carbolong chemistry: a story of carbon chain ligands and transition metals · transition metals...

Carbolong Chemistry: A Story of Carbon Chain Ligands andTransition MetalsCongqing Zhu†,‡ and Haiping Xia*,†

†State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials(iChEM), and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China‡State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry andChemical Engineering, Nanjing University, Nanjing 210023, China

CONSPECTUS: The construction of metal−carbon bonds is one of the most important issues of organometallic chemistry.However, the chelation of polydentate ligands to a metal via several metal−carbon bonds is rare. Metallapentalyne, which can beviewed as a 7-carbon (7C) chain coordinated to a metal via three metal−carbon bonds, was first reported in 2013. Althoughmetallapentalyne contains a metal−carbon triple bond in a five-membered ring (5MR) and the bond angle around the carbynecarbon is only 129.5°, metallapentalyne exhibits excellent stability to air, moisture, and heat. Metallapentalyne possesses the rareplanar Mobius aromaticity, which is in sharp contrast to the Huckel antiaromaticity in pentalyne. The metal fragment not onlyrelieves the large ring strain present in pentalyne but also results in the transformation of the antiaromaticity in pentalyne toaromaticity in metallapentalyne.With the extension of the carbon chain from 7 to 12 carbon atoms, a series of novel polycyclic frameworks were constructed viathe formation of several metal−carbon bonds. Some interesting phenomena were observed for these complexes. For instance,(1) the carbyne carbon of the 7C framework could react with both nucleophilic and electrophilic reagents, leading to theformation of 16- and 18-electron metallapentalenes; (2) σ aromaticity was first observed in an unsaturated system in the 8Cframework; (3) two classical antiaromatic frameworks, cyclobutadiene and pentalene, were simultaneously stabilized in the 9Cframework for the first time; (4) three fused 5MRs bridged by a metal are coplanar in the 10C framework; (5) the first [2 + 2 + 2]cycloaddition of a late transition metal carbyne complex with alkynes was realized during the construction of an 11C framework;(6) the largest number of carbon atoms coordinated to a metal atom in the equatorial plane was observed in the 12Cframework; and (7) sharing of the transition metal by multiple aromatic units has seldom been observed in the metalla-aromatics. Therefore, the term carbolong chemistry has been used to describe the chemistry of these novel frameworks.More interestingly, carbolong complexes exhibit diverse properties, which could lead to potential future applications. As thediscovery and creation of molecular fragments lead to advancements in chemistry, medical science, and materials chemistry,these novel polydentate carbon chain chelates might have important influences in these fields due to their facile synthesis, highstability, and unique properties.

1. INTRODUCTION

Metal−carbon bonds are one of the most important features oforganometallic chemistry.1−4 Many species that contain metal−carbon bonds have extensive applications in chemistry, materialsscience, and biology. Therefore, the central focus of organo-metallic chemistry is investigating the synthesis and reactivityof species with metal−carbon bonds.On the other hand, carbon atoms also play an important role

in coordination chemistry. Metal alkyls, metal alkenyls, andmetal alkynyls are typical representatives of compounds withcarbon-based ligands. However, polydentate chelates with metal−carbon bonds are rare.5 For example, in pincer or pincer-type

ligands, the carbon atoms in the polydentate chelates act asspectators since the coordinating atoms in these species aremainly heteroatoms.6 Even N-heterocyclic carbene ligands(two-electron donors) in polydentate pincer complexes areanalogous to heteroatoms.7

In 2013, we reported the first metallapentalyne framework,which can be viewed as a 7 carbon (7C) chain coordinated to ametal via three metal−carbon bonds.8 With the extension ofthe carbon chain, a series of novel metal bridgehead polycyclic

Received: April 20, 2018Published: June 21, 2018

Article

pubs.acs.org/accountsCite This: Acc. Chem. Res. 2018, 51, 1691−1700

© 2018 American Chemical Society 1691 DOI: 10.1021/acs.accounts.8b00175Acc. Chem. Res. 2018, 51, 1691−1700

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frameworks (from a 7C framework to a 12C framework) wereconstructed (Scheme 1).9−13 Therefore, the term carbolong

chemistry was used to describe the chemistry of planar conju-gated systems featured with a long carbon chain (≥7C) coor-dinated to a metal via at least three metal−carbon bonds.Carbolong complexes exhibit high reactivities in solution butexcellent thermodynamic stability in the solid state. In addi-tion, some of them show distinctive properties, which enabletheir potential applications in materials science and biomedicine.We will summarize our recent findings on the synthesis andstructural analysis of carbolong complexes with increasinglylong carbon chains.

