Article pubs.acs.org/JPCC
Designing Aromatic Superatoms Wei-Ming Sun, Ying Li, Di Wu,* and Zhi-Ru Li Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *
ABSTRACT: Recent developments in the field of superatom chemistry have brought forward various clusters or compounds exhibiting novel structures and special properties. By utilizing aromatic clusters as building blocks, a new class of aromatic superatom cations MLin+1+ is proposed here. Theoretical investigations reveal that most of the Mn− aromatic anions maintain their structural and electronic integrity and play the role of central core inside the MLin+1+ cations. The studied cations feature low vertical electron affinities (EAvert), large HOMO−LUMO gaps, and high thermodynamic stability with respect to loss of a Li+ ion. Besides, two kinds of MLin+1−(super)halogen compounds have been theoretically constructed and characterized, by which the feasibility of using the aromatic MLin+1+ cations as building blocks for the design of new superatom compounds is explored. The results presented in this work appear to show the existence of aromatic, and particularly organic, superatoms and may assist with further systematic researches on such species.
I. INTRODUCTION Aromaticity is a concept initially invented to account for the unusual stability of planar organic molecules with 4n + 2 delocalized electrons.1 Nowadays, it has been extended to a highly diverse number of inorganic compounds, even including three-dimensional (3D) compounds. Recently, Li et al.2 expanded this concept to include all-metal species. After this discovery, all-metal aromatic species have been the topic of numerous experimental and theoretical investigations.3 Because of the special stability of aromatic clusters, they can serve as viable building blocks for new compounds, e.g., Al62− and Al4TiAl42−.4,5 Recent experiments have confirmed that some aromatic species were found in crystalline solids as key structural units, such as C5H5−, C7H73−, and Si56−.6,7 It has been established that clusters with specific size and composition exhibiting behaviors reminiscent of atoms in the periodic table can be regarded as superatoms.8−15 Such unusual species can maintain their identity when assembled into an extended nanostructure;16 hence, they offer the exciting prospect of serving as building blocks for new nanoscale materials which may have highly tunable properties useful for a great variety of potential technologies. 17−20 Therefore, exploring novel superatomic building blocks for assembled nanomaterials is of compelling interest. One famous subset of superatoms are superalkalis,21 which feature lower ionization potentials (IPs) than those (5.4−3.9 eV)22 of alkali metal atoms. Superalkalis are of great importance in chemistry. First of all, they possess excellent reducibility and can be used in the synthesis of unusual charge-transfer salts with the counterpart possessing relatively low electron affinity.23,24 Second, superalkalis have the advantage of being © 2013 American Chemical Society
utilized in forming cluster-assembled salts, where the use of alkali metal atoms is not promising due to steric hindrance.25,26 Third, by assessing the hydrogen trapping capability of some superalkali cations, Chattaraj and co-workers27 have found that these species have the potential to become effective hydrogen storage materials. Moreover, recent investigations have demonstrated that superalkali cations can maintain their structural and electronic integrity inside a series of superatom compounds, such as (BLi6)+(X)− (X = F, LiF2, BeF3, and BF4),28 (BF4)−(M)+ (M = Li, FLi2, OLi3, NLi4),29 and (Li3)+(SH)− (SH = LiF2, BeF3, and BF4).30 Among them, BLi6X and BF4M have been predicted to be potential nonlinear optical molecules in view of their considerable first hyperpolarizabilities. Therefore, superalkalis may represent building blocks for the assembly of novel nanostructured materials with desired properties. Owing to the intriguing features mentioned above, the superalkali clusters have attracted more and more attention in recent years, and many efforts have been devoted to designing and characterizing new superalkalis. Hitherto the investigation of superalkali has been expanded from conventional mononuclear superalkalis of the MLk+1 type21 to binuclear superalkali species,31,32 and then to polynuclear superalkali cations.33−35 Very recently, Hou et al.36 designed unusual superalkali cations using hydrogen as ligands, thereby introducing nonmetallic members into this research area. These achievements support the viewpoint that the potential of creating new species Received: September 3, 2013 Revised: October 16, 2013 Published: October 22, 2013 24618
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Figure 1. Global minima of the MLin+1+ cations at the B3LYP/6-311++G(3df,3pd) level.
