Unconventional Aromaticity in Organometallics: The Power of

Article Cite This: Acc. Chem. Res. 2019, 52, 1449−1460

pubs.acs.org/accounts

Unconventional Aromaticity in Organometallics: The Power of Transition Metals Dandan Chen, Qiong Xie, and Jun Zhu*

Downloaded via BUFFALO STATE on July 22, 2019 at 08:51:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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, People’s Republic of China CONSPECTUS: Aromaticity, one of the most fundamental concepts in chemistry, has attracted considerable attention from both theoreticians and experimentalists. Much effort on aromaticity in organometallics has been devoted to metallabenzene and derivatives. In comparison, aromaticity in other organometallics is less developed. This Account describes how our group has performed quantum chemical calculations to examine aromaticity in recently synthesized novel organometallic complexes. By collaborations with experimentalists, we have extended several aromaticity concepts into organometallics to highlight the power of transition metals. In general, the transition metal could participate in delocalization either out of rings or in the rings. We examined the former by probing the possibility of transition metal substituents in hyperconjugative aromaticity, where the metal is out of the rings. Calculations on tetraaurated heteroaryl complexes reveal that incorporation of the aurated substituents at the nitrogen atom can convert nonaromaticity in the parent indolium into aromaticity in the aurated one due to hyperconjugation, thus extending the concept of hyperconjugative aromaticity to heterocycles with transition metal substituents. More importantly, further analysis indicates that the aurated substituents can perform better than traditional main-group substituents. Recently, we also probed the strongest aromatic cyclopentadiene and pyrrolium rings by hyperconjugation of transition metal substituents. Moreover, theoretical calculations suggest that one electropositive substituent is able to induce aromaticity; whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. We also probed the possibility of Craig-type Möbius aromaticity in organometallic chemistry, where the position of the transition metals is in the rings. According to the electron count and topology, aromaticity can be classified as Hückel-type and Möbius-type. In comparison with numerous Hückel aromatics containing 4n+2 π-electrons, Möbius aromatics with 4n πelectrons, especially the Craig-type species, are particularly limited. We first examined aromaticity in osmapentalynes. Theoretical calculations reveal that incorporation of the osmium center not only reduces the ring strain of the parent pentalyne, but also converts Hückel antiaromaticity in the parent pentalyne into Craig-type Möbius aromaticity in metallapentalynes. Further studies show that the transition metal fragments can also make both 16e and 18e osmapentalenes aromatic, indicating that the Craig-type Möbius aromaticity in osmapentalyne is rooted in osmapentalenes. In addition, Möbius aromaticity is also possible in dimetalla[10]annulenes, where the lithium atoms are not spectator cations but play an important role due to their bonding interaction with the diene moieties. We then examined the possibility of σ-aromaticity in an unsaturated ring. Traditional π-aromaticity is used to describe the πconjugation in fully unsaturated rings; whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σelectron conjugation. We found that the unsaturated three-membered ring in cyclopropaosmapentalene is σ-aromatic. Very recently, we extended σ-aromaticity into in a fully unsaturated ring. The concepts and examples presented here show the importance of interplay and union between experiment and theory in developing novel aromatic systems and, especially, the indispensable role of computational study in rationalization of unconventional aromaticity. All these findings highlight the strong power of transition metals originating from participation of d orbitals in aromaticity, opening an avenue to the design of unique metalla-aromatics.

■

INTRODUCTION Aromaticity has been an important concept in chemistry since the Kekulé’s structure of benzene was proposed. Although benzene is the archetypal molecule of aromaticity, the concept has been extended to the organometallic system, especially metallabenzene (the replacement of a CH moiety in benzene by a transition-metal fragment) and its derivatives.1−4 For © 2019 American Chemical Society

instance, Thorn and Hoffmann predicted three types of stable metallabenzenes5 in 1979. Three years later, experimental verification was reported by Roper and co-workers.6 The tremendous progress of metallabenzenes and related species, Received: February 24, 2019 Published: May 7, 2019 1449

DOI: 10.1021/acs.accounts.9b00092 Acc. Chem. Res. 2019, 52, 1449−1460

Article

Accounts of Chemical Research

aromatics without an out-of-plane ππ* excitation is out of the scope for the Baird’s rule, and such species exhibit unconventional excited-state aromaticity.21 We hereby summarize our recent work on novel metal-containing aromatics with hyperconjugation aromaticity, Craig−Mö bius aromaticity, and σ-aromaticity in the singlet ground state, revealing the strong power of TMs in realizing unconventional aromaticity.

including heteroatom-containing, fused-ring, dimetalla-, and metallabenzyne analogues, has been made over the past 30 years by several groups.2 Despite the controversy of the nature of aromaticity in metallabenzenes and related compounds, the aromaticity in metallabenzenes has been systematically investigated by Frenking and co-workers.1 They claimed that “From the data reported here, there is no apparent trend or pattern which indicates a correlation between aromatic stabilization and particular ligands, metals, coordination numbers, or charge”. Recently, a series of novel aromatic organometallics containing transition metals (TMs), such as metallapentalynes, metallapentalenes, and dimetalla[10]annulenes, have been synthesized.7−10 Moreover, two polyaurated complexes including a tetra-aurated indole and an octaaurated benzodipyrrole were synthesized recently.11 In this Account, we focus on these nonmetallabenzene complexes possessing unconventional aromaticity compared with traditional Hückel-type π-aromaticity in benzene. Note, that the “unconventional aromaticity” in the title is somehow arbitrary, which refers to hyperconjugative aromaticity, Mö b ius aromaticity, and σ-aromaticity in this Account. Our collaboration with experimentalists highlights the importance of computational chemistry in achieving unconventional aromaticity in organometallics. By the virtue of thorough DFT calculations and aromaticity analyses, we revealed that the TM substituents, where the metal is out of the ring, are able to participate in the delocalization over the five-membered rings (5MRs) of indoliums, pyrroliums, and cyclopentadienes through hyperconjugation11−14 (Scheme 1a). In 2013, we reported Craigtype Möbius aromaticity in synthesized metallapentalynes,7 which is in sharp contrast to conventional Hückel aromaticity and Heilbronner-type Möbius aromaticity.15−17 This novel aromaticity also holds for metallapentalenes,7 metallasilapentalynes,18 and dimetalla[10]annulenes10 (Scheme 1b). Incorporating TMs into unsaturated three-membered rings (3MRs), including cyclopropene, selenirene, and methylenecyclopropene, can lead to σ-aromaticity outweighing the π-aromaticity (Scheme 1c). Stabilization of an antiaromatic framework by TMs is not limited to the pentalene. Another antiaromatic framework, the cyclobutadiene, could be simultaneously stabilized by sharing the same metal center with the pentalene.19,20 The lowest triplet state in TM-containing

