Reactions between N-Heterocyclic Carbene and Lutetium

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Reactions between N-Heterocyclic Carbene and LutetiumMetallofullerenes: High Regioselectivity Directed by Electronic Effect in Addition to Steric Hindrance Wangqiang Shen, Le Yang, Yongbo Wu, Lipiao Bao, Ying Li, Peng Jin, Hongyun Fang, Yunpeng Xie, and Xing Lu J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02423 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 3, 2019

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The Journal of Organic Chemistry

Reactions

between

Lutetium-Metallofullerenes:

N-Heterocyclic High

Carbene

Regioselectivity

Directed

and by

Electronic Effect in Addition to Steric Hindrance Wangqiang Shen,†,# Le Yang,‡,# Yongbo Wu,†,# Lipiao Bao,† Ying Li,‡ Peng Jin,*,‡ Hongyun Fang,*,† Yunpeng Xie, *,† and Xing Lu*,†

†State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, 430074, China. ‡School of Materials Science and Engineering, Hebei University of Technology, Tianjin, 300130, China.

ABSTRACT: The Lewis acid-base pairing reaction between strained N-heterocyclic carbene (NHC) and endohedral metallofullerenes (EMFs) is an efficient strategy to get stable derivatives in a highly regioselective manner. Herein, we report an in-depth study on the reactions between 3-dimesityl-1H-imidazol-3-ium-2-ide (1) and three different EMFs, namely Lu3N@Ih(7)-C80, Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82, respectively. Only one mono-adduct is obtained for each EMF under certain conditions, demonstrating surprisingly high regioselectivity and exclusive formation of mono-adducts. X-ray results of the derivatives of Lu3N@C80 reveal that an epoxide adduct (2a) with a specific [6,6,6]-carbon atom of the C80 cage singly bonded to the normal carbene center (C2) of the NHC is obtained under ambient condition, whereas pure argon atmosphere gives 2b with an abnormal C5-bonding structure. In contrast, the derivatives of Lu2@C82 (3 and 4) are both normal C2-bonding [5,6,6]-adducts without oxygen addition even though air is involved in the reaction. Our theoretical results confirm that the remarkably high regioselectivity and the quantitative formation of mono-adducts are direct results from the distributions of molecular orbital and electrostatic potential on the cage surfaces in addition to the previously assumed steric hindrance between the fullerene cage and the NHC moiety.

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INTRODUCTION Endohedral metallofullerenes (EMFs) are a collection of hybrid molecules with metallic species encapsulated inside fullerene cages. They have attracted great attention because of their fascinating properties and broad applications in materials science, biomedicine, and energy conversion, etc.1–5 Results show that the chemical properties of EMFs are substantially determined by the nature of the internal metallic clusters, such as the cluster size and even the electronic configuration.3,6–11 For instance, Akasaka and co-workers reported that a Sc2C2 cluster, when encapsulated inside D2d(23)-C84, exerts drastic alternation on the chemical reactivity of the cage carbon atoms.6 Other reports demonstrated that the larger Y3N cluster induces a higher reactivity for the C80 cage than the smaller Sc3N, resulting in the fact that Y3N@C80 undergoes Bingel reaction easily but Sc3N@C80 appears very inert.12,13 Recently, carbene reactions with fullerenes have attracted wide interests for the advantages of getting stable derivatives useful in many areas.1–3 In general, the carbenes when reacting with fullerenes tend to donate their two unshared valence electrons to form [1+2] cyclo-adducts.6,8,14–19 However, Bazan and co-workers broke such a limit by showing that fullerenes such as C60 and C70 can behave as a Lewis base to react with a N-heterocyclic carbene (NHC), and the resultant Lewis acid-base adducts exhibit a C-C single bond linking the NHC group and the fullerene cages.20 They suggested that the bulky side group of NHC hinders the cyclopropane formation. This methodology is also applicable to EMFs in which the cages have already received a certain amount of electrons from the internal metallic species. For instance, a recent report demonstrated that the NHC group has to use its abnormal carbene center to react with Sc3N@Ih(7)-C80 which is much larger than C60 and C70, thus the steric hindrance between NHC and the cage of Sc3N@Ih(7)-C80 plays a vital role in determining the addition pattern.21 To release the steric hindrance, an oxygen atom is introduced and the normal NHC addition product was obtained.22 It thus seems that the addition pattern of Lewis acid-base complexation reaction is determined by the steric hindrance between the fullerene cage and the NHC group. However, a more recent work reported that Sc2C2@C3v(8)-C82, which has an even larger cage than Sc3N@Ih(7)-C80, can react with the same NHC to form a normal [5,6,6]-adduct. In this case, the steric congestion between the C3v(8)-C82 cage and the NHC functionality has been fully overcome.23 Accordingly, the reaction mechanism of Lewis acid-base complexation between NHC and EMFs is still


