CBe2 H5 - : Unprecedented 2σ/2π Double Aromaticity and Dynamic Structural Fluxionality in a Planar Tetracoordinate Carbon Cluster.

A 14-electron ternary anionic CBe2 H5 - cluster containing a planar tetracoordinate carbon (ptC) atom is designed herein. Remarkably, it can be stabilized by only two beryllium atoms with both π-acceptor/σ-donor properties and two hydrogen atoms, which means that the conversion from planar methane (transition state) to ptC species (global minimum) requires the substitution of only two hydrogen atoms. Moreover, two ligand H atoms exhibit alternate rotation, giving rise to interesting dynamic fluxionality in this cluster. The electronic structure analysis reveals the flexible bonding positions of ligand H atoms due to C-H localized bonds, highlighting the rotational fluxionality in the cluster, and two CBe2 3c-2e delocalized bonds endow its rare 2σ/2π double aromaticity. Unprecedentedly, the fluxional process exhibits a conversion in the type of bonding (σ bond↔π bond), which is an uncommon fluxional mechanism. The cluster can be seen as an attempt to apply planar hypercoordinate carbon species to molecular motors.

Y©B8C4 cluster: a boron-carbon molecular wheel with dodeca-coordination number in plane.

Computational evidence is reported for the largest planar molecular wheel of the Y©B8C4 cluster, featuring an yttrium atom enclosed by a highly symmetric B8C4 ring. The B8C4 ring is viable in the -(BCB)4- form with double 9π/10σ aromaticity. The centered yttrium atom is dodeca-coordinated with the peripheral B8C4 ring, which sets a record coordination number for a planar structure in chemistry heretofore.

The structure and chemical bonding in inverse sandwich B6Ca2 and B8Ca2 clusters: conflicting aromaticity vs. double aromaticity.

The typical electron-deficiency of the boron element renders fascinating architectures and chemical bonding to boron-based nanoclusters. We theoretically predict two di-Ca-doped boron clusters, B6Ca2 (D2h, 1Ag) and B8Ca2 (D8h, 1A1g), and both adopt interesting inverse sandwich geometries, showing an elongated D2h B6 or perfectly planar D8h B8 ring being sandwiched by two Ca atoms only, respectively. Natural atomic charge analyses indicate that the Ca atoms donate nearly all the 4s electrons to the B6 (or B8) ring, forming [Ca]2+[B6]4-[Ca]2+ and [Ca]2+[B8]4-[Ca]2+ charge transfer complexes. The interaction between the two Ca atoms and the boron rings is governed by robust electrostatics albeit by weaker B-Ca covalent interaction. Chemical bonding analyses show that B6Ca2 has 4σ and 6π delocalized electrons on the elongated B6 ring, leading to a conflicting aromatic system. B8Ca2, possessing 6σ and 6π delocalized electrons on the B8 ring, is doubly aromatic. Additionally, the B6Ca2 and B8Ca2 clusters show noticeable structural and electronic transmutation relative to their equivalent electronic B6Be2 and B8Mg2 clusters, respectively. The intrinsic reasons behind the transmutations are elucidated via in-depth bonding analyses.

Boron-based ternary Rb6Be2B6 cluster featuring unique sandwich geometry and a naked hexagonal boron ring.

Computational evidence is reported on a boron-based ternary Rb6Be2B6 cluster as the "Big Mac" sandwich on a subnanoscale with thickness of 0.58 nm. The core hexagonal B6 ring, occurring in the naked form due to double 6π/6σ aromaticity, is capped by two tetrahedral BeRb3 ligands. Such a B6 motif is scarce in boron clusters. The sandwich cluster has four-fold 2σ/6π/6σ/2σ aromaticity and its tetrahedral BeRb3 ligand is the simplest case of three-dimensional aromaticity (or spherical aromaticity). The sandwich can be formulated as a charge-transfer complex, [Rb3Be]3+[B6]6-[BeRb3]3+, whose components are held together by robust electrostatics, facilitating dual-mode dynamic fluxionality.

Endohedral Ca@B38: stabilization of a B38(2-) borospherene dianion by metal encapsulation.