■

HYPERCONJUGATIVE AROMATICITY TUNED BY TM SUBSTITUENTS Ubiquitous in organic molecules, the hyperconjugative interaction is commonly classified into neutral, negative, and positive types.22 Neutral hyperconjugation refers to the bidirectional interaction between a π moiety and an adjacent σ bond, with the coexistence of π−σ* and σ−π* donor− acceptor pairs. The donation of electron density is from filled π orbitals into antibonding σ* orbitals in negative hyperconjugation, and from filled σ orbitals into unfilled π* orbitals in positive hyperconjugation. Application of the hyperconjugation effect to the concept of aromaticity was first proposed by Mulliken, considering the first-order hyperconjugation between the saturated CH2 group and the unsaturated olefinic moiety in a cyclopentadiene.23,24 Schleyer and co-workers performed theoretical calculations for a series of planar cyclopolyenes (CnHn)CR2 (RH, SiH3, F,...) and showed that the saturated CR2 moiety could be treated as a pseudo-π-donor if the R is electron-donating or as a pseudo-πacceptor if the R is electron-withdrawing.25,26 The aromaticity of 5,5-disubstituted cyclopentadienes can be effectively tuned by the substituents, and particularly, when substituted with stannyls, the cyclopentadiene is almost as aromatic as furan.25 Hyperconjugative aromaticity has also been successfully applied in the studies of Diels−Alder reactions27 and is also important in triplet-state chemistry.28 Previous studies about hyperconjugative aromaticity were limited to the interaction between main-group elements. Through collaboration with experimentalists, we expanded the scope of hyperconjugative aromatics to include TM-substituted species.11−14 Hyperconjugative aromaticity caused by TM substituents was first reported in the tetra-aurated indole.11 Through hyperconjugation, electron-donating substituents on the nitrogen can prompt a pseudo-6π Hückel aromatic system in the 5MR; whereas electron-withdrawing substituents lead to a pseudo-4π antiaromatic one. The nucleus independent chemical shift (NICS) is a magnetic index for probing the (de)shielding effect in an (anti)aromatic system.29 The NICS(1)zz value calculated for the 5MR of tetra-aurated indole (with four PPh3 simplified as four PH3 ligands) is −16.6 ppm, indicating aromaticity. The parent indolium (1C) shows nonaromaticity in the 5MR (Figure 1a). Such a switch from nonaromaticity to aromaticity is caused by aurated substituents on the nitrogen atom instead of those on carbon atoms, which is demonstrated by the difference between diaurated 1A and 1B. Difluoroindolium (1D) and distannylindolium (1E) are taken into consideration because of the distinct influence of substituents (F and SnH3) on hyperconjugative aromaticity.11 Aromaticity shown in the 5MR of 1E and antiaromaticity in 5MR of 1D are solid evidence that (anti)aromaticity of these disubstituted indoliums is significantly influenced by hyperconjugative interactions. Moreover, the aurated substituents outperform the stannyls in achieving hyperconjugative aromaticity, supported by the more negative NICS(1)zz value of 5MR

Scheme 1. Unconventional Aromaticity in Metal-Containing Complexes

1450


Article


Table 1. Bond Lengths (Å), ΔBL (Å), ISE (kcal mol−1), and NICS(1)zz (ppm) Values of Substituted Pyrroliums and Indoliumsa C1−C2 C2−C3 C3−C4 ΔBL ISE NICS(1)zz C1−C2 C2−C3 ΔBL ISE NICS(1)zz

1-H

1-F

1-SnH3

1-Au

1-Ag

1-Cu

1.345 1.464 1.404 0.119 3.5 −3.6 2-H 1.344 1.470 0.126 1.3 −10.8

1.348 1.476 1.403 0.128 −0.8 17.7 2-F 1.337 1.499 0.162 −9.4 9.3

1.359 1.450 1.415 0.091 13.1 −17.9 2-SnH3 1.366 1.443 0.077 15.4 −24.9

1.366 1.446 1.419 0.080 14.8 −19.4 2-Au 1.373 1.437 0.064 17.8 −24.4

1.371 1.442 1.423 0.071 17.5 −23.1 2-Ag 1.379 1.432 0.053 21.5 −26.7

1.364 1.441 1.416 0.077 15.9 −23.5 2-Cub

a

Adapted with permission from ref 12. Copyright 2018 Wiley Online Library. Computational method, TPSS/6-311++G(d,p)(ECPnMDF)//TPSS/6-31G(d)(ECPnMDF). b The geometry optimization of 2-Cu led to an η5 copper complex.

limit the involvement of shared-electrons in the π system of 5MRs.30,31 The N(AgPPh3)2+ fragment leads to strong hyperconjugative aromaticity because of higher electrondonating ability and better orbital interaction with the C4H4 unit. On the basis of electron withdrawing and donating abilities of substituents, the push−pull effect was utilized to tune the bond length alternation in the 5MRs (Figure 3).12 The most aromatic (2-Ag-a-2CuPH3-b-2F) and the most antiaromatic (2-F-a-2F-b-2CuPH3) pyrroliums have been predicted using this strategy. Note that the inconsistency of the NICS values in Figure 3 with other aromaticity indices could be understandable, as the failure of the NICS approach on fluorinated benzenes and polycyclic hydrocarbons has been discussed previously.32 The strongest aromatic cyclopentadiene ring was also predicted by the interplay of hyperconjugation and transinfluence.13 The cyclopentadiene (3) containing the C[Pt(PH3)2SiH3]2-saturated moiety (3e) has the greatest potential to be the most aromatic one among species 3a−3e (Figure 4 and Table 2). It has been reported, that, for square-planar platinum(II) complexes, a ligand with high trans-influence could significantly lengthen the bond in its trans-position33 and the strength of trans-influence increases in the order as −C CH < −SiCl3< −CHCH2 < −SiH3 < −BCl2 < −BOCH CHO-(B) < −BMe2. Multiple aromaticity indices indicate that replacing a ligand with stronger trans-influence at the position trans to the Pt−C bond leads to stronger hyperconjugative aromaticity, and thus the BMe2 ligand performs the best (Table 2). Very recently, hyperconjugative aromaticity of monosubstituted cyclopentadienes (Figure 5) has been investigated.14 The ISE, NICS(1)zz, and multicenter index (MCI) values of 3H-R, 3-R, and 3F-R demonstrate that monosubstitution of hydrogens in the CH2 group by an electropositive substituent can also contribute “pseudo-2π electrons” by hyperconjugation, leading to the hyperconjugative aromaticity in the cyclopentadiene ring; whereas one electronegative substituent leads to nonaromaticity rather than antiaromaticity. When electropositive and electronegative substituents are considered

Figure 1. Hyperconjugative (anti)aromaticity in indolium derivatives. Lengths of delocalized C−C bonds (Å), NICS(1)zz values (ppm), and ACID plots. Adapted with permission from ref 11. Copyright 2016 Nature Publishing Group. Computational method, TPSS/6-31G(d)(ECPnMDF).