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controversial. Herein, we designed the Lewis acid-base complexation reactions between a less strained NHC with a small mesityl group and three different lutetium-containing EMFs, namely Lu3N@Ih(7)-C80, Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82. Consistently, only one mono-adduct for each EMF under certain reaction conditions is obtained quantitatively, demonstrating high regioselectivity. X-ray results of the derivatives unambiguously confirm the formation of the corresponding Lewis acid-base pairs. Computational results reveal that the orbital distribution and molecular electrostatic potential (MEP) on the cage carbon atoms synergistically affect the addition pattern of NHC to the EMFs, in addition to the steric hindrance between the carbon cage and the NHC group.

RESULTS AND DISCUSSION In a typical reaction, a toluene solution containing both Lu3N@Ih(7)-C80 and an excess amount (ca. 30-fold) of 3-dimesityl-1H-imidazol-3-ium-2-ide (1) was heated to 60 °C under either ambient or argon atmosphere to get the respective Lewis acid-base pairs, 2a or 2b (Scheme 1a). Figure 1a and 1b show the corresponding high-performance liquid chromatography (HPLC) profiles of the reaction mixture probed at different times. It is clear that only one adduct was formed even when the pristine Lu3N@Ih(7)-C80 almost vanished after 6 hrs. Procedures for the synthesis of the respectve derivative of Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82 (3 and 4) are similar (Scheme 1b).23 From the HPLC (Figure 1c and 1d) monitoring the processes, all the starting materials, Lu2@C3v(8)-C82/Lu2@C2v(9)-C82, have been consumed after 1 h, indicating a higher reactivity than that of Lu3N@Ih(7)-C80. The analytical HPLC chromatograms (Figure S2) of purified of 2a, 2b, 3 and 4 confirm their high purities. Noteworthily, there are no bis- or multi-adducts detected in all the reactions, proving that only one mono-adduct was afforded for each EMF with 100% regioselectivity and 100% conversion yield. The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra of the derivatives (2a, 2b, 3 and 4) are shown in Figure S1, which clearly indicates their identities as the mono-adducts of the corresponding EMFs. The Vis-NIR absorption spectra of 2a, 2b, 3 and 4 all differ significantly from those of the corresponding parent EMFs, confirming the markedly altered electronic properties of these EMFs after adding the NHC moiety (Figure S3). Moreover,



the nuclear magnetic resonance (NMR) technique has presented great success in confirming the cage symmetry of empty fullerenes. However, as for EMFs and their derivatives, the crucial information about the exact location of the internal metals could not be obtained by NMR spectroscopy. Accordingly, single-crystal X-ray diffraction (XRD) crystallography is currently the most reliable solution to determine the exact molecular structures for the EMFs.

Scheme 1. The reaction between (a) Lu3N@Ih(7)-C80 and (b) Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82 and 1.

Fortunately, the molecular structures of these four compounds under study (2a, 2b, 3 and 4) are all unambiguously determined by single-crystal XRD crystallography. The crystallographic data of 2a, 2b, 3 and 4 are shown in Tables S1. In detail, 2a crystallizes in the orthorhombic Pbca space group, which contains one molecule of Lu3N@Ih(7)-C80O-NHC and one carbon disulfide (CS2) molecule. Inside the cage, the Lu ions show some degree of disorder. In detail, up to 10 Lu sites with occupancies varying from 0.082 to 1.000 (fully ordered) are distinguished for the three Lu atoms. Figure 2a shows the molecular structure of 2a with the major metal sites (Lu1 of 1.000 occupancy, Lu2 of 0.471 occupancy and Lu3 of 0.384 occupancy) inside the cage. Noteworthily, it is unambiguous that an oxygen atom together with an NHC moiety attached to the Ih(7)-C80 cage. Specifically, a [6,6,6]-carbon atom (C1) is selected to link with the normal carbene center


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C2 of the NHC via a C-C single bond with a length of 1.532 Å (C1-C2). Moreover, our X-ray results confirm that the cage bond between C1 and a neighboring [5,6,6]-junction carbon atom (C6) is broken with an oxygen atom bridging them. Now, the distance between C1 and C6 is 2.355 Å, suggesting the formation of the corresponding open epoxy structure.