Based on extensive global-minimum searches and first-principles electronic structure calculations, we present the viability of an endohedral metalloborospherene Cs Ca@B38 () which contains a Cs B38(2-) () dianion composed of interwoven boron double chains with a σ + π double delocalization bonding pattern, extending the Bn(q) (q = n - 40) borospherene family from n = 39-42 to n = 38. Transition metal endohedral complexes Cs M@B38 (M = Sc, Y, Ti) (, , ) based on Cs B38(2-) () are also predicted.

Competition between quasi-planar and cage-like structures in the B29- cluster: photoelectron spectroscopy and ab initio calculations.

Size-selected boron clusters have been found to be predominantly planar or quasi-planar (2D) in the small size regime with the appearance of three-dimensional (3D) borospherene cages of larger sizes. A seashell-like B28- cluster was previously shown to be the smallest borospherene, which competes with a quasi-planar isomer for the global minimum. Here we report a study on the structures and bonding of the B29- and B29 clusters using photoelectron spectroscopy (PES) and first-principles calculations and demonstrate the continued competition between the 2D and borospherene structures. The PES spectrum of B29- displays a complex pattern with evidence of low-lying isomers. Global-minimum searches and extensive theoretical calculations revealed a complicated potential energy surface for B29- with five low-lying isomers, among which the lowest three were shown to contribute to the experimental spectrum. A 3D seashell-like Cs (2, 1A') isomer, featuring two heptagons on the waist and one octagon at the bottom, is the global minimum for B29-, followed by a 2D C1 (3, 1A) isomer with a hexagonal hole and a stingray-shaped 2D Cs (1, 1A') isomer with a pentagonal hole. However, by taking into account the entropic effects, the stingray-shaped isomer 1 was shown to be the lowest in energy at room temperature and was found to dominate the PES spectrum. Isomers 2 and 3, which have lower electron binding energies, were also found to be present in the experiment. Chemical bonding analyses showed that isomer 1 is an all-boron analogue of benzo[ghi]fluoranthene (C18H10), whereas the borospherene isomer 2 possesses 18π electrons, conforming to the 2(N + 1)2 electron counting rule for spherical aromaticity. For the B29 neutral cluster, the seashell-like borospherene isomer is the global minimum, significantly lower in energy than the stingray-shaped quasi-planar structure.

Endohedral C3 Ca@B39(+) and C2 Ca@B39(+): axially chiral metalloborospherenes based on B39(-).

Using the newly discovered borospherenes C3 B39(-) and C2 B39(-) as molecular devices and based on extensive global-minimum searches and first-principles calculations, we present herein the possibility of the first axially chiral metalloborospherenes C3 Ca@B39(+) (, (1)A) and C2 Ca@B39(+) (, (1)A), which are the global minimum and the second lowest-lying isomer of CaB39(+), respectively. These metalloborospherene species turn out to be charge-transfer complexes Ca(2+)@B39(-) in nature, with the Ca centre on the C3 or C2 molecular axis donating one electron to the B39 cage which behaves like a superhalogen. Molecular orbital analyses indicate that C3/C2 Ca(2+)@B39(-) possess the universal bonding pattern of σ plus π double delocalization, similar to their C3/C2 B39(-) parents. Molecular dynamics simulations show that both C3 Ca@B39(+) () and C2 Ca@B39(+) () are dynamically stable at 200 K, with the former starting to fluctuate structurally at 300 K and the latter at 400 K, again similar to C3/C2 B39(-). The infrared and Raman spectra of C3/C2 Ca@B39(+) (/) are simulated and compared with those of C3/C2 B39(-) to facilitate their forthcoming experimental characterization.

CBe5Hn((n-4)) (n = 2-5): Hydrogen-Stabilized CBe5 Pentagons Containing Planar or Quasi-Planar Pentacoordinate Carbons.