Figure 2. Structures of substituted pyrroliums (2-R) and indoliums (1-R) (a) and the equations used to calculate the ISE values (b).

and smaller C−C bond length alternation in the 5MR (1A 0.080 Å vs 1E 0.091 Å). The anisotropy of the current-induced density (ACID) plots (Figure 1b) indicate diatropic ring currents in the 5MRs of 1A and 1E in the π system, in line with their NICS(1)zz values. The outstanding performance of Au substituent inspired a series of further calculations.12 To examine the performance of other TM substituents, the aromaticity of substituted pyrroliums and indoliums (Figure 2) were evaluated from aspects of geometry and isomerization stabilization energy (ISE) (Table 1). Compared with the nonaromatic unsubstituted one (2-H), the difluoropyrrolium (2-F) shows antiaromaticity through its larger bond length alternation (BLA, given by ΔBL) and smaller ISE values, in contrast to the aromatic ones (2-SnH3, 2-Au, and 2-Ag). Fusing a benzene ring reduces both aromaticity and antiaromaticity in the 5MRs of indoliums (1-R), because the aromatic π-sextets tend to 1451


Article


Figure 3. C−C bond length (Å), ΔBL (Å), ELFπ bifurcation values (BV (ELFπ)), and NICS(1)zz(ppm) of furan, borole, and pyrrolium derivatives. ELF, electron localization function. The C−C bond length and ΔBL are given in black; whereas the BV (ELFπ) and ΔBV(ELFπ) are given in pink. Adapted with permission from ref 12. Copyright 2018 Wiley Online Library. Computational method, TPSS/6-311++G(d,p)(ECPnMDF)//TPSS/ 6-31G(d)(ECPnMDF).

Figure 4. Substituted cyclopentadienes (3) with various substituents (a−k).

Figure 5. Monosubstituted (3H-R and 3F-R) and disubstituted cyclopentadienes (3-R) (a), and the equation used to calculate the ISE (b).

Table 2. ΔBL Values (Å), ISE Values (kcal mol−1), NICS Values (ppm), and MCI Values of Compounds 3a−3ka 3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k

ΔBL

ISE

MCI

NICS(1)zz

0.154 0.177 0.115 0.095 0.072 0.087 0.079 0.076 0.068 0.067 0.059

−2.4 7.2 −14.6 −14.9 −19.4 −16.1 −17.3 −18.4 −20.0 −21.0 −22.0

0.028 0.016 0.048 0.056 0.066 0.059 0.061 0.065 0.067 0.069 0.072

−10.6 8.1 −23.3 −21.5 −25.9 −23.8 −23.6 −25.7 −27.8 −27.4 −28.1

topology to aromaticity was first proposed by Heilbronner in 1964,34 where the overlapping atomic orbitals are not parallel while they arrange in a twisted manner to form a phase shift and lead to aromaticity with 4n electrons. Craig proposed the parallel pπ-dπ-pπ overlapping in 1958.35 Theoretical calculations have been performed by Mauksch and Tsogoeva to demonstrate the Craig-type Möbius aromaticity in planar 4nπ metallacycles.36 In 2013, we first confirmed the Craig−Möbius aromaticity in synthesized osmapentalynes through DFT computations.7 Before that, isolation of the pentalyne molecule had never been reported due to its expected antiaromaticity and extreme ring strain. Incorporation of osmium led to successful synthesis of osmapentalynes under room temperature. The bond angle at the carbyne carbon of osmapentalyne 4 is 129.5°, larger than the computed angle in cyclopentyne (116.0°) but smaller than those in osmabenzyne derivatives (148−155°).37 On the basis of the cyclic reference compound, strain energy computed for the in-plane π bond of 4 is 24.3 kcal mol−1, much smaller than that for cyclopentyne (71.9 kcal mol−1) but still larger than that for osmabenzyne (9.6 kcal mol−1).38 These differences suggest that the incorporation of an osmium center is able to release part of the strain in the parent pentalyne. Strain-balanced ISEs calculated for a simplified model 4′ are −19.6 to −23.3 kcal mol−1, in sharp contrast to those for

a Reproduced from ref 13. Copyright 2018 American Chemical Society. Computational method, TPSS/6-31G(d)(ECPnMDF).

simultaneously, an aromatic cyclopentadiene ring could be achieved (Table 3).

■

CRAIG-TYPE MÖ BIUS AROMATICITY IN TRANSITION-METAL-INCORPORATED METALLACYCLES Möbius topology is a mathematical concept referring to a onesided surface with only one boundary. Application of Möbius 1452


Article

Accounts of Chemical Research Table 3. ΔBL (Å), ISE (kcal mol−1), NICS(1)zz(ppm), and MCI Values of 3H-R, 3-R, and 3F-Ra benzene cyclobutadiene 3H−H 3H−F 3H-SiH3 3H-GeH3 3H-SnH3 3H-[Au] 3H-[Pt] 3-F 3-SiH3 3-GeH3 3-SnH3 3-[Au] 3-[Pt] 3F-SiH3 3F-GeH3 3F-SnH3 3F-[Au] 3F-[Pt]

ΔBL

MCI

ISE

NICS(1)zz

0 0.234 0.151 0.163 0.129 0.126 0.113 0.089 0.073 0.178 0.123 0.120 0.109 0.089 0.057 0.135 0.129 0.110 0.081 0.058

0.087 0.001 0.028 0.020 0.036 0.039 0.045 0.060 0.073 0.016 0.040 0.043 0.049 0.059 0.078 0.028 0.031 0.039 0.050 0.072

−34.3 34.5 −3.2 3.0 −7.4 −7.7 −9.9 −11.6 −14.7 6.7 −11.7 −12.1 −15.8 −17 −23.1 −1.0 −0.7 −3.5 −5.3 −7.7

−29.4 61.1 −11.2 −1.8 −18.0 −18.0 −21.1 −27.4 −27.2 7.8 −21.4 −21.1 −24.4 −23.7 −30.2 −10.2 −10.6 −15.5 −19.5 −24.5

metal center (Fe, Ru, and Os) form a triple bond. Natural orbital bonding (NBO) analysis indicated the OsSi triple bond is highly polarized. Ring strain energy calculated for osmasilapentalyne is only 8.7 kcal mol−1, much smaller than that for osmapentalyne (24.8 kcal mol−1). Aromaticity of osmasilapentalyne is slightly reduced compared with osmapentalyne, with smaller ISEs and less negative NICS values (Figure 7). Moreover, the aromaticity of metallasilapentalyne reduces with less diffuse d orbitals of the metal center (Fe < Ru < Os) but could be increased by electron-withdrawing substituents (e.g., phosphonium), π-donor ligands (e.g., PMe3), and Lewis bases coordinated to the silicon atom. Syntheses of osmapentalenes with 16- and 18-valenceelectron osmium centers (Figure 8) were reported soon afterward.9 Aromaticity evaluation carried out for 5 and 6 revealed that both osmapentalenes are 8c-8e Craig−Möbius aromatic, which is also the origin of the aromaticity in osmapentalynes. Interestingly, the 16e osmapentalene (5) exhibits aromaticity in both the singlet ground state and the lowest triplet state, which has been termed as adaptive aromaticity.21 Craig−Mö b ius aromaticity has also been found in dimetalla[10]annulene species. The first aromatic dicupra[10]annulene was synthesized by Xi’s group.10 Incorporation of copper atoms releases the strain resulted from steric hindrance of two internal hydrogens, and thus, they switch the nonaromaticity of parent [10]annulene to aromaticity. Note, that Frenking and co-workers have mentioned that “electron counting in metallaaromatic compounds is far from trivial, so it may admit more than one interpretation.”40 Although Xi and co-workers claimed that the dicupra[10]annulene should be classified as a 10-electron Hückel aromatic species, we provided an alternative explanation with a series of detailed DFT calculations.41 Simplified models of dicupra[10]annulene with four (9) and two (10) lithium atoms exhibit aromaticity and antiaromaticity, respectively (Figure 9a). Complex 9 is 16π Mö bius aromatic; whereas complex 10 is 14π Mö bius antiaromatic, which were demonstrated by CMO-NICS calculations. The ACID plots show diatropic and paratropicinduced ring currents in 9 and 10, respectively.