2. CARBOLONG FRAMEWORK WITH ASEVEN-CARBON CHAIN (7C FRAMEWORK):METALLAPENTALYNE

The first carbolong complex with 7C, metallapentalyne 2a,was synthesized by the reaction of complex 114 with methylpropiolate at room temperature (RT) for only 5 min with ahigh isolated yield (Scheme 2).8 Osmapentalynes 2b and 2c

were also synthesized under similar conditions. Although theosmapentalynes contain a metal−carbon triple bond in a five-membered ring (5MR), they exhibit excellent thermal stability.The solid-state structure of 2a was characterized by X-ray

single-crystal diffraction and exhibits several features (Figure 1).

First, the osmium center is located at the bridgehead position,and the fused 5MRs in 2a are almost coplanar. Second, theC−C bond lengths in the fused 5MRs (1.377−1.402 Å) areclose to those of benzene (1.396 Å), indicating the delocalizedstructure. Third, the molecular structure of 2a contains anOsC triple bond in a 5MR and the smallest bond angle of129.5° ever observed for a carbyne carbon. Correspondingly,the length of this triple bond (1.845 Å) in the 5MR is slightlylonger than those of the OsC triple bonds in the six-membered

ring (6MR) of osmabenzyne.15−18 Structurally, the frameworkof a metallapentalyne can be viewed as a 7-carbon chain (C1−C7)coordinated to a metal center via three metal−carbon bonds.Interestingly, upon treatment of osmapentalynes 2 with

HBF4·H2O at RT, the OsC triple bond shifts position, andosmapentalynes 3 were formed (Scheme 3).8 A single-crystal

X-ray study (Figure 2) of 3a revealed that the bond length(1.808 Å) and OsC−C angle (129.3°) were similar to those

of 2a. To gain more insight into the shift in the OsC triplebond, we treated 2a with deuterated acetic acid and obtainedosmapentalyne 3a’, in which deuterium was attached to thecarbyne carbon of 2a (Scheme 3). This result suggests that thecarbyne carbon of 2a is nucleophilic.Both osmapentalynes 2 and 3 are five-membered cyclic

metal−carbyne complexes.19 Another kind of cyclic metal−carbyne complex is metallabenzyne (Chart 1), which contains

a metal−carbon triple bond in a 6MR and was first reportedby Jia and co-workers in 2001.20 Since then, a series of metall-abenzynes have been synthesized.15−18 In addition, the firstheteroatom-containing metallabenzyne, osmapyridyne, wasalso reported in 2012.21

The smaller bond angle around the carbyne carbon leads toa larger ring strain in osmapentalyne. Density functional theory(DFT) calculations were used to estimate this strain. Based onacyclic model complexes, the computed strain energy at thecarbyne carbon in 2a is approximately 24.8 kcal mol−1, which ismuch smaller than that of cyclopentyne (75.0 kcal mol−1) but stilllarger than that of osmabenzyne (9.6 kcal mol−1) (Scheme 4).22

The large ring strain in osmapentalyne seems to contradict itsexceptional stability. As we will discuss below, the remarkable

Scheme 1. Representative Frameworks of CarbolongChemistry

Scheme 2. Synthesis of the First Metallapentalynes

Figure 1. Molecular structure for the cation of 2a.

Scheme 3. Shiftable Metal−Carbyne Triple Bond

Figure 2. Molecular structures for the cation of 3a.

