Molecular Tailoring: Substituent Design for Hexagermabenzene

Article pubs.acs.org/Organometallics

Molecular Tailoring: Substituent Design for Hexagermabenzene Tibor Szilvási and Tamás Veszprémi* Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics (BUTE), Szent Gellért tér 4, 1111 Budapest, Hungary S Supporting Information *

ABSTRACT: The preparation of new, heavy element compounds with an unusual bonding situation is still a real challenge since, with the lack of solid chemical knowledge, every synthesis is unpredictable and eventual. In this paper, we analyze the general reaction scheme of reductive dehalogenation of halogermanes, which has led to the formation of several interesting structures, for example, digermyne, tetragermahedrane, hexagermaprismane, and octagermacubane, and point out that all of these syntheses can be explained by the principle of energy minimum and the special steric effect of bulky substituents. We have found that the proper choice of the bulky substituent can result in only one stable minimum on the potential energy surface, which could be realized in the synthetic works. We also demonstrate that this recognition can be exploited to design appropriate substituents for the synthesis of new organometallic structures. On the basis of our approach, we suggest Rind-type and terphenyl-type substituents for the synthesis of tetragermacyclobutadiene and hexagermabenzene, respectively.

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INTRODUCTION

The main goal of this paper is to prove that our molecular tailoring approach can be applied to interpret and to predict preparations in a wide range of chemistry. Therefore, we analyze the general reaction scheme of reductive dehalogenation of halogermanes, explain all experimental results, and, on that basis, we suggest appropriate substituents for the synthesis of the hitherto missing germanium compounds, such as tetragermacyclobutadiene or hexagermabenzene. To achieve our goal, we have explored the potentially stable (GeR)2n structures (n = 1−4) (1−16, Scheme 1) with small substituents (a, b, c; Scheme 3). To the best of our knowledge, there is no theoretical work that has dealt with this question in general, which is in wide contrast with the deep literature of the analogue silicon systems.8 Only five papers investigated germanium structures,9 with a hydrogen subtituent, and they were restricted to tetragerma structures9a,c−e or symmetric clusters.9b 1−16 structures have then been investigated, using several experimentally applied real bulky groups (d−x, Scheme 3), to explore their synthetic accessibility and to explain experimental results. Referring to structures will be done in short by using the combination of the corresponding number and letter. All

Compounds with an unusual bonding situation are always in the center of interest since they significantly broaden our chemical knowledge.1 The preparation of those compounds is, however, considerably difficult just because of the lack of the respective chemical knowledge. Therefore, the synthesis of such a compound can be the result of years or even decades of work, which makes the development of this field slow and eventual. Any possible approach that can support successful syntheses and makes the work more predictable can attract attention. In our recent work, we have introduced the concept of molecular tailoring,2 stating that every chemical reaction has one or a few optimal substituents that can be predicted effectively in silico. This concept can play an important role in the organometallic/inorganic chemistry where the synthesis of new compounds with an unusual bonding situation always requires the application of bulky substituents. We have analyzed the synthesis of several silicon compounds2 with theoretical tools and point out that all successful preparations3 can be traced back to the proper choice of bulky substituents. Several interesting syntheses have also been reported in the field of germanium chemistry. Digermyne (1, Scheme 1),4 tetragermatetrahedrane (7),5 hexagermaprismane6 (11), and octagermacubane7 (14) have also been prepared basically with the same reductive halogenation reaction scheme (Scheme 2). © 2013 American Chemical Society

Received: March 26, 2013 Published: August 16, 2013 4733

dx.doi.org/10.1021/om4002507 | Organometallics 2013, 32, 4733−4740

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Scheme 1

Conformation search was performed using the universal force field (UFF) method around the dihedral angle of Ge−R bonds with 60° resolution in the first place, exploiting the symmetry of the bulky group, reducing the possible number of conformers. It turned out, however, that the bulkiness of the group eliminated most of the conformations, and the problem was reduced to only a few conformers that were also found out by hand. After the successful search around the Ge−R bonds, similar searches were carried out for internal groups of the bulky group if it was necessary. Minima on the potential energy surface (PES) were characterized by harmonic vibrational frequency calculations at the B97-D/6-31G* level. Gibbs free energies in Table 1 are computed as the sum of the energy at the B97-D/cc-pVTZ level and the free energy correction at the B97-D/6-31G* level. Calculations were carried out using the Gaussian 0914 program. The Avogadro15 program was utilized for visualization.

