Theoretical Study on the Stability and Aromaticity of

Article pubs.acs.org/Organometallics

Theoretical Study on the Stability and Aromaticity of Metallasilapentalynes Xuerui Wang, Congqing Zhu, Haiping Xia,* and Jun Zhu* State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China S Supporting Information *

ABSTRACT: Antiaromatic compounds and small cyclic alkynes or carbynes are both challenging for synthetic chemists because of the destabilization caused by their antiaromaticity and highly distorted triple bonds, respectively. These dual destabilizations could be the reason why pentalyne (I), a highly antiaromatic and extremely strained cyclic alkyne, has never been synthesized. Recently, we have successfully synthesized the first metallapentalyne (II), benefiting from the stabilization of a metal fragment by reducing the ring strain and switching the antiaromaticity in pentalyne to the aromaticity in metallapentalyne. An interesting question is raised: can the aromaticity in metallasilapentalyne (III) be retained, considering the fact that the silicon atom is reluctant to participate in π bonding? Here we report a thorough theoretical study on the stability and aromaticity of metallasilapentalynes. The computed energies and magnetic properties reveal the reduced aromatic character of osmasilapentalyne in comparison with osmapentalyne. The effect of the ligands, substituents, and base on the aromaticity and stability of osmasilapentalyne is also discussed, thus providing an important guide to the synthesis of osmasilapentalyne.

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INTRODUCTION Transition-metal-containing aromatic compounds have attracted considerable interests from both experimentalists and theoreticians because of their fascinating reactivities, which contain both aromatic properties and the characteristics of organometallic compounds. In 1979 Thorn and Hoffmann1 first proposed theoretically three hypothetical classes of metallabenzenes. Three years later, the first isolated metallabenzene was synthetized by Roper and co-workers.2a Since then, continuing progress has been made in the chemistry of metallaaromatics. For instance, various metallabenzenes containing Os,2 Ir,3 Ru,4 Pt,5 Re,6 and Ni7 have been reported. However, in comparison, metallabicyclic complexes have been much less developed. Pentalyne (I; Scheme 1), a highly antiaromatic and strained cyclic alkyne, has never been realized due to the dual

destabilization caused by its antiaromaticity and extreme ring strain.8,9 Recently, we have successfully synthesized the first metallapentalyne (II), due to the stabilization of the transitionmetal fragment.10 A theoretical study reveals that introducing a metal fragment into the rings can not only reduce the ring strain11 but also switch the antiaromaticity in pentalyne to the aromaticity in metallapentalyne due to the additional d electrons of a transition metal. An interesting question is raised: could the aromaticity of metallasilapentalyne (III), formed by the replacement of the carbyne carbon with the silicon atom in metallapentalyne, be retained given that the silicon atom is reluctant to participate in π bonding?12 On the other hand, as the silicon is reluctant to participate in π bonding,12 the synthesis of transition-metal silylyne complexes is quite challenging. Thus, only a few examples have been synthesized so far.13 This is understandable, as the low electronegativity of the silicon atom usually leads to weak and polarized multiple bonds. However, as aromaticity is one of the most important stabilizing factors, could it compete with the reluctance of the silicon to participate in π bonding to form a stable metallasilapentalyne? Our ongoing interest in aromaticity10,14 has led us to investigate these silicon-containing

Scheme 1. Proposed Structure of Metallasilapentalyne

Received: February 16, 2014 Published: March 21, 2014 © 2014 American Chemical Society

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metallaaromatics. Here we report a thorough theoretical study on the stability and aromaticity of metallasilapentalynes.

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COMPUTATIONAL METHODS

All of the geometries of the metallasilapentalyne complexes have been fully optimized at the B3LYP level15 of density functional theory (DFT). Frequency calculations were performed at this level of theory to confirm no imaginary frequency. The LanL2DZ16 basis set was used to describe Fe, Ru, Os, Si, P, and Cl, while the standard 6-311+ +G(d,p) basis set17 was used for all other atoms. Polarization functions were added for P (ζ(d) = 0.340), Cl (ζ(d) = 0.514), Si (ζ(d) = 0.262), Fe (ζ(f) = 2.462), Ru (ζ(f) = 1.235), and Os (ζ(f) = 0.886).18 All calculations were carried out using the Gaussian 03 package.19 To examine the effect of the functional, we calculated isomerization stabilization energy (ISE) values of osmasilapentalyne at the M05/6311++G(d,p),20 TPSS/6-311++G(d,p),21 PBE/6-311++G(d,p),22 and PW91/6-311++G(d,p)23 levels. The computed ISE values are −19.4, −20.0, −19.7, and −19.6 kcal mol−1, respectively, which is comparable to that (−18.3 kcal mol−1 in Figure 5) at the B3LYP/6-311++G(d,p) level, indicating that the functional effect is very small. To gain a proper understanding of the chemical bonding, natural bond orbital (NBO) analysis24 and electron localization function (ELF)25 methods were employed. The energies (in kcal mol−1) are given including the zero-point energy corrections.

