Planar Pentacoordinate Carbon Research Articles

Revisiting the potential-energy surface of CnBe3n+2H2n+22+ (n = 2-4) clusters: are planar pentacoordinate carbon structures the global minima?

Using various exploration strategies, in this study, we investigated the potential energy surfaces (PES) of CBe5H5+ and CnBe3n+2H2n+22+ (n = 2-4) clusters. Previous studies proposed that the planar pentacoordinate carbons (ppCs) were the global minima of these clusters. However, our study identified new putative global minima and competitive isomers, refuting some previous assignments. We employed several methods, including evolutive-inspired stochastic approaches guided by "chemical criteria", and ab initio molecular dynamics simulations at elevated temperatures. Our results showed that the size of the scanned population significantly affected the evolutive method and that constrained or guided procedures showed an advantage in identifying better minima for larger systems. This study demonstrated that using multiple complementary strategies can result in a wider variety of minima in a given energy range. Our findings provide valuable insights into exploring the potential energy surfaces of clusters, mainly medium-sized clusters, which could be the connections between small clusters and nanomaterials.

Planar pentacoordinate carbon in a 16-electron neutral CCu2Be3H4 cluster.

Planar hypercoordination in carbon is a fascinating area, which departs from the usual view of coordination geometry in carbon. Herein, we propose a planar pentacoordinate carbon species in the global minimum of the CCu2Be3H4 cluster. The cluster is a 16-electron species that is thermodynamically and kinetically very stable. Bonding analyses reveal 2π/6σ double aromaticity in the cluster. A low-energy isomerization pathway also reveals that the cluster has a sufficient lifetime for its experimental detection.

Structure and Bonding in Planar Hypercoordinate Carbon Compounds

The term hypercoordination refers to the extent of the coordination of an element by its normal value. In the hypercoordination sphere, the element can achieve planar and/or non-planar molecular shape. Hence, planar hypercoordinate carbon species violate two structural rules: (i) The highest coordination number of carbon is four and (ii) the tetrahedral orientation by the connected elements and/or groups. The unusual planar orientations are mostly stabilized by the electronic interactions of the central atom with the surrounding ligands. In this review article, we will talk about the current progress in the theoretical prediction of viable planar hypercoordinate carbon compounds. Primary knowledge of the planar hypercoordinate chemistry will lead to its forthcoming expansion. Experimental and theoretical interests in planar tetracoordinate carbon (ptC), planar pentacoordinate carbon (ppC), and planar hexacoordinate carbon (phC) are continued. The proposed electronic and mechanical strategies are helpful for the designing of the ptC compounds. Moreover, the 18-valence electron rule can guide the design of new ptC clusters computationally as well as experimentally. However, the counting of 18-valence electrons is not a requisite condition to contain a ptC in a cluster. Furthermore, this ptC idea is expanded to the probability of a greater coordination number of carbon in planar orientations. Unfortunately, until now, there are no such logical approaches to designing ppC, phC, or higher-coordinate carbon molecules/ions. There exist a few global minimum structures of phC clusters identified computationally, but none have been detected experimentally. All planar hypercoordinate carbon species in the global minima may be feasible in the gas phase.

Planar pentacoordinate carbon in [XC7H2]2+(X = Be and Mg) and its derivatives

The planar pentacoordinate carbon (ppC) atom is theoretically established here in [XC7H2]2+ and [XSi2C5H2]2+, where X = Be and Mg, using density functional theory. Inclusion compounds with alkali and alkaline earth metal ions are identified with the monomer units of tricyclic C7H2 and Si2C5H2 isomers with a planar tetracoordinate carbon (ptC) atom. While all alkali and some alkaline earth metals (Ca2+, Sr2+, and Ba2+) stabilize the ptC isomer in both cases, Be2+ and Mg2+ ions form a bond directly with the ptC atom, thus making it a ppC atom. The theoretical binding energies computed at the PBE0-D3/def2-TZVP level of theory are ∼-9.68, -10.42, -5.85, and -5.47 eV for [BeC7H2]2+, [BeSi2C5H2]2+, [MgC7H2]2+, and [MgSi2C5H2]2+, respectively.

Planar Hypercoordinate Carbons in Alkali Metal Decorated CE3 2- and CE2 2- Dianions.

After exploring the potential energy surfaces of Mm CE2 p (E=S-Te, M=Li-Cs, m=2, 3 and p=m-2) and Mn CE3 q (E=S-Te, M=Li-Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE2 2- and Y-shaped CE3 2- dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals.