Chart 1. Examples for Six-Membered Cyclic Metal−CarbyneSpecies

Accounts of Chemical Research Article

DOI: 10.1021/acs.accounts.8b00175Acc. Chem. Res. 2018, 51, 1691−1700

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aromaticity in osmapentalyne is the origin of this extraordinarystability.The aromaticity in metallapentalyne was confirmed by both

experimental observations and DFT calculations. The calcu-lated isomerization stabilization energy (ISE) and nucleus-independent chemical shift (NICS)23,24 values confirm thearomaticity in metallapentalyne. Further molecular orbitalanalysis reveals that metallapentalyne is the first planar Craig-type Mobius aromatic species.8 However, pentalyne is one ofthe most typical antiaromatic species in organic chemistry.We found that the metal fragment not only relieves the largering strain present in pentalyne but also transforms the Huckelantiaromaticity in pentalyne to Craig-type Mobius aromaticityin metallapentalyne.Very recently, a new method for synthesizing 7C

osmapentalynes was realized by an aromaticity-driven assemblyof multiyne chains with a commercially available metal com-plex.25 Treatment of triyne ligands with OsCl2(PPh3)3 and PPh3at RT in air, led to the formation of osmapentalynes 4a and 4bin good yield (Scheme 5). This reaction provides a new routefor constructing carbolong complexes.

3. 7C FRAMEWORK: METALLAPENTALENEAs we mentioned above, the metal−carbon triple bond inmetallapentalyne can shift from one 5MR to another. A metall-apentalene with a 16-electron osmium center could be anintermediate.26 Fortunately, this 16-electron osmapentalene,complex 5, was captured by the reaction of osmapentalyne 2aor 3a with excess AlCl3 in wet dichloromethane (Scheme 6).X-ray study revealed that the fused 5MRs in 5 are almostcoplanar (Figure 3). Complex 5 represents a new kind of 7Cframework.According to the 18-electron transition metal rule, metal-

lapentalene 5, whose metal center has only 16 electrons, isprone to convert to the metallapentalyne with an 18-electronosmium center. For instance, when complex 5 was treated withEt2O, osmapentalynes 2a and 3a were formed in a 7:93 ratiobased on 1H NMR spectroscopy. This ratio was rationalized by

DFT computations on a simple model (Scheme 7).26 In addi-tion, the DFT calculations also show that 3a′ is thermody-namically more stable than 2a’ by 0.9 kcal mol−1, which leadsto osmapentalyne 3a being formed as the major product in thisdeprotonation process.The isolation of 16-electron osmapentalene 5 indicates that

the carbyne carbon of metallapentalyne is nucleophilic. Thereaction of osmapentalyne 3a with ICl leads to the formationof the corresponding halogenated osmapentalene 6 in excellentyield (Scheme 8).27 Moreover, 3a also reacted with elementalselenium at 60 °C, leading to the formation of Se-containingosmapentalene 7 in 84% isolated yield.28

Interestingly, the rare ambiphilic reactivity of the metal−carbon triple bond was observed in metallapentalyne. The carbynecarbon in osmapentalyne reacts not only with electrophiles togenerate 16-electron osmapentalenes but also with nucleo-philes to form 18-electron osmapentalenes.26 Nucleophiles suchas methanethiolate anion (CH3S

−) and methanolate anion(CH3O

−) first attack at the carbyne carbon of 2a to formintermediate A, which is followed by replacement of thechlorine ligand on the osmium center with carbon monoxide,leading to the formation of 18-electron osmapentalenes 8a and8b (Scheme 9). The structure of 8a was determined by X-raydiffraction (Figure 4). Both the experimental observations andthe DFT studies confirmed that the 16- and 18-electronosmapentalenes are also planar Mobius aromatic frameworks.26

In addition, the first aza-metallapentalene was alsosynthesized.29 The treatment of osmabenzene 9 with anilineled to the formation of osmapentafulvene 10 in high isolatedyield (Scheme 10). Refluxing 10 with phenylpropynol in thepresence of Ag2O and trimethylamine, yielded the metal-labicyclic product 11, which could easily convert to the aza-osmapentalene 12. The isolation of aza-osmapentaleneprovides a promising route for the synthesis of heteroatom-containing carbolong complexes.

