Cobalt-centred boron molecular drums with the highest coordination number in the CoB16− cluster

The electron deficiency and strong bonding capacity of boron have led to a vast variety of molecular structures in chemistry and materials science. Here we report the observation of highly symmetric cobalt-centered boron drum-like structures of CoB16−, characterized by photoelectron spectroscopy and ab initio calculations. The photoelectron spectra display a relatively simple spectral pattern, suggesting a high symmetry structure. Two nearly degenerate isomers with D8d (I) and C4v (II) symmetries are found computationally to compete for the global minimum. These drum-like structures consist of two B8 rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry. We show that doping of boron clusters with a transition metal atom induces an earlier two-dimensional to three-dimensional structural transition. The CoB16− cluster is tested as a building block in a triple-decker sandwich, suggesting a promising route for its realization in the solid state.

B oron, the fifth element in the periodic table, possesses such diverse chemical structures and bonding that are second only to carbon. Bulk boron consists of connected three-dimensional (3D) cages in many of its allotropes 1,2 and boron-rich borides 3,4 . However, for isolated clusters it was computationally shown 5,6 that icosahedral cage structures of B 12 and B 13 were unstable, even though they were initially proposed as possible candidates for these two clusters 7 . Over the past decade, small anionic boron clusters have been systematically characterized both experimentally and theoretically to exhibit planar or quasi-planar structures in their ground states up to B 27 À (refs [8][9][10]. Recent works show that anionic boron clusters continue to be two-dimensional (2D) at B 30 À (ref. 11), B 35 À (ref. 12) and B 36 À (ref. 13). The 2D-to-3D transition was suggested to occur at B 20 for neutral 14 , and at B 16 þ for cationic clusters 15 . Very recently it is shown that the transition from 2D to fullerene-like 3D structures occurs in negatively charged boron clusters at about 40 boron atoms in B 39 À (ref. 16) and B 40 À (ref. 17). Due to the nearly spherical shapes of these clusters, they have been named borospherenes. Doping boron clusters with a single metal atom opens a new avenue to create clusters with novel structures and chemical bonding. It has been experimentally observed that various transition metal atoms can be placed inside of monocyclic boron rings to form beautiful molecular wheel-type structures (MrB n À ) 18 , following an electronic design principle inspired by the doubly s and p aromatic B 9 À cluster 19 . It was shown that the NbrB 10 À and TarB 10 À clusters possess the record coordination number of 10 in the planar environment for the central metal atom 20 . These clusters have pushed the limits of structural chemistry.
Here we report the observation of a large metal-doped boron cluster of CoB 16 À , which is produced using a laser vaporization cluster source and characterized by photoelectron spectroscopy (PES). Extensive computational searches reveal that there are two nearly degenerate structures for CoB 16 À , which are indistinguishable at the highest level of theory employed. They both possess tubular double-ring framework and give similar photoelectron spectral patterns. The structures can be viewed as two B 8 rings sandwiching a Co atom, reminiscent of a drum and giving rise to the highest coordination number known in chemistry thus far.

Results
Experimental results. The photoelectron spectra of CoB 16 À at two photon energies are displayed in Fig. 1. The lowest binding energy band (X) represents the electron detachment transition from the anionic ground state to that of neutral CoB 16 . The higher binding energy bands, A, B, ..., denote detachment transitions to the excited states of neutral CoB 16 . The vertical detachment energies (VDEs) for all observed bands are given in Table 1, where they are compared with the calculated VDEs.
The 266 nm spectrum (Fig. 1a) reveals three well-resolved PES bands for CoB 16 À . The band X gives rise to a VDE of 2.71 eV. The adiabatic detachment energy (ADE) for band X was evaluated from its onset to be 2.48 eV, which also represents the electron affinity of neutral CoB 16 . The width of band X suggests an appreciable geometry change between the ground electronic state of CoB 16 À and the ground electronic state of CoB 16 . Following a relatively large energy gap, an intense and broad band A is observed at a VDE of 3.45 eV and a close-lying band B at a VDE of 3.78 eV. The 193 nm spectrum (Fig. 1b) shows nearly continuous signals beyond 4 eV. The sharp spikes above 5 eV in the high binding energy side of the 193 nm spectrum are due to statistical noises because of low electron counts. An intense and broad band C is clearly observed at a VDE of 4.86 eV. Two more bands can be tentatively identified at higher binding energies, D (VDE: B5.3 eV) and E (VDE: B5.6 eV). Overall, the PES spectral pattern is relatively simple, suggesting that the framework of the CoB 16 À cluster is likely to have high symmetry.
