Carbolong Chemistry: A Story of Carbon Chain Ligands and Transition

Article Cite This: Acc. Chem. Res. 2018, 51, 1691−1700

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Carbolong Chemistry: A Story of Carbon Chain Ligands and Transition Metals Congqing Zhu†,‡ and Haiping Xia*,† †

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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 and Chemical 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 be viewed as a 7-carbon (7C) chain coordinated to a metal via three metal−carbon bonds, was first reported in 2013. Although metallapentalyne contains a metal−carbon triple bond in a five-membered ring (5MR) and the bond angle around the carbyne carbon is only 129.5°, metallapentalyne exhibits excellent stability to air, moisture, and heat. Metallapentalyne possesses the rare planar Möbius aromaticity, which is in sharp contrast to the Hückel antiaromaticity in pentalyne. The metal fragment not only relieves the large ring strain present in pentalyne but also results in the transformation of the antiaromaticity in pentalyne to aromaticity in metallapentalyne. With the extension of the carbon chain from 7 to 12 carbon atoms, a series of novel polycyclic frameworks were constructed via the 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 the formation of 16- and 18-electron metallapentalenes; (2) σ aromaticity was first observed in an unsaturated system in the 8C framework; (3) two classical antiaromatic frameworks, cyclobutadiene and pentalene, were simultaneously stabilized in the 9C framework 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 12C framework; and (7) sharing of the transition metal by multiple aromatic units has seldom been observed in the metallaaromatics. 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 the discovery 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, high stability, and unique properties.

1. INTRODUCTION Metal−carbon bonds are one of the most important features of organometallic chemistry.1−4 Many species that contain metal− carbon bonds have extensive applications in chemistry, materials science, and biology. Therefore, the central focus of organometallic chemistry is investigating the synthesis and reactivity of species with metal−carbon bonds. On the other hand, carbon atoms also play an important role in coordination chemistry. Metal alkyls, metal alkenyls, and metal alkynyls are typical representatives of compounds with carbon-based ligands. However, polydentate chelates with metal− carbon bonds are rare.5 For example, in pincer or pincer-type © 2018 American Chemical Society

ligands, the carbon atoms in the polydentate chelates act as spectators since the coordinating atoms in these species are mainly heteroatoms.6 Even N-heterocyclic carbene ligands (two-electron donors) in polydentate pincer complexes are analogous to heteroatoms.7 In 2013, we reported the first metallapentalyne framework, which can be viewed as a 7 carbon (7C) chain coordinated to a metal via three metal−carbon bonds.8 With the extension of the carbon chain, a series of novel metal bridgehead polycyclic Received: April 20, 2018 Published: June 21, 2018 1691

DOI: 10.1021/acs.accounts.8b00175 Acc. Chem. Res. 2018, 51, 1691−1700

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Accounts of Chemical Research

ring (6MR) of osmabenzyne.15−18 Structurally, the framework of 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 OsC triple bond shifts position, and osmapentalynes 3 were formed (Scheme 3).8 A single-crystal

frameworks (from a 7C framework to a 12C framework) were constructed (Scheme 1).9−13 Therefore, the term carbolong Scheme 1. Representative Frameworks of Carbolong Chemistry

Scheme 3. Shiftable Metal−Carbyne Triple Bond

chemistry was used to describe the chemistry of planar conjugated systems featured with a long carbon chain (≥7C) coordinated to a metal via at least three metal−carbon bonds. Carbolong complexes exhibit high reactivities in solution but excellent thermodynamic stability in the solid state. In addition, some of them show distinctive properties, which enable their potential applications in materials science and biomedicine. We will summarize our recent findings on the synthesis and structural analysis of carbolong complexes with increasingly long carbon chains.

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

2. CARBOLONG FRAMEWORK WITH A SEVEN-CARBON CHAIN (7C FRAMEWORK): METALLAPENTALYNE The first carbolong complex with 7C, metallapentalyne 2a, was synthesized by the reaction of complex 114 with methyl propiolate at room temperature (RT) for only 5 min with a high isolated yield (Scheme 2).8 Osmapentalynes 2b and 2c Figure 2. Molecular structures for the cation of 3a.

