Our Journey to the Chemistry of Metallabenzynes - American

Review pubs.acs.org/Organometallics

Our Journey to the Chemistry of Metallabenzynes† Guochen Jia* Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ABSTRACT: This personal account summarizes our work on the chemistry of transition-metal-containing metallabenzynes, organometallic compounds derived from formal replacement of a C atom in benzyne by an isolobal transition-metal fragment. Metallabenzynes with osmium and rhenium have been synthesized and well characterized. They have aromatic character on the basis of the criteria of reactivity, geometry, aromatic stabilization energy, and magnetic properties. They can undergo typical reactions of aromatic systems (e.g., electrophilic substitution reactions) and organometallic complexes (e.g., reductive elimination reactions to form carbene complexes).

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INTRODUCTION Aromaticity is one of the oldest and most important concepts in chemistry.1 Many hydrocarbons are considered to be aromatic: for example, benzene, benzyne, cyclopentadienyl anion, cycloheptatrienyl cation, and buckminsterfullerene. Aromatic compounds are not limited to hydrocarbons but can be extended to those containing other elements. Formal replacement of a CH group or a carbon atom in aromatic hydrocarbons by an isolobal heteroatom or group can give heteroaromatics. Numerous heteroaromatics containing a main-group atom or group are known: for example, pyridines, furans, pyrroles, gallatabenzenes, 2 stannabenzenes,3 and bismabenzenes.4 There has also been much interest in the development of the chemistry of transition-metal-containing metallaaromatics, compounds derived from formal replacement of a CH group or a carbon atom in aromatic hydrocarbons or heteroaromatics containing a main-group atom or group by an isolobal transition metal fragment.5 Examples of transition-metalcontaining metallaaromatics that have received recent attention include metallabenzenes,6,7 metallathiabenzenes,8 metallapyridines,9 metallapyryliums,10 metallathiophenes,11 metallafurans,12 metallapyrroles,13 azametallahelicenes,14 metallabenzimidazolium,15 metallapyrimido[2,1-a]isoindoles,16 metalladithiolenes, 1 7 metallapyridynes, 1 8 metallapyrimidines, 1 9 metallacyclobutadienes,20 metallacyclopentadienes or metallacyclopentatrienes,21 metallacyclobutene-silacyclobutenes,22 metallapentalynes,23 aromatic transition-metal clusters,24 and 1,2-quadruply bonded dimolybdenum benzene derivatives.25 This account summarizes the work from our laboratory in the development of the chemistry of transition-metalcontaining metallabenzynes,26,27 organometallic compounds derived from formal replacement of a C atom in benzyne by an isolobal transition-metal fragment.

ISOLATION AND CHARACTERIZATION OF THE FIRST METALLABENZYNE The first metallabenzyne was obtained during our attempt to prepare osmium vinylidene complexes of the type OsCl2( Scheme 1. Synthesis of 2, the First Metallabenzyne

CCHR)(PPh3)2. It is known that ruthenium vinylidene complexes of the type RuCl2(CCHR)(PPh3)2 can be easily prepared from the reactions of terminal alkynes HCCR with the dichlororuthenium complexes RuCl2(PPh3)3.28,29 We therefore carried out reactions of the dichloroosmium complex OsCl2(PPh3)3 with HCCR with the hope of obtaining the osmium vinylidene complexes OsCl2(CCHR)(PPh3)2. When the dichloroosmium complex OsCl2(PPh3)3 (1) was allowed to react with excess HCCSiMe3 in wet benzene, a mixture of products were obtained. One of the products was identified to be the osmabenzyne complex Os(CC(SiMe3) C(CH3)C(SiMe3)CH)Cl2(PPh3)2 (2) (Scheme 1).30 An X-ray diffraction study shows that the osmabenzyne complex 2 is a planar metallacycle. Despite the difference between the OsC bond in 2 and the CC bond in benzyne,

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This article summarizes the work presented on October 22, 2012, at the 17th National Symposium on Organometallic Chemistry (Chinese Chemical Society) in receipt of the 2012 Chinese Chemical Society Yao-Zeng Huang Award in Organometallic Chemistry. © 2013 American Chemical Society

Received: July 23, 2013 Published: October 14, 2013 6852

dx.doi.org/10.1021/om400716v | Organometallics 2013, 32, 6852−6866

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Figure 1. Selected structural parameters of 2 and benzyne. All distances are in Å. Figure 2. Relative energies of optimized 2-butyne (7) and the carbyne complex Os(CCH3)(CH3)Cl2(PH3)2 (9) and their deformed forms.

