Planar tetracoordinate carbon species involving beryllium substituents

Planar tetracoordinate carbon (ptC) arrangements can be achieved by employing multiple substituents based on beryllium, despite its rather weak pi-acceptor ability. A variety of ptC-containing examples, some with more than one ptC, have been designed computationally by elaborating the planar C(BeH) 4 (2-) prototype at B3LYP/6-311++G(3df,2p) and MP2/6-311++G(3df,2p) levels of theory for some small ptC representatives. The prototype prefers a D(2h) paramagnetic triplet ground state due to Hund's rule, rather than a singlet. The highly polarized C-Be bonding weakens the rigidity of the tetrahedral carbon in T(d)C(BeH) 4 enormously, and the enhancement of both C-Be and Be 4 peripheral covalent bonding exerted by the extra electrons stabilizes the ptC eventually. The delocalization of the two p pi electrons is only modest, but their density on the most electronegative carbon atom helps stabilize the ptC arrangement. This is in contrast to the conventional strategy to delocalize p(pi) lone pairs for stabilizing the ptC arrangement. Various strategies to achieve neutral derivatives with ptCs are demonstrated.     

Understanding the planar tetracoordinate carbon atom: spiropentadiene dication

The atoms in molecule theory shows that the spiropentadiene dication has a planar tetracoordinate carbon (ptC) atom stabilized mainly through the sigma bonds and this atom has a negative charge. The bonds to the ptC atom have less covalent character than the central carbon from neutral spiropentadiene. The total positive charge is spread along the structure skeleton. The analysis of the potential energy surface shows that the dication spiropentadiene has a 2.3 kcal/mol activation barrier for ring opening.     

Planar and pyramidal tetracoordinate carbon in organoboron compounds

Using previously proposed C(BH)2(CH)2 (16, 17) and C(CH)2B2 (22) systems with a central planar tetracoordinate carbon (ptC) atom linking two three-membered rings as building blocks, a series of stable structures containing two and three ptC centers within a molecule have been designed and computationally studied with the DFT (B3LYP/6-311+G) method. Inclusion of a carbon atom ligated with pi-accepting and sigma-donating boron centers into at least one aromatic ring is critical for stabilization of a planar structure. A square pyramidal configuration at tetracoordinate carbon may be achieved in appropriately strained molecules such as [3.3.3.3]tetraborafenestrane 45 and others by surrounding the carbon with boron-centered ligands.     

包含平面四配位和五配位碳原子的特殊硼碳化合物

采用密度泛函理论(DFT), 在B3LYP/6-311+G**水平上, 研究了三类包含平面四配位碳原子(ptC)和平面五配位碳原子(ppC)的硼碳化合物. 这三类新型化合物是由C3B2H4(包含ptC)、CB4H2(包含ptC)和CB5H2(包含ppC)三种稳定结构和—CHCH—单元连接起来而得到的. 在理论上探讨了这些新型的硼碳化合物的成键特征, 光谱性质以及芳香性. 研究结果表明: 包含ptC和ppC原子的能量最低的结构, 在不受对称面限制的条件下, 具有C2v对称性的顺式立体构型比具有反式平面构型的化合物稳定. 计算的核独立化学位移(NICS)显示, 这些新型化合物的三元环中心有强的芳香性. 计算最稳定硼碳化合物的ptC和ppC原子的Wiberg键指数(WBIs)表明ptC和ppC的成键遵循八隅规则.     

Beryllium and boron decoration forms planar tetracoordinate carbon strips at the edge of graphene nanoribbons

Graphene has been viewed as one of the most promising materials in many fields. Recently, it has been found that by using Cu-decoration at the edge of zigzag graphene nanoribbons (ZGNR), a novel kind of planar tetracoordinate carbon (ptC) strip can be formed. In this paper, we investigate the edge-decoration of armchair graphene nanoribbons (AGNR) by various atom types and find that two new kinds of ptC strip can be effectively formed by using Be or B decoration. For the Be-decorated AGNR, the edge Be atoms take the form of a "zigzag-like" chain, and all the edge C atoms have a ptC nature. However, for the B-decorated AGNR, the edge B atoms form an infinite yet "fractured" chain consisting of separate B(4)-subunits, which results in only 50% of the edge C atoms being ptCs, in contrast with Be-decorated AGNR and Cu-decorated ZGNR. The high thermal stability of both types of ptC-based AGNR is indicated by isomeric sampling and molecular dynamics simulations.     