Scheme 2. Synthesis of the First Metallapentalynes

of 2a. To gain more insight into the shift in the OsC triple bond, we treated 2a with deuterated acetic acid and obtained osmapentalyne 3a’, in which deuterium was attached to the carbyne carbon of 2a (Scheme 3). This result suggests that the carbyne 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

were also synthesized under similar conditions. Although the osmapentalynes contain a metal−carbon triple bond in a fivemembered 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).

Chart 1. Examples for Six-Membered Cyclic Metal−Carbyne Species

a metal−carbon triple bond in a 6MR and was first reported by Jia and co-workers in 2001.20 Since then, a series of metallabenzynes have been synthesized.15−18 In addition, the first heteroatom-containing metallabenzyne, osmapyridyne, was also reported in 2012.21 The smaller bond angle around the carbyne carbon leads to a larger ring strain in osmapentalyne. Density functional theory (DFT) calculations were used to estimate this strain. Based on acyclic model complexes, the computed strain energy at the carbyne carbon in 2a is approximately 24.8 kcal mol−1, which is much smaller than that of cyclopentyne (75.0 kcal mol−1) but still larger than that of osmabenzyne (9.6 kcal mol−1) (Scheme 4).22 The large ring strain in osmapentalyne seems to contradict its exceptional stability. As we will discuss below, the remarkable

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

First, the osmium center is located at the bridgehead position, and the fused 5MRs in 2a are almost coplanar. Second, the C−C bond lengths in the fused 5MRs (1.377−1.402 Å) are close to those of benzene (1.396 Å), indicating the delocalized structure. Third, the molecular structure of 2a contains an OsC triple bond in a 5MR and the smallest bond angle of 129.5° ever observed for a carbyne carbon. Correspondingly, the length of this triple bond (1.845 Å) in the 5MR is slightly longer than those of the OsC triple bonds in the six-membered 1692


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Accounts of Chemical Research Scheme 4. Ring Strain in Osmapentalyne and Cyclopentyne

Scheme 6. Synthesis of Metallapentalene with 16-Electron Osmium Center

aromaticity in osmapentalyne is the origin of this extraordinary stability. The aromaticity in metallapentalyne was confirmed by both experimental observations and DFT calculations. The calculated isomerization stabilization energy (ISE) and nucleusindependent chemical shift (NICS)23,24 values confirm the aromaticity in metallapentalyne. Further molecular orbital analysis reveals that metallapentalyne is the first planar Craigtype Möbius aromatic species.8 However, pentalyne is one of the most typical antiaromatic species in organic chemistry. We found that the metal fragment not only relieves the large ring strain present in pentalyne but also transforms the Hückel antiaromaticity in pentalyne to Craig-type Möbius aromaticity in metallapentalyne. Very recently, a new method for synthesizing 7C osmapentalynes was realized by an aromaticity-driven assembly of multiyne chains with a commercially available metal complex.25 Treatment of triyne ligands with OsCl2(PPh3)3 and PPh3 at RT in air, led to the formation of osmapentalynes 4a and 4b in good yield (Scheme 5). This reaction provides a new route for constructing carbolong complexes.

Figure 3. Molecular structure for the cation of 5.

DFT computations on a simple model (Scheme 7).26 In addition, the DFT calculations also show that 3a′ is thermodynamically more stable than 2a’ by 0.9 kcal mol−1, which leads to osmapentalyne 3a being formed as the major product in this deprotonation process. The isolation of 16-electron osmapentalene 5 indicates that the carbyne carbon of metallapentalyne is nucleophilic. The reaction of osmapentalyne 3a with ICl leads to the formation of the corresponding halogenated osmapentalene 6 in excellent yield (Scheme 8).27 Moreover, 3a also reacted with elemental selenium at 60 °C, leading to the formation of Se-containing osmapentalene 7 in 84% isolated yield.28 Interestingly, the rare ambiphilic reactivity of the metal− carbon triple bond was observed in metallapentalyne. The carbyne carbon in osmapentalyne reacts not only with electrophiles to generate 16-electron osmapentalenes but also with nucleophiles to form 18-electron osmapentalenes.26 Nucleophiles such as methanethiolate anion (CH3S−) and methanolate anion (CH3O−) first attack at the carbyne carbon of 2a to form intermediate A, which is followed by replacement of the chlorine ligand on the osmium center with carbon monoxide, leading to the formation of 18-electron osmapentalenes 8a and 8b (Scheme 9). The structure of 8a was determined by X-ray diffraction (Figure 4). Both the experimental observations and the DFT studies confirmed that the 16- and 18-electron osmapentalenes are also planar Möbius aromatic frameworks.26 In addition, the first aza-metallapentalene was also synthesized.29 The treatment of osmabenzene 9 with aniline led to the formation of osmapentafulvene 10 in high isolated yield (Scheme 10). Refluxing 10 with phenylpropynol in the presence of Ag2O and trimethylamine, yielded the metallabicyclic product 11, which could easily convert to the azaosmapentalene 12. The isolation of aza-osmapentalene provides a promising route for the synthesis of heteroatomcontaining carbolong complexes.