Figure 2. Synthesized osmapentalyne 1a and proposed osmasilapentalyne 1b.

Figure 3. Optimized structure with selected bond lengths (Å) and bond angles (deg) in osmasilapentalyne 1b.

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the OsSi double bond (2.325 Å) in the first osmium silylene complex26 and longer than that in the first silylyne complex of a metal beyond group 6, [Cp*(iPr3P)(H)OsSi(Trip)] (2.176 Å),13d reported recently. This is understandable, as the elongation could release the strain in a five-membered ring to some extent. In addition, the metal−silicon triple bonds in ruthenasilapentalyne and ferrasilapentalyne are computed to be 2.280 and 2.164 Å, respectively. The computed Wiberg bond indices (Table 1) of OsC and OsSi triple bonds in 1a,b are 1.70 and 1.25, respectively,

RESULTS AND DISCUSSION Geometry and Stability of Metallasilapentalynes. As only osmapentalyne has been isolated so far, we focus our study on group 8 transition metals. We first examined the relative stabilities of the isomers of metallasilapentalynes, where the silicon atom is at different positions of the rings. The results in Figure 1 reveal one general rule: the isomers with the Si atom

Table 1. Bond Lengths (Å), Wiberg Bond Indices, and Charges of OsE Triple Bonds (E = C, Si) in 1a,b C Si

bond length (Å)

Wiberg bond index

charge (Os)

charge (E)

1.845 2.299

1.70 1.25

−0.97 −1.42

+0.17 +1.14

which is in line with the reluctance of the silicon to participate in π bonding.12 The natural charge (+1.14) on the silicon atom in 1b is significantly more positive than that (+0.17) on the carbyne carbon in 1a, leading to a highly polarized OsSi triple bond, in line with the NBO analysis that, in the Os−Si σ bond, the percentages of osmium and silicon are 27% and 73%, respectively. Moreover, for the hybridization of silicon, the components of s and p orbitals are 56% and 44%, respectively, indicating an sp hybridization. As shown in Figure 4A, the resonance structures of osmasilapentalyne 1b suggest possibly a lone pair on the silicon atom. To examine this hypothesis, we performed ELF calculations on osmapentalyne 1a and osmasilapentalyne 1b. Indeed, the domain with an isovalue of 0.85 on the silicon atom in 1b (Figure 4B) is significantly larger than that on the carbyne carbon in 1a. Our previous study showed that although introducing a transition-metal fragment into the pentalyne ring can switch the antiaromaticity in pentalyne to aromatcity in osmapentalyne 1a, the ring strain10 in 1a is still large at 24.8 kcal mol−1. When the carbyne carbon is replaced by a Si atom, we expected a reduced ring strain due to the larger size of the Si atom. Indeed, the computed ring strain in osmasilapentalyne 1b is 8.7 kcal mol−1

Figure 1. Relative stabilities of the isomers of metallasilapentalynes with the silicon atom at different positions.

directly bonded to the metal center have greater stability than those with the Si atom at other positions. Specifically, the isomer with the Si atom triply bonded to the metal center is the most stable, and that with the Si atom singly bonded to the metal center is the second most stable. This is understandable because the metal center has diffuse d orbitals. Thus, the Si atom prefers to be bonded to the metal center rather than the carbon atom position, although it is reluctant to participate in π bonding.12 Therefore, in the following study, we always choose the isomer with the Si atom triply bonded to the metal center. To compare osmapentalyne 1a with osmasilapentalyne 1b, we first optimized the structure of 1b (Figure 2), as the structure and aromaticity of 1a have been reported recently by us. Osmasilapentalyne 1b has an almost planar eight-membered metallabicycle, as reflected by particularly small dihedral angles (∠Os−Si−C1−C2 = 1.0°, ∠C5−C6−Os−Si = 0.8°, ∠C6− Os−Si−C1 = −2.1°). The C−C bond lengths (1.363−1.429 Å; Figure 3) are between those of C−C single and double bonds. The OsSi triple-bond length (2.299 Å) in 1b is shorter than 1846

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Figure 4. (A) Resonance structures of osmasilapentalyne 1b. (B) ELF calculations with an isovalue of 0.85 on osmasilapentalyne 1b and osmapentalyne 1a. (C) Strain energy of osmasilapentalyne 1b calculated on the basis of acyclic reference compounds. The correction of the zero-point energy is included in kcal mol−1. Figure 5. ISEs of metallasilapentalyne. The values are given in kcal mol−1 including the zero-point energy corrections.

