Xin-Xin Tian

Zigzag double-chain C–Be nanoribbon featuring planar pentacoordinate carbons and ribbon aromaticity

Low-dimensional materials (LDMs) involving planar hypercoordinate carbon bonding were predicted to have applications in electronic devices, energy materials, and optical materials, etc. The majority of carbon atoms in such LDMs adopt a tetracoordinate structure, while examples with a higher coordination number are extremely rare and the bonding geometries of those carbons are not perfectly planar. In this work, we designed ribbon-like clusters CnBe3n+2H2n+22+ with planar pentacoordinate carbons (ppCs) and extended the corresponding structural model under 1D periodic boundary conditions (PBCs), leading to a zigzag double-chain C–Be nanoribbon. The beryllium atoms in such a nanoribbon arrange in a cosine shape around the perfect ppCs, which are unprecedented in LDMs. Detailed analyses revealed that the perfect ppC structure in the nanoribbon was geometrically achieved by opening a Be–Be edge of small Be5 rings, thereby making the intra-ring space adjustable to fit the size of the carbons. Electronically, the structure is stabilized by a favourable sandwich type charge distribution and satisfaction of the octet rule for ppCs. Note that all the valence electrons in the nanoribbon are locally delocalized within each ppC moiety, representing a new type of ribbon aromaticity, which should be useful in nanoelectronics. The nanoribbon and its cluster precursor C2Be8H62+ are thermodynamically stable, and are promising targets for experimental realization. The nanoribbon was predicted to be an indirect band gap semiconductor; thus it has potential applications in designing light-weight electronic devices.

From Quasi-Planar B56 to Penta-Ring Tubular Ca©B56: Prediction of Metal-Stabilized Ca©B56 as the Embryo of Metal-Doped Boron α-Nanotubes.

Motifs of planar metalloborophenes, cage-like metalloborospherenes, and metal-centered double-ring tubular boron species have been reported. Based on extensive first-principles theory calculations, we present herein the possibility of doping the quasi-planar C2v B56 (A-1) with an alkaline-earth metal to produce the penta-ring tubular Ca©B56 (B-1) which is the most stable isomer of the system obtained and can be viewed as the embryo of metal-doped (4,0) boron α-nanotube Ca©BNT(4,0) (C-1). Ca©BNT(4,0) (C-1) can be constructed by rolling up the most stable boron α-sheet and is predicted to be metallic in nature. Detailed bonding analyses show that the highly stable planar C2v B56 (A-1) is the boron analog of circumbiphenyl (C38H16) in π-bonding, while the 3D aromatic C4v Ca©B56 (B-1) possesses a perfect delocalized π system over the σ-skeleton on the tube surface. The IR and Raman spectra of C4v Ca©B56 (B-1) and photoelectron spectrum of its monoanion C4v Ca©B56− are computationally simulated to facilitate their spectroscopic characterizations.

Heteroborospherene clusters Ni n ∈ B 40 (n = 1–4) and heteroborophene monolayers Ni 2 ∈ B 14 with planar heptacoordinate transition-metal centers in η 7 -B 7 heptagons

With inspirations from recent discoveries of the cage-like borospherene B40 and perfectly planar Co ∈ B18− and based on extensive global minimum searches and first-principles theory calculations, we present herein the possibility of the novel planar Ni ∈ B18 (1), cage-like heteroborospherenes Nin ∈ B40 (n = 1–4) (2–5), and planar heteroborophenes Ni2 ∈ B14 (6, 7) which all contain planar or quasi-planar heptacoordinate transition-metal (phTM) centers in η7-B7 heptagons. The nearly degenerate Ni2 ∈ B14 (6) and Ni2 ∈ B14 (7) monolayers are predicted to be metallic in nature, with Ni2 ∈ B14 (6) composed of interwoven boron double chains with two phNi centers per unit cell being the precursor of cage-like Nin ∈ B40 (n = 1–4) (2–5). Detailed bonding analyses indicate that Nin ∈ B40 (n = 1–4) (2–5) and Ni2 ∈ B14 (6, 7) possess the universal bonding pattern of σ + π double delocalization on the boron frameworks, with each phNi forming three lone pairs in radial direction (3dz22, 3dzx2, and 3dyz2) and two effective nearly in-plane 8c-2e σ-coordination bonds between the remaining tangential Ni 3d orbitals (3dx2−y2 and 3dxy) and the η7-B7 heptagon around it. The IR, Raman, and UV-vis absorption spectra of 1–5 are computationally simulated to facilitate their experimental characterizations.

Cage-like B40C30, B40C40, and B40C50: high-symmetry heterofullerenes isovalent with C60, C70, and C80

The recent discovery of the cage-like borospherenes B40−/0, composed of interwoven double chains of boron, presents the possibility of forming BmCn heterofullerenes as hybrids of borospherenes and carbon fullerenes in dual spaces. Based on extensive first-principles theory calculations, we predict herein the possible existence of the high-symmetry BmCn heterofullerenes S10 B40C30 (1), C5 B40C40 (2), and S10 B40C50 (3), which are isovalent with C60, C70, and C80, respectively. These beautiful borafullerenes with boron aggregations feature one B30 boron double-chain nanoring at the equator, two bowl-shaped C15 or C25 caps at the top and bottom, and ten quasi-planar tetracoordinate peripheral C atoms in ten B-centered B6C hexagonal pyramids that are evenly distributed around the waist in a seamless “patched” structural motif. Detailed orbital and bonding analyses indicate that, as they are isovalent with C60, C70, and C80, respectively, B40C30 (1), B40C40 (2), and B40C50 (3) possess 30, 35, and 40 π bonds, respectively, of which 20 are 5c-2e π bonds delocalized over ten hexagonal pyramids that are evenly distributed around the waist. Such structural and bonding patterns confer high stability to these B-C heterofullerenes, which may be synthesized in experiments.

NiB10, NiB11−, NiB12, and NiB13+: Half-Sandwich Complexes with the Universal Coordination Bonding Pattern of σ Plus π Double Delocalization

Transition-metal-doped boron clusters have received considerable attention in recent years. The experimentally observed planar or quasi-planar C2h B10(I), C2v B11−(II), C3v B12(III), and C2v B13+ (IV) are known to be boron analogs of benzene. Extensive global minimum searches and first-principles theory investigations performed herein indicate that doping these aromatic boron clusters with a nickel atom generates the closed-shell half-sandwich complexes C2v NiB10(1,1A1), Cs NiB11−(2, 1A′), C3v NiB12(3, 1A1), and Cs NiB13+ (4, 1A′) which are all well-defined global minima of the systems with the coordination numbers of CN = 10, 11, 12, and 13, respectively. Detailed bonding analyses indicate that these Ni-doped boron complexes are effectively stabilized by coordination interactions between the Ni center and aromatic Bn−/0/+ ligands (n = 10–13) and follow the universal coordination bonding pattern of σ plus π double delocalization. Molecular dynamics simulations show that, among these complex clusters, NiB11−(2) behaves like a Wankel motor at room temperature with the B3 inner wheel rotating almost freely inside the quasi-rotating B8 outer bearing in a concerted mechanism, revealing typical bonding fluctuations/fluxionalities in a molecular motor due to thermal vibrations. The IR, Raman and electronic spectra of the concerned species are computationally simulated to facilitate their experimental characterizations.