a Adapted with permission from ref 14. Copyright 2019 Wiley Online Library. Computational method, M06/6-31G(d)(ECPnMDF).

pentalene (8.8 kcal mol−1) and pentalyne (6.8 kcal mol−1). Four occupied π MOs reflecting the π-electron delocalization along the perimeter of the bicycle are derived from the interactions between p orbitals of the C7H5 unit and d orbitals of osmium center (Figure 6). The total diamagnetic contributions of the four π molecular orbitals are −29.4 and −25.6 ppm for rings A and B, respectively. Therefore, the 8c8e Craig−Möbius aromaticity is assigned to the metallabicycle. Aromaticity and thermodynamic stability of metallasilapentalynes were also examined.18 Silicon is more reluctant to participate in π bonding compared with carbon.39 Calculated relative stabilities of a series of ferra-, ruthena-, and osmasilapentalynes with silicon at different positions of the bicycles indicate that it is the most stable only when the silicon and the

Figure 6. Calculated strain energies of in-plane π bonds (kcal mol−1) (a), and structures of 4 and NICS(0)zz contributions of key occupied perimeter molecular orbitals of the simplified model 4′ (b). The eigenvalues of the molecular orbitals are given in parentheses. The NICS(0)zz values given before and after the “/” are those computed at the geometrical centers of rings A and B, respectively. Adapted with permission from ref 7. Copyright 2013 Nature Publishing Group. Computational method, B3LYP/6-311++G(d,p)(LanL2DZ). 1453


Article


Figure 7. ISE (kcal mol−1) and NICS(1)zz (ppm) values computed for osmapentalyne and osmasilapentalyne. Computational method, B3LYP/6311++G(d,p)(LanL2DZ).

analogues have similar (anti)aromaticity as in dicupra[10]annulenes; whereas replacing lithium in 9 with sodium or potassium will lead to reduced aromaticity.

■

σ-AROMATICITY DOMINATING IN UNSATURATED RINGS Generally, aromaticity resulted from delocalization of π electrons is termed as π-aromaticity, and similarly, the σaromaticity corresponds to delocalization of σ electrons. The concept of σ-aromaticity was first proposed by Dewar in 1979, with suggestion that cyclopropane could be considered as the σ-conjugated analogue of benzene.42,43 However, a direct

Figure 8. Simplified models of 16e (5) and 18e (6) osmapentalenes.

Similar to 10, the tetraanion analogue without lithium (11) also shows antiaromaticity with seven key π-MOs. The HOMO of 9 indicates significant bonding interaction between four lithium atoms and diene moieties (Figure 9b), which is absent in 10 and 11. Silver and gold dimetalla[10]annulene

Figure 9. Investigation of aromaticity in dicupra[10]annulenes 9 and 10. (a) Structures and ACID plots (isovalue, 0.035 au) and (b) atomic orbital contributions in the HOMO of 9. Reproduced from ref 38. Copyright 2017 American Chemical Society. Computational method, PBE0/6-311+ +G(d,p)(ECPnMDF). 1454


Article


Figure 10. Aromaticity evaluation for cyclopropaosmapentalene. (a) The isodesmic reactions designed by breaking Os−C and C−C bonds, where the ISEs are given in kcal mol−1. (b) Key occupied π molecular orbitals with their NICS(0) (ppm) contributions. (c) ACID plots (isovalue, 0.035 au). Adapted with permission from ref 53. Copyright 2015 Wiley Online Library. Computational method, B3LYP/6-311++G(d,p)(LanL2DZ).

pentalene analogues, the isodesmic reaction energies calculated for the 3MRs are in the range of +28.9−46.2 kcal mol−1, indicating considerable amount of energetic stabilization. The substituent effect on the 3MR was also studied (Figure 11).

energetic evaluation through ab initio VB computations by Wu, Schleyer, Mo, and co-workers suggested that the σaromatic stabilization energy of cyclopropane was too small to account for the small difference of strain energy between cyclopropane and cyclobutane.44 While the σ-aromaticity in cyclopropane is controversial, the concept itself has been widely accepted with great importance in many organic and inorganic species,45 including atomic clusters,46−48 transitionmetal oxide and carbonyl clusters,49−51 and species with multiple or conflicting aromaticity.52 The concept of double aromaticity refers to the coexistence of σ- and π-aromaticity.53 Representative cases of double aromaticity, such as C6H3+ and C6, were found to have larger contribution from π-aromaticity rather than σ-aromaticity.54,55 Classical unsaturated organic aromatics mainly exhibit πdelocalization. For the first time, we reported that the σaromaticity could play the dominant role in an unsaturated organometallic system.56 Cyclopropaosmapentalene (12), a tricyclic coplanar system where the three-membered ring (3MR) contains both saturated and unsaturated carbons, exhibits significant endothermicity (+29.7 and +35.0 kcal mol−1) when treated with isodesmic reactions (Figure 10a). To investigate the nature of the stabilizing effect in the 3MR, the CMO-NICS calculations were performed (Figure 10b). Five occupied π MOs (HOMO, HOMO-2, HOMO-3, HOMO-10, and HOMO-12) were separated from the rest of the σ orbitals. Since σ-aromaticity features the in-plane delocalization, the NICS(0) index is used for the evaluation of σ-aromaticity. The σ-contribution of NICS(0) reaches −34.8 ppm, which is close to the total value (−40.6 ppm), indicating strong σ-aromaticity. The ACID plots with separate σ- and π-contributions (Figure 10c) confirmed that the 3MR and fused 5MRs are σ- and π-aromatic systems, respectively. Further calculations were carried out to figure out whether such property could also exist in other cyclopropametallapentalenes.57 For Tc, Ru, Rh, Re, Os, and Ir cyclopropametalla-

Figure 11. Structures and NICS values (ppm) of complex 12 and its substituted derivatives 12-F and 12-SiH3.

