inter-molecularly formed between two di-coordinate MR2, the presently designed newtri-coordinate cyclic heavier carbene analogues should expand the higher coordinateM(II) family and await future laboratory synthesis.3. Quantum chemical calculations show that the N-heterocyclic carbene(NHC)-stabilized silavinylidene, NHCtBu�'C=SiR2is a strongly bonded complex,which has a linear arrangement of the donor and acceptor moieties. The molecule isthe energetically lowest lying isomer of the NHC-stabilized R2CSi isomers and it isstable towards dimerization when R is a bulky substituent. The silavinylidenecomplex is the only species of the silylidene homologues NHCtBu�'E=E’R2(E,E’=C–Pb) which possesses a linear arrangement. The unusual bonding situation isexplained in terms of donor–acceptor interactions between NHCtBuas s donor andC=SiR2in the doubly excited singlet state3a1�'2b2which leads to a significantlyshorter C-SiR2bond compared with free C=SiR2.4. The design and characterization of Group14E≡C (E=Si, Ge, Sn, Pb) triplebonds have continued to be a challengeable task. In this work, we report a stabilizingstrategy for E≡C (E=Si, Ge, Sn, Pb) triple bonding, i.e., Lewis acid-base pair strategy.Based on this strategy, we have successfully designed a class of E≡C species(C6F5)3Al←E≡C←NHCARvia theoretical methods. This class of compounds havebeen demonstrated to be thermally stable.5. Designing and characterizing the compounds with exotic structures and bonding that seemingly contrast the traditional chemical rules is a never-ending goal.Though the silicon chemistry is dominated by the tetrahedral picture, many exampleswith the planar tetracoordinate-Si skeletons have been discovered, among whichsimple species usually contain the17/18valence electrons. In this work, we reporthitherto the most extensive structural search for the pentaatomic ptSi with14valenceelectrons, i.e., SiXnYmq(n+m=4; q=0,±1,-2; X, Y=main group elements from H toBr). For the129studied systems,50systems have the ptSi structure as the localminimum. Promisingly,9systems, i.e., Li3SiAs2, HSiY3(Y=Al/Ga), Ca3SiAl,Mg4Si2, C2Li2Si, Si3Y2(Y=Li/Na/K), each have the global minimum ptSi. Theformer6systems represent the first prediction. Interestingly, in HSiY3(Y=Al/Ga), theH-atom is only bonded to the ptSi-center via a localized2c-2e σ bond. This sharplycontradicts the known pentaatomic planar-centered systems, in which the ligands areactively involved in the ligand-ligand bonding besides being bonded to the planarcenter. Therefore, we proposed here that to generalize the14e-ptSi, two strategies canbe applied as1) introducing the alkaline/alkaline-earth elements, and2) breaking theperipheral bonding. In light of the very limited global ptSi examples, the presentlydesigned6systems with14e are expected to enrich the exotic ptSi chemistry andwelcome future laboratory confirmation.