To Be Bridgehead or Not to Be? This is a Question of

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

To Be Bridgehead or Not to Be? This is a Question of Metallabicycles on the Interplay between Aromaticity and Ring Strain Jingjing Wu, Ke An, Tingting Sun, Jinglan Fan, 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, China S Supporting Information *

ABSTRACT: Transition-metal-containing metallaaromatics have attracted considerable interest from both experimental and computational chemists because they can display properties of both organometallic compounds and aromatic organic compounds. In general, the transition metal in a metallabicycle prefers a nonbridged position to the bridgehead one because of the larger ring strain caused by the rigidity in the bridgehead position, as exemplified by metallanaphthalene and metallanaphthalyne. On the contrary, the osmium atoms in recently synthesized osmapentalyne and osmapentalene are located at the bridgehead position. To probe the origin of the difference between these metallabicycles, we carried out density functional theory calculations. The metal-bridgehead osmanaphthalene and osmanaphthalyne are computed to be less stable by 17.9 and 26.3 kcal mol−1 than the non-metal-bridged ones, respectively. In addition, the metal-bridgehead osmapentalene and osmapentalyne are more stable by 11.8 and 22.8 kcal mol−1 than the non-metal-bridged isomers, respectively. Further study revealed that the ring strains in these paired isomers are comparable to each other. Thus, it is aromaticity rather than ring strain that determines the relative thermodynamic stabilities of these complexes.

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INTRODUCTION Since the prediction of three classes of stable metallabenzenes by Thorn and Hoffmann1 in 1979 and the first isolation of metallabenzene in 19822 and metallabenzyne in 2001,3 metallaaromatics have attracted considerable attention from both experimentalists and theoreticians.4,5 Studies in the past three decades have led to the synthesis and characterization of a number of metallabenzene6 and metallabenzyne7 complexes. Recently, the chemistry of their higher analogues, i.e., metallanaphthalenes, metallanaphthalynes, and metallaanthracene, has also been developed. Specifically, iridanaphthalenes (Figure 1a) were reported by Paneque and co-workers in 2003 and 2015.8 In addition, osmanaphthalenes, osmanaphthalynes (Figure 1b), and osmapyridynes were synthesized by the groups of Jia9 and Xia.10,11 Very recently, iridaanthracene was also isolated by Frogley and Wright.12 In general, most of the transition metals in metallabicycles prefer a nonbridged position to the bridgehead one because of the larger ring strain caused by the rigidity in the bridgehead position, as exemplified by these three classes of metallanaphthalenes, metallanaphthalynes, and metallaanthracene. However, the osmium atoms in recently synthesized osmapentalynes (Figure 1c)13 and osmapentalenes (Figure 1d)14 are located at the bridgehead position. What is the origin of the completely opposite preferences in terms of the position of the metal centers among these complexes? Our ongoing interest in © XXXX American Chemical Society

Figure 1. Examples of isolated iridanaphthalenes (a), osmanaphthalynes (b), osmanaphthalene (b), osmapentalyne (c), and osmapentalene (d).

metallaaromatics15 has led us to address this issue by investigating the aromaticity and ring strain of these metallabicyclic complexes.

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COMPUTATIONAL METHODS

All of the structures were optimized at the B3LYP16 level of density functional theory (DFT) without any constraint. In addition, frequency calculations were performed at the same level of theory to confirm that all of the stationary points were minima (no imaginary Received: October 11, 2017

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DOI: 10.1021/acs.organomet.7b00758 Organometallics XXXX, XXX, XXX−XXX

