Theoretical Studies For Six-Membered N-Heterocyclic Radicals Of Havier Group 14 Elements

The low-coordination main group element radicals are rarely reported because of the lack of steric protection due to the very low coordination number,making it difficult to be isolated as stable compounds.Until 2011,the first stable low-coordination germanium radical A?L·Ge:,L=[HC?^tBuCN-Dip?₂]^-,Dip=2,6-ⁱPr₂C₆H₃?was synthesized and isolated.Then the second of the low germanium radical ligand B(·LGe:,·L=·[PhC?PhCN-Dip?₂]^2-)were reported in 2017.The synthetic schemes of radical A and B are similar,but their molecular structures and the spin density distributions are quite different.The biggest difference between A and B is that the substituents on their ligands are typical electron donating groups and electron withdrawing groups,respectively.In order to correlate the electronic effects in germanium?-diketiminate complexes with the types of germanium radicals,we systematically investigated the nature characters of six-membered N-heterocyclic germanium radicals with different substituents on?-diketiminate ligands.And together with the experimental group,a new low-coordinate germanium radical C(·LGe:,·L=·[CH₃C?PhCN-Dip?₂]^2-)was synthesized and structurally characterized,and the formation mechanism of germanium radicals was discussed.Some radicals could not be isolated in experiment,probably due to the kinetic instability during the reduction process from the germylene chloride precursor to radical.In addition,the types and stability of other heavier group 14 element radicals were theoretically predicted.The whole text is divided into two parts.The first part is Chapter One,which mainly introduces the background of this project and briefly introduces the theoretical methods.The second part includes Chapter Two and Three.In Chapter Two,the electronic effects in six-membered N-heterocyclic germanium radicals were systematically investigated using density functional theory calculations.And Chapter Three gives further understanding of the low-coordinate radicals of group 14 elements?Si,Ge,Sn,Pb?.Chapter One is an introduction,including background of this project and the theoretical calculation methods used in this paper.The nature characters of six-membered N-heterocyclic germanium radicals with different substituents on?-diketiminate ligands are systematically investigated in Chapter Two.Based on functionalized?-diketiminato systems,there are only two types of germanium radicals that can be obtained by the reductive dehalogenation from their germylene halide.One of them is p-type radical,which has strong electron-donating substituents on side carbon atoms of the six-membered ring,resulting in its spin density distribution being concentrated around the Ge atom resembling a Ge p orbital.The other one is?-type radical,whose spin density is mainly distributed on the NC₃N backbone,corresponding to a?-anti-bonding orbital,due to the induction of strong electron-withdrawing substituents.The group electronegativity of substituent can be used to estimate the type of selected radical.For the radical with highly group electronegative substituents,such as phenyl group in B and C,the most stable geometries of corresponding germylene chloride anion LGeCl^-keep the same electronic configuration:the single electron occupies the?-type MO on the ring backbone during the formation of the radical.However,the stable geometries of LGeCl^-have two types of electronic configurations if the low group electronegative substituents are on the six-membered ring,such as the tert-butyl in A.At the beginning of the dissociation,SOMO is?-type,after which the orbital energy level changes,and p-type MO becomes SOMO until the dissociation is completed.The germanium radical,which has moderately electronegative substituents or two substituents with comparable ability for pushing and pulling electrons on the N-heterocyclic backbone,is unstable to be isolated via a reductive dehalogenation from its germylene chloride,because the advantaged geometries of six-membered radical ring cannot be maintained in any stable state.Chapter 3 shows theoretical study on a serial of N-heterocyclic carbene radicals?HC[RCN-Ph]₂E,E=Si,Ge,Sn,Pb;R=CH₃,CF₃,Ph?by density functional methods.The molecular structures,radical types and substituents electronic effects have been investigated systematically,which are enabled the further understanding of the low-coordinate radicals.