Scheme 2

calculated geometries and energies (in hartree) can be found in the Supporting Information.

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THEORETICAL SECTION According to the general experiences, DFT methods with double-ζ basis provide adequate geometries, whereas triple-ζ basis gives proper relative energies; therefore, geometries were computed at the B97-D/6-31G* level of theory and singlepoint energy calculations were performed at the optima at the B97-D/cc-pVTZ level.10 For the proper description of Ge atoms, the triple-ζ basis set with the scalar relativistic effective core potential, denoted as cc-pVTZ-PP,11 was employed for both geometry optimization and single-point calculations. The B97-D functional was chosen on the basis of our previous results; namely, it can provide adequate geometries and thermochemical results in group 14 heavy element reactions.2 The B97 functional was originally tailored to give accurate thermochemistry for small main group compounds,10c and it was completed by Grimme10d with empirical dispersion correction (−D) to obtain accurate energies for large systems.12 The resolution of identity (RI) approximation13 was also used, which enables even 10−20 times acceleration for large calculations and significant reduction in memory usage while its error is negligible. As a specific benchmark, CCSD(T)/augcc-pVTZ-PP energy calculations were carried out for hydrogensubstituted tetragerma derivatives on RI-B97-D/6-31G* optimized structures (see details in the Supporting Information), which confirmed the reliability of the selected method.

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RESULTS AND DISCUSSION Substituents H, Me, and Ph (a, b, c) as a Comparison. We have explored the low-lying minima on the (GeH)2n n = 1− 4 potential energy hypersurface (PES). The relevant structures can be seen in Scheme 1 (R = H), and the corresponding energies are in Table 1. For the sake of simplicity, we have chosen digermyne (1) as the zero level of energy and cyclic pergerma compounds are treated as the products of the formal di-, tri-, or tetramerization reaction of the corresponding digermyne (1) (ΔG). This choice allows us to compare directly the relative stability of the cyclic compounds on the same PES and also provides information between different size germanium compounds. We also computed relative Gibbs free energies (ΔGrel) as an energy gain per digermyne unit (n). Although these values are not reaction energies, they clearly indicate the thermodynamic direction of the processes, whether four-, six-, or eight-membered cyclic compounds are more stable. 4734


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Scheme 3

ΔGrel =

ΔG n

in the case of certain bulky substituents; moreover, 7w has been experimentally characterized by Wiberg et al.5 Four (GeH)6 structures have been found to be stable. Interestingly, the benzene analogue 8a is the most stable (ΔG = −467 kJ/mol); however, 12a is only less stable with 14 kJ/mol. Since our method certainly provides the error bar in that order of magnitude (we estimate it to be ±20 kJ/mol for small reference systems (a, b, c) and ±30 kJ/mol for compounds with large bulky substituents), this question cannot be answered at this level of theory. In addition, with the increasing size of the substituent, the stability order changes and 12c becomes more stable than 8c with 7 kJ/mol. Benzvalene and prismane analogues (9 and 11) are always significantly less stable than 8 and 12, independent of the substituent. We also deal with the Dewar-benzene analogue (10) in this study; though, 10a, 10b, and 10c are not stable because of their rearrangement to the analogue prismane structures, they were found to be stable in the case of larger bulky groups. We analyze four (GeH)8 structures as well that can be derived from the association of two (GeH)4 units as the synthesis of octagermacubane (14) proves.7 13a is found to be the most stable structure (ΔG = −736 kJ/mol), but 14a is only less stable with 13 kJ/mol. 16a is considerably less stable than 14a or 13a (ΔG = −690 kJ/mol); however, with the change of the substituent, it becomes as stable as 13 or 14 (13c, 14c, and 16c are −757, −745, and −746 kJ/mol, respectively). 15a is not stable since it can easily isomerize to the more stable 14a, but 15b and 15c are already stable on the PES owing to the steric hindrance of the larger bulky groups. As a result, we conclude that the formation of numerous pergerma compounds can easily occur from the analogue digermyne from the thermodynamic point of view; the energy gain per digermyne unit even slightly increases with the number