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design a new series ppC- or quasi-ppC-containing species C5v CBe5H5(+), Cs CBe5H4, C2v CBe5H3(-), and C2v CBe5H2(2-) by replacing the Al atoms in previously reported global minima planar pentacoordinate carbon (ppC) species D5h CAl5(+), C2v CAl4Be, C2v CAl3Be2(-), and C2v CAl2Be3(2-) with BeH units. The three-center two-electron (3c-2e) bonds formed between Be and bridging H atoms were crucial for the stabilization of these ppC species. The natural bond orbital (NBO) and adaptive natural density partitioning (AdNDP) analyses revealed that the central ppCs or quasi-ppCs possess the stable eight electron-shell structures. The AdNDP analyses also disclosed that these species are all 6σ+2π double-aromatic in nature. The aromaticity was proved by the calculated negative nucleus-independent chemical shifts (NICS) values. DFT and high-level CCSD(T) calculations revealed that these ppC- or quasi-ppC species are the global minimum or competitive low-lying local minimum (Cs CBe5H4) on their potential energy surfaces. The Born-Oppenheimer molecular dynamic (BOMD) simulations revealed that the H atoms in C2v CBe5H3(-) and C2v CBe5H2(2-) can easily rotate around the CBe5 cores and the structure of quasi-planar C5v CBe5H5(+) will become the planar structure at room temperature; however, these interesting dynamic behaviors did not indicate the kinetic instability as the basic ppC structures were maintained during the simulations. Therefore, it would be potentially possible to realize these interesting ppC- or quasi-ppc-species in future experiments.

Cage-Like B41 (+) and B42 (2+) : New Chiral Members of the Borospherene Family.

The newly discovered borospherenes B40 (-/0) and B39 (-) mark the onset of a new class of boron nanostructures. Based on extensive first-principles calculations, we introduce herein two new chiral members to the borospherene family: the cage-like C1 B41 (+) (1) and C2 B42 (2+) (2), both of which are the global minima of the systems with degenerate enantiomers. These chiral borospherene cations are composed of twelve interwoven boron double chains with six hexagonal and heptagonal faces and may be viewed as the cuborenes analogous to cubane (C8 H8 ). Chemical bonding analyses show that there exists a three-center two-electron σ bond on each B3 triangle and twelve multicenter two-electron π bonds over the σ skeleton. Molecular dynamics simulations indicate that C1 B41 (+) (1) fluctuates above 300 K, whereas C2 B42 (2+) (2) remains dynamically stable. The infrared and Raman spectra of these borospherene cations are predicted to facilitate their experimental characterizations.

Ribbon aromaticity in double-chain planar B(n)H2(2-) and Li2B(n)H2 nanoribbon clusters up to n = 22: lithiated boron dihydride analogues of polyenes.

We report an extensive density-functional theory and coupled-cluster CCSD(T) study on boron dihydride dianion clusters BnH2(2-) (n = 6-22) and their dilithiated Li2BnH2(0/-) salt complexes. Double-chain (DC) planar nanoribbon structures are confirmed as the global minima for the BnH2(2-) (n = 6-22) clusters. Charging proves to be an effective mechanism to stabilize and extend the DC planar nanostructures, capable of producing elongated boron nanoribbons with variable lengths between 4.3-17.0 Å. For the dilithiated salts, the DC planar nanoribbons are lowest in energy up to Li2B14H2 and represent true minima for all Li2BnH2(0/-) (n = 6-22) species. These boron nanostructures may be viewed as molecular zippers, in which two atomically-thin molecular wires are zipped together via delocalized bonds. Bonding analysis reveals the nature of π plus σ double conjugation in the lithiated DC nanoribbon Li2BnH2(0/-) (n up to 22) model clusters, which exhibit a 4n pattern in adiabatic detachment energies, ionization potentials, and second-order differences in total energies. Band structure analysis of the infinite DC boron nanoribbon structure also reveals that both π and σ electrons participate in electric conduction, much different from the monolayer boron α-sheet in which only π electrons act as carriers. A concept of "ribbon aromaticity" is proposed for this quasi-one-dimensional system, where regular π versus σ alternation of the delocalized electron clouds along the nanoribbons results in enhanced stability for a series of "magic" nanoribbon clusters. The total number of delocalized π and σ electrons for ribbon aromaticity collectively conforms to the (4n + 2) Hückel rule. Ribbon aromaticity appears to be a general concept in other nanoribbon systems as well.

Bimetallic metal-organic frameworks-based ratiometric fluorescence sensor for the quantitative detection of thiram in fruits samples.