Probing the planar tetra-, penta-, and hexacoordinate carbon in carbon-boron mixed clusters

The concept of planar hypercoordinate (e.g., penta- and hexacoordinate) carbons is intriguing [Exner, K.; Schleyer, P. v. R. Science 2000, 290, 1937] as it is neither compatible with the standard rule of three-dimensional chemical bonding nor with the maximum tetracoordination. Herein we undertake a comprehensive study of the planar tetra- (ptC), penta- (ppC), and hexacoordinate carbon (phC) by covering the whole family of carbon-boron mixed clusters C(m=1-4)B(n=4-8) and their anions. The potential energy surface of every carbon-boron cluster is sampled by using the basin-hopping global search algorithm coupled with ab initio geometry optimization. A large number of planar tetra-, penta-, and hexacoordinate carbon (local-minimum) structures are obtained. Several structures such as the phC consisting of C2B5, C2B5(-), etc. are reported for the first time. In particular, a ptC corresponding to the global minimum of CB4 is revealed, which appears to be highly stable for future synthesis. The boron-centered isomers are generally the more stable structures for planar multicoordinate carbons (ptC, ppC, and phC). The planar tetra-, penta-, and hexacoordinate boron are the prevalent structural motifs in low-lying isomers of the carbon-boron clusters. However, stability of the ptC and ppC units can be reinforced over the boron-centered isomers by attaching proper hydrocarbon unit -(CH)n- to form the so-called "hyparenes" [Wang, Z. X.; Schleyer, P. v. R. Science 2001, 292, 2465]). A new hyparene molecule is suggested for future synthesis of novel planar hypercoordinate carbon compounds.     

Design of sandwichlike complexes based on the planar tetracoordinate carbon unit CAl4(2-)

Ever being a large curiosity, a series of simple "planar tetracoordinate carbon (ptC)" molecules have been recently characterized by experiments. Incorporation of such exotic ptC units into the assembled molecular materials, which will bridge the isolated clusters in molecular beams and the potential solid materials, is very challenging. In this paper, we described the first attempt on how to assemble the fewest-number ptC unit CAl42- into molecular materials in sandwich forms on the basis of the density functional theory calculations on a series of model compounds [D(CAl4)M]q- as well as the saturated compounds [D(CAl4)Mn] ((D = CAl42-, Cp-(C5H5-); M = Li, Na, K, Be, Mg, Ca). For M = Li, Be, Mg, and Ca, the ptC unit CAl42- can only be assembled in our newly proposed "heterodecked sandwich" scheme (e.g., [Cp(CAl4)M]q- (M = Li, Na, K, q = 2; M = Be, Mg, Ca, q = 1)) so as to avoid cluster fusion. For M = Na and K, the ptC unit CAl42- can be assembled in both the traditional "homodecked sandwich" [(CAl4)2M]q- (M = Li, Na, K, q = 3; M = Be, Mg, Ca, q = 2) and the novel heterodecked sandwich schemes. Moreover, the counterions were found to have an important role in determining the type of the ground structures for the homodecked sandwich. Various assembled species in extended frameworks were designed. Notably, among all the designed sandwich species, the ptC unit CAl42- generally prefers to interact with the partner deck at the side (Al-Al bond) or corner (Al atom) site. This has not been reported in the sandwich complexes on the basis of the known decks such as Cp-, P5-, N42-, and Al42-, for which only the traditional face-face interaction type was considered. Our results for the first time showed that the ptC unit CAl42- can act as a new type of "superatom". The present results are expected to enrich the flat carbon chemistry, superatom chemistry, metallocenes, and combinational chemistry.     