Theoretical results and comparison with experiment. Extensive structural searches were initially done at the PBE0/3-21G level of theory with the follow-up calculations (D ¼ 25 kcal mol À 1 ) at the PBE0/Def2-TZVP level of theory, which led to two similar drumlike structures: isomer I (D 8d , 3 A 2 ) and isomer II (C 4v , 1 A 1 ) identified as the global minima for CoB 16 À (Fig. 2). These two highly symmetric structures, consisting of a central Co atom sandwiched by two B 8 monocyclic rings, are found to be almost degenerate at various levels of theory (Supplementary Fig. 1 and Supplementary Table 1). Clearly, the method dependency of predicting relative energies of the low-lying structures for CoB 16 À suggests the importance of comparison with experiment in determining the global minimum. We previously studied how optimized geometries of small boron clusters differed at density functional theory (DFT) and CCSD(T) levels of theory 21,22 . We found that B3LYP/6-311 þ G* geometries are quite close (within 0.03 Å between nearest boron atoms) to those at the CCSD(T)/ 6-311 þ G* level of theory. We also compared the geometries of boron clusters at PBE0/6-311 þ G* and B3LYP/6-311 þ G*, and found that they are also very close 23 . Therefore, PBE0/3-21G level of theory was used for the preliminary search and PBE0/Def2-TZVP for the final optimized geometries of CoB 16 À . The highest level of theory employed (ROCCSD(T)/6-311 þ G(2df)//PBE0/ Def2-TZVP (this abbreviation means that single-point energy calculations were performed at ROCCSD(T)/6-311 þ G(2df) using optimized UPBE0/Def2-TZVP geometries here and elsewhere) indicates 1.4 kcal mol À 1 energy difference including zero-point energy corrections (Supplementary Fig. 1). This small value is in the range of the theoretical errors for such a complex transition-metal-doped boron cluster. Therefore, isomers I and II should be considered to be degenerate based on our calculations. Figure 2 shows the small differences in bond distances between isomers I and II; the latter is not significantly distorted from the D 8d symmetry. The B-B bond lengths of the B 8 rings for both isomers are in the range of 1.55-1.63 Å, similar to the corresponding values (1.56 Å) in the CorB 8 À molecular wheel 18 . The nearest isomer III (C 2 , 1 A) is 8.7 kcal mol À 1 higher in energy at the ROCCSD(T) method and represents a distorted drum-like structure composed of two B 7 rings with two B atoms outside the drum (Supplementary Fig. 1). In fact, the majority of the low-lying isomers within 20 kcal mol À 1 (Supplementary Fig. 1) represent various derivatives (drum-like or possessing principal geometrical features of the drum-like structure) of isomers I and II, showing the stability of the drumlike structures. It should be noted that there are significant bonding interactions between the two B 8 rings and between the Co atom and all 16 B atoms in both isomers I and II (vide infra).
Interestingly, the drum structure in a quintet state (isomer XIV in Supplementary Fig. 1) appears to be the most stable one out of all other quintet isomers. It should be mentioned that there were two previous DFT calculations on similar drum-like structures of neutral boron clusters doped with transition metal atoms 24,25 .
To facilitate comparisons between the experimental and theoretical results, we calculated low-lying VDEs of isomers I and II of CoB 16 À using three methods (Table 1). We found that the VDEs computed using the two DFT methods are not very impressive; but we observed good agreement between the theoretical VDEs at ROCCSD(T)/6-311 þ G(2df) and the experimental data for the first two detachment channels ( were not able to calculate any higher VDEs because of the limitation of the ROCCSD(T) method. However, we believe that the good agreement between experiment and theory for the first two VDEs provides sufficient credence for the identified drum-like isomer I for the CoB 16 À cluster. Isomer II gives very similar theoretical VDEs as isomer I at all three levels of theory, consistent with the similarities in their geometries. Since isomer II is a closed shell species, we were able to calculate only the first VDE value at the ROCCSD(T) method as 2.61 eV, also in good agreement with the experimental data. Furthermore, the calculated ADEs of isomer I (2.45 eV) and isomer II (2.43 eV) (PBE0/Def2-TZVP) are in excellent agreement with the experimentally measured ADE value of 2.48 eV. We should point out that there is a Jahn-Teller distortion for the neutral CoB 16 drum-like structure of isomer I, consistent with the broad X band observed in the PES spectra (Fig. 1). Indeed, the calculated relaxed neutral CoB 16 structure I 0 (Supplementary Fig. 2 and Supplementary Table 1) has lower symmetry (C 2v ), as one would expect for the Jahn-Teller distorted structure due to the occupation of the doubly degenerate HOMO (4e 2 ) of isomer I by a single electron. In fact, the HOMO (4b 2 ) of isomer II originates from the HOMO (4e 2 ) of isomer I when one of the doubly degenerate orbitals is doubly occupied. Therefore, the detachment of one electron from the doubly occupied HOMO (4b 2 ) of isomer II leads to the same neutral structure I 0 . The high relative energy of isomer III, as well as its appreciably higher theoretical first VDE of 3.65 eV (Supplementary Table 2), makes this cluster unlikely to be populated in the molecular beam in any appreciable amount.