optimized geometries agree well with those reported in references.2−4,7,42−44 Note that the charged C4H42−, C5H5−, C7H73−, and C8H82− hydrocarbon molecules are quite unstable in isolated gaseous state, so we can only get their Dnh structures at the B3LYP/6-31G(d) level. Even so, the optimized structures of C4H42− and C7H73− still have imaginary frequencies. The potential energy surfaces of MLin+1+ were explored by using two different approaches. In the first one, a great number of initial geometries were artificially constructed by placing Li ligands on each possible site of the aromatic cluster gained above. For the other approach, the stochastic search procedure31,45 was used in which the geometric center of M was set as “zero” point, and then the Li atoms were tossed from the “zero” point in random directions within a spherical shell. The setting radius of the sphere varies in the 2.5−4.0 Å range for all the MLin+1+ cations. A lot of starting geometries were generated and then optimized automatically at the B3LYP/ LANL2DZ level. The search continued until no new minimum appeared. Thereafter, all the possible structures gained by the two approaches were optimized at the B3LYP/6-311++G(3df, 3pd) level with frequency calculations to check whether the optimized structures were transition states or true minima on the potential energy surfaces. The single-point calculations were carried out at the B3LYP/ 6-311++G(3df,3pd) level. The vertical electron affinities (EAvert) of the MLin+1+ cations, which was defined as the total energy difference between the cation and the neutral cluster with the same geometry as the cation, were calculated based on the single-point energies. The intramolecular interaction energies (Eint) of the superatom compounds Be3Li3−Cl, Be3Li3−BF4, Si5Li7−Cl, and Si5Li7−BF4 were
classified as superalkalis is limitless and thereby motivate us to systematically explore more diverse superalkali species to further enrich the superalkali family. In the present work, based on the MLk+1 formula proposed by Gutsev and Boldyrev,21 we have designed a novel class of aromatic superalkali cations by replacing the atomic M cores with aromatic anions, which advances the concept of aromaticity in the territory of superatom chemistry. In recent years, many ligand-protected superatom complexes have been reported,37−39 in which the ligands prevent the metal cores from coalescing.15 In this research, the L+ ligands can effectively counteract the large intramolecular Coulomb repulsion of the multiply charged aromatic cores and then help to produce more stable species. Besides, the diversity of aromatic cores conduces to create superalkalis of all sorts. Herein, we choose a series of aromatic anions, i.e., inorganic Be32−, Mg32−, B3−, Al3−, Al42−, Al62−, Si44−, Ge44−, Si56−, N42−, N5−, and B6H62− as well as organic C4H42−, C5H5−, C7H73−, and C8H82−, which have been extensively investigated by theoretical and experimental researchers,2−4,6,7,40−44 to act as the central core of superatom cations MLin+1+ (M is the aromatic core and n is the anionic charge of M). We hope that such novel superatoms not only enrich our knowledge and global view of the superatom species but also can act as building blocks for new nanomaterials in which exceptionally strong reducers are required.
II. COMPUTATIONAL DETAILS Initially, the aromatic Mn− clusters reported in previous literatures were reoptimized at the B3LYP/6-311++G(3df,3pd) level and are shown in Figure S1 of the Supporting Information. For inorganic clusters (Be32−, Mg32−, B3−, Al3−, Al42−, Al62−, Si44−, Ge44−, Si56−, N42−, N5−, B6H62−), the 24619