the C−C bond distances as well as the bond angles around the carbons in the osmabenzyne complex 2 are similar to the related parameters calculated for benzyne,31,32 as illustrated in Figure 1. The osmabenzyne complex 2 is interesting to us for the following reasons. First of all, carbyne or alkyne carbons usually have a linear geometry. However, in the osmabenzyne 2, the angle around the carbyne carbon (148.7(3)°) deviates significantly from 180°. Second, six-membered organic compounds with a CC triple bond in the six-membered ring, for example, benzyne and cyclohexyne, have low thermal stability. They cannot be isolated in the free state at room temperature and have only been detected at low temperature.33 In contrast, the osmabenzyne 2 is thermally much more stable than benzyne and remained unchanged even when it was heated to 80 °C overnight. In addition, the compound represents a new type of metallaaromatic compound. We then became interested in addressing the following questions. Why is the osmabenzyne 2 thermally more stable than benzyne? Can we synthesize other metallabenzynes? What are the chemical properties of metallabenzynes? Do metallabenzynes chemically behave like benzene or benzyne? Do metallabenzynes have aromatic character? Do metallabenzynes also show properties of typical organometallic compounds?

In the osmabenzyne complex 2, the angle at the carbyne carbon atom is 148.7(3)°, which is significantly smaller than the ideal value of 180° expected for metal−carbyne complexes. The other OsCC angle (138.6(5)°) is also slightly larger than the expected angle around an sp2-hybridized carbon. These angle bendings are expected to cause a ring strain. The ring strain of the osmabenzyne 2 caused by angle bending at the carbyne carbon and the metal was estimated to be only 9.6 kcal/mol by calculating the energy needed for the angle bending of the optimized model carbyne complex Os(CCH3)(CH3)Cl2(PH3)2 (9) to a geometry (10) similar to that of the osmabenzyne 2 (Figure 2). The strain caused by angle bending at the Os−CH carbon was found to be insignificant and is less than 2.5 kcal/mol. The results clearly indicate that the ring strain of the osmabenzyne 2 is significantly smaller than that of benzyne. We believe that the smaller ring strain of the osmabenzyne 2 is one of the major reasons for its high thermal stability. The smaller ring strain and high thermal stability of the osmabenzyne complex 2 suggest that it may be possible to isolate other metallabenzynes.

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ELECTRONIC STRUCTURES OF METALLABENZYNES To have a better understanding of the property of the osmabenzyne complex 2, we have studied the electronic structures of related model metallabenzyne complexes.32 A similar study has been carried out by Yang and his coworkers.34 In terms of bonding, the model osmabenzyne complex Os(C5H4)Cl2(PH3)2 can be viewed as a compound formed by combination of the d6 metal fragment OsCl2(PH3)2 and the carbon fragment C5H4. A schematic orbital interaction diagram between the “t2g” orbitals (left column) of the metal fragment OsCl2(PH3)2 and the π orbitals (right column) of the carbon fragment C5H4 is shown in Figure 3. The resulting molecular orbitals derived from the orbital interactions are given in the central column. The MO1, MO4, and MO6 orbitals are derived from the orbital interactions among 1π, 3π, and 5π* of C5H4 and dxz of the “t2g” orbitals. The MO2 and MO5 orbitals are derived from the interactions among 2π, 4π* of C5H4 and dyz. The MO3 orbital is derived from the

RING STRAIN OF METALLABENZYNES The low thermal stability or high reactivity of benzyne can be attributed to the large ring stain caused by bending of the formal CC triple bond in the six-membered ring. To understand why the osmabenzyne 2 is thermally much more stable than benzyne, we carried out a theoretical study to compare the ring strains of benzyne and metallabenzynes.32 In benzyne, the angle around the alkyne carbon is about 127°. In our study, we have estimated the ring stain of benzyne by calculating the relative energies of 2-butyne (7) and its deformed form (8) with a geometry of the alkyne carbon similar to that of benzyne (Figure 2). We found that when the angle around the alkyne carbon in 2-butyne is changed from 180° to 127°, the energy is raised by about 51.8 kcal/mol. The raise in energy is similar to the ring strain of benzyne (53.6 kcal/mol) estimated by calculating the energy needed for the angle bending of optimized CH2CHCHCHCCCH CHCHCH2 to a geometry similar to that of benzyne.31 6853


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Figure 3. Schematic orbital correlation diagram showing the π interaction for the all-H osmabenzyne complex Os(C5H4)Cl2(PH3)2.

