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Dual Aromaticity in Both the T and S States: Osmapyridinium with Phosphonium Substituents Ting Shen, Dandan Chen, Lu Lin, and Jun Zhu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11564 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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Journal of the American Chemical Society

Dual Aromaticity in Both the T0 and S1 States: Osmapyridinium with Phosphonium Substituents Ting Shen, Dandan Chen, Lu Lin, and Jun Zhu* State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. KEYWORDS: triplet ground state • aromaticity • density functional calculations • osmapyridinium. Supporting Information Placeholder ABSTRACT: According to Hückel’s and Baird’s rules, cyclic conjugated species are aromatic either in the ground state or in the excited state only. Thus, species with aromaticity in both states (denoted as adaptive aromaticity) are particularly rare. Here we carry out density functional theory calculations on a series of osmapyridine and osmapyridinium complexes (96 species) and find that two of them display adaptive aromaticity, which was verified by various aromaticity indices including HOMA, ELF, MCI, ACID plots and the heat of hydrogenation. Further study reveals that two osmapyridiniums containing one or two phosphonium substituents exhibit the character of the triplet ground state, which was supported by the high-level coupled cluster calculations. Our findings highlight the importance of a transition metal and phosphoniums in achieving adaptive aromaticity and triplet ground state and may aid the design of organometallics for photochemical and molecular magnetism applications.

Introduction Aromaticity, a fundamental concept in chemistry, has attracted considerable attention from both experimentalists and theoreticians due to its many fascinating and ever-increasing manifestations.1 As aromaticity is a property that cannot be observed and measured directly in an experiment, various criteria of aromaticity2-6 have been proposed. Compared with aromaticity in the singlet ground state, excited state aromaticity7 has been much less developed. The first theoretical study on aromaticity in the excited state can be traced back to Baird in 1972,8 who demonstrated that [4n] -electron annulenes are aromatic whereas [4n+2] -electron annulenes are antiaromatic in the lowest triplet state, which is reversed to the Hückel’s rule.9 Later, Gogonea, Schleyer, and Schreiner10 carried out extensive ab initio and density functional theory (DFT) studies on the lowest triplet states of annulenes with [4n] -electron to conform the existence of triplet state aromaticity supported by several criteria. Thereafter, ring-current aromaticity in triplet states of [4n] electron monocycles were investigated by Fowler, Steiner and Jenneskens.11 In addition, Ottosson12 demonstrated the analysis of the bifurcation in the -contribution to the electron localization function (ELF) for the lowest-lying triplet state of [4n] -electron monocycles, further supporting the validity of Baird’s rule. Experimentally, the first evidence on the excited-state aromaticity of [4n] -electron annulenes was demonstrated by Wan and Krogh from photochemical solvolysis of fluoren-9-ol.13 This concept was also used to explain the acid-base behavior of polycyclic conjugated hydrocarbons in their excited states.14 Afterwards, triplet state aromaticity was extended to the expanded porphyrins.15 A reversal of Hückel (anti)aromaticity in the lowest triplet state of hexaphyrins were reported by Kim et al.16-18 The spectroscopic evidence on two closely related hexaphyrins containing [26] and [28] -electron peripheries, respectively, was demonstrated to validate Baird’s rule. To date, despite numerous studies on excited-state aromaticity, to the best of our knowledge, the triplet ground state aromatic species are particularly limited.19-24 Merkt and co-workers reported that the triplet ground state of the cyclopentadienyl cation has D5h symmetry with an aromatic character by photoelectron spectroscopy.21,22 In 2017, 6-azulenyl nitrenium ion and 6-azulenyl oxenium ion were demonstrated to be a triplet ground state aromatic

species by Winter and co-workers.23 Kim and his co-workers reported the two-electron oxidized triplet biradical complexes are yinkwei triplet ground state displayed aromatic character in accord with Baird’s rule.24 Recently, Ottosson and co-workers tailored fulvenes and fulvenium with triplet ground states by utilizing the substituent effects on the singlet state Hückel-aromaticity and the triplet state Baird-aromaticity dications.25 However, all these triplet ground state species only display aromaticity in one state. In 2009, Xia and co-workers26 reported the compounds osmapyridine and osmapyridinium formed by a formal [4+2] cycloaddition reaction and mentioned that the paramagnetism found in these two azaosmabenzenes caused the unexpected NMR properties and deserved further investigation. Our ongoing interest in aromaticity27-30 stimulates us to design more models (Figure 1) and probe their aromaticity by DFT calculations. Herein, we demonstrate the first example (osmapyridinium with phosphonium substituents) with a triplet ground state and dual aromaticity in both the lowest triplet and singlet states.