The fluorine (F) substituents on the sp3 carbon weaken the strength of σ-aromaticity in the 3MR (12-F); whereas the SiH3 substituents make the ring more aromatic (12-SiH3). Such a difference could be rationalized by the electronic and hyperconjugative effects.24 Replacing the chloride ligand with H, CO, SiH3, or PH3, however, does not have significant influence on the σ-aromaticity. Osmapentaloselenirene (13), with two antiaromatic subunits (selenirene and pentalene) stabilized by TM (Figure 12a), was predicted by DFT calculations and verified experimentally.58 Similar to cyclopropametallapentalenes, this species also exhibits dominating σ-aromaticity in the 3MR. Recently, we demonstrated that σ-aromaticity dominating in a fully unsaturated ring is also possible.59 Calculations of extra cyclic resonance energy (ECRE) and NICS(0) for cyclopropane with constrained C−C bond lengths established excellent correlation between these two indices, indicating that they are suitable for evaluating σ-aromaticity in analogous 1455


Article


metallacycles and TM-substituted (hetero)cycles, respectively. The PBE0 functional serves as a fair alternative for the B3LYP in several cases. The 6-31G(d) and 6-311++G(d,p) basis sets were frequently used for main group atoms. For heavy main group atoms and TM atoms, pseudo-potential basis sets are recommended. For instance, we applied the Lan2LDZ basis set with polarization functions in the computation for metallacycles, while the ECPnMDF basis sets (n is the number of core electrons) with fully relativistic effective core potentials (ECPs) were applied to metal-substituted systems in our investigation of hyperconjugative aromaticity. Currently, there is no single aromaticity index that could be considered as perfect, which means that one should try aromaticity indices as more as possible to determine the nature and strength of (anti)aromaticity for particular systems (especially the organometallic ones). Our frequently used aromaticity indices include the NICS and ACID (magnetic), ISE and heat of hydrogenation (energy-based), ELF and MCI (electron (de)localization), harmonic oscillator model of aromaticity (HOMA) and BLA (geometry-based), and so on.

Figure 12. (a) Stabilization of two π-antiaromatic frameworks with one metal leads to complex 13 with σ-aromaticity dominating in an unsaturated Se-containing ring. (b) The isodesmic reactions for model complex 14 suggest the aromaticity in the 3MR. Electronic energies (ΔE) are given in kcal mol−1, including the zero-point energy corrections. Adapted with permission from refs 54 and 55. Copyright 2018 Wiley Online Library. Computational method, B3LYP/6-311++G(d,p)(LanL2DZ).

■

ORIGINS OF TM-INDUCED UNCONVENTIONAL AROMATICITY In Schleyer’s interpretation of hyperconjugative aromaticity in cyclopolyene containing a saturated CR2 moiety (Figure 13a),25,26 the CR2 acts like a pseudo-p orbital which could be partially filled or partially vacant, depending on the electron donating or withdrawing ability of the R substituents. A detailed illustration presented by Ottosson et al. in 2016 shows that two C-R σ bonding orbitals form a π-symmetric pattern, which could participate in the conjugation with the p orbitals of the olefin moiety.60 Our work has demonstrated that the TM substituents could also induce hyperconjugative aromaticity. Similar to the organic counterparts, the hyperconjugative aromaticity in a TM-substituted cyclopentadiene or pyrrolium ring is induced by the interaction between the π electrons of

3MRs. Methylenecyclopropene (C4H4), a complete unsaturated ring, was found to have dominant σ-aromaticity, which could be suppressed by fluoride and enhanced by silyl substituents. Further investigation of 14 (Figure 12b) revealed that the fully unsaturated 3MR incorporated with osmium could also feature the presence of dominant σ-aromaticity.

■

COMPUTATIONAL METHODS From our experience of computational simulation of aromatics containing various TMs, the B3LYP and TPSS functionals were found to satisfactorily reproduce crystal structures of

Figure 13. Schematic illustration of atomic orbital interactions in hyperconjugative aromaticity, Möbius aromaticity, and σ-aromaticity. 1456


Article


Figure 14. EDDB results for the whole system (a) and the 3MR (b) of cyclopropaosmapentalene, and key NOBDs of the 3MR (c). Isovalues are 0.015, 0.010, and 0.001 for EDDBglobal, σ-EDDB3MR, and π-EDDB3MR, respectively. Isovalue for NOBD contours is 0.020. Hydrogen atoms are excluded from the calculations for the 3MR. Computational method, PBE0/6-31G(d)(LanL2DZ).

olefin moiety and the π-symmetric TM-C(N) bondings. On the other hand, TMs use d orbitals instead of s or p orbitals to form covalent bonds with the carbon or nitrogen atom and further participate in the delocalization of the 5MR. Thus, the trans-influence can be used to modulate the hyperconjugative aromaticity in organometallic complexes. Early in 1958, Craig proposed that aromaticity could be achieved with the participation of a main-group d orbital and with a phase shift in the conjugation (Figure 13b).35 Combining Craig’s theory with the 4n Möbius aromaticity first proposed by Heilbronner,34 one could expect a planar cyclic-conjugated aromatic system with 4n delocalized electrons with the contribution of d orbitals. Such a novel type of aromaticity is commonly recognized as the Craig-type Möbius aromaticity, and it was first computationally confirmed by Mauksch and Tsogoeva.36 On the experimental side, Xia and co-workers synthesized the first example of planar Craigtype Möbius aromatic metallacycle, which was verified by DFT computations.7 The interaction between the d orbital of TM and the p orbitals from the organic moiety (C7H6 in metallapentalynes or C7H7 in metallapentalenes) is the key to realize the Craig-type Möbius aromaticity. Unlike all-metal clusters,61 the σ aromaticity in an organic system is very rare and controversial.44 Although the σaromaticity may not be present in cyclopropane due to insignificant aromatic stabilization and lack of strong diatropism. 62 One interpretation of σ-aromaticity was proposed by Cremer and Gauss in 1986, suggesting the inplane overlap of p orbitals should be the origin of 3-center 2electron (3c-2e) σ-aromaticity in cyclopropane (Figure 13c).63 Although the σ-aromaticity in the 3MRs of metallapentalenes have been demonstrated by multiple aromaticity indices as previously discussed, additional calculations based on electron density of delocalized bonds (EDDB)64,65 reveal the nature of the TM-induced σ-aromaticity (Figure 14). The global EDDB(r) contour plots display typical π delocalization along the perimeter of 5MRs and σ delocalization inside the 3MR. Electron delocalization in the 3MR is dissected into σ- and πcomponents. The number of delocalized σ electrons is 1.831 (97% of total 1.890e) in the 3MR, which is overwhelming compared with the π-counterpart (0.059e). In contrast, the

EDDB calculations performed for cyclopropane show only 0.137e of total electron delocalization in the ring (without hydrogen atoms). Notably, electron delocalization in the whole cyclopropane molecule is only 0.443e with a large contribution from hyperconjugation, and thus, the σ-aromaticity in cyclopropane is denied once again. Furthermore, natural orbitals for bond delocalization (NBODs) computed for the 3MR indicate that the σ bondings (NOBD(1−3) in Figure 14c) between the metal center and two carbon atoms account for the σ-electron delocalization involving in-plane overlap of metal d and carbon p orbitals; whereas the π delocalization is minor, as shown in NOBD(4). Interestingly, the NOBD(1) and NOBD(2) are almost degenerate, and together they contribute approximately 2 σ-delocalized electrons with significant contribution of TM d orbitals.