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Organometallics frequencies). The effective core potentials (ECPs) of Hay and Wadt with the double-ζ valence basis set (LANL2DZ) were used to describe Os, Cl, and P atoms,17 whereas the 6-311++G(d,p)18 basis set was used for C and H atoms. Polarization functions were also added for Cl (ζ(d) = 0.514), P (ζ(d) = 0.340), and Os (ζ(f) = 0.886).19 In order to examine the ligand effect of the simplified PH3, we optimized the pair of isomers 5 and 6 with PPh3. The 6-31G(d) basis set was used for C and H atoms because of the extremely large PPh3 groups, while the basis sets for the other atoms were kept unchanged. The calculated relative Gibbs free energy and electronic energy were 14.6 and 15.6 kcal mol−1, respectively, which are comparable to the values with PH3 (11.8 and 13.1 kcal mol−1; Scheme 1), indicating that the ligand effect is small. To examine the effect of the basis set for the heavier P, Cl, and Os atoms, we employed a larger 6-311++G(d,p) basis set for P and Cl and LANL2TZ(f)20 for the osmium atom in single-point calculations on complexes 1 to 8. The values computed using the larger basis sets (19.5, 27.7, 11.7, and 23.6 kcal mol−1, respectively; Scheme S1) are very close to those in Scheme 1 (18.4, 26.9, 13.1, and 24.7 kcal mol−1, respectively), suggesting a small basis set dependence. M0621 and M06L22 calculations using an ultrafine grid were performed using the Gaussian 09 package,23 whereas the B3LYP calculations were performed with the Gaussian 0324 software at 298 K. In order to examine the effect of the functional for these complexes 1 to 8, the density functionals M06 and M06L were used to do single-point calculations. The results in Scheme S1 indicate that the energies are close to those obtained at the B3LYP level. To examine the solvent effect, we optimized the structures of 1 to 8 using the polarizable continuum model (PCM)25 with benzene and acetonitrile as the solvent. The additional calculations showed that the solvent effect is particularly small. For instance, when benzene was chosen as the solvent, the relative electronic energies of 2 to 1, 4 to 3, 5 to 6, and 7 to 8 were 19.4, 27.3, 14.0, and 24.4 kcal mol−1, respectively. When acetonitrile was used as the solvent, the relative electronic energies of 2 to 1, 4 to 3, 5 to 6, and 7 to 8 were 21.5, 28.2, 14.6, and 24.1 kcal mol−1, respectively. They are comparable to those in the gas phase (18.4, 26.9, 13.1, and 24.7 kcal mol−1, respectively; Scheme 1). To examine the reliability of the DFT calculations, single-point energy calculations were performed at the CCSD26 level with the 6-311+ +G(d,p) basis set for C, H, P, and Cl atoms and LANL2TZ(f) for the metal using the Gaussian 09 package. The relative electronic energy of 3 to 4 was 25.4 kcal mol−1 at the CCSD(T) level, which is comparable to that at the B3LYP level (26.9 kcal mol−1), indicating the reliability of our DFT calculations. Nucleus-independent chemical shift (NICS) values27 were calculated at the B3LYP-GIAO level. Anisotropy of the induced current density (ACID)28 calculations were carried out at the B3LYP/LANL2DZ-6-311++G(d,p) level.

examined, as shown in Figures S1−S4. The calculated results suggested that the metal-bridgehead osmanaphthalene and osmanaphthalyne are less stable by 17.9 and 26.3 kcal mol−1 (Gibbs free energy) than the non-metal-bridged isomers, respectively. Contrary to osmanaphthalene and osmanaphthalyne, the metal-bridgehead osmapentalene and osmapentalyne are more stable than the nonbridged isomers by 11.8 and 22.8 kcal mol−1, respectively (Scheme 1). According to our previous report, osmapentalenes and osmapentalynes are aromatic, and the introduction of a metal fragment into the pentalyne ring significantly reduces the ring strain.13,14 Thus, we hypothesized that the ring strain and aromaticity may compete with each other in metallabicycles. If the aromaticity dominates over the ring strain, the more stable isomers will be the more aromatic ones. To probe the aromaticity of various metallabicycles, we employed the NICS, a key magnetic signature of aromaticity introduced by Schleyer and co-workers.27 NICS(1)zz, which describes the zz component 1.0 Å above the ring center, was proven to be a nice indicator of π aromaticity in both the ground and excited states.15c,27a As shown in Tables 1 and S1, Table 1. Computed NICS(1)zz Values (in ppm) for a Series of Metallabicyclic Compounds ([Os] = Os(PH3)2) and their HOMA Indices (for C−C Bonds) NICS(1)zz compound

