Metal-Centered Boron-Wheel Cluster of Y©B112- with Rare D11h Symmetry.

Molecules with high point-group symmetry are interesting prototype species in the textbook. As transition metal-centered boron clusters tend to have highly symmetric structures to fulfill multicenter bonding and high stability, new boron clusters with rare point-group symmetry may be viable. Through in-depth scrutiny over the structures of experimentally already observed transition metal-centered boron-wheel complexes, geometric and electronic design principles are summarized, based on which we studied M©B11k- (M = Y, La; Zr, Hf; k = 1, 2) clusters and found that a Y©B112- boron-wheel complex has an unprecedented D11h point-group symmetry. The remarkable stability of the planar Y©B112- complex is illustrated via extensive global-minimum structural search as well as comprehensive chemical bonding analyses. Similar to other boron-wheel complexes, the Y©B112- complex is shown to possess σ and π double aromaticity, indeed following the electronic design principle previously summarized. This new compound is expected to be experimentally identified, which will extend the currently known largest possible planar molecular symmetry and enrich the metal-centered boron-wheel class.

Probing the Structures and Lanthanum-Lanthanum Bonding in La2Bn- (n = 4-6) Clusters.

We report an investigation on the structures and chemical bonding in a series of di-lanthanum boron clusters, La2Bn- (n = 4-6), using photoelectron spectroscopy and theoretical calculations. Well-resolved photoelectron spectra are obtained and used to verify the global minima of the lanthanide boron clusters. The structures of La2B4- and La2B5- are found to consist of open B4 and B5 rings, respectively, around the La2 dimer equatorially. Theoretical evidence of La-La σ bonding is obtained in La2B4-, whereas the bonding in La2B5- is similar to that of an incomplete inverse sandwich without real La-La bonding. The global minimum of La2B6- is completely different, where one of the La atoms can be viewed as substituting a B atom of the B7 cluster due to the high electronic stability of the B73- borozene. The resulting lanthaborozene [LaB6]3- forms a half-sandwich structure with the second La atom, with evidence of La-La σ bonding. Lanthanide-lanthanide bonds are relatively rare in chemistry. The current work suggests that binary lanthanide boron clusters provide interesting systems to study lanthanide-lanthanide bonding.

Investigation of Pb-B Bonding in PbB2(BO)n- (n = 0-2): Transformation from Aromatic PbB2- to Pb[B2(BO)2]-/0 Complexes with BB Triple Bonds.

Boron has been found to be able to form multiple bonds with lead. To probe Pb-B bonding, here we report an investigation of three Pb-doped boron clusters, PbB2-, PbB3O-, and PbB4O2-, which are produced by a laser ablation cluster source and characterized by photoelectron spectroscopy and ab initio calculations. The most stable structures of PbB2-, PbB3O-, and PbB4O2- are found to follow the formula, [PbB2(BO)n]- (n = 0-2), with zero, one, and two boronyl ligands coordinated to a triangular and aromatic PbB2 core, respectively. The PbB2- cluster contains a BB double bond and two Pb-B single bonds. The coordination of BO is observed to weaken Pb-B bonding but strengthen the BB bond in [PbB2(BO)n]- (n = 1, 2). The anionic [PbB2(BO)2]- and its corresponding neutral closed-shell [PbB2(BO)2] contain a BB triple bond. A low-lying Y-shaped isomer is also observed for PbB4O2-, consisting of a central sp2 hybridized B atom bonded to two boronyl ligands and a PbB unit.

Probing the Nature of the Transition-Metal-Boron Bonds and Novel Aromaticity in Small Metal-Doped Boron Clusters Using Photoelectron Spectroscopy.

Photoelectron spectroscopy combined with quantum chemistry has been a powerful approach to elucidate the structures and bonding of size-selected boron clusters (Bn-), revealing a prevalent planar world that laid the foundation for borophenes. Investigations of metal-doped boron clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The current review focuses on recent advances in transition-metal-doped boron clusters, including the discoveries of metal-boron multiple bonds and metal-doped novel aromatic boron clusters. The study of the RhB- and RhB2O- clusters led to the discovery of the first quadruple bond between boron and a transition-metal atom, whereas a metal-boron triplebond was found in ReB2O- and IrB2O-. The ReB4- cluster was shown to be the first metallaborocycle with Möbius aromaticity, and the planar ReB6- cluster was found to exhibit aromaticity analogous to metallabenzenes.