Figure 1. HPLC profiles of the reaction mixtures containing (a) Lu3N@Ih(7)-C80 under ambient condition, (b) Lu3N@Ih(7)-C80 under argon condition, (c) Lu2@C3v(8)-C82 under ambient condition and (d) Lu2@C2v(9)-C82 under ambient condition and 1 probed at different reaction times. Conditions: Buckyprep column (ø = 4.6 × 250 mm); eluent = toluene, flow rate = 1.0 mL/min; detection wavelength = 330 nm.

The crystal system of 2b belongs to the monoclinic space group P2(1)/c, which contains one molecule of 2b and one hexane molecule. Inside the fullerene cage, 15 Lu disordered sites of the three Lu atoms are found with occupancies varying from 0.010 to 0.725. The molecular structure of 2b with the major metal sites (Lu1 of 0.725 occupancy, Lu2 of 0.645 occupancy and Lu3 of 0.392 occupancy) is shown in Figure 2b. It is clear that the abnormal carbene center C5 of NHC is singly bonded to a specific [6,6,6]-carbon atom (C1) of the Ih(7)-C80 cage with a bond length of 1.514 Å (C5-C1). Nevertheless, although the NHC used in this study possesses a smaller mesityl group than the previously reported one, the additional patterns and molecular structures of 2a and



2b resemble those of Sc3N@Ih(7)-C80,21,22 revealing that the regioselectivity of the Lewis acid-base complexation reaction is not altered by reducing the steric hindrance or changing the internal cluster. The crystal system of 3 falls into the orthorhombic Pnma space group and the asymmetric unit contains two halves of the C3v(8)-C82 cage. A complete C3v(8)-C82 cage is generated by combining one-half of the cage with the mirror image of the other, both having an occupancy value of 0.50. Inside the cage, the two Lu atoms prefer a motional behavior, resulting in up to 17 disordered sites with occupancies varying from 0.015 to 0.209. Figure 2c depicts the molecular structure of 3 with the major metal sites (Lu1 of 0.152 occupancy and Lu2 of 0.209 occupancy) inside the cage. It is clear that a specific [5,6,6]-carbon atom (C1) of the C3v(8)-C82 cage is singly bonded to the normal carbene site C2 of NHC via a C-C single bond with a length of 1.53 Å. Noteworthily, among the 17 kinds of nonequivalent cage carbon atoms of Lu2@C3v(8)-C82, only one has been involved in the reaction, demonstrating high regioselectivity.

Figure 2. Ortep drawings of (a) 2a, (b) 2b, (c) 3 and (d) 4 with thermal ellipsoids set at the 10 % probability level. Only one cage orientation and the major metal sites are shown. Solvent molecules, the minor metal sites and H atoms are omitted for clarity.