A

B

HOMA

1 2 3 4 5 6 7 8

−3.0 −24.4 26.8 −25.2 −24.0 −0.1 −26.7 35.9

1.6 −8.9 17.1 −7.1 −24.0 0.8 −21.3 14.8

0.665 0.675 0.595 0.787 0.912 0.440 0.935 −0.299

the NICS(1)zz values of the left (A) and right (B) rings are −24.0 and −24.0 ppm in metal-bridgehead osmapentalene (5) and −26.7 and −21.3 ppm in metal-bridgehead osmapentalyne (7), respectively. The NICS(1)zz values of their nonbridged isomers are either close to zero (6) or significantly positive (8). In general, negative NICS values indicate aromaticity and positive values antiaromaticity. It should be noted that the NICS(1)zz value of benzene is −29.3 ppm.13a In sharp contrast, the NICS(1)zz values of metal-bridgehead osmanaphthalene (1) are less negative than those of the corresponding nonbridged isomers, and osmanaphthalyne (3) even has significantly positive NICS(1)zz values. The results suggested that the metal-bridgehead osmapentalene and osmapentalyne together with the nonbridged osmanaphthalene and osmanaphthalyne are aromatic. On the contrary, the metal-bridgehead osmanaphthalene and osmanaphthalyne together with the nonbridged osmapentalene and osmapentalyne are antiaromatic or nonaromatic. Indeed, as shown in Table 1, the harmonic oscillator model of aromaticity (HOMA) indices29 calculated for the C−C bonds in aromatic metal-bridgehead osmapentalene (0.912 in 5) and osmapentalyne (0.935 in 7) and nonbridged osmanaphthalene (0.675 in 2) and osmanaphthalyne (0.787 in 4) are generally larger than those in the corresponding non-metal-bridged (0.440 and −0.299 for complexes 6 and 8) and metal-bridgehead (0.665

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RESULTS AND DISCUSSION In this work, four classes of simplified model complexes based on the experimental ones were built (Scheme 1), and the relative thermodynamic stabilities of different isomers were also Scheme 1. Calculated Relative Gibbs Free Energies at 298 K and (in Parentheses) Electronic Energies of Metallabicyclic Compounds (in kcal mol−1)

B

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osmapentalene and osmapentalyne and non-metal-bridged osmanaphthalene and osmanaphthalyne have been isolated. Thus, the synthesis and isolation of metal-bridgehead osmanaphthalene and osmanaphthalyne and non-metal-bridged osmapentalene and osmapentalyne are particularly challenging because of their thermodynamic instability. In our previous study, we reported that incorporating the osmium center into pentalyne can reduce the strain energy of the ring.13 Thus, we calculated the strain energies of osmapentalynes and osmanaphthalynes (Figure 3). The angles

and 0.595 for complexes 1 and 3), suggesting that more stable the complexes are, more delocalized their structures are. To verify the aromatic characters of different metallabicycles, we carried out ACID analysis.28 The ACID method is a versatile, intuitive, and generally applicable approach to investigate and visualize electron delocalization. In general, aromatic species exhibit clockwise diatropic circulation, whereas antiaromatics show counterclockwise paratropic circulation. The clockwise ring currents are displayed along the periphery of the fused five-membered rings of metal-bridgehead osmapentalene 5 and osmapentalyne 7 and the six-membered rings of non-metal-bridged osmanaphthalene 2 (ring A) and osmanaphthalyne 4 (ring A), indicating their aromaticity (Figure 2; see Figures S5−S12 for high-resolution ACID

Figure 3. Calculated strain energies (in kcal mol−1) of metallabicyclic complexes 3, 4, 7, and 8. Zero-point energy corrections are included.