Expanded Inverse-Sandwich Complexes of Lanthanum Borides: La2B10- and La2B11.

Inverse-sandwich structures have been observed recently for dilanthanide boride clusters, in which two Ln atoms sandwich a monocyclic Bx ring for x = 7-9. An interesting question is if larger Bx rings are possible to form such inverse-sandwich clusters. Here we address this question by investigating La2B10- and La2B11- using photoelectron spectroscopy and ab initio quantum chemical calculations. Photoelectron spectra of La2B10- and La2B11- show complicated, but well-resolved, spectral features that are used to compare with theoretical calculations. We have found that global minimum structures of the two clusters are based on the octa-boron ring. The global minimum of La2B10- consists of two chiral enantiomers with C1 symmetry, which can be viewed as adding a B2 unit off-plane to the B8 ring, whereas that of La2B11- can be viewed as adding a B3 unit in-plane to the B8 ring in a second coordination shell. Chemical bonding analyses reveal localized B-B bonds on the edge of the clusters and delocalized bonds in the expanded boron frameworks. The interactions between the La atoms and the boron frameworks include the unique (d-p)δ bonding, which was found to be the key for inverse-sandwich complexes with monocyclic boron rings. The current study confirms that the largest monocyclic boron ring to form the inverse-sandwich structures is B9 and provide insights into the structural evolutions of larger lanthanide boride clusters.

Observation of highly stable and symmetric lanthanide octa-boron inverse sandwich complexes.

While boron forms a wide range of metal borides with important industrial applications, there has been relatively little attention devoted to lanthanide boride clusters. Here we report a joint photoelectron spectroscopy and quantum chemical study on two octa-boron di-lanthanide clusters, Ln2B8- (Ln = La, Pr). We found that these clusters form highly stable inverse sandwich structures, [Ln-B8-Ln]-, with strong Ln and B8 bonding via interactions between the Ln 5d orbitals and the delocalized σ and π orbitals on the B8 ring. A (d-p)δ bond, involving the 5dδ and the antibonding π orbital of the B8 ring, is observed to be important in the Ln-B8 interactions. The highly symmetric inverse sandwich structures are overwhelmingly more stable than any other isomers. Upon electron detachment, the (d-p)δ orbitals become half-filled, giving rise to a triplet ground state for neutral La2B8 In addition to the two unpaired electrons in the (d-p)δ orbitals upon electron detachment, the neutral Pr2B8 complex also contains two unpaired 4f electrons on each Pr center. The six unpaired spins in Pr2B8 are ferromagnetically coupled to give rise to a septuplet ground state. The current work suggests that highly magnetic Ln…B8…Ln inverse sandwiches or 1D Ln…B8…Ln nanowires may be designed with novel electronic and magnetic properties.

Observation of Transition-Metal-Boron Triple Bonds in IrB2 O- and ReB2 O.

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal dπ →BR back-donation, despite the electron deficiency of boron. An electron-precise metal-boron triple bond was first observed in BiB2 O- [Bi≡B-B≡O]- in which both boron atoms can be viewed as sp-hybridized and the [B-BO]- fragment is isoelectronic to a carbyne (CR). To search for the first electron-precise transition-metal-boron triple-bond species, we have produced IrB2 O- and ReB2 O- and investigated them by photoelectron spectroscopy and quantum-chemical calculations. The results allow to elucidate the structures and bonding in the two clusters. We find IrB2 O- has a closed-shell bent structure (Cs , 1 A') with BO- coordinated to an Ir≡B unit, (- OB)Ir≡B, whereas ReB2 O- is linear (C∞v , 3 Σ- ) with an electron-precise Re≡B triple bond, [Re≡B-B≡O]- . The results suggest the intriguing possibility of synthesizing compounds with electron-precise M≡B triple bonds analogous to classical carbyne systems.

Lanthanides with Unusually Low Oxidation States in the PrB3- and PrB4- Boride Clusters.