only by comparing energy differences between the partially and fully optimized complexes (Figure 4C). For the partially optimized complex, we fixed the three angles Os−Si−C1, C3− Os−C6, and C3−Os−Si to be the same as in complex 1b. Moreover, unlike the alkynes with a linear geometry around the CC triple bond, the substituents at the SiSi group in disilyne are arranged in a trans-bent fashion.27 Similarly, in the fully optimized structure in Figure 4C, the bond angle at the silicon atom is computed to be 141.8°. The changes of the Os− Si−C angle between the fully optimized structure (141.8°) and the partially optimized structure (112.6°) is 29.2°, which is smaller than that at the carbyne carbon in the osmapentalyne (from 167.0° to 129.5°). Thus, the ring strain is significantly reduced in the osmasilapentalyne, although the bond angle at the silicon atom in 1b is much smaller than that at the carbyne carbon in 1a. Aromaticity of Osmasilapentalynes. The “isomerization stabilization energy” (ISE) method is quite effective to evaluate the aromaticity, especially for strained systems in both the ground state28 and the lowest triplet state.14f Therefore, we applied the ISE method to examine the aromaticity in metallasilapentalynes. In sharp contrast to the positive value (6.8 kcal mol−1) of the antiaromatic pentalyne,10 the ISEs values of osmasilapentalyne for the first three designed reaction (Figure 5) are computed to be −18.3, −17.5, and −16.5 kcal mol−1, respectively. Those values are smaller than those of benzene (−21.8 kcal mol−1) and osmapentalynes (−19.6 kcal mol−1),10 indicating reduced aromaticity in osmasilapentalyne. Furthermore, we also employed another strain-balanced indene−isoindene isomerization method28b to further confirm the aromaticity in osmasilapentalyne. As shown in the fourth equation, the computed ISE value is −16.9 kcal mol−1, which is consistent with those in the first three equations by the methyl−methylene isomerization method. The computed negative ISE values in the fifth and sixth equations confirmed the aromaticity in ruthenasilapentalyne and ferrasilapentalyne. The gradually reduced values of metallasilapentalynes could be attributed to the less diffuse d orbitals as one goes up the group.

In order to probe the nature of aromaticity in metallasilapentalyne, we performed canonical molecular orbital (CMO) nucleus-independent chemical shift (NICS)29 computations30 on osmasilapentalyne 1b (Figure 6). The five π

Figure 6. NICS(1)zz contributions of occupied π molecular orbitals of model complex osmasilapentalyne 1b. The eigenvalues of the molecular orbitals are given in parentheses. The NICS(1)zz values given before and after the slant are those computed at the geometrical centers of rings A and B, respectively.

molecular orbitals (HOMO, HOMO-2, HOMO-3, HOMO-8, and HOMO-12), perpendicular to the metallabicycle plane, are derived from the orbital interactions among the two d orbitals of the Os atom (5dxz and 5dyz) and the pπ atomic orbitals of the C6SiH5 unit, whereas HOMO-1 corresponds to the in-plane π bonding orbital in the OsSi triple bond. Specifically, the three Hückel-type MOs (HOMO, HOMO-2, and HOMO-12) are derived from the orbital interactions between the pzπ atomic orbitals of the C6SiH5 unit and the 5dxz orbital of the Os atom, whereas two Möbius-type MOs (HOMO-3 and HOMO-8) are formed by the orbital interactions between the pzπ atomic orbitals of the C6SiH5 unit and the 5dyz orbital of the Os atom. The computed NICS(1)zz values of rings A and B in osmasilapentalyne 1b are −19.8 and −16.2 ppm, respectively 1847

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density from the triple bond, leading to a better conjugation in the metallabicycle. Effect of Ligand on the Structure for Osmasilapentalynes. The ligand effect was also examined by changing the ligand PH3 to CO and PMe3. When PH3 was replaced by carbonyl ligands, the OsSi bond length became longer and the Wiberg bond index became smaller (Figure 9). This is

(Figure 6). In general, negative values denote aromaticity and positive values indicate antiaromaticity. These values are less negative than that of benzene (−29.3 ppm) and comparable to those (−26.2 and −20.9 ppm for rings A and B, repsectively) in 1a, further confirming the fact that the silicon atom is reluctant to participate in π bonding. Apparently, the contribution to the aromaticity in 1b from the two Möbius MOs is significantly larger than that from the three Hückel-type MOs, similar to the case for osmapentalyne 1a. Thus, the nature of the aromaticity in 1b could be described as mixed aromaticity with Möbius aromaticity dominating. The less negative NICS(1)zz values of rings A and B in ruthenasilapentalyne and ferrasilapentalyne (Figure 7) are consistent with the less negative ISE values in Figure 5.