interaction of πin‑plane and dx2−y2 and corresponds to the in-plane π bonding of the OsC triple bond. When the MO3 orbital (the in-plane π bonding in the Os C triple bond) is not included, the bonding between Os(PH3)2Cl2 and C5H4 fragments is similar to that of metallabenzenes.35,36 Excluding the electrons in the MO3 orbital (the in-plane bonding orbital in the OsC triple bond), there are eight π electrons in the metallacycle, which are housed in MO1, MO2, MO4, and MO5 orbitals. The MO1 and MO4 orbitals have Hückel character, while the MO2 and MO5 orbitals have Möbius character. The MO5 and MO6 are respectively the HOMO and LUMO of the metallacycle. The compositions of the HOMO and LUMO are similar to those of metallabenzenes.35 Figure 4 shows the spatial plots of the six π molecular orbitals calculated for the model osmabenzyne complex Os(CCH C(CH3)CHCH)Cl2(PH3)2 (11). As shown in Figure 4, the HOMO (MO5) has significant contribution from the pπ orbitals at C2 and C4 carbons, the Os(dxz) orbital, and the pπ orbitals from the chloride ligands; the LUMO (MO6) has

significant contribution from the pπ orbitals at C1, C3, and C5 carbons and the dxz(Os) orbital. For comparison, the spatial plots of the eight π molecular orbitals calculated for benzyne are shown in Figure 5. Different from the case for metallabenzynes, both the HOMO (MO4) and LUMO (MO5) of benzyne are associated with the triple bond and are mainly composed of the in-plane pπ orbitals at the alkyne carbons.

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SYNTHESIS OF METALLABENZYNES The interesting electronic and structural features of metallabenzynes and the high thermal stability of the osmabenzyne complex 2 prompted us to synthesize and isolate other metallabenzynes. The first isolated osmabenzyne was obtained from the reaction of OsCl2(PPh3)3 (1) with HCCSiMe3 in wet benzene.30 However, this method has limitations and our attempts to extend the chemistry to prepare additional osmabenzynes involving reactions of OsCl2(PPh3)3 with other alkynes were unsuccessful. For example, in the reactions of OsCl2(PPh3)3 with tert-butylacetylene37 and HCCC(OH)Ph2,38 the expected osmabenzynes were not produced in our 6854


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Figure 4. Spatial plots of the MO1−MO6 orbitals for the model complex Os(CCHC(CH3)CHCH)Cl2(PH3)2 (11).

hands. Therefore, we searched for other methodologies for the preparation of metallabenzynes. In 2005, we reported a more reliable route to prepare osmabenzynes, which involves the reactions of allenylcarbene complexes with gold(I) acetylide derivatives in the presence of HNEt3Cl.39 For example, treatment of the allenylcarbene complex 12 with (PPh3)AuCCPh and HNEt3Cl produced the osmabenzyne 15, presumably through intermediates 13 and 14 (Scheme 2). Through this methodology, we have been able to prepare a series of osmabenzynes. For example, osmabenzynes with a tolyl (16), a trimethylsilyl (17), and an n-butyl (18) group were similarly obtained from the reactions of the allenylcarbene complex 12 with the respective gold acetylides (Scheme 3). The method using allenylcarbene complexes as the starting materials also has deficiencies, as the variety of allenylcarbene complexes is limited due to the lack of synthetic routes. We have therefore searched for alternative and more versatile synthetic routes to synthesize metallabenzynes. We were particularly interested in synthesizing metallabenzynes with a starting material already having the required five-carbon chain. It is known that coordinatively unsaturated complexes can undergo oxidative addition reactions with organohalides. We therefore envisioned that osmabenzynes may be obtained from oxidative addition reactions of properly functionalized 16e osmium carbyne complexes. As a test

Figure 5. Spatial plots of the eight π molecular orbitals of benzyne.

experiment, we initially studied the zinc reduction reaction of the osmium carbyne complex OsCl3(CCHCPh2)(PPh3)2 (19), hoping that the reaction would generate the coordinatively unsaturated species 20, which may undergo an C−H oxidative addition reaction to give the hydrido metallanaphthalyne complex 21 (Scheme 4).40 Experimentally, we isolated the indenyl complex 23 from the reaction. Apparently, the hydrido metallanaphthalyne complex 21 is thermally unstable and undergoes reductive elimination to give first the metallanaphthalene complex 22 and then the indenyl complex 23. The reaction sequence was supported by DFT calculations.40 The theoretical study also suggests that the formation of the osmanaphthalyne complex 21 via C−H activation of intermediate 20 is energetically feasible. However, the intermediate can easily undergo a reductive elimination reaction involving the hydride ligand and the carbyne carbon of the metallacycle to finally give the indenyl complex 23. We envisioned that if the hydride ligand is replaced by a chloride ligand, the reductive 6855