4. CARBONLONG FRAMEWORK WITH ANEIGHT-CARBON CHAIN (8C FRAMEWORK)

The 7C framework was synthesized via the reaction of complex 1with alkyne. In organic chemistry, alkynes (two carbon atoms)

Scheme 4. Ring Strain in Osmapentalyne and Cyclopentyne

Scheme 5. Aromaticity-Driven Method for the 7CMetallapentalynes

Scheme 6. Synthesis of Metallapentalene with 16-ElectronOsmium Center

Figure 3. Molecular structure for the cation of 5.



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and allenes (three carbon atoms) are two important buildingblocks due to their high reactivities. Therefore, we tried toconstruct 8C frameworks via the reaction of complex 1 withallenes.9 As shown in Scheme 11, complex 1 reacted with allen-ylboronic acid pinacol ester, resulting in metallapentalenederivative 13 with a metallacyclopropene unit. It is worth men-tioning that complex 13 was more stable than osmapentalyne2a. This method was also successfully employed to synthesize14a and 14b with various substituents.The molecular structure of complex 13 contains an

osmacyclopropene unit, which is approximately coplanar withthe delocalized fused 5MRs (Figure 5). DFT calculations showthat the 5MRs have π aromaticity, whereas the unsaturatedthree-membered ring (3MR) is dominated by σ aromaticity.

Complexes 13 and 14 can be viewed as eight-carbon chainscoordinated to an osmium center through 4 C atoms.This kind of 8C framework could also synthesized by the

aromaticity-driven method.25 The treatment of multiyne-allenecompounds with OsCl2(PPh3)3 and PPh3 at RT formed the 8Ccomplexes 15a and 15b (Scheme 12). X-ray crystallographyanalyses show that both of these complexes are tetradentatechelates with four metal−carbon bonds.DFT calculations show that the Mulliken charge on the

sp3-hybridized carbon atom (C8 in Figure 5) in complex 13 is−0.61, which suggests that this carbon is nucleophilic.30 There-fore, we studied the reaction of 13 with tetracyanoethylene(TCNE), which has a highly positive Mulliken charge (+2.35) onthe alkene carbon. Interestingly, a 7C framework, osmapentalyne16, was formed (Scheme 13).30 In this reaction, the metal-lacyclopropene unit in 13 was opened, and the transformationfrom osmapentalene to osmapentalyne was realized.

Scheme 7. Energy profiles for the formation of 2a′ and 3a′ via 5′

Scheme 8. Electrophilic Addition of Osmapentalyne 3a

Scheme 9. Synthesis of Metallapentalenes with 18-ElectronOsmium Center

Figure 4. Molecular structure for the cation of 8a.

Scheme 10. Synthesis of Heteroatom-ContainingMetallapentalene

Scheme 11. Synthesis of 8C Framework



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In the presence of trimethylamine (NEt3), a solution ofosmapentalyne 16 changed from yellow to green, and from thesolution, osmapentalyne 17 was isolated. The structures ofboth 16 and 17 were determined by X-ray crystallography(Figure 6).30 The most significant difference between thesetwo complexes is that in 16, the four cyano groups are not con-jugated with the osmapentalyne unit, whereas in 17, all three ofthe cyano groups are conjugated with the osmapentalyne unit.This slight adjustment in the structure leads to significantdifferences in their UV−vis absorption spectra (vide infra).30

The 3MR in 8C species 13 can also react with an alleneand an alkyne, leading to the formation of 7C complexes 18and 19 via the insertion-protonation−deprotonation processes(Scheme 14).30 The relatively slow reaction is consistent withthe smaller Mulliken charges of the internal carbon atoms inallene (+0.56) and alkyne (+1.19). Through these reactions,we realized a new method for synthesizing a 7C framework andachieved the interconversion between 7C and 8C species.

