Aromatic Metal-Centered Monocyclic Boron Rings: Co©B8− and Ru©B9−

Competition between drum and quasi-planar structures in RhB18-: motifs for metallo-boronanotubes and metallo-borophenes

Two nearly degenerate isomers, one a drum and the other quasi-planar, are discovered for the gaseous RhB₁₈^– cluster, revealing a competition between the metallo-boronanotube and metallo-borophene structures.

Metal-doped boron clusters provide new opportunities to design nanoclusters with interesting structures and bonding. A cobalt-doped boron cluster, CoB₁₈^–, has been observed recently to be planar and can be viewed as a motif for metallo-borophenes, whereas the D_9d drum isomer as a motif for metallo-boronanotubes is found to be much higher in energy. Hence, whether larger doped boron drums are possible is still an open question. Here we report that for RhB₁₈^– the drum and quasi-planar structures become much closer in energy and co-exist experimentally, revealing a competition between the metallo-boronanotube and metallo-borophene structures. Photoelectron spectroscopy of RhB₁₈^– shows a complicated spectral pattern, suggesting the presence of two isomers. Quantum chemistry studies indicate that the D_9d drum isomer and a quasi-planar isomer (C_s) compete for the global minimum. The enhanced stability of the drum isomer in RhB₁₈^– is due to the less contracted Rh 4d orbitals, which can have favorable interactions with the B₁₈ drum motif. Chemical bonding analyses show that the quasi-planar isomer of RhB₁₈^– is aromatic with 10 π electrons, whereas the observed RhB₁₈^– drum cluster sets a new record for coordination number of eighteen among metal complexes. The current finding shows that the size of the boron drum can be tuned by appropriate metal dopants, suggesting that even larger boron drums with 5d, 6d transition metal, lanthanide or actinide metal atoms are possible.

Electronic Structure and Thermochemical Parameters of the Silicon-Doped Boron Clusters BnSi, with n = 8-14, and Their Anions

We performed a systematic investigation on silicon-doped boron clusters BnSi (n = 8-14) in both neutral and anionic states using quantum chemical methods. Thermochemical properties of the lowest-lying isomers of BnSi(0/-) clusters such as total atomization energies, heats of formation at 0 and 298 K, average binding energies, dissociation energies, etc. were evaluated by using the composite G4 method. The growth pattern for BnSi(0/-) with n = 8-14 is established as follows: (i) BnSi(0/-) clusters tend to be constructed by substituting B atom by Si-atom or adding one Si-impurity into the parent Bn clusters with n to be even number, and (ii) Si favors an external position of the Bn frameworks. Our theoretical results reveal that B8Si, B9Si(-), B10Si and B13Si(-) are systems with enhanced stability due to having high average binding energies, second-order difference in energies and dissociation energies. Especially, by analyzing the MOs, ELF, and ring current maps, the enhanced stability of B8Si can be rationalized in terms of a triple aromaticity.

Linear, planar, and tubular molecular structures constructed by double planar tetracoordinate carbon D2hC2(BeH)4 species via hydrogen-bridged -BeH2Be- bonds

This computational study identifies the rhombic D2hC2 (BeH)4 (2a) to be a species featuring double planar tetracoordinate carbons (ptCs). Aromaticity and the peripheral BeBeBeBe bonding around CC core contribute to the stabilization of the ptC structure. Although the ptC structure is not a global minimum, its high kinetic stability and its distinct feature of having a bonded C2 core from having two separated carbon atoms in the global minimum and other low-lying minima could make the ptC structure to be preferred if the carbon source is dominated by C2 species. The electron deficiency of the BeH group allows the ptC species to serve as building blocks to construct large/nanostructures, such as linear chains, planar sheets, and tubes, via intermolecular hydrogen-bridged bonds (HBBs). Formation of one HBB bond releases more than 30.0 kcal/mol of energy, implying the highly exothermic formation processes and the possibility to synthesize these nano-size structures.

Beyond organic chemistry: aromaticity in atomic clusters

We describe joint experimental and theoretical studies carried out collaboratively in the authors' labs for understanding the structures and chemical bonding of novel atomic clusters, which exhibit aromaticity.