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species 25, which might undergo a C−Cl oxidative addition reaction to give the metallanaphthalyne 26 or a C−H oxidative addition reaction to give the hydrido metallanaphthalyne complex 27, which might rearrange to the indenyl complex 28 (Scheme 5). The expectation was realized experimentally, and we isolated the metallanaphthalyne complex 26 and the indenyl complex 28 from the reaction. The chemistry can be extended to prepare other osmabenzynes. For example, reactions of the tert-butylsubstituted carbyne complexes 29 and 30 with zinc in THF at room temperature produced the osmabenzynes 31 and 32, respectively (Scheme 6); reactions of the adamantyl-substituted carbyne complexes 33 and 34 with zinc in THF at room temperature produced the osmabenzynes 35 and 36, respectively.41 While complexes 31 and 35 can be isolated in 62.2% and 34.7% respective yields, the complexes 32 and 36 are thermally unstable and rearranged to carbene complexes on standing in solution (see Reactivities of Metallabenzynes for details). We noted that two osmabenzynes have also been synthesized in Xia’s lab. The PPh3-substituted osmabenzyne complex 39 was obtained from the reaction of the osmium hydrido− vinylidene complex 37 with HCCCH(OEt)2 via the isoosmabenzene intermediate 38 (Scheme 7).42 The osmanaphthalyne 42 was obtained by refluxing the hydrido−carbyne complex 40 in ClCH2CH2Cl under an O2 atmosphere for 15 h, by heating a solid sample of 40 in air at 120 °C for 5 h, or by the reaction of the binuclear osmanaphthalene 41 with HCl (1 equiv), HBF4 (1 equiv), and excess PPh3 under an O2 atmosphere (Scheme 7).43 We also explored the possibility of preparing metallabenzynes of other metals. Our attempts to prepare metallabenzynes containing ruthenium have so far been unsuccessful. On the other hand, we have recently found a route to make metallabenzynes containing rhenium.44 The key starting materials for the preparation are the vinylcarbyne complexes Re{CCHC(R)CCSiMe3}Cl2(PMe2Ph)3. As illustrated in Scheme 8, treatment of the vinylcarbyne complex Re{ CCHC(CMe 3 )CCSiMe 3 }Cl 2 (PMe 2 Ph) 3 (43) with Me3CMgCl gave the hydrido−carbyne complex Re{ CCHC(CMe3)CCSiMe3}HCl(PMe2Ph)3 (44), which reacted with Bu4NF to give the hydrido−carbyne complex Re{CCHC(R)CCH}HCl(PMe2Ph)3 (45). In solution, the complex 45 first isomerized to the bicyclic complex Re{CHCHC(CMe3)CCH}Cl(PMe2Ph)3 (46) and then to the rhenabenzyne Re{CCHC(CMe3)CHCH}Cl(PMe2Ph)3 (47). The analogous rhenabenzynes 48 (with an isopropyl group) and 49 (with an adamantyl group) can be similarly prepared.

Scheme 2. Synthesis of the Osmabenzyne 15 from the Allenylcarbene Complex 12

Scheme 3. Synthesis of Osmabenzynes 16−18 from the Allenylcarbene Complex 12

Scheme 4. Reaction of the Osmium Carbyne Complex 19 with Zinc

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REACTIVITIES OF METALLABENZYNES Electrophilic Substitution Reactions. Metallabenzynes are structurally related to benzene and benzyne. We wondered whether metallabenzynes have chemical properties similar to those of benzene or benzyne. Benzene and benzyne show distinctive reactivity toward bromine. Benzene undergoes an electrophilic substitution reaction with bromine, whereas benzyne undergoes an addition reaction with bromine.45 We therefore studied the reaction of the osmabenzyne Os( CC(SiMe3)C(CH3)C(SiMe3)CH)Cl2(PPh3)2 (2) with bromine. It was found that the osmabenzyne 2 behaves like benzene in undergoing an electrophilic substitution reaction with bromine

elimination reaction might be discouraged and an osmabenzyne complex could be isolated. Therefore, the reaction of the carbyne complex OsCl3{CCHC(2-ClC6H4)2}(PPh3)2 (24) with zinc was carried out next.40 It was expected that the reaction would generate the coordinatively unsaturated 6856