Discussion
Tubular (or drum-like) boron clusters have been of interest for many years, because they can be considered as the embryos for boron nanotubes 14 . However, such drum-like structures have never   ARTICLE been observed experimentally for bare boron clusters, even though they have been shown to be stable computationally 14,[26][27][28][29] .
For instance, the B 20 cluster was first suggested as the global minimum on the basis of theoretical calculations 14 , but it has not been observed or confirmed experimentally 29 . Tubular structures were also studied for the bare B 16 þ , B 16 , B 16 À and B 16 2 À species 15,30 . For the B 16 þ cationic cluster, the tubular structure was suggested to be the global minimum 15 , whereas the tubular structures of both B 16 and B 16 À were found to be high-energy isomers 30 . Clearly, the strong coordination interactions with the Co atom significantly stabilize the tubular B 16 to give the drum-like global minima (structures I and II) for CoB 16 À . Bare anionic boron clusters are found to be 2D up to B 36 À (ref. 13), while some transitionmetal-doped anionic boron clusters are found to preserve the planar boron framework on metal doping 31,32 . The largest experimentally observed metal-doped boron cluster (CoB 12 À ) maintains a similar planar geometry for the B 12 moiety 32 . Hence, the doping of the Co atom induces an earlier 2D-to-3D transition for boron clusters, as shown by the 3D isomers I and II of CoB 16 À . In fact, the CoB 16 À drum structure represents the highest coordination number known in chemistry today.  36,37 , but the metal atom in those cases interacts with the C 60 shell primarily ionically and it does not stay in the centre of C 60 .
It is interesting to point out that the B-B distances in the B 8 rings of both isomers I and II of CoB 16 À and the bare tubular B 16 are very similar (Supplementary Table 3). To gain insight into the chemical bonding of the CoB 16 À drums, we performed chemical bonding analyses for isomers I and II using the Adaptive Natural Density Partitioning (AdNDP) method 38 , which is an extension of the popular Natural Bond Orbital method 39 . It should be noted that the bonding in some double-ring tubular boron clusters has been discussed previously 9,40,41 .
Since isomer I has two unpaired electrons, we used the unrestricted AdNDP (UAdNDP) analysis, which enables treatments of the a and b electrons separately. To obtain an averaged result for a bond (Fig. 3), we added the UAdNDP results for the a and b electrons of the same type of bonds. According to the UAdNDP analysis results, the 58 valence electrons in CoB 16 À can be divided into four sets. The first set (Fig. 3a,b) consists of localized bonding elements, while the other three sets ( Fig. 3c-g, h-j, k-o) are composed of delocalized bonding elements. In the first set, the UAdNDP analysis for isomer I revealed the following localized bonding elements: one lone pair (1c-2e bond) (Fig. 3b) of 3d z 2type on Co with an occupation number (ON) of 1.98 |e| and sixteen 2c-2e B-B s-bonds (Fig. 3a) with ON ¼ 1.84 |e| within each B 8 ring (all superimposed onto the B 16 fragment in Fig. 3), which can also be viewed as 3c-2e bonds with the ON ¼ 1.96 |e| responsible for the bonding between the boron rings. In the last case, a boron atom from the neighbouring ring contributes somewhat (0.12 |e|) to the formation of the 3c-2e s-bond. The 2c-2e B-B s-bonds are very similar to the peripheral B-B bonds found in all 2D boron clusters [8][9][10] . The second set includes five delocalized s bonds (denoted as s þ s), which are formed from delocalized s bonds between the two B 8 rings. Since the s orbitals between the two boron rings overlap positively, we designate them as s þ s in the second set, which constitutes s-aromaticity according to the 4n þ 2 (n ¼ 2) Hückel rule. The three delocalized 