Figure 1. The proposed model complexes osmapyridinium and osmapyridine for comparable study of their (aromaticity) in the S0 and T1 states. [Os]=OsCl2(PH3)2. Method Geometry optimizations of 96 species (Fig. S1) were conducted at the (U)B3LYP31 level of theory in gas phase. In addition, the frequency analysis were performed at the same level to confirm the characteristics of the calculated structures as minima. LanL2DZ basis set32 was employed to describe the heavy atom, Os, whereas 631G(d) basis set33,34 was used to describe C, H, N, O, P, F, Si, Cl atoms for singlet (spin-restricted wave functions) and triplet states (spin-unrestricted wave functions). The values of EST (Esinglet -Etriplet) were calculated by subtracting the lowest triplet energy from the lowest singlet energy. Note that for nonplanar complexes, their

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aromaticity was not evaluated. For ACID calculation,2,35 we only consider the contributions of key occupied  molecular orbitals (ACID) because of the total ACID are influenced by the  contributions. Generally, diatropic ring current indicates aromaticity whereas paratropic ring current exhibits antiaromaticity. As separation of the  and  contribution in the triplet state is not supported by the ACID program, only the ACID for compounds in their singlet state is discussed in our study. ACID calculations were carried out with the ACID program using Gaussian 03 package.36 We used the method of hydrogenation instead of the ISE method37,38 as the energetic criterion. The ISE values could be affected by some steric repulsions of the methylene group. For instance, as shown in Fig. S2, the optimized structure of compound 6c (with a dihedral angle C-C-Os-N of 28.1°) is nonplanar. In addition, the nucleus independent chemical shift (NICS) calculation39,40 is not used in our system as the applicability of NICS calculation with metallabenzenes has some limitations.41 Furthermore, when atoms of greatly different sizes are involved, the magnitudes of NICS values do not reflect on the relative “aromaticity” of different systems in the ring.42,43 Multi-center indices (MCI) and ELFπ analyses were conducted using Multiwfn.44 With the help of topology analysis performed in Multiwfn, the ELFπ bifurcation points were accurately located. All the calculations were performed in Gaussian 09 packages if not stated.45 Optimized structures were visualized by the CYLviewprogram.46 Enlarged figures of the ACID plots with high resolution are provided in Supplementary Figures. S12S42. Results and Discussion Selection of a Proper Functional. In order to choose the proper density functional, we made a comparison of the bond lengths and dihedral angles in the experimental geometry of osmapyridinium. The relative deviations in Table S1 show that the structure calculated by the B3LYP functional is closest to the experimental one, indicating the outperformance of this method. Therefore, in the following calculation, we chose the B3LYP functional together with LanL2DZ-6-31G(d) basis set as our computational method. As the OPBE functional was reported to perform well for the accuracy of spin-state energies,47-49 this functional was chosen for single point calculations. We first examined the geometries of complexes 1 and 2 in both the singlet and triplet states in our calculation (Figure 2 and S3). Compound 1 in both states and compound 2 in the triplet state are almost planar whereas compound 2 in the singlet state has a considerable dihedral angle of C-C-Os-N (28.7°). Compound 1 is aromatic in the singlet state and nonaromatic in the triplet state while compound 2 is nonaromatic in both states, according to their bond length alternation (ΔBL, the bond length difference between the longest and shortest C-C bonds) and the harmonic oscillator model of aromaticity (HOMA)50 values considering the C-C and C-N bonds. Note that compound 2 in the singlet state has a considerable dihedral angle, its aromaticity has been significantly reduced. The EST values (-1.7 and 0.4 kcal mol-1, respectively) are close to zero calculated by the OPBE density functional, indicating the triplet ground state character could be achieved or enhanced by tuning the substituents or ligands. The Effects of Substituents and Ligands. Substituent effect was first considered for these two compounds. As shown in Table S2, the fully substituted and partially substituted compounds cannot possess a higher positive EST values than complexes 1 or 2. Therefore, we consider the monosubstituted species (Figure S1). Compared with the EST values of compounds 6 (-6.3 kcal mol-1) and 13 (-4.1 kcal mol-1) without any substituent, respectively, the results in Table S3 suggest that complexes 22 (-1.8 kcal mol-1), 23 (1.9 kcal mol-1), 41 (0.6 kcal mol-1), 42 (0.1 kcal mol-1) are quite

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potential to help design triplet ground state species. When the ligands effect is considered, the calculated EST values (Table S4) for the proposed complexes (Figure S1) are all negative. Hence, we only consider the substituent effect for this system. Five complexes are screened out (23, 89, 41, 42 and 90) to have a positive EST value (Tables S2-S4, Figure S4). When the calculations at the CCSD(T) level were considered, only compounds 23 and 89 have a triplet ground state character with EST values of 2.6 kcal mol-1 and 5.5 kcal mol-1, respectively. For comparison, detailed studies on compound 6 (the EST value is -9.5 kcal mol-1 at CCSD(T) level) are also considered.

