Peng Shao*a,
Li-Ping Dinga,
Cheng Lub,
Jiang-Tao Caia,
Bo Liua and
Chang-Bo Suna
aCollege of Science, Shaanxi University of Science & Technology, Xi'an 710021, China. E-mail: scu_sp@163.com
bDepartment of Physics, Nanyang Normal University, Nanyang 473061, China
First published on 12th October 2015
Equilibrium structures of auropolyboroenes [Au2(B4)xB3]−, [Au2(B4)xB2]2− and [Au2(B4)xB]+ (x = 2, 3) are obtained from density functional theory-based calculations. Results show that the ground states of Au2B9+, Au2B13+ and Au2B142− can be obtained by adding two Au atoms to the corresponding ground-state pure boron clusters. For Au2B102−, Au2B11−, Au2B142− and Au2B15−, the ladder structures are proven to be the ground states at TPSS, OVGF, and CCSD(T) levels, which is similar to that of dihydride polyboroenes. AdNDP analysis indicates that the two rows of boron atoms in these auropolyboroenes are bonded by delocalized three-, four-, or five-center σ and π bonds. Especially, the dominant bonding patterns in Au2B11− and Au2B142− bear similarities to those of dihydride polyboroenes. The photoelectron spectroscopy (PES) spectra for anionic clusters were simulated to facilitate the experimental PES spectra. In addition, the fragmentation energies and products against different decay channels are estimated and discussed.
In previous studies, Au/H similarity is well supported by surprising experimental discovery of H/AuPR3 analogy.20 Further analogy between a bare Au atom and H has also been discovered in gas-phase binary Au clusters.21–23 The analogy extended the structures and bonding in variety of boron-hydrides to B–Au compounds. Such as the heptaboron aurides B7Au2− anion22 was first studied by Zhai et al. using photoelectron spectroscopy and ab initio calculations in 2006. The results showed that B7Au2− possesses an extremely stable planar structure, which is identical to that of B7H2−. What's more, the B–Au bonding was shown to be covalent, similar to the B–H bonding, indicating the Au mimics H in its bonding to boron. A subsequent study23 via density functional theory calculations showed that the closo-auroboranes BxAux2− (x = 7–14) dianions possess structure and bonding analogous to the famous deltahedral closo-borane cages, BxHx2−, demonstrating the Au indeed similar to H. In 2010, Zhai groups24 found that the highly covalent B–Au bond in B10Au− is also similar to the B–H bond in B10H−. The BAun0/− (n = 1–4)25 and B2Aun0/− (n = 1, 3, 5)26 auroboranes have also been proved to possess similar geometrical structures and bonding patterns with corresponding boron hydrides. More recently, Chen et al.27 studied B6Aun0/− (n = 1–3), which provide new examples for the Au/H analogy in Au alloy clusters. These studies on B–Au clusters all indicate that Au serve as monovalent σ ligand apes hydrogen.
The investigations mentioned above unquestionably provide precious information on Au/H analogy, as well as the structure and chemical bonding of boron-gold alloy clusters. However, the previous studies only limited to neutral and anionic B–Au compounds. In the current study, we report a new specie cations with the stable closed structures, such as [Au2(B4)xB]+ in combination with [Au2(B4)xB3]− and [Au2(B4)xB2]2− (x = 2, 3) auropolyboroenes. In previous investigation, Li et al.28 have conjectured that [H2(B4)xB3]−, [H2(B4)xB2]2− and [H2(B4)xB]+ should possess similar characteristics with the stable closed-shell H2B7−, H2B102−, H2B9+ species, respectively. Taking Au/H analogy into account, we think the corresponding auropolyboroenes should also show interesting properties. In this case, we performed quantum chemical calculations on the auropolyboroenes [Au2(B4)xB3]−, [Au2(B4)xB2]2−, [Au2(B4)xB]+ (x = 2, 3) using density functional theory (DFT) method to address several issues. (1) What patterns of structure and chemical bonding exist in them? (2) Whether they are high stable molecules? (3) Whether their bonding pattern bear similarities to those in dihydride polyboroenes especially for cations. Additionally, in order to facilitate the synthesis of auropolyboroenes in the bulk or deposited on surfaces, we have calculated vertical detachment energy (VDE) for the concerned anions and adiabatic ionization potentials (AIP) for neutrals. Meanwhile, the most preferred fragmentation channels and products were also predicted.