Stabilization of fullerene-like boron cages by transition metal encapsulation

The stabilization of fullerene-like boron (B) cages in the free-standing form has been long sought after and a challenging problem. Studies that have been carried out for more than a decade have confirmed that the planar or quasi-planar polymorphs are energetically favored ground states over a wide range of small and medium-sized B clusters. Recently, the breakthroughs represented by Nat. Chem., 2014, 6, 727 established that the transition from planar/quasi-planar to cage-like Bn clusters occurs around n = ∼38-40, paving the way for understanding the intriguing chemistry of B-fullerene. We herein demonstrate that the transition demarcation, n, can be significantly reduced with the help of transition metal encapsulation. We explore via extensive first-principles swarm-intelligence based structure searches the free energy landscapes of B24 clusters doped by a series of transition metals and find that the low-lying energy regime is generally dominated by cage-like isomers. This is in sharp contrast to that of bare B24 clusters, where the quasi-planar and rather irregular polyhedrons are prevalent. Most strikingly, a highly symmetric B cage with D3h symmetry is discovered in the case of Mo or W encapsulation. The endohedral D3h cages exhibit robust thermodynamic, dynamic and chemical stabilities, which can be rationalized in terms of their unique electronic structure of an 18-electron closed-shell configuration. Our results indicate that transition metal encapsulation is a feasible route for stabilizing medium-sized B cages, offering a useful roadmap for the discovery of more B fullerene analogues as building blocks of nanomaterials.

Strong chemisorption of CO on M@Bn− (M = Co, Ir, Rh, Ru, Ta, Nb, n = 8–10) clusters: an implication for wheel boron clusters as CO gas detectors

The potential applications of wheel M@B_n⁻ clusters in CO detection have been proposed theoretically.

Exploring the geometrical structures of X©BnHnm [(X, m) = (B, +1), (C, +2) for n = 5; (X, m) = (Be, 0), (B, +1) for n = 6] by an electronic method

The (quasi-)planar wheel-type structures can be obtained by adding electrons.

Planar wheel-type M©BnHn2−/−/0 clusters (M = Cr, Mn and Fe for dianion, anion and neutral, respectively; n = 6 and 7)

A new “electronic” strategy that adds two electrons into the d_z2 orbital of the central M atom to form a lone pair, in contrast to Hoffmann’s “electronic” strategy to turn the bowl-type MB_nH_n^0/+ (M = Cr and Mn; n = 6 and 7) clusters into planar wheel-type clusters.

How can carbon favor planar multi-coordination in boron-based clusters? Global structures of CBxEy2−(E = Al, Ga, x + y = 4)

CB₂E₂Mg (E = Al, Ga) designed in the present study represents the first successful design of a boron-based planar penta-coordinate carbon (ppC) structures as the global minima.

Unconventional charge distribution in the planar wheel-type M©B6H6−/0/+ (M = Mn, Fe and Co): central M with negative charges and peripheral boron ring with positive charges

Unconventional charge distribution exists in the planar wheel-type M©B₆H₆^−/0/+ (M = Mn, Fe and Co).

Planar pentacoordinate carbons in CBe54− derivatives

Ab initio computations show that the global minimum structure of the CBe₅Li_nⁿ⁻⁴ clusters (n = 1 to 5) contains a planar pentacoordinate carbon atom.

An insight into the structures, stabilities, and bond character of B(n)Pt (n=1∼6) clusters

We perform a systematical investigation on the geometry, thermodynamic/kinetic stability, and bonding nature of low-lying isomers of BnPt (n=1-6) at the CCSD(T)/[6-311+G(d)/LanL2DZ]//B3LYP/[6-311+G(d)/LanL2DZ] level. The most stable isomers of BnPt (n=1-6) adopt planar or quasi-planar structure. BnPt (n=2-5) clusters can be generated by capping a Pt atom on the B-B edge of pure boron clusters. However, For B6Pt with non-planar structure, a single doped Pt atom significantly affects the shape of the host boron cluster. The dopant of the Pt atom can improve the stability of pure boron clusters. The valence molecular orbital (VMO), electron localization function (ELF), and Mayer bond order (MBO) are applied to gain insight into the bonding nature of BnPt (n=2-6) isomers. The aromaticity for some isomers of BnPt (n=2-6) is analyzed and discussed in terms of VMO, ELF, adaptive natural density partitioning (AdNDP), and nucleus-independent chemical shift (NICS) analyses. Results obtained from the energy and cluster decomposition analyses demonstrate that B2Pt and B4Pt exhibits as highly stable. Importantly, some isomers of BnPt (n=2-5) are stable both thermodynamically and kinetically, which are observable in future experiment.