The identification of residual thiram (Tr) in foods is vital in view of its harmful effects on human health. Herein, a ratiometric fluorescence sensor (I435/I590) based on rhodamine B/NH2-MIL-53(Al0.75Fe0.25) was constructed for the detection of Tr. Interestingly, the probe RhB/NH2-MIL-53(Bim) assisted by Cu2+ could rapidly and sensitively recognize Tr with a low detection limit of 0.11 µg/mL in 10 min. The fluorescence sensing mechanism was investigated using fluorescence spectra, UV-Vis absorption spectra, the fluorescence lifetime and quantum yield. The results showed that the excellent sensing performance was attributed to fluorescence resonance energy transfer, electrostatic interaction, and photoinduced electron transfer. In addition, the practical application of this platform showed acceptable relative recoveries for Tr (84.03-107.81 %), and precisions were also achieved (relative standard deviation ≤ 8.69 %, n = 3). These results show that the presented herein can be applied to monitor the Tr content in real fruit samples.

The dodeca-coordinated La©B8C4 +/0/- molecular wheels: conflicting aromaticity versus double aromaticity.

The transition-metal centered boron molecular wheels have attracted the attention of chemists. The highest deca-coordination number for central metal atoms was observed in D 10h Ta©B10 - and Nb©B10 - molecular wheels. Here, we report a theoretical study of La©B8C4 q (q = +1, 0, -1) clusters with the dodeca-coordinated La atom. The La©B8C4 q clusters adopt fascinating molecular wheel structures, showing a La atom enclosed by a perfect B8C4 monocyclic ring. The cationic La©B8C4 + cluster has a C 4v symmetry with the distinctly out-of-plane distortion of the La atom (0.70 Å), which is gradually flattened by the sequential reduction reaction. The distortion of the La atom from the plane in the neutral La©B8C4 cluster decreases to 0.46 Å. The La©B8C4 - species turns out to be perfectly planar. Chemical bonding analyses indicate that the neutral La©B8C4 and anionic La©B8C4 - possess 10σ and 9π/10π double aromaticity, respectively, obeying the principle of double aromaticity. However, the cationic La©B8C4 + has 10σ and 8π conflicting aromaticity, representing a counterexample in planar hyper-coordinated molecular wheels. The dodeca-coordination number in La©B8C4 q (q = +1, 0, -1) clusters is unprecedented, which provides a new idea and concept for searching planar hyper-coordinated systems.

The unique sandwich K6Be2B6H6 cluster with a real borozene B6H6 core.

Theoretical evidence is reported for a boron-based K6Be2B6H6 sandwich cluster, showing a perfectly D 6h B6H6 ring, being capped by two tetrahedral K3Be ligands. Due to the comfortable charge transfer, the sandwich is viable in [K3Be]3+[B6H6]6-[BeK3]3+ ionic complex in nature. The [B6H6]6- core with 6π aromaticity vividly imitates the benzene (C6H6), occurring as a real borozene. In contrast, the tetrahedral [K3Be]3+ ligand is 2σ three-dimensional aromatic, acting as the simple superatom. Thus, this complex possesses a collectively three-fold 2σ/6π/2σ aromaticity. The interlaminar interaction is governed by the robust electrostatic attraction. The unique chemical bonding gives rise to interesting dynamic fluxionality.

Perfectly planar concentric π-aromatic B18H3-, B18H4, B18H5+, and B18H6(2+) with [10]annulene character.

Based upon extensive density functional theory and wave function theory investigations, we predict the existence of the perfectly planar concentric π-aromatic D(3h) B(18)H(3)(-)(6), D(2h) B(18)H(4)(8), C(2v) B(18)H(5)(+)(10), and D(6h) B(18)H(6)(2+)(12) which are the smallest boron hydride clusters composed of a hybrid of the triangular and hexagonal motifs with a hexagonal hole at the center. These partially hydrogenated B(18) clusters, tentatively referred to as borannulenes in this work, prove to possess [10]annulene character with 10 delocalized π-electrons. Detailed adaptive natural density partitioning (AdNDP) analyses unravel the bonding patterns of the π plus σ doubly aromatic D(3h) B(18)H(3)(-)(6) and C(2v) B(18)H(5)(+)(10) and the π aromatic and σ antiaromatic D(2h) B(18)H(4)(8) and D(6h) B(18)H(6)(2+)(12). Borannulenes prove to possess negative nucleus-independent chemical shifts (NICS(zz)) comparable with that of [10]annulene and huge negative anisotropies of the magnetic susceptibility (AMS) much bigger than the latter. The slightly non-planar C(s) B(18)H(3)(-)(15) (which is essentially the same as D(3h) B(18)H(3)(-)) with a high first vertical detachment energy of 3.71 eV and the perfectly planar D(2h) B(18)H(4) neutral with a huge first excitation energy of 1.89 eV are predicted to be the most possible borannulenes to be targeted in future experiments.