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Scheme 5. Synthesis of the Metallanaphthalyne 26 from Osmium Carbyne Complex 24

electrophiles, the reactions also occur at the carbon attached to silicon. To see if the regiochemistry of the electrophilic substitution reactions is caused by the more reactive C−Si bonds or other factors, we have studied the reactions of electrophiles with the osmabenzyne complex Os(CCH C(CH3)CHCH)Cl2(PPh3)2 (52),47 which does not contain a trimethylsilyl group. It was found that this osmabenzyne can also undergo electrophilic substitution reactions such as H/D exchange (with DOTf), bromination, nitration, and chlorination at the carbons meta to the metal (Scheme 10), suggesting that the regiochemistry is not related to the C−Si bond. In the reactions of electrophiles with osmabenzynes 2 and 52, electrophiles selectively attack at the carbons of the metallacycle meta to the metal. In principle, the selectivity can be charge controlled or frontier orbital controlled. To understand the origin of the regioselectivity, the NBO (natural bond orbital) atomic charge distributions of the model osmabenzyne Os(CC(SiH3)HC(CH3)C(SiH3)CH)Cl2(PH3)2 (60) were calculated.49 As shown in Figure 6, the C5 carbon of the metallacycle has the largest negative charge. If the reaction is charge controlled, the electrophile should attack at the C5 carbon. However, experimentally, electrophiles do not attack at this carbon but attack at the C2 and C4 carbons. Thus, the regioselectivity of the electrophilic substitution reactions is not related to the charge density. On the other hand, the regioselectivity correlates well the electron clouds of the HOMO of the osmabenzyne 60.46 Therefore, we believe that the regioselectivity of the electrophilic substitution reactions is frontier orbital or HOMO controlled.

Scheme 6. Synthesis of Osmabenzynes via Zinc Reduction of Osmium Vinyl−Carbyne Complexes

at the carbons meta to osmium (Scheme 9).46,47 Further experiments show that this metallabenzyne can undergo a variety of electrophilic substitution reactions, for example, protonation, nitration, nitrosylation, and chlorination, as illustrated in Scheme 9. The result indicates that osmabenzynes chemically behave like benzene more than benzyne. In the reactions of electrophiles with trimethylsilylbenzene, the substitution reactions usually occur at the carbon attached to silicon.48 In the reactions of the osmabenzyne 2 with

Scheme 7. Synthesis of the Osmabenzyne Complex 39 and the Osmanaphthalyne Complex 42

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Scheme 8. Synthesis of Rhenabenzyne Complexes

Scheme 10. Electrophilic Substitution Reactions of the Osmabenyne Complex 52

electrophilic substitutions reactions have been reported for metallabenzene complexes such as Os{C5H4(SMe)}I(CO)(PPh3)250 and Ir{C5H4(SMe)}Cl2(PPh3)2.51 These complexes reacted regioselectively with nucleophiles at the carbon para to the SMe group. Electrophilic substitution reactions of an osmabenzofuran52 and iridabenzofurans53 have also been reported. Nucleophilic Addition Reactions. We also carried out the reactions of osmabenzynes with various nucleophiles, in order to see if metallabenzynes would undergo charge controlled or frontier orbital controlled reactions with nucleophiles. The neutral osmabenzyne Os(CC(SiMe3)C(CH3)C(SiMe3)CH)Cl2(PPh3)2 (2) was found to be unreactive toward nucleophiles such as NaBH4, amines, and phosphines. The dicationic complex [Os(CC(SiMe 3)C(CH3 )C(SiMe3)CH)Cl(bipy)(PPh3)2]OTf (62), which can be synthesized from a ligand substitution reaction of 2, was found to readily react with nucleophiles. For example, it reacted with water and methanol in the presence of K 2 CO 3 regioselectively at the carbyne carbon (C1) to give the osmabenzenes 64 and 63, respectively (Scheme 11).54 The dicationic osmabenzyne 62 also reacted with NaBH4 to give the cyclopentadienyl complex 66, presumably via the metallabenzene intermediate 65. The observed regiochemistry of the nucleophilic addition reactions cannot be explained in terms of the charge distribution of the metallacycle. As suggested by the NBO atomic charge distribution of the model osmabenzyne Os( CC(SiH3)HC(CH3)C(SiH3)CH)Cl2(PH3)2 (60) shown in Figure 6, the order of preferential sites of the metallacycle for nucleophilic attack is C3 > C4 > C1 > C2 > C5; therefore, C3 rather than the carbyne carbon should be attacked by a nucleophile, if the regioselectivity of the reactions is charge controlled. On the other hand, the regiochemistry of the nucleophilic addition reactions is understandable if we assume that the reactions are LUMO controlled. The lowest unoccupied molecular orbital (LUMO) of osmabenzynes is mainly made up of a d orbital of the metal and p orbitals of the