CAl4Mg0/−: Global Minima with a Planar Tetracoordinate Carbon Atom

Isomers of CAl4Mg and CAl4Mg− have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl4Be case, which contains a planar pentacoordinate carbon atom as the global minimum geometry, replacing beryllium with magnesium makes the ptC isomer the global minimum due to increased ionic radii of magnesium. However, it is relatively easier to conduct experimental studies for CAl4Mg0/− as beryllium is toxic. While the neutral molecule containing the ptC atom follows the 18 valence electron rule, the anion breaks the rule with 19 valence electrons. The electron affinity of CAl4Mg is in the range of 1.96–2.05 eV. Both the global minima exhibit π/σ double aromaticity. Ab initio molecular dynamics simulations were carried out for both the global minima at 298 K for 10 ps to confirm their kinetic stability.

Highly Stable Two-Dimensional Iron Monocarbide with Planar Hypercoordinate Moiety and Superior Li-Ion Storage Performance.

Stable planar hypercoordinate motifs have been recently demonstrated in two-dimensional (2D) confinement systems, while perfectly planar hypercoordinate motifs in 2D carbon-transition metal systems are rarely reported. Here, by using comprehensive ab initio computations, we discover two new iron monocarbide (FeC) binary sheets stabilized at 2D confined space, labeled as tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), which are energetically more favorable compared with the previously reported square and honeycomb lattices. The proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with four ambient iron atoms, considered as the quasi-planar tetragonal lattice. Strikingly, the o-FeC monolayer is an orthorhombic phase with a perfectly planar pentacoordinate carbon moiety and a planar seven-coordinate iron moiety. These monolayers are the first example of a simultaneously pentacoordinate carbon and planar seven-coordinate Fe-containing material. State-of-the-art theoretical calculations confirm that all these monolayers have significantly dynamic, mechanical, and thermal stabilities. Among these two monolayers, the t-FeC monolayer shows a higher theoretical capacity (395 mAh g-1) and can stably adsorb Li up to t-FeCLi4 (1579 mAh g-1). The low migration energy barrier is predicted as small as 0.26 eV for Li, which results in the fast diffusion of Li atoms on this monolayer, making it a promising candidate for lithium-ion battery material.

Planar Hypercoordinate Motifs in Two-Dimensional Materials.

ConspectusAs one of the most important and versatile elements, carbon renders itself as one of the most fundamental and cutting-edge topics in chemistry, physics, and materials science. Many carbon-based chemical rules were established accordingly. While the tetrahedral predilection of tetracoordinate carbon has been a cornerstone of organic chemistry since 1874, almost a century later tetracoordinate carbon was found to be able to adopt planar structures known as planar tetracoordinate carbon (ptC), which are stabilized electronically by good π-acceptor (delocalization of a lone electron pair of ptC) or σ-donor (promoting electron transfer to electron-deficient bonding) substituents or mechanically by appropriate steric enforcement. The experimental and theoretical achievements for the rule-breaking ptC species totally refreshed our understanding of chemical bonding and triggered exploration of peculiar molecules featuring planar pentacoordinate carbon (ppC) and planar hexacoordinate carbon (phC) as well as other outlandish species such as planar hypercoordinate silicon.While the planar hypercoordinate carbon chemistry has been gradually established for molecules in the past five decades, there is growing interest in pursuing their extension systems, especially in two-dimensional (2D) space as a result of the recent extensive studies of graphene and its analogues. Though the natural 2D layered crystals do not contain any planar hypercoordinate carbon or silicon, several 2D nanosheets featuring planar or quasi-planar hypercoordinate ones have been theoretically suggested. Encouragingly, these unique planar configurations possess decent stabilities, and some of them are even the global minimum structure, exhibiting great potential for experimental realization. As the nature of a material is mainly determined by its structural characteristics (e.g., dimensionality, crystallography, and bonding), the combination of planar hypercoordinate chemistry and 2D nanoscience not only endows these rule-breaking systems with the merits of 2D materials but also may offer various promising properties and applications. For example, an unusual negative Poisson's ratio can be found in ppC-containing Be5C2 and planar pentacoordinate silicon (ppSi)-containing CaSi monolayers, of which the former has an anisotropic Dirac cone and the latter is a semiconductor with a desirable band gap for the semiconductor industry. Specially, shortly after the theoretical prediction, a planar hexacoordinate silicon (phSi)-containing Cu2Si monolayer was experimentally synthesized and characterized with the 2D Dirac nodal line fermion, which offers a platform to achieve high-speed, low-dissipation nanodevices.In this Account, we review the recent progress, mostly by density functional theory (DFT) computations, in designing 2D materials with planar hypercoordinate motifs. We describe the key achievements in this field, paying special attention to the "bottom-up" and "isoelectronic substitution" design strategies. In addition, the fundamental stabilization mechanisms of planar hypercoordinate motifs in an infinite layer are discussed. We hope that this Account will inspire more experimental and theoretical efforts to explore nanomaterials with such unconventional chemical bonding.