Deciphering the mystery of hexagon holes in an all-boron graphene α-sheet.

Boron could be the next element after carbon capable of forming 2D-materials similar to graphene. Theoretical calculations predict that the most stable planar all-boron structure is the so-called α-sheet. The mysterious structure of the α-sheet with peculiar distribution of filled and empty hexagons is rationalized in terms of chemical bonding. We show that the hexagon holes serve as scavengers of extra electrons from the filled hexagons. This work could advance rational design of all-boron nanomaterials.

On the analogy of B-BO and B-Au chemical bonding in B11O- and B10Au- clusters.

During photoelectron spectroscopy experiments, the spectra of B(11)O(-) and B(10)Au(-) clusters are found to exhibit similar patterns except for a systematic spectral shift of ∼0.5 eV, hinting that they possess similar geometric structures. The electron affinities are measured to be 4.02 ± 0.04 eV for B(11)O and 3.55 ± 0.02 eV for B(10)Au. DFT calculations at the B3LYP level show that B(11)O(-) and B(10)Au(-) adopt similar C(1) ((1)A) ground states, which are based on the quasiplanar B(10) cluster interacting with a BO unit and Au, respectively. The B(11)O(-) and B(10)Au(-) clusters are thus valent isoelectronic because both BO and Au can be viewed as monovalent units, forming highly covalent B-BO and B-Au bonds analogous to the B-H bond in B(10)H(-). For B(10)Au(-), we also find a highly symmetric D(10h) ((1)A(1g)) planar molecular wheel as a minimum on the potential energy surface. However, it is 45 kcal/mol above the ground state at the B3LYP level and not viable for experimental observation. Natural bond orbital analyses reveal interesting covalent versus ionic B-Au bonding in the C(1) B(10)Au(-) and D(10h) B(10)Au(-) structures, respectively, providing insight for the design of D(nh) MB(n) molecular wheels.

Tetradecker transition metal complexes containing double planar hexacoordinate carbons and double planar heptacoordinate borons.

A theoretical investigation on tetradecker transition metal complexes of Cp-Fe-CB6-Fe-CB6-Fe-Cp (1) containing double planar hexacoordinate carbons and Cp-Fe-BB7-Fe-BB7-Fe-Cp (2) containing double planar heptcoordinate borons has been performed in this work at density functional theory level. [CpFe]+ monocations prove to effectively stabilize these unusual complexes, which are mainly maintained by effective d-pi coordination interactions between the partially filled Fe 3d orbitals and the delocalized pi molecular orbitals (MOs) of the four planar deckerlike ligands. The results obtained in these model computations expand the domain of ferrocene chemistry and could provide a new approach for synthesizing planar hyper-coordinate carbons and borons in transition metal complexes.

Transition metal-boron complexes BnM: from bowls (n = 8-14) to tires (n = 14).

Transition metal-boron complexes BnM have been predicted at density functional theory level to be molecular bowls (n = 8-14) hosting a transition metal atom (M) inside or molecular tires (n = 14) centered with a transition metal atom. Small Bn clusters prove to be effective inorganic ligands to all the VB-VIIIB transition metal elements in the periodic table. Density functional evidences obtained in this work strongly suggest that bowl-shaped fullerene analogues of Bn units exist in small BnM complexes and the bowl-to-tire structural transition occur to the first-row transition metal complexes BnM (M = Mn, Fe, Co) at n = 14, a size obviously smaller than n = 20 where the 2D-3D structural transition occurs to bare Bn. The half-sandwich-type B12Cr (C3v), full sandwich-type (B12)2Cr (D3d), bowl-shaped B14Fe (C2), and tire-shaped B14Fe (D7d) and B14Fe- (C7v) are the most interesting prototypes to be targeted in future experiments. These BnM complexes may serve as building blocks to form extended boron-rich BnMm tubes or cages (m > or = 2) or as structural units to be placed inside carbon nanotubes with suitable diameters.