All stable (GeH)4 structures (2a−5a) have a butterfly-shaped germanium core, although only 4a has a cross-bond over the four-membered ring (Ge−Ge distance is 2.58 Å; Wiberg bond index (WBI) = 0.91), whereas this distance is more than 3 Å in the other compounds. Despite the large distance, the Wiberg bond index suggests interaction between these Ge atoms as well (corresponding WBIs in 2a, 3a, and 5a are 0.45, 0.47, and 0.59, respectively). The natural charges, Wiberg bond indices, and total atomic Wiberg bond indices of hydrogen-substituted compounds can be found in the Supporting Information (Tables S8−S19). The main difference between 2a and 5a is the direction of the substituents compared to the Ge ring, which also determines their stability. 5a is the most stable (ΔG = −286 kJ/mol), where substituents are situated in a symmetrical axial position. The turnover of a hydrogen to the other side of the Ge ring (3a) destabilizes the molecule with 50 kJ/mol (ΔG = −236 kJ/mol); then, another turnover results in further destabilization (ΔG = −222 kJ/mol). Nevertheless, the least stable four-membered ring is 4a, where the close germanium distance over the ring indicates significant destabilization (ΔG = −157 kJ/mol). The steric crowding in 2a−4a predicts that these structures can be hardly stable using large bulky groups. In the case of the flat phenyl substituent (c), these structures are all stable, even slightly more stable than 2a−4a (see Table 1) because of the stacking interaction between the phenyl rings. However, the more bulky methyl group significantly decreases the stability of 2b with 75 kJ/mol (ΔG = −147 kJ/mol) compared to 2a, and 4b does not even exist. We also consider two further four-membered structures (6 and 7) in this study, though, they are transition states with substituents a, b, or c. We have found that 6 and 7 can be stable 4735


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Table 1. Gibbs Free Energies of Substituted Pergerma Compoundsa

a

Gibbs free energies (ΔG) in kJ/mol are computed as a formal di-, tri-, or tetramerization reaction of the corresponding substituted digermyne (1). Relative Gibbs free energies (ΔGrel) are given as Gibbs free energies (ΔG) per digermyne units (n) (italics). Bold values indicate the experimentally characterized compounds.

of digermyne units. On the other hand, several germanium structures are stable, which are close to each other in energy; therefore, one cannot predict the formation of a certain structure in the experiments at this point. Interpretation of the Synthetic Results. To draw general conclusions about the role of substituents, it is necessary to understand previous experimental results. Seven stable pergerma compounds (1g, 1j, 1x, 7w, 11u, 14d, and 14v) have been isolated so far, all with different substituents; therefore, all possible germanium structures have to be investigated with these bulky groups. 2,6-Diethyl-phenyl group d is barely larger than c; therefore, it is not surprising that all structures are stable on the PES.

which are stable with c as well. Moreover, 6d and 10d are also minima thanks to the increased steric hindrance of d. However, the increased steric hindrance causes considerable changes in the relative energies of the structures. We have seen that the methyl substituent (b) can significantly decrease the relative stability of structures or even eliminate them as true minima. Substituent d also destabilizes several structures, changing the relative energies of the structures radically. The relative energies are close to each other in the case of the phenyl substituent, while 14d (see Figure 1) is significantly more stable than any other structure using substituent d. 14d is more stable than the second best 16d with 112 kJ/mol (the difference between 14c and 16c is only 1 kJ/mol for 16c). The relative Gibbs free 4736