B3LYP | B3PW91 | TPSS | PBE | BPW91 | PW91 | M06 | CCSD | Exp./theo. | |
---|---|---|---|---|---|---|---|---|---|
a Experiment ref. 42–44.b Theoretical studied bond length ref. 25. | |||||||||
Au2 (1∑gD∞h) | |||||||||
r | 2.58 | 2.55 | 2.55 | 2.56 | 2.56 | 2.56 | 2.60 | 2.57 | 2.47a |
ω | 163 | 169 | 171 | 166 | 166 | 167 | 160 | 172 | 191a |
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B2 (5∑uD∞h) | |||||||||
r | 1.52 | 1.52 | 1.53 | 1.53 | 1.53 | 1.53 | 1.52 | 1.53 | 1.59a |
ω | 1280 | 1285 | 1240 | 1244 | 1246 | 1245 | 1298 | 1263 | 1052a |
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AuB (1∑+C∞v) | |||||||||
r | 1.95 | 1.95 | 1.95 | 1.94 | 1.94 | 1.94 | 1.98 | 1.93 | 1.95b |
ω | 634 | 646 | 655 | 660 | 653 | 660 | 589 | 661 | — |
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AuB− (2∑+C∞v) | |||||||||
r | 2.01 | 2.00 | 2.00 | 1.99 | 1.99 | 1.99 | 2.04 | 1.97 | 1.99b |
ω | 525 | 543 | 544 | 568 | 558 | 567 | 497 | 574 | — |
Our recipe for finding the equilibrium structures of auropolyboroenes is as follows. The geometries of pure B9+, B102−, B11−, B13+, B142− and B15− clusters were first optimized referring to various previous reported structures.45–47 To search for the lowest energy structure of auropolyboroenes, a large number of initial structures were obtained by placing the gold atoms on each possible site of corresponding pure boron clusters as well as by substituting two B atoms using Au in Bn+2+/− clusters. Then, the simulated photoelectron spectroscopy spectra for obtained anionic isomers were performed and compared with available experimental spectra.28 In the geometry optimization procedure, different possible spin multiplicities were also taken into account for each of these isomers. All the structure optimizations were carried out without any symmetry constraints. Finally, we performed the vibrational frequency computations to ensure that the optimized geometry corresponds to a local minimum in potential energy. Afterwards, the first two lowest-energy isomers obtained at TPSS theory method were recalculated by high-quality methods such as CCSD(T)48 and the outer valence Green's function (OVGF).49–51 The vertical detachment energy and adiabatic ionization potentials (VDE and AIP) were calculated by the TPSS function. The chemical bonding analysis was also performed using the adaptive natural density partitioning (AdNDP) method52,53 as implemented in the Multiwfn 3.1 program.54 Molecular orbital visualization was done using the Gaussview 5.0.8 program.55
Species | ||||||
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Au2B9+ | Au2B13+ | Au2B102− | Au2B142− | Au2B11− | Au2B15− | |
a The energy required to remove one electron from doubly charged Au2B102− and Au2B142− clusters.b The energy required to remove one electron from singly charged Au2B10− and Au2B14−. | ||||||
AIP | 7.11 | 6.26 | — | — | — | — |
VDE | — | — | 0.43a, 3.76b | 0.31a, 4.21b | 3.82 | 3.92 |
As is shown in Fig. 2, our most stable Au2B9+-a structure has Cs symmetry with two Au atoms bind associatively (dimer like) to different binding sites. It lies 0.13, 1.31 and 0.16 eV lower than the next lowest isomer (Au2B9+-b) at TPSS, OVGF and CCSD(T) levels, respectively. The calculated Au–Au bond length (2.72 Å) of Au2B9+-a is longer than the equilibrium Au–Au distance (2.47 Å) in isolated Au2, whereas it is shorter than the Au–Au distance (2.884 Å) in bulk Au. The second low-lying isomer Au2B9+-b has two Au atoms separately connected to the ladder structure of pure boron B9+-b isomer. This structure is similar to the ground state of H2B9+ (Fig. S4†) obtained by us at TPSS/6-311+G* level. For the cationic Au2B13+ auropolyboroene, the ground state can be obtained by adding two Au atoms on the apexes of