Scheme 9. Electrophilic Substitution Reactions of the Osmabenyne Complex 2

As shown in Figure 6, the HOMO of the osmabenzyne 60 also has contribution from the pπ orbitals of the chloride ligands. Thus, electrophiles could also attack at the chloride ligands. Indeed, the replacement of the chloride ligands with bromide ligands in the reactions of bromine with osmabenzynes 2 and 52 to give 50 is consistent with this expectation. An additional example of such reactivity was observed in the reactions of the osmabenzyne 2 with 2 equiv of HBF4 in wet dichloromethane. The reactions produced the cationic osmabenzyne complex 61 (eq 1).46 It should be noted that 6858


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Figure 6. Structure (a), NBO charge distribution (b), and spatial plot of the HOMO (c) of the osmabenzyne Os(CC(SiH3)C(CH3)C(SiH3) CH)Cl2(PH3)2 (60).

Scheme 11. Nucleophilic Addition Reactions of the Osmabenzyne 62

Scheme 13. Conversion of Osmabenzynes to Carbene Complexes

expected to attack at the carbyne (C1), C3, and C5 carbons of the metallacycle, if the reactions are LUMO controlled. The preferred attack of nucleophiles at the carbyne carbon in the reactions of 62 is not surprising, as the ring strain is relieved in the process. Regioselective attack of nucleophiles at the C3 carbon (para to metal) of metallabenzynes was recently reported by Xia et al.42 They found that the osmabenzyne complex 39 reacted with ethyllithium and sodium methanethiolate (NaSMe) to give isoosmabenzenes 67 and 68, respectively (Scheme 12). Interestingly, the primary amines NH 2 (n-Bu) and NH2CH3CCH reacted with 39 to give the corresponding ring-opened products 69 and 70. In these reactions, both electronic and steric effects play a role in determining the regioselectivity. Formation of Carbene Complexes. A previous discussion shows that metallabenzynes can undergo typical reactions of aromatic compounds. We were also interested in knowing if metallabenzynes also show properties of typical organometallic compounds. Hydrido− or alkyl−carbyne complexes LnM(R) CR′ can undergo reductive elimination reactions to give carbene complexes LnMCRR′.55 Metallabenzenes can undergo rearrangement reactions to give cyclopentadienyl complexes.56 We have therefore studied the thermal reactions of various metallabenzynes to see if they would undergo rearrangement reactions to give carbene complexes. It was found that most of the metallabenzynes we synthesized are thermally stable and do not undergo reductive elimination reactions to give carbene complexes. The expected rearrangement reactions to give the corresponding carbene complexes were observed in the following two cases.41 The tertbutyl-containing osmabenzyne 32 and the adamantyl-containing osmabenzyne 36, which can be generated in situ and

Scheme 12. Nucleophilic Addition Reactions of Osmabenzyne 39

carbyne carbon (C1) and carbons at the C3 and C5 of the metallacycle (see Figure 4). Therefore, nucleophiles are 6859


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characterized spectroscopically, were found to undergo rearrangement reactions to give osmium carbene complexes 71 and 72, respectively, when their benzene solutions stood at room temperature for 24 h (Scheme 13). We noticed that substituents on the metallacycle can have a significant effect on the transformation. Thus, while complexes 32 and 36 undergo rearrangement reactions to give the corresponding carbene complexes 71 and 72, the related osmabenzynes Os{CCHC(CMe3)CHCH(n-pentyl)}Cl2(PPh3)2 (31) and Os{CCHC(1-adamantyl)CH CH(n-propyl)}Cl2(PPh3)2 (35) as well as the osmabenzyne Os{≡C−CHCMeCHCH}Cl2(PPh3)2 (52) are stable and do not rearrange to the corresponding carbene complexes under similar conditions. Our theoretical study suggests that the difference in the stability of these osmabenzynes is of thermodynamic origin and is related to steric effects.41 As shown in Figure 7, the conversion of 32 to the carbene complex 71 is thermally favorable, while that of complexes 52 and 35 is thermally unfavorable. We have also studied the effect of ligands and metals on the transformation by DFT calculations. The calculations indicate that the osmium-containing metallabenzyne 75 is thermally stable, while the ruthenium-containing metallabenzyne 77 is thermally unstable with respect to the formation of the corresponding carbene complexes (Figure 8). As expected, ligands can also have a dramatic effect on the transformation. For example, the theoretical study suggests that the bisphosphine osmabenzyne complex 75 is thermally stable, while the CO-containing osmabenzyne complex 79 is thermally unstable with respect to the formation of the corresponding carbene complexes (Figure 8). We have also compared the energy changes in the rearrangement reactions of metallabenzynes and metallabenzenes with the same metal fragment. It is interesting to note that the conversion of the osmabenzyne 81 to the carbene complex 82 is thermally unfavorable, while the conversion of the osmabenzene 83 to the cyclopentadienyl complex 84 is thermally favorable (Figure 9).54

Figure 7. Energy profile calculated for the rearrangement of metallabenzynes to carbene complexes. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/ mol.