Figure 2. Some proposed structures of osmapyridinium and osmapyridine. [Os] = OsCl2(PH3)2 Aromaticity Analysis of Complexes 6, 23 and 89. The geometries of complexes 6, 23 and 89 in both two states (Figure 3) are planar. The C-C bond lengths in both two states are in the range of 1.3801.465 Å, which are shorter than a single bond (1.531 Å in CH3-CH3) and significantly longer than a double bond (1.331 Å in CH2=CH2) at the same computation level, indicating delocalized C-C bonds and thus suggesting their aromaticity. The ΔBL values for these three complexes in the singlet state demonstrating the strongest delocalization in complex 23. In the triplet state, complex 6 has the smallest ΔBL value, revealing the reduced delocalization from complexes 6 to 23 and further to 89. In accord with BL, the HOMA values of these three compounds exhibiting the most aromaticity in the lowest singlet state of complex 23 and in the lowest triplet state of complex 6.


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Journal of the American Chemical Society Figure 3. The ΔBL (Å, left, black), HOMA (middle, blue) and ΔBV (right, purple) values of compounds 6 (a), 23 (b) and 89 (c) in the lowest singlet and triplet states. The multicenter bond order values (red) are labelled on the center of the ring. The bond lengths (black) and ELF bifurcation values (purple) are annotated along the bond. The hydrogenation method was also used to evaluate the aromaticity in both the lowest singlet and triplet states. Note that the hydrogenation of the osmium-nitrogen double bonds will lead to the change of the coordination number of the metal center, we consider the hydrogenation of two C=C double bonds only. For comparison, we also consider the hydrogenation of two C=C double bonds in benzene. We use the difference (ΔΔH = ΔHexpected – ΔHcomputed) between the expected change of enthalpies for the hydrogenation of two C=C double bonds (ΔHexpected) and the computed change of enthalpies (ΔHcomputed) to evaluate the aromaticity. A negative value indicates aromaticity whereas a positive one suggests anti-aromaticity. As shown in Figure 4, the results of the hydrogenation for benzene display the aromatic stabilization (ΔΔH= -36.1 kcal mol-1) in the singlet state whereas anti-aromatic destabilization (ΔΔH= +20.2 kcal mol-1) in the triplet state. In sharp contrast, the ΔΔH values for compounds 6 and 23 in both the lowest singlet and triplet states are all negative, indicating the aromatic stabilization. The degree of aromaticity shows a similar trend with structural criteria, indicating the aromaticity for compounds 6 and 23 in both the lowest singlet and triplet states. The smaller ΔΔH values (-7.3 kcal mol1) for compound 89 in the singlet state suggest weaker aromaticity whereas the loss of the aromaticity to nonaromaticity in the triplet state is supported by the close-to-zero values (-2.1 kcal mol-1).