Transition metal Mo-doped boron clusters: A computational investigation

Geometries associated with relative stabilities and energy gaps of the Mo -doped boron clusters have been investigated systematically by using density functional theory. The critical size of Mo -encapsulated B n structures emerges as n = 10, the evaluated relative stabilities in term of the calculated fragmentation energies reveal that the MoB 6 has enhanced stabilities over their neighboring clusters. Furthermore, the calculated polarities of the MoB n reveal that the hypercoordinated planar MoB 10 wheel is a weakened polar molecule and MoB 11 ring is a nonpolar molecule, and aromatic properties are discussed. Additionally, the MoB 10 cluster with smaller highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap is supposed to be stronger chemical activity and smaller chemical hardness. Moreover, the recorded natural populations show that the charges transfer from boron framework to Mo atom. It should be pointed out that the remarkable charge-transfer features of MoB n clusters are distinctly similar to those of transitional metal (TM)-doped Si n clusters; growth-pattern of the TMBn depends on the doped TM impurity.

Pi and sigma double conjugations in boronyl polyboroene nanoribbons: B(n)(BO)2- and B(n)(BO)2 (n = 5-12)

A series of boron dioxide clusters, B(x)O2(-) (x = 7-14), have been produced and investigated using photoelectron spectroscopy and quantum chemical calculations. The dioxide clusters are shown to possess elongated ladder-like structures with two terminal boronyl (BO) groups, forming an extensive series of boron nanoribbons, B(n)(BO)2(-) (n = 5-12). The electron affinities of B(n)(BO)2 exhibit a 4n periodicity, indicating that the rhombic B4 unit is the fundamental building block in the nanoribbons. Both π and σ conjugations are found to be important in the unique bonding patterns of the boron nanoribbons. The π conjugation in these clusters is analogous to the polyenes (aka polyboroenes), while the σ conjugation plays an equally important role in rendering the stability of the nanoribbons. The concept of σ conjugation established here has no analogues in hydrocarbons. Calculations suggest the viability of even larger boronyl polyboroenes, B16(BO)2 and B20(BO)2, extending the boron nanoribbons to ~1.5 nm in length or possibly even longer. The nanoribbons form a new class of nanowires and may serve as precursors for a variety of boron nanostructures.

Understanding boron through size-selected clusters: structure, chemical bonding, and fluxionality

Boron is an interesting element with unusual polymorphism. While three-dimensional (3D) structural motifs are prevalent in bulk boron, atomic boron clusters are found to have planar or quasi-planar structures, stabilized by localized two-center-two-electron (2c-2e) σ bonds on the periphery and delocalized multicenter-two-electron (nc-2e) bonds in both σ and π frameworks. Electron delocalization is a result of boron's electron deficiency and leads to fluxional behavior, which has been observed in B13(+) and B19(-). A unique capability of the in-plane rotation of the inner atoms against the periphery of the cluster in a chosen direction by employing circularly polarized infrared radiation has been suggested. Such fluxional behaviors in boron clusters are interesting and have been proposed as molecular Wankel motors. The concepts of aromaticity and antiaromaticity have been extended beyond organic chemistry to planar boron clusters. The validity of these concepts in understanding the electronic structures of boron clusters is evident in the striking similarities of the π-systems of planar boron clusters to those of polycyclic aromatic hydrocarbons, such as benzene, naphthalene, coronene, anthracene, or phenanthrene. Chemical bonding models developed for boron clusters not only allowed the rationalization of the stability of boron clusters but also lead to the design of novel metal-centered boron wheels with a record-setting planar coordination number of 10. The unprecedented highly coordinated borometallic molecular wheels provide insights into the interactions between transition metals and boron and expand the frontier of boron chemistry. Another interesting feature discovered through cluster studies is boron transmutation. Even though it is well-known that B(-), formed by adding one electron to boron, is isoelectronic to carbon, cluster studies have considerably expanded the possibilities of new structures and new materials using the B(-)/C analogy. It is believed that the electronic transmutation concept will be effective and valuable in aiding the design of new boride materials with predictable properties. The study of boron clusters with intermediate properties between those of individual atoms and bulk solids has given rise to a unique opportunity to broaden the frontier of boron chemistry. Understanding boron clusters has spurred experimentalists and theoreticians to find new boron-based nanomaterials, such as boron fullerenes, nanotubes, two-dimensional boron, and new compounds containing boron clusters as building blocks. Here, a brief and timely overview is presented addressing the recent progress made on boron clusters and the approaches used in the authors' laboratories to determine the structure, stability, and chemical bonding of size-selected boron clusters by joint photoelectron spectroscopy and theoretical studies. Specifically, key findings on all-boron hydrocarbon analogues, metal-centered boron wheels, and electronic transmutation in boron clusters are summarized.