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Figure 1. van der Waals representation of synthesized pergerma compounds. Red, gray, green, and white colors refer to germanium, carbon, silicon, and hydrogen atoms, respectively.

energy of 14d (ΔGrel = −175 kJ/mol) also shows the prominence of 14d compared to other, smaller structures; therefore, the isolation of 14d is clearly understandable7 and its 1% yield is consistent with the several possible side reactions. Tokitoh et al. applied the extremely bulky substituent g in their experiments.4c Investigating the previously determined germanium structures with this substituent, it suggests that only digermyne analogue 1g is stable on the PES; other complex structures are not because of the considerable steric hindrance. Therefore, the synthesis of 1g was simply inevitable, which is shown by the “almost quantitative”4c yield. The digermyne structure has also been synthesized with two other substituents (1j and 1x),4a,b,d and we have found that, in those cases, only the digermyne structure is stable on the PES as well, owing to the large bulky groups. Therefore, their syntheses can also be easily explained. Three other syntheses have been reported5−7 in which substituents u, v, and w resemble each other in shape; only their size varies somewhat, resulting in different products. In the case of the smallest u, which can be derived as a substituted analogue of the methyl group, all germanium structures are stable with the methyl substituent. The increased steric properties of u, however, change the relative energies of the structures similarly to the difference between phenyl (c) and 2,6-diethyl-phenyl (d) groups that has been previously analyzed. 14u has a distinct stability (ΔG = −766 kJ/mol) over other germanium structures; based on Gibbs free and relative Gibbs free energies as well, 14u is even more stable than that using the methyl substituent 14b (ΔG = −715 kJ/ mol). Therefore, the 3% yield of 14u can be explained analogously to 14d. When the bulkiness of the substituent (v) is increased, most of the germanium structures are eliminated from the PES and only four remain stable (1v, 4v, 8v, and 11v), but 8v and 11v are considerably more stable (ΔG = −401 and −389 kJ/mol,

respectively) than the other two. The Gibbs free energy difference between 8v and 11v is only 12 kJ/mol; therefore, we cannot give an exact global minimum at this level of theory (see previous section). Only 11v has been experimentally characterized,6 which can be the consequence of steric effects; substituent v provides full cover of 11 (see Figure 1) that enhances the isolation of 11v. The huge tert-butyl substituted silyl group (w) reduces further the number of stable structures; only three remain, 1w, 4w, and 7w. The stability of 4w and 7w is close to each other at this level of theory; the formation of 7w5 can be explained by steric effects of substituent w. 7 is not stable with any other substituent but w because it tends to isomerize to 4. In the case of 7w, however, the shape and the size of the substituent are ideal; w hinders the isomerization process because of its perfect cover of the Ge atoms (see Figure 1). This fact also enhances the isolation of 7w, hindering any other reaction as well. Interestingly, 7w ought to reflect Td symmetry, the symmetry of the tetrahedrane structure. The calculations suggest, however, that the molecule has no symmetry at all, in accordance with the experimental results;5 the Si−Ge−Ge bond angles are not identical in the molecule. Wiberg and coworkers attributed this feature to the attractive van der Waals interaction between the bulky tri-t-butylsilyl groups and the flexibility of the Si−Ge−Ge bond angles.5 Our results, however, suggest another solution. 7a with Td symmetry is a higher-order transition state where the H−Ge−Ge bending motions indicate the imaginary frequencies. Though, w hinders the isomerization of 7w, it is still unstable. One possible motion to eliminate that structure is the Si−Ge−Ge bending until the steric hindrance of the other substituents allows it. Substituent Design for Hitherto Unknown Pergerma Compounds. We have observed that the synthesis of pergerma compounds using the experimentally applied bulky substituents can be explained well by calculations. Appropriate 4737