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The crystal system of 4 falls into the monoclinic C2/m space group, which also contains two halves of the fullerene cage and a symmetry-related NHC group. Inside the cage, up to 10 disordered sites of the two Lu atoms are found with occupancies varying from 0.041 to 0.422. The structure of 2b with the major metal sites (Lu1 of 0.422 occupancy and Lu2 of 0.392 occupancy) is shown in Figure 2d. The addition also occurs at a [5,6,6]-junction carbon (C1) out of the 24 different types of nonequivalent cage carbon atoms of Lu2@C2v(9)-C82. The length of the new bond is 1.527 Å, corresponding to a C-C single bond. It should be mentioned that our crystallographic results have unambiguously assigned this new Lu2C82 isomer as Lu2@C2v(9)-C82 for the first time. Noteworthily, although the reaction between Lu2@C3v(8)-C82/Lu2@C2v(9)-C82 and 1 was conducted under ambient atmosphere, no epoxide derivatives have been detected. The above crystallographic results suggest that the steric hindrance between the fullerene cage and the NHC functionality is not a decisive effect for the Lewis acid-base complexation reaction between the NHC and EMFs. Thus, density functional theory (DFT) calculations are carried out to rationalize the high regioselectivity and preferential formation of mono-adducts. First, the geometries of 2a, 2b, 3 and 4 obtained at the M06-2X/6-31G*~SDD level of theory perfectly reproduce the corresponding X-ray structures (Figure S5). The calculated bridge bond lengths of 2a, 2b, 3 and 4 are 1.539, 1.513, 1.527 and 1.532 Å , with Wiberg bond index (electron occupancy) values of 0.94 (1.97 e), 0.98 (1.97 e), 0.97 (1.97 e) and 0.96 (1.96 e), respectively. These results clearly confirm that a C-C single bond is formed between the NHC moiety and the respective fullerene cage. For the formation mechanism of 2a, our previous study has demonstrated that the oxygen in the solution was first activated by the NHC, and the reaction involves the formation of an intermediate product Sc3N@Ih(7)-C80O.22 Similarly, as for the reaction between Lu3N@Ih(7)-C80 and 1 in this study, our computational results for the same formation pathway also indicate that all the related reactions are extremely exothermic (Scheme S1 and Figure S6). Moreover, the high reactivity of these lutetium-containing EMFs towards 1 was investigated by using the frontier molecular orbital theory. A high reactivity of the neutralization reaction can be expected if the energy difference between the lowest unoccupied molecular orbital (LUMO) of a Lewis acid and the highest occupied molecular orbital (HOMO) of a Lewis base is small, since the effective orbital interaction lead to a large stabilizing energy. Indeed, as clearly shown in



Figure 3, our DFT calculations indicate that Lu3N@Ih(7)-C80O, Lu3N@Ih(7)-C80, Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82 all have low LUMO energy levels (-2.42 eV, -2.27 eV, -2.86 eV and -3.31 eV, respectively), implying their high electron-accepting abilities and high reactivity towards 1.

Figure 3. Orbital diagrams of C2-bound NHC, C5-bound NHC, Lu3N@Ih(7)-C80O, Lu3N@Ih(7)-C80, Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82.

The high regioselectivity is further interpreted by considering the LUMO and MEP distributions of these endohedrals. For all these lutetium-containing EMFs under study, the addition sites (C1 in Figure 4) all possess the largest LUMO distributions, which ensure that they can behave as good Lewis acids to accept the electron from the Lewis base 1. In addition, the MEP distribution suggest that the reaction sites have rather positive values and thus favorable for the nucleophilic attack of 1 (Figure 4). The abnormal C5 bonding in 2b can be understood by the MEP plots as well. Figure 4 shows that Lu3N@Ih(7)-C80O bears a large region of positive electrostatic potential around the addition site to interact perfectly with the negative parts of the normal C2-bound NHC. In contrast, Lu3N@Ih(7)-C80 exhibits a large area of zero and even negative electrostatic potential around the addition site where the LUMO is abundant, thus the alternative C5-binding was chosen in order to reduce the electrostatic repulsion between the fullerene cage and the NHC moiety. As for Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82, they both bear large regions of positive electrostatic potential around the additional sites, which ensure they can interact effectively with the negative parts of the normal C2-bound NHC.


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The quantitative formation of pure mono-adducts is also noteworthy in the Lewis acid-base complexation reactions without the formation of any bis- or multi-adducts. Figure S7 reveals that the bulky NHC group cover all the cage carbons featuring both large LUMO distribution and positive MEP values, and thus strongly prevent further additions due to the steric repulsion.

Figure 4. Molecular electrostatic potential (MEP) distributions of the C2/C5-binding NHCs (two views), Lu3N@Ih(7)-C80O, Lu3N@Ih(7)-C80, Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82 with the schematic projections of the NHCs on the cage surfaces in their corresponding mono-adducts. For each endohedral, the five cage carbons with the largest LUMO distribution (%) are marked by numbers.