Perfect planar tetracoordinate carbon in neutral unsaturated hydrocarbon cages: A new strategy utilizing three‐dimensional electron delocalization

A new series of unsaturated pure and boron‐substituted hydrocarbons containing a perfect planar tetracoordinate carbon (ptC) have been proposed by performing density functional computations. The ptC is effectively stabilized through three‐dimensional delocalization of ptC's lone pair into π‐conjugated systems, by utilizing a new strategy opening a brand new way of designing ptC structures. Compared to previously proposed ptC‐containing hydrocarbon cage compound, a neutral hydrocarbon designed here might be a more viable target for synthetic attempts. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009     

A theoretical prediction of potentially observable lithium compounds with planar tetracoordinate carbons

Several potentially experimentally accessible lithiated heterocyclic and heteroatom compounds with planar tetracoordinate carbons (ptC) have been predicted computationally. These utilize the strong electron-donating ability and the bridging proclivity of lithium to achieve the ptC preferences. As the p orbitals on the central carbons are only partially occupied, their electronic structures are similar to those of the related carbenes, e.g. imidazole-2-ylidene, rather than to the other ptC compounds such as dilithiocyclopropane.     

The theoretical design of neutral planar tetracoordinate carbon molecules with C(C)(4) substructures

Using a new charge-compensation strategy, we designed neutral molecules with perfectly planar C(C)(4)-type tetracoordinate carbon arrangements (ptC) employing DFT computations. These designs, based on the planar preference of methane dications, replace two remote carbons in spiroalkaplanes by borons or two remote hydrogens by BH(3) groups; the two formally anionic boron units which result compensate the formal double positive charge on the central ptC's. The LUMOs correspond to the "wasted" lone pair HOMOs of the alkaplanes. As compared to the latter, pi occupancies on the central carbon are much smaller (less than 0.7e), and the IPs are much larger. The newly predicted compounds utilize all of the electrons more effectively. There are no lone pairs, and the ptC-C bond lengths are ca. 1.50 A. The Wiberg bond index sums of the ptC's are near 3.2, and the boron sums are close to 4.     

CAl4X (X=Si,Ge): molecules with simultaneous planar tetracoordinate carbon, aluminum, and silicon/germanium

The authors report the first theoretical study on the hexa-atomic molecules CAl(4)X (X=Si,Ge) at the B3LYP/6-311++G(2d), MP2/6-311++G(2d), and CCSD(T)/6-311++G(3df) (single point) levels. Three low-lying isomers (within 2.0 kcal/mol) can be formally viewed as constructed by one Al+ interacting with the planar CAl3X- at the side Al-X bond (X-1), side Al-Al bond (X-2), and central C atom (X-3). The isomers X-1 and X-2 both have planar structures that include the planar tetracoordinate carbon, aluminum, and silicon/germanium, while the three-dimensional isomer X-3 has the pentacoordinate carbon. The planarity of X-1 and X-2 is ascribed to the ligand five-center two-electron bonding molecular orbital, similar to the orbital responsible for the planarity of CAl3X- (X=Si,Ge). Kinetically, the two planar structures X-1 and X-2 can be easily interconverted to each other via the intermediate X-3, indicative of their coexistence. Of particular interest, isomer X-1 represents the first example that simultaneously contains three types of planar centers in a single molecule, to the best of our knowledge. The three low-lying and structurally interesting isomers X-1, X-2, and X-3 await future experimental verification. The present results could enrich the planar chemistry.     

Mono-silicon isoelectronic replacement in CAl4: van't hoff/le bel carbon or not?