the low-lying isomer of B13+ (ref. 46). In other words, the B13+ moiety retains intact except for minor distortions. It is followed by a ladder structure (Au2B13+-e) with high C2v point symmetry, which is 0.72 eV higher in energy at TPSS level. However, for the four negatively charged auropolyboroenes Au2B102−, Au2B11−, Au2B142− and Au2B15−, the ladderlike structures were proved to be the ground states at three theory levels TPSS, OVGF, and CCSD(T). Among these ground state structures, there is a very interesting phenomenon. For even numbered boron clusters, the two Au atoms attached terminally to the corresponding pure B102− and B142− dianions in a trans fashion; while for odd numbered ones, the two Au atoms are in the cis position. In addition, the structures of these lowest-energy isomers are nearly identical to the ground state structures of H2B102−, H2B11−, H2B142− and H2B15− as shown in Fig. S4.† This further provides new examples for Au/H analogy in Au alloy clusters. Thus, comparing with the stable closed-shell H2B102−, H2B9+ species28 and our obtained dihydride polyboroenes at TPSS level, we can conclude that these closed-shell auropolyboroenes considered in present work are all stable. It is worth mentioning that among these four auropolyboroenes, only the ground state of Au2B142− can be obtained by adding two Au atoms to the lowest-energy structure of B142−, while the ground states of Au2B102−, Au2B11− and Au2B15− is obtained by bonding two Au atoms to the second lowest-energy isomer of pure boron clusters. Moreover, for Au2B102−, Au2B11−, Au2B142− and Au2B15− clusters, we found that the structures with two Au atoms binding associatively are higher in energy than that with two Au atoms connected to the boron framework separately. This indicates that the Au atoms prefer to bind atomically to negatively charged boron clusters.
Furthermore, the adiabatic ionization potential (AIP) for Au2B9+, Au2B13+ and vertical detachment energy (VDE) for the concerned anions Au2B102−, Au2B11−, Au2B142− and Au2B15− are calculated. The values are given at TPSS/Au/SDD/B/6-311+G* level, unless mentioned otherwise. The AIP is computed by following formula:
AIP = Eoptimized cation − Eoptimized neutral | (1) |
The vertical detachment energy (VDE) is defined as the energy difference between the anion and its unoptimized neutral counterpart both at the ground state geometry of the anionic cluster. The ith detachment energy (DE) is the energy required to remove the ith electron corresponding to i-valence anion change into (i-1)-valence ion. Then, it might be given by the following definition:
DEi = Eni−1 − Eni | (2) |
The DEi is vertical if the Eni−1 of corresponding cluster is the total energy of the unoptimized singly charged (neutral) cluster at the equilibrium geometry of the doubly (singly) charged anions. The VDE values for Au2B102−, Au2B11−, Au2B142− and Au2B15− clusters are listed in Table 2. From Table 2, we found that the energies required to remove one-electron from doubly charged Au2B102− and Au2B142− are 0.43 and 0.31 eV, respectively. These are much smaller than those (3.76 and 4.21 eV) of removing one electron from singly charged Au2B10− and Au2B14−, suggesting that removing one electron from doubly charged anion is easier than from singly charged anion. In other words, the “second extra electron” is only weakly attached on these single charged anions. In addition, we also calculated the energies required to remove two-electrons from doubly charged Au2B102− and Au2B142−. Their respective values are 3.23 and 4.46 eV, which implies that Au2B102− and Au2B142− dianions are more likely to lose one electron. For the singly charged Au2B11− and Au2B15−, the calculated VDEs are 3.82 and 3.92 eV, respectively.