Scheme 5. Aromaticity-Driven Method for the 7C Metallapentalynes

3. 7C FRAMEWORK: METALLAPENTALENE As we mentioned above, the metal−carbon triple bond in metallapentalyne can shift from one 5MR to another. A metallapentalene with a 16-electron osmium center could be an intermediate.26 Fortunately, this 16-electron osmapentalene, complex 5, was captured by the reaction of osmapentalyne 2a or 3a with excess AlCl3 in wet dichloromethane (Scheme 6). X-ray study revealed that the fused 5MRs in 5 are almost coplanar (Figure 3). Complex 5 represents a new kind of 7C framework. According to the 18-electron transition metal rule, metallapentalene 5, whose metal center has only 16 electrons, is prone to convert to the metallapentalyne with an 18-electron osmium center. For instance, when complex 5 was treated with Et2O, osmapentalynes 2a and 3a were formed in a 7:93 ratio based on 1H NMR spectroscopy. This ratio was rationalized by

4. CARBONLONG FRAMEWORK WITH AN EIGHT-CARBON CHAIN (8C FRAMEWORK) The 7C framework was synthesized via the reaction of complex 1 with alkyne. In organic chemistry, alkynes (two carbon atoms) 1693


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

Scheme 8. Electrophilic Addition of Osmapentalyne 3a

Scheme 10. Synthesis of Heteroatom-Containing Metallapentalene

Scheme 9. Synthesis of Metallapentalenes with 18-Electron Osmium Center

Scheme 11. Synthesis of 8C Framework

Complexes 13 and 14 can be viewed as eight-carbon chains coordinated 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-allene compounds with OsCl2(PPh3)3 and PPh3 at RT formed the 8C complexes 15a and 15b (Scheme 12). X-ray crystallography analyses show that both of these complexes are tetradentate chelates 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 Therefore, we studied the reaction of 13 with tetracyanoethylene (TCNE), which has a highly positive Mulliken charge (+2.35) on the alkene carbon. Interestingly, a 7C framework, osmapentalyne 16, was formed (Scheme 13).30 In this reaction, the metallacyclopropene unit in 13 was opened, and the transformation from osmapentalene to osmapentalyne was realized.

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

and allenes (three carbon atoms) are two important building blocks due to their high reactivities. Therefore, we tried to construct 8C frameworks via the reaction of complex 1 with allenes.9 As shown in Scheme 11, complex 1 reacted with allenylboronic acid pinacol ester, resulting in metallapentalene derivative 13 with a metallacyclopropene unit. It is worth mentioning that complex 13 was more stable than osmapentalyne 2a. This method was also successfully employed to synthesize 14a and 14b with various substituents. The molecular structure of complex 13 contains an osmacyclopropene unit, which is approximately coplanar with the delocalized fused 5MRs (Figure 5). DFT calculations show that the 5MRs have π aromaticity, whereas the unsaturated three-membered ring (3MR) is dominated by σ aromaticity. 1694


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

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


Scheme 12. Synthesis of 8C Framework

Scheme 15. Synthesis of the 8C Framework

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

group could coordinate to the osmium center to give IN5 followed by isocyanide insertion and coordination of a third isocyanide to form IN6. The exocyclic imine group of IN6 could coordinate to the osmium center, and then aromatization afforded another kind of 8C complex, 21. Both complexes 20 and 21 possess a novel polycyclic framework in which a carbon chain with eight atoms is coordinated to the metal center forming only two metallacycles. In addition, complex 21 represents the first discovered metallaindene framework with the metal in the bridge position.

