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 (B_n^-), 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 triple bond 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. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

How O2-Binding Affects Structural Evolution of Medium Even-Sized Gold Clusters Aun- (n = 20-34)

We report the first joint anion photoelectron spectroscopy and theoretical study on how O₂-binding affects the structures of medium even-sized gold clusters, Au_n^- (n = 20-34), a special size region that entails a variety of distinct structures. Under the temperature conditions in the current photoelectron spectroscopy experiment, O₂-bound gold clusters were observed only for n = 22-24 and 34. Nevertheless, O₂ binding with the clusters in the size range of n = 20-34 can be still predicted based on the obtained global-minimum structures. Consequently, a series of structural transitions, from the pyramidal to fused-planar to core-shell structures, are either identified or predicted for the Au_nO₂^- clusters, where the O₂-binding is in either superoxo or peroxo fashion. The identified global-minimum structures of Au_nO₂^- (n = 20-34) also allow us to gain improved understanding of why the clusters Au_n^- (n = 26-32) are less reactive with O₂ in comparison to others.

Observation of Transition-Metal-Boron Triple Bonds in IrB2 O- and ReB2 O. Angew Chem Int Ed Engl 2020;59:15260-15265. [PMID: 32424965 DOI: 10.1002/anie.202006652]

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal d_π →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 (C_s , 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.

High-resolution photoelectron imaging of MnB3 -: Probing the bonding between the aromatic B3 cluster and 3d transition metals

The B₃ triangular unit is a fundamental bonding motif in all boron compounds and nanostructures. The isolated B₃ ^- cluster has a D_3h structure with double σ and π aromaticity. Here, we report an investigation of the bonding between a B₃ cluster and a 3d transition metal using high-resolution photoelectron imaging and computational chemistry. Photoelectron spectra of MnB₃ ^- are obtained at six different photon energies, revealing rich vibrational information for the ground state detachment transition. The electron affinity of MnB₃ is determined to be 1.6756(8) eV, and the most Franck-Condon-active mode observed has a measured frequency of 415(6) cm^-1 due to the Mn-B₃ stretch. Theoretical calculations show that MnB₃ ^- has a C_2v planar structure, with Mn coordinated to one side of the triangular B₃ unit. The ground states of MnB₃ ^- (⁶B₂) and MnB₃ (⁵B₂) are found to have high spin multiplicity with a significant decrease in the Mn-B bond distances in the neutral due to the detachment of an Mn-B₃ anti-bonding electron. The Mn atom is shown to have weak interactions with the B₃ unit, which maintains its double aromaticity with relatively small structural changes from the bare B₃ cluster. The bonding in MnB₃ is compared with that in 5d MB₃ clusters, where the strong metal-B₃ interactions strongly change the structures and bonding in the B₃ moiety.

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 4d_xz/4d_yz and B 2p_x/2p_y orbitals and two σ-bonds between the Rh 4d_z² and B 2s/2p_z 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.

Planar B41- and B42- clusters with double-hexagonal vacancies

Since the discovery of the B₄₀ borospherene, research interests have been directed to the structural evolution of even larger boron clusters. An interesting question concerns if the borospherene cages persist in larger boron clusters like the fullerenes. Here we report a photoelectron spectroscopy (PES) and computational study on the structures and bonding of B₄₁^- and B₄₂^-, the largest boron clusters characterized experimentally thus far. The PE spectra of both clusters display broad and complicated features, suggesting the existence of multiple low-lying isomers. Global minimum searches for B₄₁^- reveal three low-lying isomers (I-III), which are all related to the planar B₄₀^- structure. Isomer II (C_s, ¹A') possessing a double hexagonal vacancy is found to agree well with the experiment, while isomers I (C_s, ³A'') and III (C_s, ¹A') both with a single hexagonal vacancy are also present as minor isomers in the experiment. The potential landscape of B₄₂^- is found to be much more complicated with numerous low-lying isomers (VII-XII). The quasi-planar structure VIII (C₁, ²A) containing a double hexagonal vacancy is found to make major contributions to the observed PE spectrum of B₄₂^-, while the other low-lying isomers may also be present to give rise to a complicated spectral pattern. Chemical bonding analyses show isomer II of B₄₁^- (C_s, ¹A') and isomer VIII of B₄₂^- (C₁, ²A) are π aromatic, analogous to that in the polycyclic aromatic hydrocarbon C₂₇H₁₃⁺ (C_2v, ¹A₁). Borospherene cage isomers are also found for both B₄₁^- and B₄₂^- in the global minimum searches, but they are much higher energy isomers.

ReB6-: A Metallaboron Analog of Metallabenzenes

Metallabenzenes are a class of molecules in which a CH unit in benzene is replaced by a functionalized transition-metal atom. While all-boron analogues of aromatic and antiaromatic hydrocarbons are well-known, there have not been any metallaboron analogs. We have produced and investigated two metal-doped boron clusters, ReB₆^- and AlB₆^-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra have been obtained and compared with the theoretical results. The ReB₆^- cluster is found to be perfectly planar with a B-centered hexagonal structure (C_2v, ¹A₁), while AlB₆^- is known to have a similar structure, but with a slightly out-of-plane distortion (C_s, ¹A'). Chemical bonding analyses show that the closed-shell ReB₆^- is doubly σ- and π-aromatic, while AlB₆^- is known to be σ-aromatic and π-antiaromatic. The out-of-plane distortion in AlB₆^- is due to antiaromaticity, akin to the out-of-plane distortion of the prototypical antiaromatic cyclooctatetraene. The π-bonding in ReB₆^- is compared with that in both benzene and rhenabenzene [(CO)₄ReC₅H₅], and remarkable similarities are found. Hence, ReB₆^- can be viewed as the first metallaboron analog of metallabenzenes and it may be viable for syntheses with suitable ligands.