In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology. The well-known penta-atomic 18 valence electron (ve) species and its Al⁻/Si or Al/Si⁺ isoelectronically replaced clusters CAl₃Si⁻, CAl₂Si₂, , and , all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure. In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl₄ with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl₃X (X = Ga/In/Tl). At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl₂SiX⁺ (X = Al/Ga/In/Tl) maintains thC as the parent CAl₄ does. Instead, X = Al/Ga globally favors an unusual ptC structure that has one long C─X distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon. The frustrated formation of thC in these clusters is ascribed to the CSi bonding that prefers a planar fashion. Inclusion of chloride ion would further stabilize the ptC of CAl₂SiAl⁺ and CAl₂SiGa⁺. The unexpectedly disclosed CAl₂SiAl⁺ and CAl₂SiGa⁺ represent the first type of 16ve-cationic ptCs with multiple bonds. © 2019 Wiley Periodicals, Inc.     

B4O分子异构化稳定性的理论研究

采用CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d)方法, 系统研究了B₄O体系各个异构体的结构和能量, 以及重要异构体的解离和异构化稳定性. 结果表明, 单态平面三角形含-BO单元的异构体cB₃-BO ¹1能量最低, 其次是带状的异构体B₄O ¹2(10.9 kJ/mol), 并且¹1和¹2结构都具有良好的动力学稳定性. 因此对于B₄O体系, ¹1和¹2都是有可能存在的. 而文献报道的三态直线型结构BBBBO(146.0 kJ/mol)的能量比异构体¹1和¹2高得多.     

Tetrahedrane--dossier of an unknown

To probe whether tetrahedrane should be isolable the thermodynamics and kinetics of C4H4 singlet and triplet structures were studied extensively at the CCSD(T)/cc-pVTZ//CCSD(T)/cc-pVDZ, CCSD(T)/cc-pVDZ, CCSD(T)/cc-pVDZ//B3 LYP/6-311G**, and B3 LYP/6-311G** levels of theory. The reaction of cyclopropene with atomic carbon, which was previously suggested to involve tetrahedrane as a reactive intermediate, was re-examined experimentally with low-temperature matrix-isolation techniques. While experimental and theoretical results exclude the intermediacy of tetrahedrane in the above reaction, it is predicted to be an isolable molecule. Among the many C4H4 species, we pay special attention to the electronic effects on the ground state multiplicity of the respective carbenes.     

Sn和Pb单掺杂Al4小团簇的理论研究

在CCSD(T)/aug-cc-pVTZ&;CEP-121G//B3LYP/6-311+G(d)&;LANL2DZ水平上, 研究了由更高周期的Sn和Pb单掺杂Al4团簇形成的五原子含铝体系XAl4(X=Sn, Pb), 确定了体系的低能异构体, 分析了关键异构体的结构和稳定性. 研究结果表明, 与SiAl4及GeAl4的基态平面四配位Si/Ge结构所不同, 等价电子的SnAl4和PbAl4体系的基态结构不是平面四配位Sn/Pb, 而是平面四配位Al, 其中杂原子Sn/Pb采取二配位方式, 此外, Sn/Pb采取三配位方式的非平面结构的稳定性也要优于平面四配位Sn/Pb结构.     

Structure and Bonding in CE5− (E=Al–Tl) Clusters: Planar Tetracoordinate Carbon versus Pentacoordinate Carbon

The structure, bonding, and stability of clusters with the empirical formula CE₅^? (E=Al–Tl) have been analyzed by means of high‐level computations. The results indicate that, whereas aluminum and gallium clusters have C_2v structures with a planar tetracoordinate carbon (ptC), their heavier homologues prefer three‐dimensional C_4v forms with a pentacoordinate carbon center over the ptC one. The reason for such a preference is a delicate balance between the interaction energy of the fifth E atom with CE₄ and the distortion energy. Moreover, bonding analysis shows that the ptC systems can be better described as CE₄^?, with 17‐valence electrons interacting with E. The ptC core in these systems exhibits double aromatic (both σ and π) behavior, but the σ contribution is dominating.     