In the presence of trimethylamine (NEt3), a solution of osmapentalyne 16 changed from yellow to green, and from the solution, osmapentalyne 17 was isolated. The structures of both 16 and 17 were determined by X-ray crystallography (Figure 6).30 The most significant difference between these two complexes is that in 16, the four cyano groups are not conjugated with the osmapentalyne unit, whereas in 17, all three of the cyano groups are conjugated with the osmapentalyne unit. This slight adjustment in the structure leads to significant differences in their UV−vis absorption spectra (vide infra).30 The 3MR in 8C species 13 can also react with an allene and an alkyne, leading to the formation of 7C complexes 18 and 19 via the insertion-protonation−deprotonation processes (Scheme 14).30 The relatively slow reaction is consistent with the smaller Mulliken charges of the internal carbon atoms in allene (+0.56) and alkyne (+1.19). Through these reactions, we realized a new method for synthesizing a 7C framework and achieved the interconversion between 7C and 8C species.

5. CARBOLONG FRAMEWORK WITH A NINE-CARBON CHAIN (9C FRAMEWORK) If two carbons are added to the 7C framework, a 9C carbolong framework could be synthesized. As shown in Scheme 16, the treatment of 3a with HCCCOOH or HCCOEt afforded complexes 22 and 23, respectively.10 X-ray diffraction shows that this 9C framework contains a metallapentalene and a metallacyclobutadiene unit (Figure 7). Both pentalene and cyclobutadiene are antiaromatic and unstable in organic chemistry. We found that using one transition metal could stabilize these two antiaromatic frameworks simultaneously.

Figure 6. Molecular structures for the cation of 16 and 17. 1695


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Accounts of Chemical Research Scheme 16. Synthesis of the 9C Carbolong Framework

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


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 3a with phenylacetylene in the presence of water and oxygen led to the first α-metallapentalenofuran 24, whereas a lactonefused metallapentalyne 25 was formed if acid was also present (Scheme 17).32 18O labeling experiments suggested that the


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

Scheme 17. Cyclization of the 7C Framework with Alkynes Scheme 19. Syntheses of the 10C Framework 29

oxygen atoms in both 24 and 25 were derived from water. Interestingly, complex 25 could be used as a precursor for the synthesis of a model to investigate charge transport, which represents the first example of charge transport employing a metallacycle as a single molecule junction.33 A new kind of β-metallapentalenofuran (i.e., 26) was synthesized by the reaction of complex 3a with arene nucleophiles (Scheme 18).34−36 Experimental observations and theoretical calculations revealed that the three 5MRs around the osmium center are aromatic. As shown in Figure 8, the molecular structure of 27 features pentagonal bipyramid geometry with a metal center that is shared by three 5MRs. These reactions provide a new possibility for constructing planar metal-bridged polycyclic aromatics and even for synthesizing metallananographene.

13 was expanded to a 5MR in this reaction. Further treatment of 28 with tert-butyl isocyanide generated 10C species 29. Both the experimental observations and DFT results confirmed that the three fused 5MRs in 28 are aromatic, whereas these rings are nonaromatic in the 10C framework of 29. This result suggests that the ligand on the metal center could change the aromaticity 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 constitute a 10-carbon chain coordinated to the metal center. In addition, these three fused 5MRs show good thermal stability and planarity, which is in contrast to the nonplanar organic analogue I′. Complex 29 represents a novel kind of carbolong

6. CARBOLONG FRAMEWORK WITH A TEN-CARBON CHAIN (10C FRAMEWORK) Similar to the construction strategy for the 9C framework, adding two carbon atoms to the 8C framework is a promising 1696


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under the same conditions, only the elimination product was isolated from the reaction of 30 with an electron-withdrawing alkyne, such as 3-butyn-2-one.12 These results revealed that the electron-donating groups play an important role in the stabilization of this polydentate carbon chain chelate.37 The [2 + 2] cycloaddition of alkynes with metal carbyne complexes has been proposed as a key step in alkyne metathesis. Thus, far, the cycloaddition reaction of an alkyne with a late transition metal carbyne complex remains rare. This reaction represents the first example of a [2 + 2 + 2] cycloaddition of an alkyne with a late transition metal carbyne as well as a cyclic metal carbyne complex. The reactions shown in Schemes 16 and 20 reveal the potential for the catalysis of alkyne metathesis or alkyne polymerization by late transition metal carbyne complexes.