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with the third Li ligand coordinated to one side of the Be3 ring. By comparison, the Mg3Li3+ structure possesses lower Cs symmetry as the third Li ligand located on one face of the Mg3Li2 segment. The structural discrepancy between Be3Li3+ and Mg3Li3+ can be well explained by their different electronic structures. As shown in Figure S7, each Li atom in Be3Li3+ carries over +0.8|e| NBO charge, so the three Li ligands tend to locate far from each other to reduce the repulsive interaction. In comparison, the Li positive charges in the Mg3Li3+ cation are much smaller, and the upper Li ligand carries only +0.323|e| charge. Consequently, a Li−Li bond forms between the side Li ligand and the upper one in the Mg3Li3+ cation. In addition, it is found that the incorporation of Li+ cations results in the shortening of Be−Be (Mg−Mg) bond compared with those in isolated Be32− (Mg32−) cluster. For example, the Mg−Mg bond lengths of 2.969 and 3.075 Å in Mg3Li3+ are much shorter than that of 3.364 Å in isolated Mg32−. This indicates that the introduction of Li+ cations helps to reduce the internal Coulomb repulsion of the charged aromatic ring. From Figure S2, the second low-energy structure (A) of Be3Li3+ is generated by the staggered fusion of Li3 and Be3 rings, and the case is similar for isomer A of Mg3Li3+. However, the Mg3 triangle is cleaved by Li ligands in isomer B of Mg3Li3+. The global minimum structure of B3Li2+ is a regular trigonal bipyramid with D3h symmetry, while Al3Li2+ has a Cssymmetrical geometry with a Li ligand bound to the apex Al atom of Al3Li tetrahedron (Figure 1). As can be seen from Figure 1 and Figures S1 and S3, the entities of B3 and Al3 cores are preserved in both lowest-energy and low-lying structures of these two species. Take B3Li2+ as an example. The B−B bond length of 1.530 Å is nearly equal to that of 1.536 Å for isolated B3− (Figure S1), and the total charge on the B3− unit is −0.831| e|, which is close to −1|e| (Figure S7). For Al4Li3+, its lowestenergy structure could be viewed as two Li ligands being attached to the square-pyramidal LiAl4 unit on two adjacent Al−Al edges. Similar to B3Li2+ and Al3Li2+, the aromatic Al4 core retains its structural integrity inside the Al4Li3+ conformers though the Al4 fragment distorts from square to quasi-rhombus in isomers D−G. It is known that the octahedron-Al6-derived structure is the global minimum for Al6Li− and the capped trigonal prism structure is a low-lying isomer,4 but the case is reversed for the Al6Li3+ cation. From Figure 1 and Figure S3, capping the three square faces of the Al6 trigonal prism with three Li ligands yields the most stable structure of Al6Li3+ (D3h), while isomers A−C containing octahedral Al6 core are higher in energy by 2.57−6.88 kcal/mol than the global minimum. A pentacapped tetrahedron is found to be the most stable structure for both Si4Li5+ and Ge4Li5+. As for Si5Li7+, its global minimum has a perfect seven-peak star-like structure with D5h symmetry, where the regular five-membered Si5 ring is face- and edge-capped by two and five Li ligands, respectively. It is worthy to mention that this structure has also been reported by Tiznado et al.51 in their recent work on the Si5Linn−6 (n = 5, 6, 7) systems. Their investigation also indicates that the significant electron delocalization enhances the stability of such star-like structures. From Figure S4, the geometric integrity of the Si44−, Ge44−, and Si56− anions is clearly seen not only in the global minimum structures but also in several low-lying isomers of the resulting MLin+1+ cations. For Si5Li7+, isomers A, C, and D were also found by Tiznado et al.51 Herein, much more isomers are located and presented.
calculated at the B3LYP/6-311+G(3df) level. We used the counterpoise procedure46 to eliminate the basis set superposition error (BSSE) effect given as follows:47 E int = EAB(XAB) − EA (XAB) − E B(XAB)
where the same basis set, XAB, was used for the subunit energy (EA and EB) calculations as for the complex energy (EAB) calculation. Natural bond orbital (NBO)48 analysis was also performed at the B3LYP/6-311++G(3df,3pd) level. The NICS values [NICS(0)] at the geometrical centers of the aromatic cores of the MLin+1+ cations and designed compounds have been calculated by using the gauge-independent atomic orbital (GIAO) procedure49 at the B3LYP/6-311++G(3df,3pd) level. All the calculations were performed using the GAUSSIAN 09 program package.50 Dimensional plots of molecular structures were generated with the GaussView program (Gaussian, Inc., Pittsburgh, PA).