Lanthanide elements typically exhibit a +III oxidation state (OS) in chemical compounds with a few in +IV or even +V OS. Although lanthanides with +II OS have been observed recently in organometallic compounds, +I OS is extremely rare. Using a joint photoelectron spectroscopy and quantum theoretical study, we have found two low OS lanthanides in doped boron clusters, PrB3- and PrB4-. These two clusters are shown to have planar structures, in which the Pr atom is bonded to the aromatic boron clusters via two Pr-B σ bonds. Chemical bonding and electronic structure analyses reveal that the Pr atom is in a very low OS in the two boride clusters: +II in PrB3- and +I in PrB4-. The current finding suggests that there should exist a whole class of boride complexes featuring rather low-valent lanthanides and expands the frontier of lanthanide chemistry.

Re©B8- and Re©B9-: New Members of the Transition-Metal-Centered Borometallic Molecular Wheel Family.

Transition-metal-centered monocyclic boron wheel clusters (M©B n q) represent a family of interesting borometallic compounds with double aromaticity. A variety of early and late transition metal atoms have been found to form such structures with high symmetries and various B n ring sizes. Here we report a combined photoelectron spectroscopy and quantum-chemistry theoretical study of two M©B n- clusters from the middle of the transition metal series: Re©B8- and Re©B9-. Global minimum structure searches revealed that ReB8- adopts a pseudo- C8 v structure while ReB9- is a perfectly planar D9 h molecular wheel. Chemical bonding analyses showed that both clusters exhibit σ and π double aromaticity and obey the electronic design principle for metal-centered borometallic molecular wheels. The central Re atoms are found to possess unusually low oxidation states of +I in Re©B8- and +II in Re©B9-, i.e., the Re atom behaves similarly to late transition metal elements (Ru, Fe, Co, Rh, Ir) in the M©B n- molecular wheels. These two clusters become new members of the family of transition-metal-centered monocyclic borometallic molecular wheels, which may be viable for chemical syntheses with appropriate ligands.

Bismuth-Boron Multiple Bonding in BiB2 O- and Bi2 B.

Despite its electron deficiency, boron is versatile in forming multiple bonds. Transition-metal-boron double bonding is known, but boron-metal triple bonds have been elusive. Two bismuth boron cluster anions, BiB2 O- and Bi2 B- , containing triple and double B-Bi bonds are presented. The BiB2 O- and Bi2 B- clusters are produced by laser vaporization of a mixed B/Bi target and characterized by photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra are obtained and interpreted with the help of ab initio calculations, which show that both species are linear. Chemical bonding analyses reveal that Bi forms triple and double bonds with boron in BiB2 O- ([Bi≡B-B≡O]- ) and Bi2 B- ([Bi=B=Bi]- ), respectively. The Bi-B double and triple bond strengths are calculated to be 3.21 and 4.70 eV, respectively. This is the first experimental observation of Bi-B double and triple bonds, opening the door to design main-group metal-boron complexes with multiple bonding.

PrB7- : A Praseodymium-Doped Boron Cluster with a PrII Center Coordinated by a Doubly Aromatic Planar η7 -B73- Ligand.

The structure and bonding of a Pr-doped boron cluster (PrB7- ) are investigated using photoelectron spectroscopy and quantum chemistry. The adiabatic electron detachment energy of PrB7- is found to be low [1.47(8) eV]. A large energy gap is observed between the first and second detachment features, indicating a highly stable neutral PrB7 . Global minimum searches and comparison between experiment and theory show that PrB7- has a half-sandwich structure with C6v symmetry. Chemical bonding analyses show that PrB7- can be viewed as a PrII [η7 -B73- ] complex with three unpaired electrons, corresponding to a Pr (4f2 6s1 ) open-shell configuration. Upon detachment of the 6s electron, the neutral PrB7 cluster is a highly stable PrIII [η7 -B73- ] complex with Pr in its favorite +3 oxidation state. The B73- ligand is found to be highly stable and doubly aromatic with six delocalized π and six delocalized σ electrons and should exist for a series of lanthanide MIII [η7 -B73- ] complexes.

The Planar CoB18 (-) Cluster as a Motif for Metallo-Borophenes.