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

7. CARBOLONG FRAMEWORK WITH AN ELEVEN-CARBON CHAIN (11C FRAMEWORK) We tried to construct an 11C framework by adding two carbon atoms to the 9C framework (e.g., complexes 22 and 23) or adding four carbon atoms (two molecules of alkyne) to the 7C framework (e.g., complexes 2a and 3a) but were unsuccessful. However, a new 7C complex, osmapentalyne 30, which was formed by the reaction of complex 1 with dimethyl acetylenedicarboxylate, could react with two molecules of alkyne to generate an 11C framework (Scheme 20).12 There are two

8. CARBOLONG FRAMEWORK WITH A TWELVE-CARBON CHAIN (12C FRAMEWORK) Using this carbon chain-growing strategy, a 12C framework was constructed by adding four carbon atoms (two molecules of alkyne) to the 8C framework. As shown in Scheme 21,

Scheme 20. Synthesis of an 11C Framework 31

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

features that make complex 30 more reactive than 2a and 3a. First, the bond angle around the carbyne carbon of 30 (only 127.9°) is the smallest example reported thus far. Second, the ligands on the osmium center are one phosphine and two chlorides, which provide less steric protection of the OsC triple bond than that in osmapentalynes 2a and 3a reported previously (with two PPh3 and one chlorine ligands). As shown in Scheme 20, osmapentalyne 30 reacts with two molecules of an electron-donating alkyne, HCC−OEt, to generate an 11C framework.12 X-ray diffraction of complex 31 shows that the 11C framework is nonplanar, and the 6MR unit is extremely distorted (Figure 10). Nevertheless, complex 31 can also be viewed as an 11-carbon chain coordinated to an osmium center. Complex 31 can undergo an elimination reaction to generate the thermally more stable complex 32 under reflux conditions, which is analogous to the conversion of metallabenzene to the cyclopentadienyl complex. However,

complex 13 reacts with two molecules of phenylacetylene or allene in the presence of AgBF4, leading to the formation of complex 33 or 34.13 X-ray structural analysis shows that the coordination sites in the equatorial plane are all carbon atoms, and therefore, this complex represents the highest number of coordinated carbon atoms in a planar species observed thus far (Figure 11). The mechanism of this reaction was further confirmed by the isolation of a β-agostic Os···H−C(sp3) species in the reaction of complex 13 with 3-hexyne.38 The 12-carbon chain and the metal center of complex 33 are almost coplanar. This is the first example of a pentadentate chelate in which all of the binding atoms are carbons. Both the experimental observations and the calculated NICS and ISE values confirm the aromaticity of the 12C framework. More importantly, the 12C species exhibits a broad absorption band 1697


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424 nm, respectively, whereas the maximum absorptions of osmapentalenes 8a (λmax = 505 nm) and 8b (λmax = 465 nm) have an obvious red-shift.8,26 This result suggests that the substituent group on the framework has an important effect on the absorption spectrum. As we mentioned above, the three conjugated cyano groups in complex 17 lead to an obvious red-shift in the spectrum compared with that of complex 16 (Figure 13).30 Specifically,


and excellent photothermal properties, which enable their application in photothermal cancer therapy (vide infra). Traditionally, the term “pincer” has been restrictively used for planar tridentate ligands (I).39 We propose that the analogous planar tetradentate (II) and pentadentate (III) chelates can be included in the family of pincer-type complexes (Chart 2). Chart 2. Examples of Pincer-Type Frameworks

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

the maximum absorption of 16 is observed at 452 nm (log ε = 3.84 M−1 cm−1, ε: molar extinction coefficient), whereas the low-energy absorption band of 17 is located at 694 nm (log ε = 4.67 M−1 cm−1). The molar extinction coefficient of the lowenergy absorption band of 17 is more than three times that of ruthenium complex (N3 dye). The multiple conjugated cyano groups reduced the HOMO−LUMO gap of complex 17 as the conjugation became larger. Therefore, a larger conjugated framework could also lead to a red-shift in the UV−vis absorption spectra. As shown in Figure 14, the low-energy absorption bands of the 12C

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

9. PROPERTIES AND APPLICATIONS OF CARBOLONG SPECIES The properties of the carbolong complexes were also investigated.8,10,13,40 We first examined the photoluminescence properties of osmapentalyne 2a. As shown in Figure 12, excitation at 440 nm