Si6C18: A bispentalene derivative with two planar tetracoordinate carbons

Here we show that substituting the ten protons in the dianion of a bispentalene derivative (C₁₈H₁₀²⁻) by six Si²⁺ dications produces a minimum energy structure with two planar tetracoordinate carbons (ptC). In Si₆C₁₈, the ptCs are embedded in the terminal C₅ pentagonal rings and participate in a three-center, two-electron (3c-2e) Si-ptC-Si σ-bond. Our exploration of the potential energy surface identifies a triphenylene derivative as the putative global minimum. Nevertheless, robustness to Born–Oppenheimer molecular dynamics (BOMD) simulations at 900 and 1500 K supports bispentalene derivative kinetic stability. Chemical bonding analysis reveals ten delocalized π-bonds, which, according to Hückel's 4n + 2 π-electron rule, would classify it as an aromatic system. Magnetically induced current density analysis reveals the presence of intense local paratropic currents and a weakly global diatropic current, the latter agreeing with the possible global aromatic character of this specie.     

NXAl(3)+ (X = N, P, As): penta-atomic planar tetracoordinate nitrogen with N-X multiple bonding

Planar tetracoordinate nitrogen (ptN) has been successfully extended as a new branch of planar chemistry. As the simplest type of ptN, penta-atomic species (pptN, e.g., NAl(4)(-)) are known to have a "delocalized" molecular environment with a single bond between the central N and the ligand. In this paper, through an extensive isomeric search of a series of group V-based systems NXAl(3)(+) (X = N, P and As) in both singlet and triplet electronic states at the B3LYP/6-311+G(d) level, we report a class of novel pptN with unique chemical bonding, i.e., the central nitrogen and the connected ligand X (X = N, P and As) effectively form a highly "localized" N-X multiple bond, as confirmed by the aug-cc-pVTZ-B3LYP and MP2 calculations. The high-level CCSD(T)/aug-cc-pVTZ energetic calculations show that the three pptN species each have appreciable kinetic stability against structural transformation and fragmentation, which is confirmed by the Born-Oppenheimer molecular dynamics calculations. Possible formation pathways of the three pptNs are discussed. In particular, the pptN isomer with X = P, i.e., NPAl(3)(+), is the global minimum, making the pptN-based NPAl(3)(+) the most accessible via mass spectroscopic characterization. The present work demonstrates that the frequently used concept "localization vs. delocalization" in organic chemistry can also be transplanted to exotic planar chemistries like pptN.     

一种新型包含平面四配位碳二硼有机化合物的理论研究

采用B3LYP/6-311+G**方法, 研究了一种新型的包含平面四配位碳(ptC)二硼有机化合物C9B2H6的结构、稳定性和振动频率. 计算结果表明, C9B2H6结构的稳定性和两个硼原子的位置有很大关系, 硼原子起给予σ电子和接受π电子的作用. 在C9B2H6的15个异构体中, 最稳定的结构是具有C2v对称性的异构体(1,5), 在异构体(1,5)中, 两个硼原子位于同一个六元环中且与ptC相邻. 而且占据的π轨道说明异构体(1,5)具有10个π电子, 满足4n+2规则. 计算的核独立化学位移(NICS)值显示异构体(1,5)强的芳香性位于C9B2H6的两个三元环而不是两个六元环上.     

包含平面五配位和平面四配位碳化合物的结构和稳定性

在B3LYP/6-311+G**水平上,研究了包含平面五配位碳(ppC)和平面四配位碳(ptC)的化合物CB5C2H2(C3B2)nC2H2CB5(n=1-5)的结构、能隙、IR光谱、电子光谱、Wiberg键指数以及芳香性.计算结果表明,这五种化合物的能量最低结构均位于势能面的极小值点,HOMO-LUMO能隙在0.5到1.2eV之间,第一电子的激发能在1780到2910nm之间,且与分子尺寸呈现非单调变化趋势.Wiberg键指数和键长表明这五种化合物的能量最低结构均包含平面五配位和平面四配位碳.C3B2单元和右侧CB5单元中三元环中心的核独立化学位移(NICS(0))值均为负值,而左侧CB5单元中的三元环中心的NICS(0)值中两个为正值,另两个为负值,NICS(1)值与NICS(0)值也一致,因此电子的局域离域对结构的稳定是很重要的.