Another method for synthesizing 8C framework is addingone carbon to the 7C framework of metallapentalyne.31 There-fore, the treatment of complex 2a with an equivalent ofcyclohexyl isocyanide led to the formation of 8C complex 20(Scheme 15). Interestingly, a second cyclohexyl isocyanide

group could coordinate to the osmium center to give IN5followed by isocyanide insertion and coordination of a thirdisocyanide to form IN6. The exocyclic imine group of IN6could coordinate to the osmium center, and then aromatiza-tion afforded another kind of 8C complex, 21. Both complexes20 and 21 possess a novel polycyclic framework in which acarbon chain with eight atoms is coordinated to the metalcenter forming only two metallacycles. In addition, complex 21represents the first discovered metallaindene framework withthe metal in the bridge position.

5. CARBOLONG FRAMEWORK WITH A NINE-CARBONCHAIN (9C FRAMEWORK)

If two carbons are added to the 7C framework, a 9C carbolongframework could be synthesized. As shown in Scheme 16, thetreatment of 3a with HCCCOOH or HCCOEt affordedcomplexes 22 and 23, respectively.10 X-ray diffraction showsthat this 9C framework contains a metallapentalene and ametallacyclobutadiene unit (Figure 7). Both pentalene andcyclobutadiene are antiaromatic and unstable in organicchemistry. We found that using one transition metal couldstabilize these two antiaromatic frameworks simultaneously.


Scheme 12. Synthesis of 8C Framework

Scheme 13. Reaction of the 8C Framework 13 with TCNE

Figure 6. Molecular structures for the cation of 16 and 17.

Scheme 14. Reactions of the 8C Framework of 13 with anAllene and an Alkyne

Scheme 15. Synthesis of the 8C Framework



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In addition, this reaction is also the first example of a [2 + 2]cycloaddition of a late transition metal carbyne with alkynes.We further investigated the cyclization of the 7C framework

with alkynes under different conditions. The treatment of 3awith phenylacetylene in the presence of water and oxygen ledto the first α-metallapentalenofuran 24, whereas a lactone-fused metallapentalyne 25 was formed if acid was also present(Scheme 17).32 18O labeling experiments suggested that the

oxygen atoms in both 24 and 25 were derived from water.Interestingly, complex 25 could be used as a precursor for thesynthesis of a model to investigate charge transport, whichrepresents the first example of charge transport employing ametallacycle as a single molecule junction.33

A new kind of β-metallapentalenofuran (i.e., 26) was synthe-sized by the reaction of complex 3a with arene nucleophiles(Scheme 18).34−36 Experimental observations and theoreticalcalculations revealed that the three 5MRs around the osmiumcenter are aromatic. As shown in Figure 8, the molecularstructure of 27 features pentagonal bipyramid geometry with ametal center that is shared by three 5MRs. These reactionsprovide a new possibility for constructing planar metal-bridgedpolycyclic aromatics and even for synthesizing metalla-nanographene.

6. CARBOLONG FRAMEWORK WITH A TEN-CARBONCHAIN (10C FRAMEWORK)

Similar to the construction strategy for the 9C framework,adding two carbon atoms to the 8C framework is a promising

route for constructing a 10C framework. Treatment of 8Ccomplex 13 with 3-butyn-2-one and AgBF4 resulted in 3MR theformation of 10C complex 28 (Scheme 19).11 The 3MR in

13 was expanded to a 5MR in this reaction. Further treatmentof 28 with tert-butyl isocyanide generated 10C species 29. Boththe experimental observations and DFT results confirmed thatthe three fused 5MRs in 28 are aromatic, whereas these ringsare nonaromatic in the 10C framework of 29. This resultsuggests that the ligand on the metal center could change thearomaticity of the three fused 5MRs.The X-ray crystal structure (Figure 9) shows that the

osmium center in 29 is shared by three fused 5MRs that con-stitute a 10-carbon chain coordinated to the metal center.In addition, these three fused 5MRs show good thermal stabilityand planarity, which is in contrast to the nonplanar organicanalogue I′. Complex 29 represents a novel kind of carbolong