Planar or tetrahedral? A ternary 17-electron CBe5H4+ cluster with planar pentacoordinate carbon.

A 17-electron CBe5H4+ cluster features planar pentacoordinate carbon, owing to the 2π/6σ double aromaticity. The neutral CBe5H4 cluster has a tetrahedral configuration despite its 18-electron counting. The latter species is governed by σ conjugation.

Influence of stepwise oxidation on the structure, stability, and properties of planar pentacoordinate carbon species CAl5.

Computational design has played an important role in planar hyper-coordinate carbon (phC) chemistry. However, none of numerous computationally predicted phC species were subsequently successfully synthesized in the condensed phase, perhaps due to the frustrating issue of oxidation. In the present work, we studied the influence of stepwise oxidation on the structure, stability, and properties of phC species using the milestone planar pentacoordinate carbon (ppC) species CAl5+ as an example. Our results indicated that the ppC structure of CAl5+ would be directly destroyed with one, two, or six O atom(s) per molecule present and indirectly with three or four O atoms, but maintained with five O atoms due to the ppC isomer of CAl5O5+ being a kinetically stable global energy minimum displaying σ and π double aromaticity. Moreover, the magnitudes of the first to fifth vertical oxygen affinities (VOAs) for CAl5+ were determined to be very high (-85.5 to -116.3 kcal mol-1), probably due to the existence of peripheral diffuse Al-Al bond(s). However, the sixth VOA was reduced significantly to -50.2 kcal mol-1, consistent with the absence of any diffuse Al-Al bond in the corresponding CAl5O5+ species. So CAl5O5+ may be insensitive to oxidation. Therefore, the ppC species D5h CAl5O5+ might be resistant to being degraded under a delicate control of oxidation level (producing five O atoms per CAl5+ molecule).

Avoided spin coupling: an unexpected σ-σ diradical in global planar pentacoordinate carbon.

We present a global planar pentacoordinate carbon (ppC) featuring a hitherto unreported σ-σ diradical characteristic. Using the multi-reference approach combined with the CCSD(T)/aug-cc-pVTZ method, the ppC C3Li3- was found to be an intriguing triplet ground state, in which the unpaired density is mostly located at three Li ligands. Chemical bonding analysis reveals that the 2pzπ electrons of C3Li3- are fully located at the C3 ring formed by C-C multiple bonds, in contrast to the perfect 2pzπ-delocalization found in the well-known ppCs.

TMC (TM = Co, Ni, and Cu) monolayers with planar pentacoordinate carbon and their potential applications

A class of stable TMC (TM = Co, Ni, and Cu) monolayers with entirely planar pentacoordinate carbon in their ground states and their promising functional applications in spintronics, electronics, and mechanics are identified through the swarm-intelligence structure search method and first-principles calculations.

Planar Pentacoordinate versus Tetracoordinate Carbons in Ternary CBe4Li4 and CBe4Li42- Clusters.

Planar hypercoordinate carbon molecules are exotic species, for which the 18-electron counting has been considered a rule. We report herein computational evidence of perfectly planar C2 v CBe4Li4 (1) and D4 h CBe4Li42- (3) clusters. These ternary species contain 16 and 18 electrons, respectively. The dianion is highly symmetric with a planar tetracoordinate carbon (ptC), whereas the neutral features a planar pentacoordinate carbon (ppC). Thus, charge-state alters the coordination environments of a cluster. Chemical bonding analysis shows that both clusters have 2π and 6σ delocalization around the C center, suggesting that ppC or ptC clusters are governed by double π/σ aromaticity, rather than the 18-electron rule. The outer Be4Li4 ring in 1 and 3 also supports 2σ aromaticity, collectively leading to 3-fold π/σ aromaticity for these ppC/ptC clusters. Structural transformation from ptC (3) to ppC (1) is discussed, in which the 16-electron quasi-ptC CBe4Li4 (2) cluster serves as an intermediate. Cluster 2 as a local minimum has severe out-of-plane distortion. Flattening of 2 leads to reorganization of Be4 ring around the C center, which offers space for the fifth atom to coordinate and facilitates ppC formation. The latter arrangement optimizes π aromaticity and better manages intramolecular Coulomb repulsion. This work highlights the geometric factor (and unconventional electron counting) in the design of planar hypercoordinate carbons.