CONCLUSIONS In summary, we have investigated the reactions between an NHC with a small mesityl group and

three

different

lutetium-containing

EMFs

(Lu3N@Ih(7)-C80,

Lu2@C3v(8)-C82 and

Lu2@C2v(9)-C82). Only one mono-adduct is obtained for each EMF under specific reaction



conditions, demonstrating surprisingly high regioselectivity. Crystallographic results of the derivatives (2a, 2b, 3 and 4) unambiguously reveal the formation of a C-C single bond between the NHC group and a specific cage carbon atom of the corresponding EMF. Theoretical results confirm that such a high regioselectivity and preferential formation for mono-adducts are mainly determined by the electronic effect (molecular orbitals and electrostatic interactions) of these fullerene cages in addition to the steric clash between the NHC and EMFs. Our current work provides new insights into the reaction mechanism between NHC and EMFs, which may be applied in the future for the quantitative synthesis of fullerenes/EMF derivatives with applications in fields such as catalysis and materials science.

EXPERIMENTAL SECTION General Reagents and Instruments. Lu3N@Ih(7)-C80, Lu2@C3v(8)-C82 and Lu2@C2v(9)-C82 were synthesized with a direct-current arc discharge method and were isolated with high-performance liquid chromatography (HPLC). All other solvents and chemicals are commercially available and were used as received. The reaction processes were monitored with an analytical HPLC (CTO-16, Shimadzu Co. Ltd.) equipped with an analytical Buckyprep column (ø = 4.6 × 250 mm, Cosmosil Nacalai Tesque). The separations were conducted on a preparative HPLC machine (LC-908; Japan Analytical Industry Co. Ltd.) equipped with a preparative Buckyprep column (ø = 20 × 250 mm, Cosmosil Nacalai Tesque) with toluene as the eluent. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was measured on a Bruker BIFLEX Ⅲ

spectrometer (Bruker Daltonics, Germany) using

1,1,4,4-tetraphenyl- 1,3-butadiene (TPB) as matrix. Vis-NIR absorption spectra were recorded on a LAMBDA 750 UV/Vis/NIR spectrophotometer (PerkinElmer, US) in CS2. Attempts to record the 1H NMR and

13C

NMR spectra of 2a, 2b, 3 and 4 all failed due to the limited amounts of

samples and the poor solubility in the common organic solvent. Theoretical Calculations. DFT calculations were conducted with the Gaussian 09 package24 using the M06-2X functional with the 6-31G* basis set for non-metal atoms and SDD basis set and corresponding effective core potential for Lu atoms. 25–27 Synthesis of 2a/2b. 2.0 μmol of Lu3N@Ih(7)-C80 and 65.9 μmol of NHC (1) were dissolved in 20 mL anhydrous toluene. The solution was heated to 60 °C under ambient/argon conditions. After


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stirring for 1 h, a new peak was observed at 13.6/21.3 min. The reaction was terminated after stirring for 6 h when the peak at 32.8 min corresponding to pristine Lu3N@Ih(7)-C80 almost disappeared. The reaction mixture was concentrated and subjected to further HPLC separations. m/z MALDI-TOF-MS Calcd for 2a (Lu3C101H24ON3) 1819.014; Found 1820.356 [Lu3C101H24ON3 + H]+. m/z MALDI-TOF-MS Calcd for 2b (Lu3C101H24N3) 1803.019; Found 1804.228 [Lu3C101H24N3 + H]+. Synthesis of 3/4. 1.5 μmol of Lu2@C3v(8)-C82/Lu2@C2v(9)-C82 and 49.4 μmol of NHC (1) were dissolved in 20 mL anhydrous toluene. The solution was stirred at ambient conditions. After stirring for 0.5 h, a new peak was observed at 17.3/19.8 min. The reaction was terminated after stirring