16c-2e s þ s bonds (Fig. 3c-e) with ON ¼ 1.82-1.86 |e| involve only s-bonding within the boron rings, whereas the two delocalized 17c-2e s þ s bonds (Fig. 3f,g) come primarily from the 3d xy and 3d x 2 -y 2 AOs of Co interacting with the boron rings. It should be noted that the direct covalent interactions between Co and the B 16 unit via the 3d xy and 3d x 2 -y 2 AOs of Co are found to be around 0.6 |e| according to the AdNDP analysis. The third set (Fig. 3h-j) shows three delocalized s-s bonds, which represent bonding interactions within each ring, but anti-bonding interactions between the two boron rings. This set of delocalized bonds also constitutes s-aromaticity according to the 4n þ 2 (n ¼ 1) Hückel rule. In the third set, the 16c-2e s-s bond (Fig. 3h) involves mainly the two boron rings, whereas the two 17c-2e s-s bonds (Fig. 3i,j) involve interactions between the 3d xz and 3d yz AOs of Co with the boron rings. The direct covalent interaction of the 3d xz and 3d yz AOs of Co with the boron kernel is assessed to be around 0.5 |e|. The last set includes five delocalized bonds, which represent p-p interactions between the boron rings: three 16c-2e p-p bonds (Fig. 3k-m) with ON ¼ 1.98-2.00 |e| and two 16c-1e p-p bonds (Fig. 3n,o) with ON ¼ 1.00 |e| (one unpaired electron on each bond). The eight p electrons in the last set suggest p-aromaticity according to the 4n rule (n ¼ 2) for triplet states. Therefore, the stability of isomer I of CoB 16 À can be considered to be due to the double sand p-aromaticity and bonding interactions of the 3d AOs of Co with the B 8 rings.
As expected, isomer II of CoB 16 À , which is close in energy and geometry to isomer I, has almost the same bonding pattern as that of isomer I (Supplementary Fig. 3). All the bonding elements found in isomer I are also found in isomer II except for the last set ( Supplementary Fig. 3k-n). Since isomer II is closed shell, eight electrons in the last set are observed to form four 16c-e p-p bonds with ON ¼ 1.98-2.00 |e|, rendering this isomer p-antiaromatic. Hence, isomer II exhibits conflicting aromaticity (s-aromatic and p-antiaromatic), which leads to some distortion to C 4v symmetry compared to the D 8d symmetry of the doubly aromatic isomer I. As was mentioned earlier, the HOMO (4b 2 ) of isomer II originates from the HOMO (4e 2 ) of isomer I when one of the doubly degenerate orbitals is doubly occupied. Indeed, occupation of only one degenerate MO by two electrons causes the electronic instability, which causes the geometric rearrangement of isomer II lowering the D 8d symmetry to C 4v .
To understand the interactions between Co and the tubular B 16 host, we have performed AdNDP analyses for the neutral B 16 tubular isomer (Supplementary Fig. 4). Similar to isomers I and II of CoB 16 À , the AdNDP analyses give 16 2c-2e B-B s-bonds with ON values of 1.70 |e| within the two B 8 rings. The encapsulation of Co strengthens the B-B s-bonds within each B 8 ring in CoB 16 À , but weakens the inter-ring interactions, compared with the bare B 16 , as reflected by their ON values ( Fig. 3 and Supplementary  Fig. 4) and the B-B bond lengths (Supplementary Table 3). The remaining 16 electrons in B 16 participate in delocalized bonding: five 16c-2e s þ s bonds and three 16c-2e p-p bonds, rendering the tubular B 16 doubly sand p-aromatic. The major difference in chemical bonding between the drum-like B 16 and CoB 16 À comes from two factors: (1) the formation of an additional set (Fig. 3h-j) of the delocalized s-s bonds in CoB 16 À ; and (2) participation of Co 3d AOs in the two 17c-2e s þ s bonds (Fig. 3f,g). Both factors are consistent with structural changes between CoB 16 À and B 16 . There are strong bonding interactions between Co and the B 16 host in CoB 16 À to stabilize the tubular B 16 structure, because the global minimum of B 16 is planar 30 .