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AROMATIC PROPERTIES OF METALLABENZYNES An interesting question is whether metallabenzynes are aromatic or not. The aromatic character of a system can be evaluated with various criteria, including electronic structure, reactivity, geometry, energetics, and magnetic properties. In terms of electronic structure and reactivity, metallabenzynes can be considered to be aromatic, as they have an electronic structure similar to that of metallabenzenes and they can undergo electrophilic aromatic substitution reactions (see previous discussion). The aromatic character of metallabenzynes is also reflected by their structural data. Tables 1 and 2 give the selected bond distances and angles of the metallacycles of selected structurally characterized metallabenzynes. As shown in Table 2, all of the complexes contain an essentially planar six-membered metallacycle with the sums of internal angles in the six-membered ring being close to the ideal value of 720° required for a planar hexagon. The M−Cl−C2 angles are in the range 148.3(6)− 155.0(3)°, which are significantly smaller than the 180° expected for carbyne complexes. The M−Cl bond distances (1.726(5)−1.815(4) Å) are at the high end of those observed for typical osmium and rhenium carbyne complexes LnMCR and at the low end of those observed for typical osmium and

Figure 8. Relative energies of metallabenzynes and their corresponding carbene isomers.

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Figure 9. Relative energies of metallabenzyne 81 and metallabenzene 83 and their rearranged products.

al.58 In this method, the ASE is approximated by the energy difference of a methyl-substituted aromatic compound (e.g., toluene) and the isomeric compound with an exocyclic double bond (e.g., methylenecyclohexadiene). We have used the isomerization method to evaluate the aromatic stabililization energies of metallabenzynes. With the method, the ASE values of the model osmabenzyne 11 and the rhenabenzyne complex 93 were estimated to be 19.5 and 14.3 kcal/mol, respectively,44b at the B3LYP/6-311+G** level (Figure 10). For comparison, the ASE values of benzene and benzyne were estimated to be 33.9 and 26.8 kcal/mol, respectively, and the ASE values of the platinabenzene 9559 and iridabenzene 9760 were estimated to be 23.4 and 16 kcal/mol, respectively. The results suggest that the model complexes osmabenzyne 11 and rhenabenzyne 93 have some aromatic character, although the aromatic character is less than that of benzene and benzyne. The nuclear independent chemical shift (NICS) is another commonly used criterion for aromaticity.61 An aromatic compound such as benzene has a negative NICS value, while an antiaromatic compound such as cyclobutadiene has a positive NICS value. Our calculations at the B3LYP/6311+G** level show that the NICS(0) and NICS(1) values of the model osmabenzyne complex 11 are −6.8 and −6.6 and those of the model rhenabenzyne complex 93 are −5.8 and −6.0, respectively.44b The negative NICS values further suggest that osmabenzynes and rhenabenzynes are aromatic. Protons of aromatic compounds usually have a relatively large downfield NMR spectroscopic shift due to aromatic ring current. This characteristic has been widely used as an aromaticity criterion. Figure 11 shows selected 1H NMR data of representative metallabenzynes. On the basis of the NMR criterion, rhenabenzynes can be considered as aromatic compounds. For example, the rhenabenzynes 47 and 48 show 1H signals of ReCCH at ca. 4.9 ppm, which is downfield from those of ReCCH of related rhenium carbyne complexes, the C4-H signals at ca. 8 ppm, which is downfield from those of typical vinyl proton signals of rhenium vinyl