Scheme 16. Synthesis of the 9C Carbolong Framework


Scheme 17. Cyclization of the 7C Framework with Alkynes

Scheme 18. Synthesis of a Heterometal Bridgehead-FusedThree 5MR Framework


Scheme 19. Syntheses of the 10C Framework 29



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framework, although related heteroatom-containing species(e.g., complexes 24 and 26) were also synthesized.32

7. CARBOLONG FRAMEWORK WITH ANELEVEN-CARBON CHAIN (11C FRAMEWORK)

We tried to construct an 11C framework by adding two carbonatoms to the 9C framework (e.g., complexes 22 and 23) oradding four carbon atoms (two molecules of alkyne) to the 7Cframework (e.g., complexes 2a and 3a) but were unsuccessful.However, a new 7C complex, osmapentalyne 30, which wasformed by the reaction of complex 1 with dimethyl acetylene-dicarboxylate, could react with two molecules of alkyne togenerate an 11C framework (Scheme 20).12 There are two

features that make complex 30 more reactive than 2a and 3a.First, the bond angle around the carbyne carbon of 30 (only127.9°) is the smallest example reported thus far. Second, theligands on the osmium center are one phosphine and twochlorides, which provide less steric protection of the OsCtriple bond than that in osmapentalynes 2a and 3a reportedpreviously (with two PPh3 and one chlorine ligands).As shown in Scheme 20, osmapentalyne 30 reacts with two

molecules of an electron-donating alkyne, HCC−OEt, togenerate an 11C framework.12 X-ray diffraction of complex 31shows that the 11C framework is nonplanar, and the 6MR unitis extremely distorted (Figure 10). Nevertheless, complex 31can also be viewed as an 11-carbon chain coordinated to anosmium center. Complex 31 can undergo an elimination reac-tion to generate the thermally more stable complex 32 underreflux conditions, which is analogous to the conversion ofmetallabenzene to the cyclopentadienyl complex. However,

under the same conditions, only the elimination product wasisolated from the reaction of 30 with an electron-withdrawingalkyne, such as 3-butyn-2-one.12 These results revealed that theelectron-donating groups play an important role in the stabi-lization of this polydentate carbon chain chelate.37

The [2 + 2] cycloaddition of alkynes with metal carbynecomplexes has been proposed as a key step in alkyne metathe-sis. Thus, far, the cycloaddition reaction of an alkyne with a latetransition metal carbyne complex remains rare. This reactionrepresents the first example of a [2 + 2 + 2] cycloaddition of analkyne with a late transition metal carbyne as well as a cyclic metalcarbyne complex. The reactions shown in Schemes 16 and 20reveal the potential for the catalysis of alkyne metathesis oralkyne polymerization by late transition metal carbyne complexes.

8. CARBOLONG FRAMEWORK WITH ATWELVE-CARBON CHAIN (12C FRAMEWORK)

Using this carbon chain-growing strategy, a 12C frameworkwas constructed by adding four carbon atoms (two moleculesof alkyne) to the 8C framework. As shown in Scheme 21,

complex 13 reacts with two molecules of phenylacetylene orallene in the presence of AgBF4, leading to the formation ofcomplex 33 or 34.13 X-ray structural analysis shows that thecoordination sites in the equatorial plane are all carbon atoms,and therefore, this complex represents the highest number ofcoordinated carbon atoms in a planar species observed thus far(Figure 11). The mechanism of this reaction was further con-firmed by the isolation of a β-agostic Os···H−C(sp3) species inthe reaction of complex 13 with 3-hexyne.38

The 12-carbon chain and the metal center of complex 33 arealmost coplanar. This is the first example of a pentadentatechelate in which all of the binding atoms are carbons. Both theexperimental observations and the calculated NICS and ISEvalues confirm the aromaticity of the 12C framework. Moreimportantly, the 12C species exhibits a broad absorption band


Scheme 20. Synthesis of an 11C Framework 31


Scheme 21. Syntheses of the 12C Framework 33 and 34



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and excellent photothermal properties, which enable theirapplication in photothermal cancer therapy (vide infra).Traditionally, the term “pincer” has been restrictively used

for planar tridentate ligands (I).39 We propose that the analo-gous planar tetradentate (II) and pentadentate (III) chelatescan be included in the family of pincer-type complexes (Chart 2).