ΔEnQ = ΔEn−iQ + Xi − EnQ, X = Au or H, i = 1 or 2, | (3) |
For the sake of brevity, the ΔE of two channels for Au2B9+, Au2B102−, Au2B11−, Au2B13+, Au2B142−, Au2B15− and the corresponding dihydride polyboroenes (H2B9+, H2B102−, H2B11−, H2B13+, H2B142− and H2B15−) as a function of the cluster size are plotted in Fig. 3a and b, respectively. It can be seen that all the auropolyboroenes possess high fragmentation energies (about ≥3 eV) against the two type fragmentation channels. This appears to be comparable with the values of corresponding dihydride polyboroenes at the same theoretical level. The channels Au2BnQ = AuBnQ + Au for auropolyboroenes Au2B9+, Au2B102−, Au2B13+ and Au2B142− have the lower fragmentation energies. However, for both singly charged anions Au2B11− and Au2B15−, the fragmentation pathway Au2BnQ = BnQ + Au2 has the lower energies. Thus for n = 11 and 15, the negative charge mainly resides on the B11− and B15− clusters during the dissociation. Additionally, the ΔE exhibits a maximum at n = 14, indicating that Au2B142− is less probable dissociation into B142− and Au2 due to one need to supply relatively high energy 5.40 eV. In the case of dihydride polyboroenes H2B9+, H2B102−, H2B11−, H2B13+, H2B142− and H2B15−, we found that removing two H atoms of the ground state dihydride polyboroenes H2BnQ to produce an H2 molecule plus a BnQ cluster is proved to have the lowest fragmentation energies. This may be due to the H2 molecule is the most stable form for H atoms. Meanwhile, for all the dihydride boron clusters, the fragmentation energies of the fragmentation channel H2BnQ = HBnQ + H have very close values. Most strikingly, in the case of H2B142−, the fragmentation energies of pathway HB142− + H is higher than that of B142− + H2 by only 0.04 eV (see Table S1†).
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Fig. 3 (a) Dissociation energies for two fragmentation channels of various auropolyboroenes Au2BnQ. (b) Dissociation energies for two fragmentation channels of dihydride polyboroenes H2BnQ. |
According to the AdNDP analysis, the ground state Au2B9+ isomer (see Fig. 4) has ten lone pair (1c-2e) Au atomic 5d-orbitals with occupy number (ON) in the range of 1.81–1.99|e|, The remaining 26 electrons form seven 2c-2e localized B–B σ-bonds (ON = 1.80–1.92|e|), one 3c-2e Au–Au–B σ-bonds (ON = 1.98|e|), three 4c-2e B–B–B–B σ-bonds and two 4c-2e B–B–B–B delocalized over four boron atoms' π-bonds (ON = 1.84–1.88|e|). We found Au2B9+ isomer possesses two 4c-2e π (four π electrons) bonds. However, it can be seen that these 3c-2e and 4c-2e bonds are very weak as revealed by the calculated bond orders, and hence the bonds in this cluster are mainly described as 2c-2e localized B–B σ-bonds and Au d-orbitals. This may be used to explain the reason why the structure of this cluster is nonplanar and different from the ground state of H2B9+. Additionally, this structure may provide some occupation of s- and p-AOs of boron, avoiding the presence of any unoccupied atomic orbital.
For the anionic Au2B11− (see Fig. 5), AdNDP analysis reveals clearly the existence of ten 1c-2e Au 5d lone-pairs (dxy, dxz, dyz, dx2y2 and dz2 on each Au atom), which is similar to the 1c-2e bonding picture of Au2B9+. In addition, there are two 2c-2e Au–B σ-bonds, six 2c-2e B–B localized σ-bonds, two 4c-2e B–B–B–B π-bonds with the adjacent B2 pair in each B4 and one 5c-2e B–B–B–B–B π-bond. Three π-bonds (six π-electrons) in the boron framework is π aromatic according to the Hückel 4n + 2 rule.60 This is similar to the H2B11− presented by Li et al.,28 and three π orbitals should be also compared with those of 1,3,5-hexatriene. However, the elongated shape of Au2B11− is not consistent with π aromaticity. More interestingly, we found that there are five 3c-2e B–B–B and two 5c-2e B–B–B–B–B σ-bonds in this cluster. The latter bonding features give rise to double aromaticity for Au2B11− cluster because each number of π or σ electrons satisfies the Hückel 4n + 2 rule. Thus, this cluster possesses structure stabilized by electron delocalization both in the π or σ framework. By calculating the bond order, we found that three 3c-2e B–B–B σ-bonds in the center of boron framework are much stronger comparing with the others in two sides. Among the three 5c-2e B–B–B–B–B bonds, only one 5c-2e σ-bond has large bond order 0.11, whereas others are nearly zero. Moreover, the bonding patterns of 2c-2e, 3c-2e, 4c-2e and 5c-2e for Au2B11− cluster are very similar to those of H2B11− (see Fig. S6 in ESI†). This further supports the Au/H analogy in Au alloy clusters.