Isomer I of CoB 16 À is open shell with two unpaired electrons, whereas isomer II can be viewed as a result of Jahn-Teller distortion from isomer I. Addition of two electrons to isomers I or II would create a closed shell and doubly aromatic CoB 16 3 À species with D 8d symmetry. Our calculations indeed confirmed this hypothesis: CoB 16 3 À was found to be a minimum on the potential energy surface with very similar bond distances as in isomer I (Supplementary Table 3 42 with the double antiaromatic B 6 2 À unit was previously suggested to be stable and viable experimentally. We found that the [CoB 16 (CaCp) 2 ] À triple-decked complex was a minimum on the potential energy surface with high electronic stability. All the B-B and Co-B bond lengths were found to be almost the same as in isomers I and II of CoB 16 À (Supplementary Table 3). We have further performed AdNDP analyses and found that the triple-decked sandwich complex exhibits exactly the same chemical bonding pattern as the parent CoB 16 À ( Supplementary Figs 6-8). The Natural Population Analysis (NPA) charge on Ca was found to be þ 1.54, consistent with the initial hypothesis and the charge-transfer nature of the triple-decked [CoB 16 (CaCp) 2 ] À sandwich complex. Thus, the CoB 16 À molecular drum can serve as a building block for the design of novel cluster-assembled nanomaterials. The high stability of the CoB 16 À drum structures may also help the search for new metal-boride phases containing various boron ring units 43 .
We have produced and characterized a large Co-doped boron cluster, CoB 16 À , using photoelectron spectroscopy and quantumchemical calculations. Extensive computational searches established two high symmetry (D 8d and C 4v ) drum-like structures with Co sandwiched by two B 8 rings as nearly degenerate global minima. The CoB 16 À molecular drums represent the highest coordination for a metal atom known in chemistry and opens new possibilities for designing novel boron-based nanomaterials. First, the CoB 16 À drums may be considered as the embryo to make filled boron nanotubes due to the significant B-B bonding between the two B 8   ARTICLE observe larger doped-boron clusters with even higher coordination number to further push the limit of coordination number in chemistry. Third, we have demonstrated one possibility to use CoB 16 À as a building block of new cluster-assembled nanomaterials in a triple-decked complex.

Methods
Experimental methods. The experiment was carried out using a magnetic-bottle PES apparatus equipped with a laser vaporization cluster source 44 . Briefly, the CoB 16 À anion clusters were produced by laser vaporization of a cold-pressed target composed of Co and isotopically enriched 11 B. Bismuth was added as a binder and it also provided a convenient calibrant (Bi À ) for the PES experiment. Clusters formed in the nozzle were entrained in a He carrier gas and underwent a supersonic expansion to form a collimated cluster beam. The He carrier gas was seeded with 5% Ar for better cooling of the entrained clusters 22 . The anionic clusters were extracted from the collimated cluster beam and analysed by a time-offlight mass spectrometer. The CoB 16 À anion clusters were mass selected and decelerated before being photodetached by a laser beam at 193 nm (6.424 eV) from an ArF excimer laser or 266 nm (4.661 eV) from a Nd:YAG laser. Photoelectrons were collected at nearly 100% efficiency by a magnetic bottle and analysed in a 3.5 m long flight tube. The resolution of the apparatus, DEk/Ek, was about than 2.5%, that is, B25 meV for 1 eV electrons.
Theoretical methods. Search for the global minimum of CoB 16 À was performed using the Coalescence Kick program 45 at the PBE0/3-21G level of theory 46,47 . The Coalescence Kick algorithm generated B10,000 trial structures for each spin multiplicity (singlet, triplet and quintet), followed by geometry optimization. Low-lying isomers within 25 kcal mol À 1 were further refined at a more expansive basis set, Def2-TZVP 48 . For each structure, vibrational frequencies were calculated and imaginary frequencies were followed to ensure that the isomer corresponded to a true minimum on the potential energy surface. Spin contamination was found to be o10% in all DFT calculations. For selected isomers, we performed additional geometry optimization at various DFT levels, as well as more accurate single-point coupled-cluster calculations [ROCCSD(T)/6-311 þ G(2df)], to reliably establish the relative energy ordering. Vertical detachment energies of the three lowest energy structures were calculated at three different methods (UPBE0, UB3LYP and ROCCSD(T)) to compare with the experimental data. The VDEs were obtained as the difference in energy between the ground state of the anion and selected low-lying electronic states of the neutral molecule at the geometry of the anion. All calculations were done using GAUSSIAN-09 (ref. 49).
To understand the chemical bonding, we carried out electron localization analyses using the AdNDP method 38 at the PBE0/6-31G(d) level of theory. Previously, AdNDP results have been shown to be insensitive to the level of theory or basis set used 50 . The AdNDP analysis is based on the concept of electron pairs as the main elements of chemical bonds. It represents the molecular electronic structure in terms of n-centre two-electron (nc-2e) bonds, recovering the familiar lone pairs (1c-2e) and localized 2c-2e bonds or delocalized nc-2e bonds (3rnrtotal number of atoms in the system). The MOLEKEL 5.4.0.8 program 51 is used for molecular structure and AdNDP bond visualizations.