Theoretical Studies On The Structure And Stability Of The 6-vertex Dicarboranes And Dicarbalanes Clusters

Carboranes,carbalanes and their derivatives have attracted many excellent researchers’interest in the fields of organic,organic-metallic,inorganic chemistry and material science,due to their unique structures and special reaction activity.Currently,they have been applied successfully in many fields.Due to the electron deficiency of boron atom,various electronic rules have been formed during the development of borane chemisty and carborane chemistry.Among them,the most famous one is Wade-Mingos rules,which were originally established for the boron hydrides（boranes）,carbon-boron hydrides（carboranes）and some transition metal carbonyl compounds,and later extended to the fields of other electron deficient compounds.The Wade-Mingos rules,based on a simple electron count,have provided a direct and rational explanation for the stability of the electron deficient compounds,such as boranes,carboranes and their derivatives.For nearly half a century,the Wade-Mingos rules have become an important part of inorganic chemistry.Aluminum and boron are in the same group of the periodic table,and they are similar in many aspects.In order to improve the stability and obtain the unique features of the compounds,substituents other than hydrogen atom usually are introduced in most synthesis and applications of the carboranes and carbalanes.The interaction between the electron deficient skeletons,i.e.,C-B or C-Al skeletons,and subtituents,in some case,is large enough to break the rules.The discovery of these“rule breakers”not only enriched the carborane chemistry and carbalane chemistry,but also tested the application scope of the Wade-Mingos rules.But there are few researches on this aspect.In this paper,we chose the six-vertex dicarborane and dicarbalane clusters as the research objects and studied the substituent effects.In addition,we designed the aluminum-containing clusters with the character of the frustrated Lewis pairs and the aluminum-containing clusters with bent carbon-carbon triple bond.These studies would provide effective and reliable basic data for experimental research and meaningful reference value for the research of compounds with other group 13 elements.The main achievements of this paper are as follows:1.The discovery of new global minima structures of the six-vertex dicarboranes.As one of the central carboranes families and the topological model of compounds containing the group 13 elements,the six-vertex dicarborane（C₂B₄H₆）has the most stable closo octahedron structure predicted by the Wade-Mingos rules.In this paper,two new non-closo global minima structures,i.e.,the unexpected trigonal bipyramid（R=SH）and the butterfly structure（R=Cl,NH₂,OH and F）were firstly predicted among the lowest-energy structures of C₂B₄R₆ with nine different substituents（i.e.,R=SiH₃,H,PH₂,CH₃,SH,Cl,NH₂,OH and F）.The butterfly structure has never been reported in previous studies.But in case of the nine substituents,the long expected benzvalene-like structure is not the global minimum structure for any one of them.This discovery provides a more abundant theoretical basis for the experimental study of these new structures.2.Scrutinizing the application scope of the Wade-Mingos rules in the substituted carborane systems.The Wade-Mingos rules are the famous concepts in inorganic chemistry and have become a regular teaching content in universities.In this chapter,the substitution effects on the famous（n+1）six-vertex carboranes family are systematically studied.The results show that the closo octahedron structure which satisfies the（n+1）Wade-Mingos rule only accounts for 31.5%among the 235 systems,i.e.,C₂B₄R_nR’_6-n（n=1-6,R and R’=SiH₃,H,PH₂,CH₃,SH,Cl,NH₂,CN,OH and F）,while the non-Wade-Mingos structure is dominant（68.5%）,which is obviously different from the traditional concepts.These non-Wade-Mingos structures with normal tri-coordinated boron atoms,include the trigonal bipyramid structures（52.3%）,butterfly structures（15.3%）and benzvalene-like structures（0.9%）.The four types of lowest-energy structures are located in three regions,i.e.,I,II and III regions,which are highly dependent on the type of substituents and the number of different substituents.The number of lone electron pairs and the electronegativity of the substituents are the main factors for the distribution.The distribution of the lowest-energy structures in the six-vertex dicarboranes family is of great significance to the structural design of dicarboranes in the future.The current work does not mean to reduce the importance of the Wade-Mingos rules,just wants to remind everyone to pay attention to the scope of the rules（at least for the six-vertex dicarboranes）.3.The design of the Al/Al type frustrated Lewis pair moleculars.Frustrated Lewis pairs（FLPs）have high reactivity and can be used to activate and split small molecules,catalyze olefin polymerization and ring opening polymerization.Al（III）has an empty orbital that can be considered as Lewis acid,while Al（I）with one lone electron pair has the property of Lewis base.Therefore,when the Al（III）and Al（I）centers are close to each other,they have a high probability to conduct disproportionation.It is a big challenge to find the Al/Al type frustrated Lewis pairs.In this chapter,we reported that the global minimum,C₂Al₄H₄-01,is an unprecedented structure with well-separated Al（I）and Al（III）atoms.C₂Al₄H₄-01 can be considered as the first Al/Al type frustrated Lewis pair molecular.In addition,through quantum chemical calculations,we also found that suitable substituents,such as R=SiH₃,Si（C₆H₅）₃,SiiPrDis₂ and SiMe（SitBu₃）₂,can well promote for the detection of the Al/Al type FLP,i.e.,C₂Al₄R₄-01,in gas phase and the laboratory synthesis in condensed phase.It is expected that these new Al/Al type frustrated Lewis pair moleculars could be used in laboratories in future.4.The founding of the new bent carbon-carbon triple bond with Al₄F₆scaffolding.Since the bent carbon-carbon triple bond（C≡C）was firstly proposed more than 100 years ago,the compounds with bent C≡C have become the focus of many theoretical and experimental studies due to their unique bonding and reactivity.Up to now,there are only two carbon-based bent C≡C types either in organic（I）（i.e.,cycloalkyne systems）or all-carbon form（II）（i.e.,cyclo[n]carbon）.By screening from 28600 C₂Al₄F₆ isomers,a novel bent C≡C compound,C₂Al₄F₆-01,was discovered.As the global minimum at the CBS-QB3 level,C₂Al₄F₆-01 contains a bent C≡C stabilized by the main-group metal-inorganic scaffolding Al₄F₆,which is obviously different from the known carbon-based scaffolding types.So it can be considered as the new（III）type bent C≡C compound,which has never been reported previously.The study shows that the C₂Al₄F₆-02 with linear C≡C is more active than C₂Al₄F₆-01 with bent C≡C,which is in sharp contrast with the traditional view.The scaffolding Al₄F₆ in C₂Al₄F₆-01 can be viewed as a kind of salt compound,i.e.,（[-AlF₃]^-Al⁺）₂,and the two kinds of aluminum atoms with characteristics of sp³ and sp²,respectively.The new carbon-carbon triple bond stabilized by aluminum fluoride scaffolding is expected to be a useful chemical agent in synthetic chemistry.In addition,in order to realize the laboratory synthesis of the new compound,we also proposed a possible strategy for the synthesis of C₂Al₄F₆-01.This study also shows that it is possible to control the shape of the lowest-energy dicarbalanes by substituent engineering and the strong tendency of forming C≡C implies that in the similar dicarbon metal fluorides,i.e.,C₂M_xF_y,（M=elements of 13 group）,there may be various bent C≡C with metal-inorganic scaffolding.