Photoelectron spectroscopy of boron-gold alloy clusters and boron boronyl clusters: B3Au(n)(-) and B3(BO)n(-) (n = 1, 2)

Photoelectron spectroscopy and density-functional theory are combined to study the structures and chemical bonding in boron-gold alloy clusters and boron boronyl clusters: B3Au(n)(-) and B3(BO)n(-) (n = 1, 2). Vibrationally resolved photoelectron spectra are obtained for all four species and the B-Au and B-BO clusters exhibit similar spectral patterns, with the latter species having higher electron binding energies. The electron affinities of B3Au, B3Au2, B3(BO), and B3(BO)2 are determined to be 2.29 ± 0.02, 3.17 ± 0.03, 2.71 ± 0.02, and 4.44 ± 0.02 eV, respectively. The anion and neutral clusters turn out to be isostructural and isovalent to the B3H(n)(-)∕B3H(n) (n = 1, 2) species, which are similar in bonding owing to the fact that Au, BO, and H are monovalent σ ligands. All B3Au(n)(-) and B3(BO)n(-) (n = 1, 2) clusters are aromatic with 2π electrons. The current results provide new examples for the Au∕H and BO∕H isolobal analogy and enrich the chemistry of boronyl and gold.

Stabilization of diborane(4) by transition metal fragments and a novel metal to π Dewar-Chatt-Duncanson model of back donation

The feasibility of using transition metal fragments to stabilize B2H4 in planar configuration by donating 2 electrons to the boron moiety is investigated. Building upon the existing theoretical and experimental data and aided by the isolobal analogy, the model transition metal complexes Cr(CO)4B2H4 (6), Mn(CO)CpB2H4 (7), Fe(CO)3B2H4 (8) and CoCpB2H4 (9) are chosen to illustrate this unique bonding feature--bond strengthening with π-back donation. Other possible types of complexes with B2H4 and the metal fragment are also explored and the energies are compared. One of the low energy isomers wherein the planar B2H4 interacts with the metal fragment in an in-plane fashion represents a unique case study for the Dewar-Chatt-Duncanson model. In this complex the back-donation from the metal fills the π bonding orbital between the two boron atoms thus forming a B=B double bond.

Transition-metal-centered monocyclic boron wheel clusters (M©Bn): a new class of aromatic borometallic compounds