Monolayer-boron (borophene) has been predicted with various atomic arrangements consisting of a triangular boron lattice with hexagonal vacancies. Its viability was confirmed by the observation of a planar hexagonal B36 cluster with a central six-membered ring. Here we report a planar boron cluster doped with a transition-metal atom in the boron network (CoB18 (-) ), suggesting the prospect of forming stable hetero-borophenes. The CoB18 (-) cluster was characterized by photoelectron spectroscopy and quantum chemistry calculations, showing that its most stable structure is planar with the Co atom as an integral part of a triangular boron lattice. Chemical bonding analyses show that the planar CoB18 (-) is aromatic with ten π-electrons and the Co atom has strong covalent interactions with the surrounding boron atoms. The current result suggests that transition metals can be doped into the planes of borophenes to create metallo-borophenes, opening vast opportunities to design hetero-borophenes with tunable chemical, magnetic, and optical properties.

Cavity-enabled enhancement of ultrafast intramolecular vibrational redistribution over pseudorotation.

Vibrational strong coupling (VSC) between molecular vibrations and microcavity photons yields a few polaritons (light-matter modes) and many dark modes (with negligible photonic character). Although VSC is reported to alter thermally activated chemical reactions, its mechanisms remain opaque. To elucidate this problem, we followed ultrafast dynamics of a simple unimolecular vibrational energy exchange in iron pentacarbonyl [Fe(CO)5] under VSC, which showed two competing channels: pseudorotation and intramolecular vibrational-energy redistribution (IVR). We found that under polariton excitation, energy exchange was overall accelerated, with IVR becoming faster and pseudorotation being slowed down. However, dark-mode excitation revealed unchanged dynamics compared with those outside of the cavity, with pseudorotation dominating. Thus, despite controversies around thermally activated VSC modified chemistry, our work shows that VSC can indeed alter chemistry through a nonequilibrium preparation of polaritons.

Boron-lead multiple bonds in the PbB2O- and PbB3O2- clusters.

Despite its electron deficiency, boron can form multiple bonds with a variety of elements. However, multiple bonds between boron and main-group metal elements are relatively rare. Here we report the observation of boron-lead multiple bonds in PbB2O- and PbB3O2-, which are produced and characterized in a cluster beam. PbB2O- is found to have an open-shell linear structure, in which the bond order of B☱Pb is 2.5, while the closed-shell [Pb≡B-B≡O]2- contains a B≡Pb triple bond. PbB3O2- is shown to have a Y-shaped structure with a terminal B = Pb double bond coordinated by two boronyl ligands. Comparison between [Pb≡B-B≡O]2-/[Pb=B(B≡O)2]- and the isoelectronic [Pb≡B-C≡O]-/[Pb=B(C≡O)2]+ carbonyl counterparts further reveals transition-metal-like behaviors for the central B atoms. Additional theoretical studies show that Ge and Sn can form similar boron species as Pb, suggesting the possibilities to synthesize new compounds containing multiple boron bonds with heavy group-14 elements.

The smallest 4f-metalla-aromatic molecule of cyclo-PrB2 - with Pr-B multiple bonds.

The concept of metalla-aromaticity proposed by Thorn-Hoffmann (Nouv. J. Chim. 1979, 3, 39) has been expanded to organometallic molecules of transition metals that have more than one independent electron-delocalized system. Lanthanides, with highly contracted 4f atomic orbitals, are rarely found in multiply aromatic systems. Here we report the discovery of a doubly aromatic triatomic lanthanide-boron molecule PrB2 - based on a joint photoelectron spectroscopy and quantum chemical investigation. Global minimum structural searches reveal that PrB2 - has a C 2v triangular structure with a paramagnetic triplet 3B2 electronic ground state, which can be viewed as featuring a trivalent Pr(III,f2) and B2 4-. Chemical bonding analyses show that this cyclo-PrB2 - species is the smallest 4f-metalla-aromatic system exhibiting σ and π double aromaticity and multiple Pr-B bonding characters. It also sheds light on the formation of the rare B2 4- tetraanion by the high-lying 5d orbitals of the 4f-elements, completing the isoelectronic B2 4-, C2 2-, N2, and O2 2+ series.