■

SUMMARY AND FUTURE PROSPECTS TM substituents can achieve better hyperconjugative interaction than the main group ones. Aurated substituents are potent to switch the nonaromaticity in the 5MR of indolium to hyperconjugative aromaticity. The most aromatic and antiaromatic pyrroliums have been probed with the combination of hyperconjugation and push−pull effects. The predicted most aromatic cyclopentadiene ring reported so far can be achieved by combining trans-influence and the hyperconjugation effect. Monosubstitution with an electropositive substituent can also induce hyperconjugative aromaticity in a cyclopentadiene. Introducing a TM fragment into pentalyne can not only release partial ring strain but also switch the antiaromaticity to Craig−Möbius aromaticity. Calculations on osmapentalenes reveal that the aromaticity of osmapentalynes is rooted in osmapentalenes. Metallasilapentalynes have reduced aromaticity compared with metallapentalyne counterparts, and Craig− Möbius aromaticity has been confirmed among dimetalla[10]annulene species. σ-Aromaticity dominating in unsaturated rings has been found in cyclopropametallapentalenes. The dominant influence of σ-aromaticity in methylenecyclopropene and the 3MR of its osmacyclic counterpart was revealed, suggesting that the delocalization of σ electrons could also be possible to be the 1457


Article

Accounts of Chemical Research major contribution in fully unsaturated systems. The osmapentaloselenirene is an interesting case of not only dominant σ-aromaticity but also the simultaneous stabilization of antiaromatic selenirene and pentalene by TM. The participation of more than one electron in d orbitals of cyclic-conjugated systems is the origin of novel aromaticity in TM-containing aromatics. Aromaticity of some d-block metallacycles could also be classified as hybrid Hückel− Möbius or quasi-aromatic types because such systems are very difficult to unambiguously associate their aromaticity with the “4n+2” (Hückel) and “4n” (Möbius) rules.66 Compared with main-group aromatics, the TM-incorporated ones possess unconventional aromaticity that has been attracting both experimentalists and theoreticians. The power of TMs highlighted in this Account enables further development of novel concepts of aromaticity, e.g., adaptive aromaticity21,67 and spiro-aromaticity,68 in organometallic chemistry. Computational chemistry69,70 is expected to again play an important role in bridging theoreticians and experimentalists in understanding the structure and bonding in TM containing aromatics and antiaromatics. Theoretical calculations would be used for not only rationalizing the experimental results but also predicting interesting structures and reaction mechanisms for experimental verification or falsification.71,72

■

via quantum calculations with an emphasis on aromaticity, and its application in the C−F and NN bond activations.

■

ACKNOWLEDGMENTS We thank all the collaborators and co-workers for their contribution to the work reported here, especially these experimental collaborators, Professors Haiping Xia, Liang Zhao, and Huan Cong, who gave me reminders and corrections to too-simple and naive thoughts as a theoretician. We also thank Dr. Dariusz W. Szczepanik at the University of Girona for the discussion on the EDDB result and Professors Roald Hoffmann, Paul v. R. Schleyer, Xin Lu, and Zhenfeng Xi for their help on the discussions on aromaticity. We gratefully acknowledge the National Natural Science Foundation of China (21873079, 21573179, 21172184, and 21103142) and the Top-Notch Young Talents Program of China for their financial support.

■

REFERENCES

(1) Fernandez, I.; Frenking, G. Aromaticity in metallabenzenes. Chem. - Eur. J. 2007, 13, 5873−5884. (2) Fernandez, I.; Frenking, G.; Merino, G. Aromaticity of metallabenzenes and related compounds. Chem. Soc. Rev. 2015, 44, 6452−6463. (3) Cao, X.-Y.; Zhao, Q.; Lin, Z.; Xia, H. The Chemistry of Aromatic Osmacycles. Acc. Chem. Res. 2014, 47, 341−354. (4) Bleeke, J. R. Metallabenzenes. Chem. Rev. 2001, 101, 1205− 1228. (5) Thorn, D. L.; Hoffmann, R. Delocalization in metallocycles. Nouv. J. Chim. 1979, 3, 39−45. (6) Elliott, G. P.; Roper, W. R.; Waters, J. M. Metallacyclohexatrienes or ’metallabenzenes.’ Synthesis of osmabenzene derivatives and X-ray crystal structure of [Os(CSCHCHCHCH)(CO)(PPh3)2]. J. Chem. Soc., Chem. Commun. 1982, 811−813. (7) Zhu, C.; Li, S.; Luo, M.; Zhou, X.; Niu, Y.; Lin, M.; Zhu, J.; Cao, Z.; Lu, X.; Wen, T.; Xie, Z.; Schleyer, P. v. R.; Xia, H. Stabilization of anti-aromatic and strained five-membered rings with a transition metal. Nat. Chem. 2013, 5, 698−703. (8) Zhu, C.; Xia, H. Carbolong Chemistry: A Story of Carbon Chain Ligands and Transition Metals. Acc. Chem. Res. 2018, 51, 1691−1700. (9) Zhu, C.; Luo, M.; Zhu, Q.; Zhu, J.; Schleyer, P. v. R.; Wu, J. I.; Lu, X.; Xia, H. Planar Möbius aromatic pentalenes incorporating 16 and 18 valence electron osmiums. Nat. Commun. 2014, 5, 3265. (10) Wei, J.; Zhang, Y.; Chi, Y.; Liu, L.; Zhang, W. X.; Xi, Z. Aromatic Dicupra[10]annulenes. J. Am. Chem. Soc. 2016, 138, 60−63. (11) Yuan, J.; Sun, T.; He, X.; An, K.; Zhu, J.; Zhao, L. Synthesis of tetra- and octa-aurated heteroaryl complexes towards probing aromatic indoliums. Nat. Commun. 2016, 7, 11489. (12) Sun, T.; Xie, Q.; Zhao, L.; Zhu, J. Probing the Most Aromatic and Antiaromatic Pyrrolium Rings by Maximizing Hyperconjugation and Push-Pull Effect. Chem. - Asian J. 2018, 13, 1419−1423. (13) Xie, Q.; Sun, T.; Zhu, J. Probing the Strongest Aromatic Cyclopentadiene Ring by Hyperconjugation. Organometallics 2018, 37, 3219−3224. (14) Xie, Q.; Sun, T.; Orozco-Ic, M.; Barroso, J.; Zhao, Y.; Merino, G.; Zhu, J. Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes. Asian J. Org. Chem. 2018, 8, 123−127. (15) Hückel, E. Quantentheoretische Beiträge zum Benzolproblem. Eur. Phys. J. A 1931, 70, 204−286. (16) Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Synthesis of a Möbius aromatic hydrocarbon. Nature 2003, 426, 819. (17) Fan, Y.-Y.; Chen, D.; Huang, Z.-A.; Zhu, J.; Tung, C.-H.; Wu, L.-Z.; Cong, H. An isolable catenane consisting of two Möbius conjugated nanohoops. Nat. Commun. 2018, 9, 3037. (18) Wang, X.; Zhu, C.; Xia, H.; Zhu, J. Theoretical Study on the Stability and Aromaticity of Metallasilapentalynes. Organometallics 2014, 33, 1845−1850.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jun Zhu: 0000-0002-2099-3156 Notes