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Figure 2. van der Waals representation of synthesized pergerma compounds. Red, gray, and white colors refer to germanium, carbon, and hydrogen atoms, respectively.

is still more stable than 4i (ΔG = +53 kJ/mol). Substituent h, the 2,6-diphenylphenyl group, can be regarded as the basic terphenyl-type substituent. Only 1h, 4h, and 8h are stable, and 8h (ΔG = −456 kJ/mol) is significantly more stable than 4h (which is also more stable than 1h). The Gibbs free energy of 4h and 8h is similar to that of 8c and 4c, indicating that there is no steric repulsion of the substituents in 4h or 8h. In the case of 8h, the substituents provide cover for the hexagermabenzene moiety (see Figure 2), suggesting that 8h has also kinetic stability. 8h fulfills all criteria that have been raised based on previous experiments: a structure that dominates the PES serves as the undisputed global minimum, all other competitive structures are eliminated from the PES, and the substituents provide almost perfect cover of the germanium core to hinder any side reactions. Therefore, 8h can be suggested for preparation. 8h also gives us a hint to reach the perfect cover of the hexagermabenzene moiety by meta substitution of phenyl rings (substituent k). In that case, the same structures are stable than that in substituent h (1k, 4k, and 8k). The Gibbs free energy of 4k and 8k (ΔG = −212 and −434 kJ/mol, respectively) is similar to that of 8h and 4h (ΔG = −200 and −456 kJ/mol, respectively), indicating that the additional methyl groups do not cause steric repulsion in these molecules but increase the steric cover of the hexagermabenzene moiety (see Figure 2). Therefore, 8k can also be a potential synthetic target; however, we have to note that substituent k has not been reported in the literature. It is based on our considerations. The Rind substituent family is known for its flexibility; the alteration of the alkyl chain can essentially change the shape and size of the bulky substituent.17 Recently, Tamao and coworkers have synthesized the first stable germanone with the proper choice of Rind substituent.18 Substituent l can be regarded as the basic structure of Rind groups, without alkyl chains, which marks the limit of the family. All studied structures are stable with substituent l but 7l. 14l has a distinct thermodynamic stability (ΔG = −900 kJ/mol) compared to other structures; 14l is more stable than the second best 16l (ΔG = −814 kJ/mol) with 86 kJ/mol. In this sense, substituent l resembles substituents d and u, where 14d and 14u have been

substituents can eliminate competitive structures from the PES, resulting in significantly stable molecules, and these global minima have been achieved in experiments. Steric effects can enhance the formation; the full cover of the germanium core hinders further reactions. These simple observations can be exploited to design substituents for the synthesis of hitherto unknown pergerma compounds. In this section, we analyze frequently used bulky group families to find substituents that stabilize hitherto unknown pergerma structures similarly to that in the previous section, and therefore, they may be realized in experiments. We have seen that, in the case of the 2,6-diethylphenyl group (d), all investigated structures are stable on the PES but 7d. Increasing the bulkiness of the substituent may stabilize other structures than 14d. 2,6-Diisopropylphenyl group e is one of the most frequently used bulky substituents in organometallic chemistry.16 Applying substituent e, four further structures are eliminated from the PES compared to substituents d. According to relative Gibbs free energies, 11e is the most stable, but 8e and 14e cannot be excluded based on thermodynamic results. The 2,6-di-tert-butylphenyl group (f) completely changes the stability of pergerma compounds; only five structures are stable (1f, 3f, 5f, 6f, and 8f). Two of them have positive Gibbs free energies (5f and 8f, ΔG = +95 and +47 kJ/mol, respectively), whereas these values are large negative numbers in the case of substituent e (5e and 8e, ΔG = −249 and −514 kJ/mol, respectively). The other three show very small stability. 3f and 6f are somewhat more stable than 1f (3f and 6f, ΔG = −18 and −50 kJ/mol, respectively), but their relative stability decreases significantly compared to their substituent e analogues (3e and 6e, ΔG = −250 and −244 kJ/mol, respectively). On the basis of these results, the extreme steric substituent f obstructs their formation. Substituent g can be derived from substituent f with the increasing of bulky groups where only digermyne 1g is stable. j is the widely applied terphenyl-type group;16 less steric analogues are also known, which may stabilize other pergerma structures but digermyne. Ortho-diisopropyl substitution of j can be changed to methyl groups, resulting in substituent i. In that case, 4i becomes stable on the PES besides 1i; however, 1i 4738


