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.

Observation of Möbius Aromatic Planar Metallaborocycles.

Möbius aromaticity was developed for twisted annulenes with electron counting rules opposite to those of Hückel aromaticity. The introduction of transition metals makes it possible for planar cyclic systems to exhibit Möbius aromaticity. Here we report the first planar monocyclic metallaboron systems with Möbius aromaticity. The structures and bonding of two rhenium-boride clusters are studied by high-resolution photoelectron imaging and ab initio calculations. The ReB3- cluster is shown to have a near-pyramidal structure, while ReB4- is found to be a planar pentagonal ring. Chemical bonding analyses show that both ReB4- and ReB4 possess four delocalized π-electrons, including two π-electrons in an orbital of Möbius topology. NICS calculations reveal strong aromatic characters in ReB4- and ReB4, consistent with the 4n electron counting rule for Möbius aromaticity.

MnB6-: An Open-Shell Metallaboron Analog of 3d Metallabenzenes.

We report a study of the structure and bonding of a transition-metal-doped boron cluster, MnB6-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra indicate a significant geometry change between the anionic and neutral ground states of MnB6. The electron affinity of MnB6 is measured to be 2.4591(5) eV, and vibrational frequencies for five of its vibrational modes were determined. The experimental data are combined with theoretical calculations to determine the structure and bonding of MnB6-, which is found to be planar with a B-centered hexagonal structure (C2v symmetry) and a quintet spin state (5A2). Nuclear-independent chemical shift calculations indicate that MnB6- is aromatic. Molecular orbital analyses reveal that MnB6- contains three π orbitals, one of which is singly occupied. Hence, MnB6- can be considered as an open-shell metallaboron analog of 3d metallabenzenes.

The nature of the chemical bonding in 5d transition-metal diatomic borides MB (M = Ir, Pt, Au).

Boron can form strong bonds with transition metals in diatomic metal borides (MB), but the nature of the chemical bonding has not been well understood. Recently, a quadruple bond was discovered in Rh≣B, consisting of two σ bonds formed between the Rh 4dz2 and B 2s/2p orbitals and two π bonds between the Rh 4dxz/4dyz and the B 2px/2py orbitals. The bonding between the 5d transition metals and boron is expected to be even stronger. Here, we report an investigation on the electronic structure and chemical bonding of the 5d transition metal diatomic borides (IrB, PtB, and AuB) using high-resolution photoelectron imaging on the corresponding anions (MB-) and theoretical calculations. Vibrationally resolved photoelectron spectra are obtained for all three anions, and the electron affinities are measured for IrB, PtB, and AuB to be 1.995(1), 2.153(3), and 0.877(6) eV, respectively. It is found that the weakly anti-bonding 3σ molecular orbital (mainly of M 6s and B sp characters) is singly occupied in IrB (3Δ) and PtB (2Σ+), resulting in a bond order of three and half for these two diatomic borides. The 3σ orbital is doubly occupied in AuB (1Σ+), giving rise to a weak triple bond. Despite the lower bond order, the bonding in IrB and PtB is only slightly weaker than that in RhB due to the more favorable interactions between the M 5d orbitals and the B sp orbitals.

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 D3h 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 C2v 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 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.

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 (C2v, 1A1), while AlB6- is known to have a similar structure, but with a slightly out-of-plane distortion (Cs, 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.

High resolution photoelectron imaging of boron-bismuth binary clusters: Bi2Bn - (n = 2-4).

Bismuth boride is a heavy member of the III-V semiconductors. Although there have been some theoretical interests in this material, it has not been synthesized experimentally. Here, we report a high-resolution photoelectron imaging study on a series of boron-bismuth binary clusters, Bi2Bn - (n = 2-4), produced by laser vaporization of a B/Bi mixed target. Vibrationally resolved photoelectron spectra are obtained for all three clusters, and the measured vibrational and electronic information is used to compare with theoretical calculations to understand their structures and bonding. Bi2B2 - is found to be linear (D∞h, 2Πg) with a B2 unit and two terminal Bi atoms, while Bi2B3 - is found to be planar (C2v, 1A1), consisting of a B3 triangle with two bridging Bi atoms. Interestingly, the spectra of Bi2B4 - reveal two co-existing isomers; both are found to be planar and contain a rhombus B4 unit with two bridging Bi atoms in a trans (C2h, 2Au) and cis (C2v, 2B1) fashion separated only by 0.03 eV in energy. The interactions between the two Bi atoms and the Bn motifs are understood using chemical bonding analyses. This study shows that the Bi-B bonding is weak enough so that the Bn units maintain their structural integrity with the Bi atoms bonded to the cluster periphery only.