The multi-center bond index (MCI), known as an excellent indicator to evaluate aromaticity based on molecular electronic structure, was originally proposed by Giambiagi.51-53 The higher the value, the stronger the aromaticity of a species. As shown in Figure 3, the MCI values for the two complexes 6 and 23 are comparable in the singlet and triplet states whereas the values calculated for traditional organic compounds (Figure S5) are significantly different in both states, further supporting the aromatic character for the compounds 6 and 23 in both states. The smaller MCI values of compound 89 indicate the weaker aromaticity and non-aromaticity in the lowest singlet and triplet states, respectively. Note that the MCI values for complexes 6 and 23 in both states are comparable and even larger than a series of heterometallcycles which were previously reported by Solà54 and co-workers to have some aromatic character. In addition, the clockwise ring currents in Figure S6 demonstrate the aromatic character for these three compounds in the singlet state. Topological analysis of the electron localization function (ELF),5556 especially the -contribution (ELF ), is another excellent indica tor to evaluate antiaromaticity based on molecular electronic structure.12,57-58 Therefore, it was performed to examine the delocalization of  electrons along the perimeter of osmacycles. The key  molecular orbitals for compounds 6, 23 and 89 in both states are shown in Figure S7. Notably, in line with BL and HOMA values, the BV(ELF)s values (Figure 3) further confirm the aromaticity of compounds 6 and 23 in both the singlet and triplet states whereas compound 89 has lower aromaticity in the singlet state and nonaromaticity in the triplet state. Analyses from Molecular Orbitals and Spin Density. To probe the origin of triplet ground state and adaptive aromaticity in compounds 6 and 23, we analyzed their key molecular orbitals and compared them in the singlet and triplet states (Figure 5). For comparison, complex 89 is also examined. The energies of two singly occupied molecular orbitals (SOMOs) in the triplet state are close to each other for the two compounds 23 and 89 (energy differences are 0.62 and 0.28 eV, respectively). However, the energy differences is 1.23 eV for compound 6. Compared with traditional organic aromatics, it is more complicated for the excitation pattern of metallaaromatics in the triplet state, which are less predictable as the formation of the triplet state is not always out-of-plane * excitation. According to Baird’s rule, the compound which is aromatic in the singlet state will become antiaromatic in the lowest triplet state or vice versa. The excited state should be formed by an out-of-plane * excitation. Different from Baird’s rule in the lowest triplet state, the lowest triplet state of these two compounds was formed by the electronic excitation from the in-plane HOMO to the out-of-plane LUMO of the singlet state. The HOMO in the singlet state for the two compounds are in-plane  obtitals, therefore, their triplet states are not formed by out-of-plane * excitation. As the second highest singly occupied molecular orbital (HSOMO-1) is an in-plane orbital, its contribution for the out-of-plane aromaticity should be minor.

Figure 4. The change of enthalpies (ΔH, kcal mol-1), ΔΔH values (ΔHexpected – ΔHcomputed) for the hydrogenation of the C=C double bonds in benzene, compounds 6, 23 and 89.



to the paratropic ring currents, leading to the aromaticity reversal in benzene. This aromaticity change from the S0 to T1 state could be attributed to the reduced diatropic ring current of HSOMO-1 and newly generated significant paratropic ring current of HSOMO orbitals. In sharp contrast, for both complexes 6 and 23, when an electron is excited from HOMO in the singlet state (which is an in-plane molecular orbital and does not contribute to the aromaticity in the singlet state) to form the lowest triplet state, the HSOMO orbital has a contribution to the paratropic ring current (Figure S10). This change cannot completely compensate the original aromaticity from the  molecular orbitals, thus leading to a reduction of aromaticity in their triplet states. However, compound 89 has lower aromaticity in the singlet state according to BV(ELF) values due to the two phosphonium substituents. Thus, the contributions to the paratropic ring currents from newly formed HSOMO in compound 89 is comparable to that of the diatropic ring currents from other unchanged  molecular orbitals, resulting in the nonaromaticity of compound 89 in its lowest triplet state. Analyses of Phosphonium Substituent Effect.To probe the origin of the unexpected substituent effect by the phosphonium groups on compounds 23 and 89, we examined the changes on their geometric and electronic structures in comparison with benzene. As shown in Figure 6a, the phosphonium substituents slightly reduce the aromaticity in the benzene ring in the singlet state, according to the smaller HOMA and larger ΔBL values. In the triplet state, the antiaromaticity of benzene is reduced when one phosphonium substituent was introduced. However, the structure of benzene becomes nonplanar when two phosphonium substituents are introduced (Figure 6b). On the contrary, the change of aromaticity on osmapyridinium by the phosphonium substituents in the singlet state is increased when introducing one phosphonium substituent rather than decreased in benzene, in line with our previous finding that the phosphonium substituent can enhance the aromaticity of osmasilapentalynes.59 The difference is understandable because benzene has a zero ΔBL value whereas osmapyridinium 6 has relatively large ΔBL value. For the triplet state, complex 6 has a very small ΔBL (0.03 Å). Thus introducing a phosphonium substituent leads to a larger ΔBL in complex 23 and it becomes further larger in complex 89 when the second phosphonium substituent was introduced. That is why the aromaticity in complex 23 is reduced and complex 89 even becomes nonaromatic in their triplet states.

Figure 5. The key frontier molecular orbitals of compounds 6, 23 and 89 in the lowest singlet and triplet states. The isovalues are 0.03 a.u. All the qualitative analyses are further supported by the induced current density (ACID) calculations on these key molecular orbitals of the two compounds. The  molecular orbitals of these three compounds have diatropic ring currents in the singlet state (Figure S8), supporting their aromaticity. For benzene, when an electron from an occupied out-of-plane π orbital (HOMO) was excited to form the T1 state, its contribution to the diatropic ring currents will be reduced, as indicated by the HSOMO-1 of ACID plots (Figure S9). More importantly, the HSOMO orbital has significant contribution

Figure 6. The BL (Å), bond lengths (Å) and HOMA (red, on the center of the ring) values in the lowest singlet and triplet states for benzene with and without substituents at the same computational level.