Monovalent lanthanide(I) in borozene complexes.

Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB8-, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB8- species changes from Cs to C7v symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C7v-LnB8- clusters can be viewed as a monovalent Ln(I) coordinated by a η8-B82- doubly aromatic ligand. The B73-, B82-, and B9- series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C5H5-, C6H6, and C7H7+, respectively, with similar trends of size and charge state and they are named collectively as "borozenes". Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes.

Transition-metal-like bonding behaviors of a boron atom in a boron-cluster boronyl complex [(η7-B7)-B-BO].

Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B9O-) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B9O-: the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B9O- is found surprisingly to be an umbrella-like structure (C 6v, 1A1) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer (C s, 1A') consisting of a BO unit bonded to a disk-like B8 cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B9O- can be viewed as a central boron atom coordinated by a η7-B7 ligand on one side and a BO ligand on the other side, [(η7-B7)-B-BO]-. The central B atom is found to share its valence electrons with the B7 unit to fulfill double aromaticity, similar to that in half-sandwich [(η7-B7)-Zn-CO]- or [(η7-B7)-Fe(CO)3]- transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron.

B48-: a bilayer boron cluster.

Size-selected negatively-charged boron clusters (Bn-) have been found to be planar or quasi-planar in a wide size range. Even though cage structures emerged as the global minimum at B39-, the global minimum of B40- was in fact planar. Only in the neutral form did the B40 borospherene become the global minimum. How the structures of larger boron clusters evolve is of immense interest. Here we report the observation of a bilayer B48- cluster using photoelectron spectroscopy and first-principles calculations. The photoelectron spectra of B48- exhibit two well-resolved features at low binding energies, which are used as electronic signatures to compare with theoretical calculations. Global minimum searches and theoretical calculations indicate that both the B48- anion and the B48 neutral possess a bilayer-type structure with D2h symmetry. The simulated spectrum of the D2h B48- agrees well with the experimental spectral features, confirming the bilayer global minimum structure. The bilayer B48-/0 clusters are found to be highly stable with strong interlayer covalent bonding, revealing a new structural type for size-selected boron clusters. The current study shows the structural diversity of boron nanoclusters and provides experimental evidence for the viability of bilayer borophenes.

Observation of Four-Fold Boron-Metal Bonds in RhB(BO-) and RhB.

The maximum bond order between two main-group atoms was known to be three. However, it has been suggested recently that there is quadruple bonding in C2 and analogous eight-valence electron species. While the quadruple bond in C2 has aroused some debates, an interesting question is: are main-group elements capable of forming quadruple bonds? Here we use photoelectron spectroscopy and computational chemistry to probe the electronic structure and chemical bonding in RhB2O- and RhB- and show that the boron atom engages in quadruple bonding with rhodium in RhB(BO)- and neutral RhB. The quadruple bonds consist of two π-bonds formed between the Rh 4dxz/4dyz and B 2px/2py orbitals and two σ-bonds between the Rh 4dz2 and B 2s/2pz orbitals. To confirm the quadruple bond in RhB, we also investigate the linear Rh≡B-H+ species and find a triple bond between Rh and B, which has a longer bond length, lower stretching frequency, and smaller bond dissociation energy in comparison with that of the Rh≣B quadruple bond in RhB.

Spherical trihedral metallo-borospherenes.

The discovery of borospherenes unveiled the capacity of boron to form fullerene-like cage structures. While fullerenes are known to entrap metal atoms to form endohedral metallofullerenes, few metal atoms have been observed to be part of the fullerene cages. Here we report the observation of a class of remarkable metallo-borospherenes, where metal atoms are integral parts of the cage surface. We have produced La3B18- and Tb3B18- and probed their structures and bonding using photoelectron spectroscopy and theoretical calculations. Global minimum searches revealed that the most stable structures of Ln3B18- are hollow cages with D3h symmetry. The B18-framework in the Ln3B18- cages can be viewed as consisting of two triangular B6 motifs connected by three B2 units, forming three shared B10 rings which are coordinated to the three Ln atoms on the cage surface. These metallo-borospherenes represent a new class of unusual geometry that has not been observed in chemistry heretofore.