Planar pentacoordinate carbons

Carbon centres in typical organic molecules have a coordination number that can reach a maximum of four, in which case the bonded atoms are situated at the vertices of a tetrahedron. Exceptions to those two structural rules have been posited and examined for decades, and planar tetracoordinate carbon (ptC) species are notable molecules that violate the second rule. There is continued interest in experimental and theoretical studies of ptCs, as well as emerging molecules that contain planar pentacoordinate carbon (ppC) and planar hexacoordinate carbon (phC) atoms, species that violate both structural rules. This Review describes recent progress in the theoretical prediction of viable entities that contain ppC centres. The first such molecule reported, the D5h-symmetric ppC species CAl5+, was followed by a series of predicted ppC species that could be obtained by substituting the Al centres for other heteroatoms. More complicated ppC systems have also been suggested, including metallocene-stabilized ppCs and quasi-ppCs embedded within cage structures or 2D materials. To date, computational studies have identified at least 65 local and 39 global minimum energy structures that contain ppCs or quasi-ppCs. The general design principles for ptC-centred candidate structures include delocalization of the central C 2pz lone electron pair, ensuring an 18 valence electron count and allowing for strong electron delocalization. These principles have been extended to ppC systems with some success. It is hard to predict the extent to which the coordination number of planar C can be increased because it depends not only on the valence and size of C but also on the size of the atoms bonded to it and the mode of bonding. Although a few energetically low-lying planar hexacoordinate and heptacoordinate C species have been identified computationally, none have been observed experimentally. Building on the theoretical prediction and experimental realization of planar tetracoordinate carbons, there has been great progress in the theoretical prediction of species containing planar pentacoordinate carbon (ppC) centres. From simple clusters such as CAl5+ to complicated ppCs in 2D materials, this Review summarizes the ppC-containing species predicted to date and the design principles behind them.

Star-Like CBe5Au5+ Cluster: Planar Pentacoordinate Carbon, Superalkali Cation, and Multifold (π and σ) Aromaticity.

We report on the computational design of star-like CBe5Au5+ cluster with planar pentacoordinate carbon (ppC), which is also classified as a superalkali cation. Relevant isovalent CBe5Aunn-4 (n = 2-4), BBe5Au5, and NBe5Au52+ clusters with ppC/B/N are studied as well. Global-minimum structures of the clusters are established via computer global searches. The species feature a pentacoordinate pentagonal XBe5 (X = C, B, N) core, with Au occupying outer bridging positions. Molecular dynamics simulations indicate that they are dynamically stable. Bonding analysis reveals 3-fold (π and σ) aromaticity in CBe5Au5+, a key concept that overrides the 18-electron rule and should be applicable for (or help revisit existing models of) other planar hypercoordinate systems. Vertical electron affinities of CBe5Au5+ and its lighter counterparts (CBe5Cu5+ and CBe5Ag5+) are calculated to be unusually low, which are below 3.89 eV, the smallest atomic ionization potential of any element in the periodic table. Thus, these three clusters belong to superalkali cations. The merge of ppC and superalkali characters makes them unique chemical species.

CB3E2q (q = ±1): a family of "hyparene" analogues with a planar pentacoordinate carbon.

A CB3 moiety extracted from the building units of milestone "hyparenes" (families of species with a planar pentacoordinate carbon (ppC)) was found to be a more basic building block, which can be employed to design a family of "hyparene" analogues CB3E2q (q = ±1) also with a ppC. The majority of main group elements can feasibly serve as the E atom. Despite the number of valence electrons, the ppC atoms in the CB3E2q (q = ±1) species were involved in three delocalized σ orbitals and a delocalized π orbital, so the carbon atom obeys the octet rule. The NICS studies indicated that these ppC structures are σ and π double aromatic. Given that most of them are less favourable in energy than their boron-centered isomers, it is remarkable that the global minimum of CB3Mg2- adopts the ppC arrangement. Such a ppC structure is also kinetically stable. Compared to previously reported anionic ppC global minima, CB3Mg2- does not contain hyper toxic beryllium and thus is much more attractive to our experimental colleagues for realizing the ppC species using negative ion photoelectron detachment spectroscopy.