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 (Cs , 2 A'), and the second isomer features a tetrahedral structure (C3v , 2 A1 ). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C3v 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.

A high-resolution photoelectron imaging and theoretical study of CP- and C2P.

The discovery of interstellar anions has been a milestone in astrochemistry. In the search for new interstellar anions, CP- and C2P- are viable candidates since their corresponding neutrals have already been detected astronomically. However, scarce data exist for these negatively charged species. Here we report the electron affinities of CP and C2P along with the vibrational frequencies of their anions using high-resolution photoelectron imaging. These results along with previous spectroscopic data of the neutral species are used further to benchmark very accurate quartic force field quantum chemical methods that are applied to CP, CP-, C2P, and two electronic states of C2P-. The predicted electron affinities, vibrational frequencies, and rotational constants are in excellent agreement with the experimental data. The electron affinities of CP (2.8508 ± 0.0007 eV) and C2P (2.6328 ± 0.0006 eV) are measured accurately and found to be quite high, suggesting that the CP- and C2P- anions are thermodynamically stable and possibly observable. The current study suggests that the combination of high-resolution photoelectron imaging and quantum chemistry can be used to determine accurate molecular constants for exotic radical species of astronomical interest.

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.

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, Aun- (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 AunO2- clusters, where the O2-binding is in either superoxo or peroxo fashion. The identified global-minimum structures of AunO2- (n = 20-34) also allow us to gain improved understanding of why the clusters Aun- (n = 26-32) are less reactive with O2 in comparison to others.

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.

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 (2Π1/2 and 2Π3/2). 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.

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.

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 (Cs, 1A') possessing a double hexagonal vacancy is found to agree well with the experiment, while isomers I (Cs, 3A'') and III (Cs, 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- (Cs, 1A') and isomer VIII of B42- (C1, 2A) are π aromatic, analogous to that in the polycyclic aromatic hydrocarbon C27H13+ (C2v, 1A1). Borospherene cage isomers are also found for both B41- and B42- in the global minimum searches, but they are much higher energy isomers.

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

Among the series of stable closo-borate dianions, [B n H 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 ( n Bu4N)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)22- 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)22-, 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)22- dianion is estimated to be 1.96 eV and is investigated theoretically using the electron density difference between (C59N)22- and (C59N)2.

Nb2©Au6: a molecular wheel with a short Nb[triple bond, length as m-dash]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[triple bond, length as m-dash]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©Au6 shows that the Au6 ring can serve as a bridging ligand to facilitate multiple bonding in transition metal dimers via delocalized σ bonding.

Vibrational State-Selective Resonant Two-Photon Photoelectron Spectroscopy of AuS(-) via a Spin-Forbidden Excited State.

Vibrational state-selective resonant two-photon photoelectron spectra have been obtained via a triplet intermediate state ((3)Σ(-)) of AuS(-) near its detachment threshold using high-resolution photoelectron imaging of cryogenically cooled AuS(-) anions. Four vibrational levels of the (3)Σ(-) excited state are observed to be below the detachment threshold. Resonant two-photon absorptions through these levels yield vibrational state-selective photoelectron spectra to the (2)Σ final state of neutral AuS with broad and drastically different Franck-Condon distributions, reflecting the symmetries of the vibrational wave functions of the (3)Σ(-) intermediate state. The (3)Σ(-) excited state is spin-forbidden from the (1)Σ(+) ground state of AuS(-) and is accessed due to strong relativistic effects. The nature of the (3)Σ(-) excited state is confirmed by angular distributions of the photoelectron images and quantum calculations.