The authors declare no competing financial interest. Biographies Dandan Chen was born in 1994 in Fujian (China) and studied at Sun Yat-Sen University, where she received her B. Eng. degree in Macromolecular Materials and Engineering in 2016. In September 2016, she joined the research group of Prof. Jun Zhu at Xiamen University as a full-time PhD student. Her research interests include excited state aromaticity and reaction mechanisms in organic and organometallic species. Qiong Xie was born in 1993 in Fujian (China) and studied at Fujian Normal University, where she received a B.S. in Chemistry in 2016. She is currently completing her master degree studies with Prof. Jun Zhu at Xiamen University. Her studies have primarily focused on hyperconjugative aromaticity and reaction mechanisms in organometallic chemistry. Jun Zhu received his B.Sc. in Chemistry from Xiamen University in 2000 and his M.Sc. degree under the supervision of Professor Zexing Cao in Physical Chemistry in 2003 from the same university. His doctoral research was carried out from 2003 to 2007 in the Theoretical Inorganic Chemistry Laboratory of the Hong Kong University of Science and Technology under supervision of Prof. Zhenyang Lin. After his work with Prof. Dan Yang at the University of Hong Kong and Dr. Henrik Ottosson at Uppsala University as a postdoctoral research associate, he joined the faculty of the Department of Chemistry at Xiamen University in 2010, where he was promoted to be a full Professor of Chemistry in 2018. He was supported by the Top-Notch Young Talents Program of China in 2015. His current research is mainly focused on the structure and bonding, reaction mechanisms in organic/organometallic chemistry 1458


Article


(44) Wu, W.; Ma, B.; I-Chia Wu, J.; Schleyer, P.; Mo, Y. Is cyclopropane really the sigma-aromatic paradigm? Chem. - Eur. J. 2009, 15, 9730−9736. (45) Popov, I. A.; Starikova, A. A.; Steglenko, D. V.; Boldyrev, A. I. Usefulness of the σ-Aromaticity and σ-Antiaromaticity Concepts for Clusters and Solid-State Compounds. Chem. - Eur. J. 2018, 24, 292− 305. (46) Havenith, R. W. A.; De Proft, F.; Fowler, P. W.; Geerlings, P. σAromaticity in H3+ and Li3+: Insights from ring-current maps. Chem. Phys. Lett. 2005, 407, 391−396. (47) Alexandrova, A. N.; Boldyrev, A. I. σ-Aromaticity and σAntiaromaticity in Alkali Metal and Alkaline Earth Metal Small Clusters. J. Phys. Chem. A 2003, 107, 554−560. (48) Yong, L.; Wu, S. D.; Chi, X. X. Theoretical study of aromaticity in small hydrogen and metal cation clusters X3+ (X = H, Li, Na, K, and Cu). Int. J. Quantum Chem. 2007, 107, 722−728. (49) Huang, X.; Zhai, H. J.; Kiran, B.; Wang, L. S. Observation of dorbital aromaticity. Angew. Chem., Int. Ed. 2005, 44, 7251−7254. (50) Xu, Q.; Jiang, L.; Tsumori, N. Cyclo-Ti3[η2(μ2-C,O)]3: a sideon-bonded polycarbonyl titanium cluster with potentially antiaromatic character. Angew. Chem., Int. Ed. 2005, 44, 4338−4342. (51) Foroutan-Nejad, C.; Shahbazian, S.; Rashidi-Ranjbar, P. The critical re-evaluation of the aromatic/antiaromatic nature of Ti3(CO)3: a missed opportunity? Phys. Chem. Chem. Phys. 2011, 13, 4576−4582. (52) Zubarev, D. Y.; Boldyrev, A. I. Chapter 5: Multiple Aromaticity, Multiple Antiaromaticity, and Conflicting Aromaticity in Planar Clusters. In Science and Technology of Atomic, Molecular, Condensed Matter & Biological Systems; Jena, P., Castleman, A. W., Eds.; Elsevier, 2010; Vol. 1, pp 219−267. (53) Chandrasekhar, J.; Jemmis, E. D.; von Rague Schleyer, P. Double aromaticity: aromaticity in orthogonal planes. The 3,5dehydrophenyl cation. Tetrahedron Lett. 1979, 20, 3707−3710. (54) Schleyer, P. v. R.; Jiao, H.; Glukhovtsev, M. N.; Chandrasekhar, J.; Kraka, E. Double Aromaticity in the 3,5-Dehydrophenyl Cation and in Cyclo[6]carbon. J. Am. Chem. Soc. 1994, 116, 10129−10134. (55) Wodrich, M. D.; Corminboeuf, C.; Park, S. S.; Schleyer, P. v. R. Double aromaticity in monocyclic carbon, boron, and borocarbon rings based on magnetic criteria. Chem. - Eur. J. 2007, 13, 4582−4593. (56) Zhu, C.; Zhou, X.; Xing, H.; An, K.; Zhu, J.; Xia, H. σAromaticity in an unsaturated ring: osmapentalene derivatives containing a metallacyclopropene unit. Angew. Chem., Int. Ed. 2015, 54, 3102−3106. (57) Hao, Y.; Wu, J.; Zhu, J. σ Aromaticity Dominates in the Unsaturated Three-Membered Ring of Cyclopropametallapentalenes from Groups 7−9: A DFT Study. Chem. - Eur. J. 2015, 21, 18805− 18810. (58) Zhou, X.; Wu, J.; Hao, Y.; Zhu, C.; Zhuo, Q.; Xia, H.; Zhu, J. Rational Design and Synthesis of Unsaturated Se-Containing Osmacycles with σ-Aromaticity. Chem. - Eur. J. 2018, 24, 2389−2395. (59) Wu, J.; Liu, X.; Hao, Y.; Chen, H.; Su, P.; Wu, W.; Zhu, J. σAromaticity in a Fully Unsaturated Ring. Chem. - Asian J. 2018, 13, 3691−3696. (60) Denisova, A. V.; Emanuelsson, R.; Ottosson, H. Expanding the (cross-)hyperconjugation of 1,4-disilacyclohexa-2,5-dienes to larger monomers and oligomers: a computational investigation. RSC Adv. 2016, 6, 36961−36970. (61) Feixas, F.; Matito, E.; Duran, M.; Poater, J.; Solà, M. Aromaticity and electronic delocalization in all-metal clusters with single, double, and triple aromatic character. Theor. Chem. Acc. 2011, 128, 419−431. (62) Pelloni, S.; Lazzeretti, P.; Zanasi, R. Assessment of σDiatropicity of the Cyclopropane Molecule. J. Phys. Chem. A 2007, 111, 8163−8169. (63) Cremer, D.; Gauss, J. Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane carbon-carbon and carbon-hydrogen bond energies, 1,3 interactions, and σ-aromaticity. J. Am. Chem. Soc. 1986, 108, 7467−7477.