given in boldface were fixed in the partial optimizations. All of these fixed angles were taken from the optimized counterparts. For instance, the osmium carbyne angles were fixed at 129.7°, 131.0°, 150.8°, and 152.2°, respectively, which were taken from metal-bridgehead osmapentalyne 7, non-metal-bridged osmapentalyne 8, metal-bridgehead osmanaphthalyne 3, and nonmetal-bridged osmanaphthalyne 4. The computed strain energy of osmapentalyne 7 (−24.3 kcal mol−1) is particularly close to the previous value (−24.8 kcal mol−1) based on the X-ray structure of osmapentalyne,13 suggesting the reliability of our calculations. The strain energy in the first equation is larger than the second one, which could be attributed to the smaller carbyne angle in osmapentalyne 7. Although osmapentalyne 7 has higher ring strain than osmapentalyne 8, it is aromatic in both of the five-membered rings. In sharp contrast, osmapentalyne 8 is antiaromatic in both of the five-membered rings, as evidenced by the positive NICS(1)zz values and paratropic ring currents. Therefore, it is aromaticity rather than ring strain that determines the higher thermodynamic stability of 7 over 8. For osmanaphthalynes 3 and 4, their strain energies are comparable to each other. It should be noted that the strain energies in 3 and 4 are significantly smaller than those in 7 and 8, indicating a remarkable reduction in the strain energy caused by the nonlinear osmium carbyne angles in going from the fivemembered ring to the six-membered ring. Again, according to the NICS(1)zz values and ACID plots, osmanaphthalyne 3 is antiaromatic in both of the six-membered rings, whereas osmanaphthalyne 4 is aromatic in ring A and nearly nonaromatic in ring B. Thus, again aromaticity rather than ring strain dominates in the relative thermodynamic stabilities

Figure 2. ACID plots of complexes 1−8 from the total contribution with an isosurface value of 0.03. The molecular planes are placed perpendicular to the magnetic field vector.

plots for the eight metallabicycles). In sharp contrast, the anticlockwise ring currents are displayed along the periphery of the five-membered rings of non-metal-bridged osmapentalyne 8 and the fused six-membered rings of metal-bridgehead osmanaphthalyne 3, indicating their antiaromaticity. This could be one of the reasons why metal-bridgehead C

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Innovative Research Team in University (IRT1263) is gratefully acknowledged.

of 3 and 4. When the OsC triple bond becomes a double bond, a significantly reduced strain energy is expected. Therefore, we could draw a similar conclusion that it is aromaticity rather than ring strain that determines the relative thermodynamic stabilities of 1 and 2 and also 5 and 6. Indeed, remarkable differences in the NICS(1)zz values and ACID plots are found among these complexes (Table 1, Figure 2, and Figures S5, S6, S9, and S10). Moreover, we also investigated the relative stabilities and aromaticities of the complexes with metal centers at the βposition (Figures S1−S4 and Table S1). The results suggested that these isomers have thermodynamic stabilities comparable to those of non-metal-bridged ones with metal centers at the αposition. In addition, the aromaticity in the metal-containing rings of these isomers with metal centers at the β-position is comparable to those of the non-metal-bridged ones with metal centers at the α-position. It should be note that some nonmetal-bridged metallapentalenes have been synthesized.30

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CONCLUSION A systematic study of the relative stabilities of metallabicycles was carried out by DFT calculations. Our results reveal that metal-bridgehead osmapentalyne and osmapentalene are thermodynamically more stable than the non-metal-bridged ones. In sharp contrast, non-metal-bridged osmanaphthalyne and osmanaphthalene are thermodynamically more favorable than the metal-bridgehead ones. Further studies show that the ring strain of osmanaphthalynes (−4.4 to −6.9 kcal mol−1) is significantly reduced in comparison with that of osmapentalynes (−19.8 to −24.3 kcal mol−1). As the ring strains in these paired isomers are comparable to each other and the difference in aromaticity of these isomers is remarkable, we can safely draw the conclusion that it is aromaticity rather than ring strain that determines the relative stabilities of these complexes. Thus, realizing thermodynamically unfavorable metallabicycles, such as 1, 3, 6, and 8, is particularly challenging.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00758. Relative stabilities, NICS values, and ACID plots of isomers for metallabicycles (PDF) Cartesian coordinates for all of the complexes (XYZ)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail for J.Z.: [email protected]. ORCID

Jun Zhu: 0000-0002-2099-3156 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This article is dedicated to Prof. Yundong Wu on the occasion of his 60th birthday. Financial support from the Top-Notch Young Talents Program of China, the National Natural Science Foundation of China (21573179 and 21172184), the Program for New Century Excellent Talents in University (NCET-130511), and the Program for Changjiang Scholars and D

DOI: 10.1021/acs.organomet.7b00758 Organometallics XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.organomet.7b00758 Organometallics XXXX, XXX, XXX−XXX