Resonant Two-Photon Photoelectron Imaging and Intersystem Crossing from Excited Dipole-Bound States of Cold Anions

We report the observation of a dipole-bound state (DBS) 659 cm^-1 below the electron detachment threshold of cryogenically cooled deprotonated 4,4'-biphenol anion (bPh^-) and 19 of its lowest vibrational levels. Resonant two-photon photoelectron imaging (R2P-PEI) via the vibrational levels of the DBS displays a sharp peak with a constant binding energy. This observation indicates vertical detachment from the vibrational levels of the DBS to the corresponding neutral levels with the conservation of the vibrational energy, suggesting that the highly diffuse electron in the DBS has little effect on the neutral core. The R2P-PEI spectra also exhibit two features at lower binding energies, which come from intersystem crossings from the DBS to two lower-lying valence-bound triplet excited states of bPh^-. The current study discloses the first R2P-PEI spectra from vibrational excited states of a DBS and direct spectroscopic evidence of transitions from a DBS to valence-bound states of anions.

High-Resolution Photoelectron Imaging of IrB3 - : Observation of a π-Aromatic B3 + Ring Coordinated to a Transition Metal

In a high-resolution photoelectron imaging and theoretical study of the IrB₃ ^- cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B₃ ring coordinated with the Ir atom. The isomer with the higher EA consists of a B₃ ring with a bridge-bonded Ir atom (C_s , ² A'), and the second isomer features a tetrahedral structure (C_3v , ² A₁ ). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C_3v isomer involves significant covalent Ir-B bonding and weak ionic bonding with charge transfer from B₃ to Ir, and can be viewed as an Ir-(η³ -B₃ ⁺ ) complex. This study provides the first example of a boron-to-metal charge-transfer complex and evidence of a π-aromatic B₃ ⁺ ring coordinated to a transition metal.

Probing the coupling of a dipole-bound electron with a molecular core

A dipolar molecule can weakly bind an electron in a diffuse orbital. However, the spin-orbit coupling between this weakly bound electron and the electrons in the molecular core is not known. Here we probe this coupling using the linear C₂P^- anion with the ³Σ⁺ ground state, which possesses dipole-bound excited states because neutral C₂P (²Π) has a sufficiently large dipole moment. Photodetachment spectroscopy and resonant photoelectron spectroscopy are used to probe the nature of the dipole-bound states. Two dipole-bound excited states are observed with a binding energy of 37 cm^-1, corresponding to the two spin-orbit states of neutral C₂P (²Π_1/2 and ²Π_3/2). The current study demonstrates that the weakly bound electron in the dipole-bound excited states of C₂P^- is not spin-coupled to the electrons in the C₂P core and can be considered as a quasi-free electron.

[(Cp2M)2B9H11] (M = Zr or Hf): early transition metal 'guarded' heptaborane with strong covalent and electrostatic bonding

We have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp₂M)₂B₉H₁₁] (Cp = η⁵-C₅H₅; M = Zr or Hf).

Among the series of stable closo-borate dianions, [B_nH_n]^2–, the X-ray crystallographic structure of [B₇H₇]^2– was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp₂M)₂B₉H₁₁] (Cp = η⁵-C₅H₅; M = Zr or Hf). The structures of [(Cp₂M)₂B₉H₁₁] contain a pentagonal bipyramidal B₇ core, coordinated by two {Cp₂M} and two {BH₂} units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp₂M)₂B₉H₁₁] complexes are substantially more stable than the parent dianion, in either [B₇H₇]^2– or (ⁿBu₄N)₂[B₇H₇]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two exo-{Cp₂M} units, as well as electrostatic interactions between the {Cp₂M} units and the B₇ core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.

High-Resolution Photoelectron Imaging of Cryogenically-Cooled C59N- and (C59N)22- Azafullerene Anions

We report a photoelectron imaging study of cryogenically cooled C₅₉N^- and (C₅₉N)₂^2- anions produced from electrospray ionization. High-resolution photoelectron spectra are obtained for C₅₉N^- for the first time, allowing seven vibrational frequencies of the C₅₉N azafullerene to be measured. The electron affinity of C₅₉N is determined accurately to be 3.0150 ± 0.0007 eV. The observed vibrational features are understood on the basis of calculated frequencies and compared with those of C₆₀ and C₅₉HN. The photoelectron image of (C₅₉N)₂^2-, which has the same mass/charge ratio as C₅₉N^-, is also observed, allowing the second electron affinity of the (C₅₉N)₂ azafullerene dimer to be measured as 1.20 ± 0.05 eV. The intramolecular Coulomb repulsion of the (C₅₉N)₂^2- dianion is estimated to be 1.96 eV and is investigated theoretically using the electron density difference between (C₅₉N)₂^2- and (C₅₉N)₂.

Nb2©Au6: a molecular wheel with a short Nb 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Nb triple bond coordinated by an Au6 ring and reinforced by σ aromaticity

We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic Nb₂Au₆^- cluster. Theoretical calculations, in conjunction with the experimental data, reveal that Nb₂Au₆^-/0 possess high-symmetry D_6h structures featuring a Nb-Nb axis coordinated equatorially by an Au₆ ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb₂ moiety in Nb₂©Au₆, as well as five totally delocalized σ bonds. The Nb 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Nb triple bond is strengthened significantly by the delocalized σ bonds, resulting in an extremely short Nb-Nb bond length comparable to the quintuple bond in gaseous Nb₂. The totally delocalized σ bonding in Nb₂©Au₆ is reminiscent of σ aromaticity, representing a new bonding mode in metal-ligand systems. The unusually short Nb-Nb bond length in Nb₂