III. RESULTS AND DISCUSSION The resulting lowest-energy and low-lying structures of the MLin+1+ cations are shown in Figure 1 and Figures S2−S6 (Supporting Information), respectively. The NBO charges of the most stable MLin+1+ structures are shown in Figure S7 (Supporting Information). The lowest vibrational frequencies, highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gaps, NICS(0) values, and EAvert values of these cations are gathered in Table 1. In the following subsections, the structural features, aromaticity, thermodynamic stability, and the EAvert values as well as the assembly feasibility of the MLin+1+ cations are discussed in detail. A. Structural Features. From Figure 1, Be3Li3+ has a C2vsymmetrical geometry, featuring a Be3Li2 trigonal bipyramid Table 1. Lowest Vibrational Frequencies (v1, in cm−1), NICS(0) Values (in ppm),a HOMO−LUMO Gaps (in eV), and the Vertical Electron Affinities (EAvert, in eV) of the Most Stable MLin+1+ Cations as Well as the Zero-PointCorrected Dissociation Energies (De, in kcal/mol)a of the MLin+1+ → MLin + Li+ Channel for the Most Stable MLin+1+ Cations cation
v1
NICS(0)
gap
De
EAvert
Be3Li3+ Mg3Li3+ B3Li2+ Al3Li2+ Al4Li3+ Al6Li3+ Si4Li5+ Ge4Li5+ Si5Li7+ N4Li3+ N5Li2+ B6H6Li3+ C4H4Li3+ C5H5Li2+ C7H7Li4+ C8H8Li3+
115 80 95 84 62 124 60 46 40 39 102 34 127 276 18 78
−42.5 −30.5 −70.5 −32.6 −32.4 −73.3 −65.8 −64.9 −18.1
2.48 2.42 3.68 1.94 2.41 2.75 3.57 3.46 3.81 2.31 7.95 6.03 4.02 6.65 3.16 3.92
53.2 47.2 42.6 45.0 58.0 56.1 60.3 59.0 74.9
4.74 4.06 4.78 4.74 4.11 4.34 3.49 3.51 2.88 3.70 3.96 3.39 4.20 3.65 3.56 3.75
−19.9 −43.8 −18.2 −13.4 −16.1
53.0 57.9 57.0 64.5 52.6
a
Excluding the N4Li3+ and C4H4Li3+ cations where the N42− and C4H42− rings are greatly destroyed. 24620
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Different from the above-mentioned cases, the aromatic N42− ring lost its geometric integrity in the C2v-symmetrical global minimum structure of N4Li3+ and was cleaved into two N2 units by Li cations. The N−N bond lengths of the two N2 units are 1.237 and 1.091 Å, respectively, and the latter is equal to that in bare N2 at the same computational level. Thus, this structure can be viewed as constituted by a N2Li3+ cation and a N2 molecule, i.e., (N2Li3)+(N2). This description can be confirmed by NBO analysis. That is the charges on the N2 and N2Li3+ units are 0.058|e| and 0.942|e|, respectively (see Figure S7). As is shown in Figure S5, isomers A−E of N4Li3+ also contain two separate N2 units, while the N4 ring only occurs in higherenergy isomers. As for N5Li2+, a scoop-shaped C2v structure is found to be its lowest-energy configuration, which has two nearly isoenergetic isomers, namely, A and B. Apparently, the local structures of the N5− units appear to be similar in these three structures. For the most stable N5Li2+, the average N−N bond length of 1.315 Å is nearly equal to the N−N bond length (1.322 Å) of N5−, and the NBO charge of −0.891|e| on the N5 unit is close to −1|e|, indicating that the structural and electronic integrity of N5− is completely retained in this structure. When the aromatic borohydride B6H62− serves as the central core, a C3-symmetrical conformation has been identified to be the most stable structure of the B6H6Li3+ cation. In this structure, the Li ligands occupy three discrete bridge sites of the B6 octahedron, respectively. The intact B6H62− anion is observed in this structure, which is also reflected by the −1.850|e| NBO charges on the B6H6 unit (see Figure S7). We have also obtained some local minimum structures of B6H6Li3+, which contain a severely distorted B6H6 unit. However, these structures are not presented and discussed here because they are much higher in total energy than the global minimum. Furthermore, several prototypical aromatic hydrocarbons, i.e., C4H42−, C5H5−, C7H73−, and C8H82−, were also taken into consideration. From Figure 1, the C4 ring of C4H42− is cut off in the global minimum structure of C4H4Li3+. In contrast, the most stable C5H5Li2+ contains an almost intact C5H5− anion. The −0.790|e| charge on the C5H5 unit suggests that the electronic integrity of C5H5− is also preserved in this configuration (see Figure S7). For C7H7Li4+, a mass of isomers were obtained based on the experimentally known C7H73− ligand.6 The most stable one has a C2 structure with four Li cations evenly distributed on the two sides of the C7H7 ring. As for C8H8Li3+, it possesses a Cs structure involving a bridge-like C8H8 unit. Although the C7H73− trianion and C8H82− dianion maintain their geometric integrity in C7H7Li4+ and C8H8Li3+, respectively, their structures are distorted from the regular polygon.6,43 Clearly, the distortion is caused by the unbalanced C−Li interactions. The low-lying isomers of these organic cations are shown in Figure S6. For C4H4Li3+, isomer A containing a chainlike C4H4 unit is more stable than isomers B−G containing cyclic C4H4 segment, and isomer H with two separate C2H2 units is the least favorable. In contrast, the cyclic structure of C5H5− is superior to the chain-like one for constructing a more stable C5H5Li2+ cation. For C7H73−, the complexation with Li cations leads to three different results: (i) the cyclic structure of C7H7 is maintained; (ii) the C7H7 ring is cut off (in isomers A, E, and F); (iii) one hydrogen atom is split from C7H7 (in C and D). Similarly, the same is true for the identified structures of C8H8Li3+.