for

1

h

when

the

peak

at

38.9/38.1

min

corresponding

to

pristine

Lu2@C3v(8)-C82/Lu2@C2v(9)-C82 disappeared. The reaction mixture was concentrated and subjected to further HPLC separations. m/z MALDI-TOF-MS Calcd for 3 (Lu2C103H24N2) 1838.056; Found 1839.431 [Lu2C103H24N2 + H]+. m/z MALDI-TOF-MS Calcd for 4 (Lu2C103H24N2) 1838.056; Found 1804.382 [Lu2C103H24N2 + H]+. Single crystal XRD measurements of 2a, 2b, 3 and 4. Crystalline blocks of 2a, 2b, 3 and 4 were obtained by layering anhydrous n-hexane over a nearly saturated solution of the respective derivative in CS2 in a glass tube. Over a 20-day period, the two solutions diffused together and black crystals formed. XRD measurements were performed at 173 K on a Bruker D8 QUEST machine equipped with a CMOS camera (Bruker AXS Inc., Germany). The multiscan method was used for absorption corrections. The structures were solved by direct method and refined with SHELXL-2014/7.28 Crystal data for 2a: black blocks, 0.20  0.10  0.10 mm, orthorhombic, space group Pbca, a = 25.423(3) Å, b = 15.6051(15) Å, c = 28.953(3) Å, V = 11487(2) Å 3, Fw = 1896.27, λ = 0.71703 Å, Z = 8, Dcalc = 2.193 Mgm-3, μ = 5.261 mm-1, T = 173 K; 126856 reflections, 10357 unique reflections; 8968 with I >2σ(I); R1 = 0.0789 [I >2s(I)], wR2 = 0.1716 (all data), GOF (on F2) = 1.151. The maximum residual electron density is 1.80 eÅ -3. CCDC number: 1574703. Crystal data for 2b: black blocks, 0.15  0.10  0.08 mm, monoclinic, space group P21/c, a = 20.132(5) Å, b = 20.735(5) Å, c = 14.696(5) Å, β = 90.598(5), V = 6134(3) Å 3, Fw = 1890.31, λ = 0.71073 Å, Z = 4, Dcalc = 2.047 Mgm-3, μ = 4.859 mm-1, T = 173 K; 38043 reflections, 11239 unique reflections; 7879 with I >2σ(I); R1 = 0.0631 [I >2s(I)], wR2 = 0.1705 (all data), GOF (on



F2) = 1.019. The maximum residual electron density is 1.42 eÅ -3. CCDC number: 1574704. Crystal data for 3: black blocks, 0.20  0.10  0.10 mm, orthorhombic, space group Pnma, a = 21.244(3) Å, b = 19.830(3) Å, c = 14.957(2) Å, V = 6301(15) Å 3, Fw = 3648., λ = 0.71073 Å, Z = 4, Dcalc = 1.969 Mgm-3, μ = 3.385 mm-1, T = 173 K; 62938 reflections, 5968 unique reflections; 5123 with I >2σ(I); R1 = 0.1581 [I >2s(I)], wR2 = 0.3810 (all data), GOF (on F2) = 1.088. The maximum residual electron density is 1.46 eÅ -3. CCDC number: 1509533. Crystal data for 4: black blocks, 0.20  0.15  0.10 mm, monoclinic, space group C2/m, a = 26.199(7) Å, b = 11.220(3) Å, c = 20.553(5) Å, β = 107.986(7), V = 5746(2) Å 3, Fw = 3442, λ = 0.71069 Å, Z = 1, Dcalc = 2.039 Mgm-3, μ = 3.064 mm-1, T = 173 K; 30080 reflections, 5548 unique reflections; 4796 with I >2σ(I); R1 = 0.1301 [I >2s(I)], wR2 = 0.3021 (all data), GOF (on F2) = 1.115. The maximum residual electron density is 2.10 eÅ -3. CCDC number: 1509532.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acg.org. Additional crystal data for 2a (CIF) Additional crystal data for 2b (CIF) Additional crystal data for 3 (CIF) Additional crystal data for 4 (CIF) MALDI-TOF mass spectra, Vis-NIR spectra and optimized geometries of 2a, 2b, 3, 4, plausible formation mechanism of 2a, energy profiles for the possible formation mechanism of 2a and molecular electrostatic potential distributions of 2a, 2b, 3 and 4.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions # Equal contribution. Notes


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The authors declare no competing financial interests.

ACKNOWLEDGMENTS Financial support from NSFC (Nos. 51472095, 51672093, 51602112, 51602097 and 21103224) and Program for Changjiang Scholars and Innovative Research Team in University (IRT1014) is gratefully acknowledged. We thank the Analytical and Testing Center in Huazhong University of Science and Technology for all related measurements.

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Table of Contents


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Reactions between N-Heterocyclic Carbene and Lutetium

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