As for the Au2B142−, ten 1c-2e Au 5d-orbitals are similar to those observed in both Au2B9+ and Au2B11− clusters. The two rows of boron atoms in the ladder structure are bonded via multicenter σ and π bonds. The bond pattern between the two Au atoms and the boron framework is σ bond. Additionally, eight 2c-2e B–B σ-bonds (ON = 1.82–1.99|e|), six 3c-2e B–B–B σ-bonds (ON = 1.97|e|), and two 4c-2e B–B–B–B σ-bonds (ON = 1.90|e|) are observed in this cluster. It is worth pointing out that the four middle 3c-2e B–B–B σ-bonds are much stronger than the two others with bond order −1.68. Meanwhile, the two 4c-2e B–B–B–B σ-bonds with bond order 0.14 are also significantly strong compared with the two 4c-2e π-bonds. Two 5c-2e B–B–B–B–B π-bonds and two 5c-2e B–B–B–B–B σ-bonds are all very weak according to the calculated bond order. Just like Au2B11−, we found the Au2B142− cluster with three delocalized π bonds (six π electrons) is also π aromatic, while Au2B142− is an σ antiaromatic due to it has eighteen σ bonds fulfilling the Hückel 4n rule for antiaromaticity. This may be the reason why Au2B142− gains major stabilization to the ground state. By the comparison between chemical bonding of Au2B142− and H2B142− (Fig. S6†), we found that their major bonds are the same especially for the bonds of Au–B and H–B.
In conclusion, the π bonds in auropolyboroenes Au2B9+, Au2B11− and Au2B142− are delocalized only over parts of the boron frameworks. There is on longer a complete ring of 2c-2e B–B σ bonds found in all boron clusters, whereas there are some 3c-2e B–B–B σ bonds exist. Au2B11− and Au2B142− anions can be considered as covalent complexes between π aromatic B11− and B142− core and two monovalent σ-radicals Au, which are connected by two B–Au σ-bonds. Additionally, the bonding patterns of auropolyboroenes bear similarities to those in dihydride polyboroenes which have the similar structures with auropolyboroenes, especially for the Au–B and H–B bondings. This further supports the Au/H analogy in our studied complexes.
(1) Based on the geometry optimization, the ladderlike structures are proved to be the ground states for Au2B102−, Au2B11−, Au2B142− and Au2B15− at TPSS, OVGF and CCSD(T) levels, which are similar to the corresponding dihydride polyboroenes H2B102−, H2B11−, H2B142− and H2B15−. These auropolyboroenes and dihydride polyboroenes could be potential nanowires.
(2) To facilitate the experimental PES spectra, the simulated photoelectron spectroscopy spectra for anionic clusters were performed. We also calculated the AIPs for cations and VDEs for the concerned anions. The calculated VDEs for the Au2B102− and Au2B142− dianions show that removing one electron from dianions is easier than from singly charged Au2B10− and Au2B14−. That is to say, these two dianions are more likely to lose one electron. Meanwhile, the fragmentation channels and products are deeply discussed.
(3) Using the AdNDP method, we take Au2B9+, Au2B11− and Au2B142− as examples to analyze their electronic structure and chemical bonding. The results reveal that the π-bonds are delocalized only over parts of the boron frameworks. Au2B11− and Au2B142− can be regarded as covalent complexes between a π aromatic core (B11− or B142−) and two monovalent σ-radicals Au. Noteworthy is the fact that the chemical bonding patterns of auroboroenes bear similarities to these in dihydride polyboroenes especially for Au–B and H–B bonding.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15940g |
This journal is © The Royal Society of Chemistry 2015 |