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prepared, and these compounds have significant stability compared to other structures. In this study, we considered two types of modification of the basic structure of Rind substituents, the systematic increasing of the alkyl chain on one or both sides of substituent l. Substituent m is the methyl derivative of substituent l in the R2 position. This modification eliminates 12m, 13m, 15m, and 16m from the PES, the competitive structures of 14, while the stability of 14m remains (ΔG = −882 kJ/mol) compared to 14l. Ethyl modification of substituent l terminates structures 2 and 4 from the PES and also slightly decreases the stability of 14n (ΔG = −816 kJ/mol) compared to 14l, while isopropyl substitution, substituent o, eliminates 14o. On the basis of these results, substituent m can be suggested for an alternative synthesis of octagermacubane, 14. t-Butyl substitution in the R2 position, in p, significantly reduces the stability of the remaining structures 3p, 5p, and 8p (ΔG = −144, −92, and −210 kJ/mol, respectively) compared to 3o, 5o, and 8o, respectively (3o, 5o, and 8o ΔG = −329, −342, and −568 kJ/mol, respectively), which marks the limit of the one-sided modification of the Rind substituent l. Substituent q, the methyl substitution of the Rind group in the R1 and R2 positions, immediately destabilizes most of the investigated structures; only 1q, 3q, 6q, 8q, and 11q are stable on the PES. The relative Gibbs free energy of 6q, 8q, and 11q (ΔGrel = −112, −108, and −105 kJ/mol, respectively) is commensurable with each other at this level of theory; there is no favorable structure at this point. However, ethyl modification of l (substituent r) differentiates these structures. 11r is no longer stable on the PES, and the relative Gibbs free of 6r is almost twice as large as that of 8r (ΔGrel = −101 and −56 kJ/mol, respectively). Therefore, 6r can be regarded as the global minimum of the PES. Figure 2 shows that the substituents provide perfect cover of the germanium ring, indicating that 6r also has considerable kinetic stability, which means that 6r fulfills all criteria that have been raised based on the previous successful synthesis: 6r can be suggested as an experimental target. Substituent s, bearing isopropyl alkyl chains, destabilizes all studied structures but 1s, whereas, in case of tert-butyl modified substituent t, even 1t is not stable on the PES, which closes the possible two-sided modifications of the Rind substituent l. Figure 3 grasps the essence of our results. In all experiments, the most stable structure has been synthesized, which is understandable since the thermodynamic driving force is huge. On the basis of the results of (GeH)2n structures, however, we have found that several low-lying minima, potential products, are situated on the PES (P1 and P2) with similar stability, and one cannot conclude which structure may be synthesized. The top picture of Figure 3 shows this scenario when two reaction paths are possible without the use of bulky substituents. (We note that this picture is a serious simplification; numerous reaction routes can be imagined to several stable structures in practice, not to mention its high dimensionality.) With the application of the proper bulky groups, most or even all of the competitive structures can be eliminated from the PES, one structure remaining, which is the global minimum. The bottom pictures of Figure 3 show these cases. With the proper choice of the bulky group, one can terminate all competitive structures (in this case, the picture represents them with only one minimum), and with the proper change of the bulky group, another structure can be prepared with high yield.