Atomic clusters have intermediate properties between that of individual atoms and bulk solids, which provide fertile ground for the discovery of new molecules and novel chemical bonding. In addition, the study of small clusters can help researchers design better nanosystems with specific physical and chemical properties. From recent experimental and computational studies, we know that small boron clusters possess planar structures stabilized by electron delocalization both in the σ and π frameworks. An interesting boron cluster is B(9)(-), which has a D(8h) molecular wheel structure with a single boron atom in the center of a B(8) ring. This ring in the D(8h)-B(9)(-) cluster is connected by eight classical two-center, two-electron bonds. In contrast, the cluster's central boron atom is bonded to the peripheral ring through three delocalized σ and three delocalized π bonds. This bonding structure gives the molecular wheel double aromaticity and high electronic stability. The unprecedented structure and bonding pattern in B(9)(-) and other planar boron clusters have inspired the designs of similar molecular wheel-type structures. But these mimics instead substitute a heteroatom for the central boron. Through recent experiments in cluster beams, chemists have demonstrated that transition metals can be doped into the center of the planar boron clusters. These new metal-centered monocyclic boron rings have variable ring sizes, M©B(n) and M©B(n)(-) with n = 8-10. Using size-selected anion photoelectron spectroscopy and ab initio calculations, researchers have characterized these novel borometallic molecules. Chemists have proposed a design principle based on σ and π double aromaticity for electronically stable borometallic cluster compounds, featuring a highly coordinated transition metal atom centered inside monocyclic boron rings. The central metal atom is coordinatively unsaturated in the direction perpendicular to the molecular plane. Thus, chemists may design appropriate ligands to synthesize the molecular wheels in the bulk. In this Account, we discuss these recent experimental and theoretical advances of this new class of aromatic borometallic compounds, which contain a highly coordinated central transition metal atom inside a monocyclic boron ring. Through these examples, we show that atomic clusters can facilitate the discovery of new structures, new chemical bonding, and possibly new nanostructures with specific, advantageous properties.

Probing the structures and chemical bonding of boron-boronyl clusters using photoelectron spectroscopy and computational chemistry: B4(BO)(n)- (n = 1-3)

The electronic and structural properties of a series of boron oxide clusters, B(5)O(-), B(6)O(2)(-), and B(7)O(3)(-), are studied using photoelectron spectroscopy and density functional calculations. Vibrationally resolved photoelectron spectra are obtained, yielding electron affinities of 3.45, 3.54, and 4.94 eV for the corresponding neutrals, B(5)O, B(6)O(2), and B(7)O(3), respectively. Structural optimizations show that these oxide clusters can be formulated as B(4)(BO)(n)(-) (n = 1-3), which involve boronyls coordinated to a planar rhombic B(4) cluster. Chemical bonding analyses indicate that the B(4)(BO)(n)(-) clusters are all aromatic species with two π electrons.

[CTi7(2+)]: Heptacoordinate Carbon Motif?

A heptacoordinate carbon motif [CTi7(2+)] is predicted to be a highly stable structure (with D5h point group symmetry) based on ab initio computation. This motif possesses a sizable HOMO-LUMO gap along with the lowest vibrational frequency greater than 95 cm(-1). An investigation of the motif-containing neutral species [CTi7(2+)][BH4(-)]2 further confirms the chemical stability of the heptacoordinate carbon motif. In view of its structural stability, a quasi-one-dimensional (quasi-1D) nanowire [CTi7]n[C16H8]n is built from the carbon motifs. This organometallic nanowire is predicted to be metallic based on density functional theory computation.

LARGE-SCALE FIRST PRINCIPLES CONFIGURATION INTERACTION CALCULATIONS OF OPTICAL ABSORPTION IN BORON CLUSTERS

We have performed systematic large-scale all-electron correlated calculations on boron clusters B n(n = 2 - 5), to study their linear optical absorption spectra. Several possible isomers of each cluster were considered, and their geometries were optimized at the coupled-cluster singles doubles (CCSD) level of theory. Using the optimized ground-state geometries, the excited states of different clusters were computed using the multi-reference singles-doubles configuration–interaction (MRSDCI) approach, which includes electron correlation effects at a sophisticated level. These CI wave functions were used to compute the transition dipole matrix elements connecting the ground and various excited states of different clusters, eventually leading to their linear absorption spectra. The convergence of our results with respect to the basis sets, and the size of the CI expansion were carefully examined. The contribution of configurations to many body wave-function of various excited states suggests that the excitations involved are collective, plasmonic type.

A special conjugated model around sp3 carbon atoms: density functional theory study on the homoaromatic electron delocalization and applications of benzo-fused tetra(triptycene)porphyrins

The three-unit homoaromatic electron-delocalizing nature of the benzo-fused tetra(triptycene)porphyrins (TTPs) with a three-dimensional conjugated model is clarified using density functional theory studies. Due to the electron delocalization, the unidirectional photon-induced current of this kind of TTP molecular skeleton with a highest efficiency of about 90% in the range between 350 and 500 nm gives them great potential as efficient solar antenna collectors. In addition, their active triptycene cups fused at the central porphyrin core render possible potential application in host-guest chemistry.