According to the above discussion, almost all the aromatic Mn− anions preserve the geometric integrity in the most stable MLin+1+ cations. Besides, as can be seen from Figure S7, the Li ligands carry +0.570|e| to +0.961|e| NBO charge, and the average Li charge exceeds +0.664|e| in the lowest-energy MLin+1+ structures (except for Mg3Li3+), which presents good charge separation in these species. That is to say, the Li ligands in these cations donate n electrons to the M cores, so the aromatic Mn− anions are also expected to keep the electronic integrity in the MLin+1+ cations. As to the Mg3Li3+ cation, it exhibits relatively small Li positive charges because of the similar electronegativities of magnesium and lithium. B. Aromaticity and Stability. As pointed out above, most aromatic cores maintain their integrity in the designed MLin+1+ cations, so it is reasonable to expect the resulting cations to be aromatic. The nucleus-independent chemical shift (NICS), proposed by Schleyer and co-workers,52 is an efficient probe of aromaticity in a variety of molecules. Negative NICSs denote aromaticity, whereas positive NICSs denote antiaromaticity. As shown in Table 1, the calculated NICS(0) values at the geometrical center of the aromatic cores in the MLin+1+ cations are all negative (−13.4 to −73.3 ppm). Hence, the studied cations, except for N4Li3+ and C4H4Li3+, can also be regarded as aromatic species. As is well-known, the HOMO−LUMO gap is considered to be an important criterion in terms of the electronic stability of clusters. To some degree, it represents the ability of a molecule to participate in chemical reaction. A large value of the HOMO−LUMO energy gap is related to an enhanced chemical stability.53 From Table 1, the gaps of the most stable MLin+1+ cations are in the range of 1.94−7.95 eV. These values are not only larger than the experimental HOMO−LUMO gap of 1.9 eV for the kinetically stable C6054 but also exceed that of 1.87 eV for the chemically inert Al13−,55 suggesting the chemical stability and superatom character of these MLin+1+ cations. To explore the thermodynamic driving force for dissociation for these cationic systems, the zero-point-corrected dissociation energies (De) of selected fragmentation pathways are obtained and displayed in Table 1 and Table S1. It is found that all the dissociation reactions are endothermic. Besides, the preferred channel, namely MLin+1+ → MLin + Li+, requires large reaction energies of 42.6−74.9 kcal/mol, indicating the high stability of the MLin+1+ cations upon loss of Li+. This also gives evidence of the superatom identity of the proposed aromatic cations. From Table 1, it is interesting to find that the Si5Li7+ cation with the lowest EAvert value requires the largest dissociation energy to lose a Li+ while the opposite is true for the B3Li2+ cation. C. Vertical Electron Affinities. We now turn to the key question of this research: do the aromatic MLin+1+ cations belong to the superalkali group? As shown in Table 1, the EAvert values of these cations are 2.88−4.78 eV, which are lower than the IP = 5.14 eV of the Na atom.22 Especially, the EAvert values of Si4Li5+ (3.49 eV), Ge4Li5+ (3.51 eV), Si5Li7+ (2.88 eV), B6H6Li3+ (3.39 eV), C5H5Li2+ (3.65 eV), C7H7Li4+ (3.56 eV), and C8H8Li3+ (3.75 eV) are even lower than the IP = 3.89 eV of the Cs atom.22 Thus, these cations should be classified as superalkali cations. As for the rest, they may be named as pseudoalkali cations, just as the definition of well-known pseudohalogen (CN and SCN, etc.). It is also interesting to compare these results with those of previously reported superalkalis. Among the monoanion-based cations, organic C5H5Li2+ possess the lowest EAvert value, which is near to the IP value (3.64 eV)21 of mononuclear superalkali 24621
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Figure 2. Equilibrium structures with symmetry and relative energy (Erel, in kcal/mol) of the Be3Li3−Cl, Be3Li3−BF4, Si5Li7−Cl, and Si5Li7−BF4 compounds at the B3LYP/6-311+G(3df) level.