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Journal of the American Chemical Society In addition to changing the geometric structures of osmapyridinium and osmapyridine, the phosphonium group can also change their electronic structures significantly. As shown in Figure 7, introduction of the phosphonium substituents can significantly lower the energy of the frontier molecular orbitals including the HOMO, LUMO, and singly occupied molecular orbitals (HSOMO and HSOMO-1), leading to reduced energy gaps between singly occupied molecular orbitals (HSOMO and HSOMO-1). Thus, the phosphonium substituents could be a key factor to account for the triplet ground state character of complexes 23 and 89. Therefore, structurally, one phosphonium substituent can improve the aromaticity of osmapyridinium whereas two phosphonium substituents overadjust the structure and reduce the aromaticity in the singlet state. However, in the triplet state, introducing one and two phosphonium substituents will gradually reduce the aromaticity on osmapyridinium. Furthermore, the phosphonium substituents play a crucial role for complexes 23 and 89 to be triplet ground state species.

of transition metal. These results are similar to pentalene incorporating 16 valence electron osmium.61 Further Confirmation by Bulky Phosphorus Ligands. As the trimethyl phosphine (PMe3) and triphenyl phosphine (PPh3) ligands are commonly used in the experiment, we examined the model complexes based on these two bulky ligands to ensure the simplification of phosphorus ligands does not change the nature of triplet ground state in compounds 23 and 89 and adaptive aromaticity in compounds 6 and 23. The computed BL, HOMA and EST values (Figure S12) are essentially same as the simplified models, further verifying the triplet ground state and adaptive aromaticity species in this study. Conclusion In summary, we demonstrate for the first time that compound 23 can possess both aromaticity in the lowest singlet and triplet states (adaptive aromaticity) and triplet ground state character. Compound 6 is a singlet ground state species but displays dual aromaticity in both states. Compound 89 has a triplet ground state character with weaker aromaticity in the singlet state and nonaromaticity in the triplet state, supported by the BL, HOMA, ELF values, together with MCI values, the computed H values, the clockwise ring current in ACID plots and the positive singlet-triplet energy gap. A general strategy to obtain adaptive aromatic species could be that the lowest triplet state is formed by excitation from in-plane molecular orbital rather than out-of-plane molecular orbital. In addition, aromaticity in the lowest singlet state should be high enough to overwhelm the paratropic ring currents contributed by two newly formed highly simply occupied molecular orbitals (HSOMO and HSOMO-1). In general, organometallic species have a high possibility to achieve this goal. Our findings emphasize the important role of transition metal and phosphoniums, which might help the design of organometallics for photochemical and molecular magnetism applications.

ASSOCIATED CONTENT Supporting Information The proposed structures of complexes, the ISE values of complex 6, the comparison of methods, bond lengths and HOMA values of complexes 1 and 2, effects of substituent and ligand on osmapyridine and osmapyridinium, the EST values for some compounds, the MCI analysis for benzene and 4n species, Key  molecular orbitals of compounds 6, 23 and 89 in both states, ACID plots for complexes 6, 23 and 89, ACID plots for individual molecular orbitals of benzene, compounds 6, 23 and 89, the spin densities of complexes 6, 23 and 89 the BL, HOMA and EST values for the complexes with bulkier phosphorus ligands in both states, high resolution ACID plots and cartesian coordinates for all the complexes.

AUTHOR INFORMATION Corresponding Author Figure 7. (a) The LUMO, HOMO and HOMO-LUMO gap (b) HSOMO, HSOMO-1, HSOMO-HSOMO-1 gap (eV) in the lowest singlet and triplet states for unsubstituted and substituted benzenes, 6, 23 and 89. Baird demonstrated that the * transition tends to be “localized” within a subunit of a molecule in his original paper.8 A recent study shows that in the triplet state, the spin density for an aromatic organic compound is delocalized whereas it is localized for antiaromatic species.60 Unlike this observation in the main-group systems, we found the localization of spin density for complexes 6, 23 and 89, the electrons were mainly localized on the metal osmium in the triplet state (Figure S11), which could be attributed to the influence

*E-mail for J.Z.: [email protected]. ORCID Jun Zhu: 0000-0002-2099-3156

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Financial support by the National Science Foundation of China (21873079 and 21573179) and the Top-Notch Young Talents Program of China is gratefully acknowledged.



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