Planar pentacoordinate carbon in CGa5+ derivatives.

We report a family of systems having a planar pentacoordinate carbon (ppC) based on the next heavier analogue of CAl5+, the ppC system par excellence. Although because of the larger size of Ga, the ppC isomer is not even a local minimum in CGa5+, a single isoelectronic substitution of Ga by smaller sized Be maximizes the bonding in the ppC form. Retaining the 18 valence electron rule, the global minimum structures of CGa4Be, CGa3Be2-, CGa2Be32-, and CGaBe43- clusters and their corresponding lithium salts have a ppC.

Stabilization of beryllium-containing planar pentacoordinate carbon species through attaching hydrogen atoms.

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design nine new planar and quasi-planar pentacoordinate carbon (ppC) species CAlnBemHxq (n + m = 5, q = 0, ±1, x = q + m − 1) (1a–9a) by attaching H atoms onto the Be atoms in CAl4Be, CAl3Be2−, CAl2Be32−, and CAlBe43−. These ppC species are σ and π double aromatic. In comparison with their parents, these H-attached molecules are more stable electronically, as can be reflected by the more favourable alternative negative–positive–negative charge-arranging pattern and the less dispersed peripheral orbitals. Remarkably, seven of these nine molecules are global energy minima, in which four of them are kinetically stable, including CAl3Be2H (2a), CAl2Be3H− (4a), CAl2Be3H2 (5a), and CAlBe4H4+ (9a). They are the promising target for the experimental realization of species with a ppC.

Planar pentacoordinate carbon atoms embedded in a metallocene framework.

Viable planar pentacoordinate carbon (ppC) systems with a ppC bonded to a transition metal and embedded in a metallocene framework are reported. Our detailed global minima search shows that CAl4MX2 (M = Zr and Hf; X = F-I and C5H5) clusters with ppCs are appropriate candidates for experimental realization in the gas phase. The fulfillment of the 18 electron rule and electron delocalization is found to be crucial for the stabilization of these ppC arrangements.

Star-like superalkali cations featuring planar pentacoordinate carbon.

Superalkali cations, known to possess low vertical electron affinities (VEAs), high vertical detachment energies, and large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps, are intriguing chemical species. Thermodynamically, such species need to be the global minima in order to serve as the promising targets for experimental realization. In this work, we propose the strategies of polyhalogenation and polyalkalination for designing the superalkali cations. By applying these strategies, the local-minimum planar pentacoordinate carbon (ppC) cluster CBe5 can be modified to form a series of star-like superalkali ppC or quasi-ppC CBe5X5 (+) (X = F, Cl, Br, Li, Na, K) cations containing a CBe5 moiety. Polyhalogenation and polyalkalination on the CBe5 unit may help eliminate the high reactivity of bare CBe5 molecule by covering the reactive Be atoms with noble halogen anions and alkali cations. Computational exploration of the potential energy surfaces reveals that the star-like ppC or quasi-ppC CBe5X5 (+) (X = F, Cl, Br, Li, Na, K) clusters are the true global minima of the systems. The predicted VEAs for CBe5X5 (+) range from 3.01 to 3.71 eV for X = F, Cl, Br and 2.12-2.51 eV for X = Li, Na, K, being below the lower bound of the atomic ionization potential of 3.89 eV in the periodic table. Large HOMO-LUMO energy gaps are also revealed for the species: 10.76-11.07 eV for X = F, Cl, Br and 4.99-6.91 eV for X = Li, Na, K. These designer clusters represent the first series of superalkali cations with a ppC center. Bonding analyses show five Be-X-Be three-center two-electron (3c-2e) σ bonds for the peripheral bonding, whereas the central C atom is associated with one 6c-2e π bond and three 6c-2e σ bonds, rendering (π and σ) double aromaticity. Born-Oppenheimer molecular dynamics simulations indicate that the CBe5 motif is robust in the clusters. As planar hypercoordination carbon species are often thermodynamically unstable and highly reactive, the superalkali cation characters of these ppC species should be highlighted, which may be suitable for experimental realization.