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

in the visible region leads to near-infrared (NIR) emission by 2a, in which the Stokes shift reaches 320 nm. The long emission lifetimes and large Stokes shifts of osmapentalynes make these 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, adding large amounts of water (a poor solvent for 2a) into an ethanol solution of 2a results in a remarkable enhancement in the emission intensity (Figure 12).41 Consistent with the above analysis, an intense red emission is observed for the crystals of 2a (inset of Figure 12). We also examined the ultraviolet−visible absorption spectra of a series of carbolong complexes. The low-energy absorption bands of the 7C complexes 2a and 3a are located at 438 and

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

complexes 33 and 34 are located at 810 and 740 nm, respectively; both of these bands are obviously red-shift relative to those of all the other carbolong frameworks. The NIR absorption spectra of the 12C complexes reveal their potential application in photothermal therapy (PTT). The photothermal effect of 33 was first examined. As shown in Figure 15, under NIR laser irradiation, the temperature of a water−ethanol solution containing 0.1 mg mL−1 complex 33 1698


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facilitate the rapid development of carbolong chemistry. Therefore, 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) will be an interesting project, although osmium is the only metal that has currently been reported. Additionally, the reactivity studies of these unique species need further investigation, and the isolation of other metalla-aromatics or even metallananographene is also expected. The unique structure leads to novel properties. Metallpentalyne exhibits unique near-infrared photoluminescence with particularly large Stokes shifts, long lifetimes and an aggregation-enhanced effect. A series of carbolong complexes show broad absorption bands in the UV−vis-NIR region, which enable their potential applications in materials science and biomedicine. Carbolong chemistry not only extends our perception of the chelating ability of carbon chains but may also lead to the formation of novel organometallic frameworks using carbon chains as ligands.

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

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increased from 28 to 52 °C within 10 min, whereas the temperature of the water−ethanol solution without complex remained nearly unchanged.13 Previous researches have shown that maintaining the temperature above 50 °C for 4−6 min could result in irreversible tumor cell ablation.42 To increase the water solubility of the carbolong complex, an amphiphilic polymer (alkyl-PEI2K-PEG2k) was used to load complex 33. The resulting micelles, 33@NPs, showed good biocompatibility, a photothermal effect equal to that of 33, and low cell cytotoxicity. In addition, a series of organometallic macromolecules synthesized by the click reaction of an alkynecontaining 12C framework with methoxypolyethylene glycol azides exhibited excellent photothermal properties under 808 nm laser irradiation.43 Therefore, the in vivo PTT of 33@NPs was further investigated using SCC7 tumor-bearing mice after intravenous injection. We found that the tumor volume irreversibly decreased in the group of mice that were treated with 33@NPs and NIR laser irradiation, whereas the tumor volumes in the control groups increased exponentially (Figure 16). This is the first

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

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

The authors declare no competing financial interest. Biographies Congqing Zhu was born in Anhui, China, in 1986. He received his B.S. degree from Anqing Teachers College in 2008 and his Ph.D. from Xiamen University in 2014 under the supervision of Prof. Haiping Xia. After finishing his iChEM fellow research in collaborative innovation center of chemistry for energy materials (iChEM) of China, he continued his research as postdoctoral associate in the group of Prof. Richard R. Schrock at MIT. He started his independent career at Nanjing University in late 2016. His research interests include rareearth 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 Xiamen University. He started to work in Xiamen University in 1986, and he was promoted to associate professor in 1991, and professor in 1999. He received the National Natural Science Funds for Distinguished Young Scholar of China in 2009 and the Huang Yao-Zeng organometallic chemistry award of the Chinese Chemical Society in 2016. His group’s research interests are focused on carbolong chemistry.

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

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example of the use of organometallics for photothermal therapy. In addition, further studies showed that the 12C complex could also be used for photoacoustic imaging-guided cancer phototherapy.44

ACKNOWLEDGMENTS We thank all the co-workers who have contributed to this project. We gratefully acknowledge the National Natural Science Foundation of China (Nos. 21332002, 21490573, and U1705254) for their financial support.

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10. SUMMARY AND FUTURE PROSPECTS A series of carbolong frameworks were formally constructed by coordinating an extended carbon chain (from seven to 12 carbon atoms) to a metal center. The effective chelation of a carbon chain ligand to a transition metal forms aromatic metal bridgehead polycyclic frameworks. The strong chelation and the remarkable aromaticity make the carbolong complexes quite stable. This property of carbolong complexes will hopefully

REFERENCES

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