Figure 3. The effect of bulky groups on the PES. The top picture represents the case without the use of bulky groups. From the initial state R (R refers to reactants, 2n digermyne units), digermynes can react toward several low-lying stable minima that are situated on the PES with similar stability, and one cannot conclude which structure may be synthesized. For the sake of simplicity, only two reaction paths are displayed, P1 and P2, where P refers to products. The bottom left picture shows that, with the proper choice of substituents, one can eliminate the competitive structure P2; that is why the reaction has to proceed toward P2. With the appropriate change of the bulky group, only the other structure remains stable, P1 on the bottom right picture, which can be prepared with high yield. The color bar follows the standard cartographical representation; q1 and q2 are generalized reaction coordinates. Red lines represent the minimal energy paths (MEPs) from the reactants to the products.

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CONCLUSION In this study, we analyzed the general reaction scheme of reductive dehalogenation of halogermanes (Scheme 2) by theoretical tools. First, we explored the possible stable (GeR)2n structures up to octagermanes (n = 4) with small substituents. We then investigated previously reported experimental results, applying real bulky substituents, and found that all preparations can be easily explained by the principle of energy minimum and the special steric effect of the bulky substituents. We found, with the proper choice of the bulky groups, all structures could be destabilized, owing to steric hindrance, but one of which the stability remained almost the same. These findings can be applied to design substituents for still unknown compounds, such as tetragermacyclobutadiene and hexagermabenzene. We found that substituent r is perfect for the synthesis of tetragermacyclobutadiene, whereas substituents h and k for hexagermabenzene; they fulfill all criteria that have been raised based on the discussion of previous successful experimental results. These results are in accordance with our recent molecular tailoring concept,2 which states that every chemical reaction has one or a few optimal substituents that can be found based on in silico. We note that, although this molecular tailoring concept has been demonstrated originally in the case of silicon compounds2 and now in germanium compounds, we think that it is more universal. It can be easily adapted to other organometallic synthesis families; moreover, it can be used for 4739


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optimizing a catalyst19 to accelerate the desired reactions and eliminate side reactions, simply with the stabilization or destabilization of the product or the intermediate during the reaction paths. Since the synthesis of hitherto unknown reactive compounds is a slow, expensive, and unpredictable procedure, our approach may bring a fundamental breakthrough in this area.

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(10) (a) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (b) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007. (c) Becke, A. D. J. Chem. Phys. 1997, 107, 8554. (d) Grimme, S. J. Comput. Chem. 2006, 27, 1787. (11) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045. (12) (a) Antony, J.; Grimme, S. Phys. Chem. Chem. Phys. 2006, 8, 5287. (b) Grimme, S.; Antony, J.; Schwabe, T.; Mück-Lichtenfeld, C. Org. Biomol. Chem. 2007, 5, 741. (c) Mück-Lichtenfeld, C.; Grimme, S.; Kobrynb, L.; Sygula, A. Phys. Chem. Chem. Phys. 2010, 12, 7091. (13) Vahtras, O.; Almlöf, J.; Feyereisen, M. W. Chem. Phys. Lett. 1993, 213, 514. (14) Frisch, M. J.; et al. Gaussian 09, Revision B.01; Gaussian Inc.: Wallingford, CT, 2010. (15) Avogadro: An Open-Source Molecular Builder and Visualization Tool, Version 1.00. http://avogadro.openmolecules.net/. (16) Power, P. P. J. Organomet. Chem. 2004, 689, 3904. (17) Matsuo, T.; Kobayashi, M.; Tamao, K. Dalton Trans. 2010, 39, 9203. (18) Li, B.; Fukawa, T.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. Nat. Chem. 2012, 4, 361. (19) Szilvási, T.; Veszprémi, T. ACS Catal. 2013, 3, 1984.

ASSOCIATED CONTENT

S Supporting Information *

Benchmark study, natural charges, Wiberg bond indices, optimized geometries, and computed energies. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Hungarian Scientific Research Foundation (OTKA) for financial support under Grant No. 76806 K. The New Széchenyi Plan TAMOP-4.2.2/B-10/1-2010-0009 is also gratefully acknowledged.

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REFERENCES

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