Figure 9. Computed bond lengths (Å), Wiberg bond indices, and NICS values (ppm) in osmasilapentalynes with different ligands.

understandable, because there is a bonding interaction between the metal and metal-bonded carbon and silicon in the HOMO (Figure 6). Therefore, π-acceptor ligands can decrease the electron density of the metal center, leading to a reduced metal−silicon triple bond. Indeed, when PMe3 is used to replace PH3, the OsSi triple bond becomes stronger, which is indicated by the smaller bond length and larger bond order. The more negative NICS values are consistent with the fact that π-donor ligand can stabilize the osmasilapentalynes. Effect of Lewis Base on the Stabilization for Osmasilapentalyne. As we discussed above, the silicon atom carries a significantly positive charge (+1.14), leading to a highly polarized metal−silicon triple bond (Table 1). Thus, the Lewis base is expected to be able to stabilize osmasilapentalyne. Actually, the first isolated silylene complex33 was favored by a Lewis base as highly polarized toward Mδ‑ Siδ+ in the MSi double bond.34 In addition, Cp(CO)3MoSi(2,6-Trip2-C6H3), the neutral silylyne from a base-stabilized halosilyene adduct, was reported by Filippou et al.13c As shown in Figure 10, the bidentate base can stabilize osmasilapentalyne more effectively than the monodentate base, as indicated by the greater exothermicity of the reaction.

Figure 7. NICS(1)zz values (ppm) for rings A and B in metallasilapentalynes.

Effect of the Phosphonium Substituent on the Aromaticity in Osmasilapentalynes. Substituents play an important role in the stabilization of metallabenzene and metallabenzyne.31 Therefore, we examined the phosphonium substituent effect on the aromaticity in 1a,b. A previous study has shown that the phosphonium substituent plays a crucial role in stabilizing iso-osmabenzene complexes14c and osmapyridine.32 In addition, the first osmapentalyne also contains the phosphonium substituent in the ring. Therefore, the phosphonium substituent is introduced into the ring of 1a,b. Comparing the ISE value (−17.5 kcal mol−1) in the second equation in Figure 5 with that (−25.1 kcal mol−1) in the first equation in Figure 8, one can conclude that the phosphonium

Figure 8. Effect of the phosphonium substituent on the stabilization of osmasilapentalyne 1b and osmapentalyne 1a evaluated by the ISE values (kcal mol−1).

Figure 10. Base effects on the stabilization of osmasilapentalyne 1b indicated by the exothermic reactions. The reaction energies are given in kcal mol−1.

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CONCLUSION We have performed DFT calculations to examine the stability and aromaticity in metallasilapentalyne. The computed negative ISE and NICS(1)zz values reveal the aromatic character of osmasilapentalyne, although the silicon atom is reluctant to participate in π bonding. However, the aromaticity in osmasilapentalyne is reduced in comparison with that of osmapentalyne. Further CMO-NICS calculations suggest that the nature of the aromaticity in osmasilapentalynes could be

substituent leads to an increase of aromaticity in osmasilapentalyne. To further confirm the stabilization of the phosphonium substituent, we also calculated the ISE value (−31.0 kcal mol−1) of osmapentalyne with the phosphonium substituent, which is more negative than that (−23.3 kcal mol−1) of osmapentalyne without the phosphonium substituent.10 The increased aromaticity in 1a,b caused by the phosphonium substituent is understandable, because as a strong electron-withdrawing group, it can decrease the electron 1848

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described as mixed aromaticity with Möbius aromaticity dominating. In addition, the phosphonium substituent, πdonor ligands, and Lewis base can enhance the aromaticity or stability of osmasilapentalynes. All of these findings could be helpful for the synsthesis of the first metallasilapentalyne.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

An .xyz file containing the computed Cartesian coordinates of all of the species reported in this study. This material is available free of charge via the Internet at http://pubs.acs.org. The .xyz file may be opened as a text file to read the coordinates or opened directly by a molecular modeling program such as Mercury (version 3.3 or later, http://www. ccdc.cam.ac.uk/pages/Home.aspx) for visualization and analysis.

Corresponding Authors

*H.X.: e-mail, [email protected]; web, http://chem.xmu.edu. cn/group/hpxia/index.htm. *J.Z.: e-mail, [email protected]; web, http://junzhu.chem8. org. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Chinese National Natural Science Foundation (21172184 and 21133007), the National Basic Research Program of China (2011CB808504), the Program for New Century Excellent Talents in University (NCET-13-0511), the Program for Changjiang Scholars and Innovative Research Team in University, and the Fundamental Research Funds for the Central Universities (2012121021).

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dx.doi.org/10.1021/om500170w | Organometallics 2014, 33, 1845−1850