Thus, the concept of a pincer complex can be extended fromtridentate to polydentate chelates.

9. PROPERTIES AND APPLICATIONS OF CARBOLONGSPECIES

The properties of the carbolong complexes were also investi-gated.8,10,13,40 We first examined the photoluminescence propertiesof osmapentalyne 2a. As shown in Figure 12, excitation at 440 nm

in the visible region leads to near-infrared (NIR) emission by2a, in which the Stokes shift reaches 320 nm. The long emis-sion lifetimes and large Stokes shifts of osmapentalynes makethese new metalla-aromatics promising novel NIR dyes.In addition, the NIR emission of the osmapentalynes is

enhanced in aggregates in the solid state. For example, addinglarge amounts of water (a poor solvent for 2a) into an ethanolsolution of 2a results in a remarkable enhancement in theemission intensity (Figure 12).41 Consistent with the aboveanalysis, an intense red emission is observed for the crystals of2a (inset of Figure 12).We also examined the ultraviolet−visible absorption spectra

of a series of carbolong complexes. The low-energy absorptionbands of the 7C complexes 2a and 3a are located at 438 and

424 nm, respectively, whereas the maximum absorptions ofosmapentalenes 8a (λmax = 505 nm) and 8b (λmax = 465 nm)have an obvious red-shift.8,26 This result suggests that thesubstituent group on the framework has an important effect onthe absorption spectrum.As we mentioned above, the three conjugated cyano groups

in complex 17 lead to an obvious red-shift in the spectrumcompared with that of complex 16 (Figure 13).30 Specifically,

the maximum absorption of 16 is observed at 452 nm (log ε =3.84 M−1 cm−1, ε: molar extinction coefficient), whereas thelow-energy absorption band of 17 is located at 694 nm (log ε =4.67 M−1 cm−1). The molar extinction coefficient of the low-energy absorption band of 17 is more than three times that ofruthenium complex (N3 dye). The multiple conjugated cyanogroups reduced the HOMO−LUMO gap of complex 17 as theconjugation became larger.Therefore, a larger conjugated framework could also lead to

a red-shift in the UV−vis absorption spectra. As shown inFigure 14, the low-energy absorption bands of the 12C

complexes 33 and 34 are located at 810 and 740 nm, respec-tively; both of these bands are obviously red-shift relative tothose of all the other carbolong frameworks. The NIR absorp-tion spectra of the 12C complexes reveal their potentialapplication in photothermal therapy (PTT).The photothermal effect of 33 was first examined. As shown

in Figure 15, under NIR laser irradiation, the temperature of awater−ethanol solution containing 0.1 mg mL−1 complex 33


Chart 2. Examples of Pincer-Type Frameworks

Figure 12. Photoluminescence of the 7C complex 2a in ethanol/water.

Figure 13. UV−vis absorption spectra of complexes 16 and 17 at RT.

Figure 14. UV−vis-NIR absorption spectra of the 12C complexes 33and 34.



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increased from 28 to 52 °C within 10 min, whereas the tem-perature of the water−ethanol solution without complex remainednearly unchanged.13 Previous researches have shown thatmaintaining the temperature above 50 °C for 4−6 min couldresult in irreversible tumor cell ablation.42 To increase thewater solubility of the carbolong complex, an amphiphilicpolymer (alkyl-PEI2K-PEG2k) was used to load complex 33.The resulting micelles, 33@NPs, showed good biocompati-bility, a photothermal effect equal to that of 33, and low cellcytotoxicity. In addition, a series of organometallic macro-molecules synthesized by the click reaction of an alkyne-containing 12C framework with methoxypolyethylene glycolazides exhibited excellent photothermal properties under 808 nmlaser irradiation.43