FLi2. As to B3Li2+, Al3Li2+, and N5Li2+, their EAvert values are a bit larger. Recently, several kinds of superalkali cations involving dianion cores have been reported.32−35 In the present work, Be3Li3+, Mg3Li3+, Al4Li3+, Al6Li3+, B6H6Li3+, and C8H8Li3+ belong to this group. Herein, the EAvert value of B6H6Li3+ is comparable to those of SO3Li3+ and SO4Li3+ (3.28 and 3.34 eV,33 respectively) and is lower than the IP value of 3.59 eV56 for the known superalkali OLi3. From Table 1, the trianionbased C7H7Li4+ cation also has a pretty low EAvert value of 3.56 eV, which is the same as the IP of traditional NLi4 superalkali.21 Note that the low-lying isomers of MLin+1+ also exhibit low EAvert values of 2.89−4.99 eV (except for 6.36 eV for isomer A of C5H5Li2+, see Table S2), demonstrating that they are potential members for the superalkali family. D. Possibility of Forming Superatom Compounds. As was previously reported, superalkalis can be combined with (super)halogens and maintain their structural integrity inside the resulting ionic salts.28−30 Hence, we also considered the possibility that the aromatic MLin+1+ cations serve as building blocks for the design of new superatom compounds. In the present work, we chose Be3Li3+ and Si5Li7+ cations as representatives to combine with the Cl− and typical superhalogen BF4− anions. All the resulting structures presented in Figure 2 are minima on the potential energy surface at the B3LYP/6-311+G(3df) level. To evaluate the stability of the designed compounds, their HOMO−LUMO gaps and bond energies Eb are calculated and collected in Table 2, together with the NBO charge Q on the Cl/BF4 subunit. The Eb values are defined as the negative of the intramolecular interaction energies between the Cl/BF4 and Be3Li3/Si5Li7 moieties. The larger the Eb value, the stronger the interaction between two subunits.
Table 2. NICS(0) Values (in ppm), HOMO−LUMO Gaps (eV), Bond Energies Eb (kcal/mol), and the NBO Charge Q on the Cl/BF4 Subunit of the Be3Li3−Cl, Be3Li3−BF4, Si5Li7−Cl, and Si5Li7−BF4 Compounds compounds
isomer
NICS(0)
gap
Eb
Q
Be3Li3−Cl
A B C D E A B C D E F G A B A B C D E
−36.8 −36.0 −40.3 −36.0 −31.2 −37.3 −41.3 −35.5 −40.4 −41.4 −37.0 −33.7 −18.0 −17.4 −17.8 −18.1 −18.6 −18.4 −18.6
2.48 2.48 2.39 2.05 1.90 2.60 2.42 2.10 2.38 2.43 2.22 2.12 2.91 2.78 2.87 2.75 2.95 2.99 2.63
121.6 121.3 123.2 117.7 110.9 189.8 188.3 186.3 187.5 186.0 185.3 178.5 134.7 133.8 209.5 208.3 204.5 203.0 208.9
−0.462 −0.465 −0.699 −0.493 −0.489 −0.753 −0.825 −0.741 −0.823 −0.869 −0.770 −0.695 −0.716 −0.601 −0.780 −0.744 −0.837 −0.877 −0.726
Be3Li3−BF4
Si5Li7−Cl Si5Li7−BF4
From Figure 2, the Cl atom is either bound to an apex Be atom or side-on bound to the Be3Li3 unit, which yields five different ground-state geometries of Be3Li3−Cl. It is found that isomer A with the former bonding pattern is more stable than the others. As for Be3Li3−BF4, seven minimum structures were obtained, and in most cases the BF4 superhalogen was side-on bound to Be3Li3. Isomer C, however, is an exception, where the 24622
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BF4 subunit caps a BeLi2 triangle face. From Table 2, there is a charge transfer of 0.462−0.699 e− from Be3Li3 unit to Cl atom, resulting in ionically bound compound (Be3Li3)+Cl−. Because of the high electron affinity of superhalogen BF4, the negative charges (−0.695|e| to −0.869|e|) on the BF4 subunits in Be3Li3−BF4 are very close to −1. The ionic bonding character between Be3Li3 and Cl/BF4 guarantees the electronic integrity of Be3Li3+ inside the Be3Li3−Cl and Be3Li3−BF4 compounds. Besides, from Figures 1 and 2, the structural identity of the Be3Li3+ cation is also maintained