rhenium vinylidene complexes LnMCCRR′. The M−C5 bond distances (1.939(5)−2.110(6) Å) are within the range of M−C(vinyl) bonds and at the high end of those observed for typical osmium and rhenium carbene complexes LnMCHR. The ring C−C distances are in the range 1.363−1.457 Å, which are intermediate between single and double carbon−carbon bonds. The structural data of the six-membered rings suggest that the metallacycle of metallabenzyne has a delocalized structure with contributions from the resonance structures 85 and 86 (Scheme 14). Since the M−Cl bond distances are at the high end of those observed for typical carbyne complexes and at the low end of those observed for typical vinylidene complexes LnMCCRR′ and the M−C5 bond distances are at the high end of those observed for typical carbene complexes LnM CHR, the structure of 85 should be more important. The contribution of structure 86 to the overall structure can be inferred by the fact that the C2−C3 and C3−C4 bonds have similar bond distances and that the C1−C2 bonds are shorter than C2−C3 bonds in “symmetrically substituted” complexes such as 2, 62, 52, and 50 (see Table 1). We have also investigated the aromatic character of metallabenzynes on the basis of the criterion of energetics. We initially estimated32 the conjugation energy of the model osmabenzyne complex Os(CCHC(CH3 )CHCH)Cl2(PH3)2 (11) on the basis of an isodesmic reaction.57 The conjugation energy of 11 was estimated to be 44.35 kcal/mol. For comparison, the conjugation energies of benzyne and benzene were estimated to be 37.47 and 46.66 kcal/mol, respectively. Estimation of the ASE (aromatic stabilization energy) from isodesmic reactions has the complications that the choice of reference reactions can have a large effect on the result and that it requires the separation of the ring strain from the reaction energy. An effective and simpler method to evaluate the aromatic stabililization energy of a strained system is the isomerization method, which was introduced by Schleyer et 6861


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Table 1. Selected Bond Distances of Metallabenzyne Ringsa

a

L = PPh3, except for compound 47, in which L = PMe2Ph. See Scheme 14 for the atom-labeling scheme. 6862


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Table 2. Selected Bond Angles of Metallabenzyne Ringsa

a

L = PPh3, except for compound 47, in which L = PMe2Ph. See Scheme 14 for the atom labeling scheme.

osmabenzynes. In particular, the 1H signals of OsCCH appear in the region 3.9−4.3 ppm, which are typical of those of

complexes. However, the NMR data of osmabenzynes do not give a clear indication of the aromatic character of 6863


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Biography

Scheme 14. Two Resonance Structures of Metallabenzynes

OsCCH of related osmium carbyne complexes, and the C4H signals appear in the region 5.6−6.8 ppm, which are typical of vinyl proton signals of osmium vinyl complexes. We noted that Schleyer et al. recently pointed out that proton chemical shifts are not generally reliable aromaticity indicators.62

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Guochen Jia received his B.S. degree from Wuhan University in 1983 and his Ph.D. degree in 1989 from The Ohio State University. After his postdoctoral stays at University of Toronto and University of Western Ontario, he joined The Hong Kong University of Science and Technology in 1992 as an Assistant Professor. He was promoted to an Associate Professor in 1998 and to a full Professor in 2005. He is now a Chair Professor of Chemistry at The Hong Kong University of Science and Technology. His research interest is in the areas of organometallic chemistry and homogeneous catalysis.

SUMMARY AND PERSPECTIVES

We have given great effort to developing the chemistry of metallabenzynes. Stable metallabenzynes are now known for those with Os and Re. It has been demonstrated that metallabenzynes have aromatic character in terms of reactivity, geometry, aromatic stabilization energy, and magnetic properties. Metallabenzynes can undergo typical reactions of aromatic systems (e.g., electrophilic substitution reactions) as well as organometallic complexes (e.g., formation of carbene complexes). There are many opportunities in further developing the chemistry of metallabenzynes, especially in the syntheses of metallabenzynes with other metals, investigation of their physical and chemical properties, and exploration of their potential applications. The development of methodologies to construct the metallabenzyne ring is of critical importance for new advances.

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ACKNOWLEDGMENTS Many of the results and ideas described in this article resulted from research performed with former and current members of the group as cited. Special thanks are given to my collaborators Professor Zhenyang Lin and his students for their work on computational chemistry and Professors Ian D. Williams and Zhong Yuan Zhou for X-ray structural determinations. This work was supported by the Hong Kong Research Grant Council and The Hong Kong University of Science and Technology.

AUTHOR INFORMATION

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Corresponding Author

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

REFERENCES

(1) Examples of recent reviews: (a) Lee, V. Y.; Sekiguchi, A. Angew. Chem., Int. Ed. 2007, 46, 6596. (b) Thematic issue “Aromaticity”: Chem. Rev. 2001, 101 (5) (Schleyer, P. v. R., Ed.). (c) Thematic issue

Notes

The authors declare no competing financial interest.

Figure 10. ASE values (in kcal/mol) of benzene, benzyne, metallabenzynes, and metallabenzenes derived from the isomerization method. 6864


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