Therefore, the in vivo PTT of 33@NPs was further investi-gated using SCC7 tumor-bearing mice after intravenous injec-tion. We found that the tumor volume irreversibly decreased inthe group of mice that were treated with 33@NPs and NIRlaser irradiation, whereas the tumor volumes in the controlgroups increased exponentially (Figure 16). This is the first

example of the use of organometallics for photothermaltherapy. In addition, further studies showed that the 12Ccomplex could also be used for photoacoustic imaging-guidedcancer phototherapy.44

10. SUMMARY AND FUTURE PROSPECTSA series of carbolong frameworks were formally constructedby coordinating an extended carbon chain (from seven to 12carbon atoms) to a metal center. The effective chelation of acarbon chain ligand to a transition metal forms aromatic metalbridgehead polycyclic frameworks. The strong chelation andthe remarkable aromaticity make the carbolong complexes quitestable. This property of carbolong complexes will hopefully

facilitate the rapid development of carbolong chemistry. There-fore, further exploration of carbolong chemistry is still needed.From a synthetic perspective, the coordination of carbon

chains to other transition metals (e.g., Re, Ru, Rh, and Ir) willbe an interesting project, although osmium is the only metalthat has currently been reported. Additionally, the reactivitystudies of these unique species need further investigation,and the isolation of other metalla-aromatics or even metalla-nanographene is also expected.The unique structure leads to novel properties. Metal-

lpentalyne exhibits unique near-infrared photoluminescencewith particularly large Stokes shifts, long lifetimes and anaggregation-enhanced effect. A series of carbolong complexesshow broad absorption bands in the UV−vis-NIR region,which enable their potential applications in materials scienceand biomedicine. Carbolong chemistry not only extends ourperception of the chelating ability of carbon chains but mayalso lead to the formation of novel organometallic frameworksusing carbon chains as ligands.

■ AUTHOR INFORMATIONCorresponding Author

*E-mail: [email protected]

Congqing Zhu: 0000-0003-4722-0484Haiping Xia: 0000-0002-2688-6634Notes

The authors declare no competing financial interest.

Biographies

Congqing Zhu was born in Anhui, China, in 1986. He received hisB.S. degree from Anqing Teachers College in 2008 and his Ph.D. fromXiamen University in 2014 under the supervision of Prof. Haiping Xia.After finishing his iChEM fellow research in collaborative innovationcenter of chemistry for energy materials (iChEM) of China, hecontinued his research as postdoctoral associate in the group ofProf. Richard R. Schrock at MIT. He started his independent career atNanjing University in late 2016. His research interests include rare-earth metals and actinides chemistry.

Haiping Xia was born in Fujian, China, in 1964. He received his B.S.degree in 1983, M.S. in 1986, and Ph.D. in 2002 from XiamenUniversity. He started to work in Xiamen University in 1986, and hewas promoted to associate professor in 1991, and professor in 1999.He received the National Natural Science Funds for DistinguishedYoung Scholar of China in 2009 and the Huang Yao-Zengorganometallic chemistry award of the Chinese Chemical Society in2016. His group’s research interests are focused on carbolong chemistry.

■ ACKNOWLEDGMENTSWe thank all the co-workers who have contributed to thisproject. We gratefully acknowledge the National NaturalScience Foundation of China (Nos. 21332002, 21490573,and U1705254) for their financial support.

■ REFERENCES(1) Crabtree, R. H. The Organometallic Chemistry of the TransitionMetals, 3rd ed.; Wiley: New York, 2001.(2) Spitler, E. L.; Johnson, C. A.; Haley, M. M. Renaissance ofannulene chemistry. Chem. Rev. 2006, 106, 5344−5386.(3) Mindiola, D. J. Oxidatively induced abstraction reactions. Asynthetic approach to low-coordinate, reactive, early-transition metal

Figure 15. Photothermal effects of a water−ethanol solution andsolutions of complex 33 with different concentrations upon irradiationby a 1 W cm−2 laser (808 nm).

Figure 16. Photographs of the SCC7 tumor-bearing mice on 0 and 7days after PTT treatment.



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http://orcid.org/0000-0002-2688-6634


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