(19) Zhu, C.; Yang, Y.; Luo, M.; Yang, C.; Wu, J.; Chen, L.; Liu, G.; Wen, T.; Zhu, J.; Xia, H. Stabilizing Two Classical Antiaromatic Frameworks: Demonstration of Photoacoustic Imaging and the Photothermal Effect in Metalla-aromatics. Angew. Chem., Int. Ed. 2015, 54, 6181−6185. (20) Wu, J.; Hao, Y.; An, K.; Zhu, J. Unexpected higher stabilisation of two classical antiaromatic frameworks with a ruthenium fragment compared to the osmium counterpart: origin probed by DFT calculations. Chem. Commun. 2016, 52, 272−275. (21) Chen, D.; Shen, T.; An, K.; Zhu, J. Adaptive aromaticity in S0 and T1 states of pentalene incorporating 16 valence electron osmium. Commun. Chem. 2018, 1, 18. (22) Alabugin, I. V.; Gilmore, K. M.; Peterson, P. W. Hyperconjugation. WIREs: Comput. Mol. Sci. 2011, 1, 109−141. (23) Mulliken, R. S. Intensities of Electronic Transitions in Molecular Spectra IV. Cyclic Dienes and Hyperconjugation. J. Chem. Phys. 1939, 7, 339−352. (24) Mulliken, R. S.; Rieke, C. A.; Brown, W. G. Hyperconjugation*. J. Am. Chem. Soc. 1941, 63, 41−56. (25) Nyulászi, L.; Schleyer, P. v. R. Hyperconjugative π-Aromaticity: How To Make Cyclopentadiene Aromatic. J. Am. Chem. Soc. 1999, 121, 6872−6875. (26) Fernández, I.; Wu, J. I.; Von Ragué Schleyer, P. v. R. Substituent Effects on “Hyperconjugative” Aromaticity and Antiaromaticity in Planar Cyclopolyenes. Org. Lett. 2013, 15, 2990−2993. (27) Levandowski, B. J.; Zou, L.; Houk, K. N. Schleyer hyperconjugative aromaticity and Diels-Alder reactivity of 5-substituted cyclopentadienes. J. Comput. Chem. 2016, 37, 117−123. (28) Sun, H.; An, K.; Zhu, J. Triplet State Aromaticity: NICS Criterion, Hyperconjugation, and Charge Effects. Chem. - Asian J. 2016, 11, 234−240. (29) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema Hommes, N. J. R. Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe. J. Am. Chem. Soc. 1996, 118, 6317−6318. (30) Clar, E. The Aromatic Sextet; Wiley: New York, 1972. (31) Solà, M. Forty years of Clar’s aromatic π-sextet rule. Front. Chem. 2013, 1, 22. (32) Kaipio, M.; Patzschke, M.; Fliegl, H.; Pichierri, F.; Sundholm, D. Effect of Fluorine Substitution on the Aromaticity of Polycyclic Hydrocarbons. J. Phys. Chem. A 2012, 116, 10257−10268. (33) Zhu, J.; Lin, Z.; Marder, T. B. Trans Influence of Boryl Ligands and Comparison with C, Si, and Sn Ligands. Inorg. Chem. 2005, 44, 9384−9390. (34) Heilbronner, E. Hückel molecular orbitals of Möbius-type conformations of annulenes. Tetrahedron Lett. 1964, 5, 1923−1928. (35) Craig, D. P.; Paddock, N. L. Nature 1958, 181, 1052−1053. (36) Mauksch, M.; Tsogoeva, S. B. Demonstration of “Möbius” aromaticity in planar metallacycles. Chem. - Eur. J. 2010, 16, 7843− 7851. (37) Jia, G. Recent progress in the chemistry of osmium carbyne and metallabenzyne complexes. Coord. Chem. Rev. 2007, 251, 2167−2187. (38) Ng, S. M.; Huang, X.; Wen, T. B.; Jia, G.; Lin, Z. Theoretical studies on the stabilities of metallabenzynes. Organometallics 2003, 22, 3898−3904. (39) Kutzelnigg, W. Chemical Bonding in Higher Main Group Elements. Angew. Chem., Int. Ed. Engl. 1984, 23, 272−295. (40) Grande-Aztatzi, R.; Mercero, J. M.; Matito, E.; Frenking, G.; Ugalde, J. M. The aromaticity of dicupra[10]annulenes. Phys. Chem. Chem. Phys. 2017, 19, 9669−9675. (41) An, K.; Shen, T.; Zhu, J. Craig-Type Möbius Aromaticity and Antiaromaticity in Dimetalla[10]annulenes: A Metal-Induced Yinand-Yang Pair. Organometallics 2017, 36, 3199−3204. (42) Dewar, M. J. S. σ-Conjugation and σ-Aromaticity. Bull. Soc. Chim. Belg. 1979, 88, 957−967. (43) Dewar, M. J. S.; McKee, M. L. Aspects of cyclic conjugation. Pure Appl. Chem. 1980, 52, 1431. 1459


Article

Accounts of Chemical Research (64) Szczepanik, D. W.; Andrzejak, M.; Dyduch, K.; Zak, E.; Makowski, M.; Mazur, G.; Mrozek, J. A uniform approach to the description of multicenter bonding. Phys. Chem. Chem. Phys. 2014, 16, 20514−20523. (65) Szczepanik, D. W.; Andrzejak, M.; Dominikowska, J.; Pawelek, B.; Krygowski, T. M.; Szatylowicz, H.; Sola, M. The electron density of delocalized bonds (EDDB) applied for quantifying aromaticity. Phys. Chem. Chem. Phys. 2017, 19, 28970−28981. (66) Szczepanik, D.; Solà, M. Electron delocalization in planar metallacycles: Hückel or Möbius aromatic? ChemistryOpen 2019, 8, 219−227. (67) Shen, T.; Chen, D.; Lin, L.; Zhu, J. Dual Aromaticity in Both the T 0 and S 1 States: Osmapyridinium with Phosphonium Substituents. J. Am. Chem. Soc. 2019, 141, 5720−5727. (68) Zhang, Y.; Wei, J.; Chi, Y.; Zhang, X.; Zhang, W.-X.; Xi, Z. Spiro Metalla-aromatics of Pd, Pt, and Rh: Synthesis and Characterization. J. Am. Chem. Soc. 2017, 139, 5039−5042. (69) Lin, Z. Interplay between Theory and Experiment: Computational Organometallic and Transition Metal Chemistry. Acc. Chem. Res. 2010, 43, 602−611. (70) Cheng, G.-J.; Zhang, X.; Chung, L. W.; Xu, L.; Wu, Y.-D. Computational Organic Chemistry: Bridging Theory and Experiment in Establishing the Mechanisms of Chemical Reactions. J. Am. Chem. Soc. 2015, 137, 1706−1725. (71) Sun, T.; Guo, P.; Xie, Q.; Zhao, L.; Zhu, J. Bonded to Carbon or Nitrogen? This is a Question on the Regioselectivity in Hyperconjugative Aromaticity. J. Org. Chem. 2019, 84, 3881−3886. (72) Zhu, J. Rational Design of a Carbon-Boron Frustrated Lewis Pair for Metal-free Dinitrogen Activation. Chem. - Asian J. 2019, 14, 1413−1417, DOI: 10.1002/asia.201900010.

1460


Unconventional Aromaticity in Organometallics: The Power of

Recommend Documents