in these species. Two structures were identified for Si5Li7−Cl. From Figure 2, the Cl atom binding with two Li ligands of Si5Li7 generates isomer A, which is 0.84 kcal/mol more stable than isomer B with three Li−Cl bonds. As to superhalogen BF4, it can combine with Si5Li7 via two, three, or four Li−Cl connections, leading to isomer D, isomers A and C, and isomers B and E, respectively. The 0.601−0.877|e| NBO charges on the Si5Li7 subunits demonstrate that there is an electron transfer from Si5Li7 to Cl/BF4. Hence, superhalogen BF4 and superalkali Si5Li7 exhibit the characteristics of single halogen atom and alkali metal atom, respectively, when they are interacting with each other in the cluster assemblies. Besides, the Si5Li7+ cation also keeps its structural integrity in these superatom compounds in spite of the fact that several Li ligands deviated from the original position to facilitate the combination of two subunits. From Figure 2, the metallic Be3 core directly participates in the interaction with (super)halogen while the nonmetallic Si5 core is linked with Cl/BF4 via Li ligands. In any case, the aromatic cores retain their structural identity in the designed superatom compounds. Thus, these compounds are also aromatic, which is supported by the negative NICS(0) values of −17.4 to −40.3 ppm (Table 2). As shown in Table 2, the HOMO−LUMO gaps of 1.90− 2.99 eV for these species are considerably large compared with those of 1.24 and 1.29 eV for the previously reported Al13− K3O27 and Al13−K,57 respectively, indicating the enhanced chemical stability of the investigated superatom compounds. In addition to the large gaps, these ionic compounds also exhibit quite large bond energies of 110.9−209.5 kcal/mol, which are comparable to or much larger than conventional ionic bond energies. Thus, the aromatic MLin+1+ cations and (super)halogens anions are expected to be tightly bound together and consequently form stable superatom compounds.
Article
ASSOCIATED CONTENT
S Supporting Information *
Geometries of the isolated aromatic cores M; structural and electronic information for the local minima of the MLin+1+ cations; NBO charges and Cartesian coordinates as well as the dissociation energies of several dissociation channels for the most stable MLin+1+ cations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (D.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21173095 and 21303066) and Program for New Century Excellent Talents in University of the Ministry of Education.
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REFERENCES
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IV. CONCLUSIONS Our systematic calculations on the polynuclear MLin+1+ cations show that atomatic superatoms can be designed by combination of suitable aromatic anions and alkali metal cations. Herein, the selected aromatic anions include metallic, nonmetallic, and organic species. It is found that, except for N42− and C4H42−, all the aromatic anions retain their identities inside the resulting MLin+1+ cations. The aromatic MLin+1+ proposed here possess low electron affinities and represent a new kind of superalkali or pseudoalkali cation. They also show pretty large HOMO−LUMO gaps as well as thermodynamic stability against dissociation. Besides, it is also possible for the MLin+1+ cations to combine with (super)halogens anions and consequently form stable superatom compounds. The theoretical characterization of these aromatic superatoms may open a new branch of superatom chemistry and provide meaningful references to the further design of novel superatoms with aromatic building blocks. 24623
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