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Evidence of Oxygen Activation in the Reaction Between an N-heterocyclic Carbene and M3N@Ih(7)-C80 (M = Sc, Lu): An Unexpected Way of Steric Hindrance Release Muqing Chen, Wangqiang Shen, Ping Peng, Lipiao Bao, Shasha Zhao, Yunpeng Xie, Peng Jin, Hongyun Fang, Fang-Fang Li, and Xing Lu J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b03004 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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The Journal of Organic Chemistry

Evidence of Oxygen Activation in the Reaction Between an N-heterocyclic Carbene and M3N@Ih(7)-C80 (M = Sc, Lu): An Unexpected Way of Steric Hindrance Release Muqing Chen,ǁ,†,‡Wangqiang Shen,†,‡ Ping Peng,† Lipiao Bao,† Shasha Zhao,† Yunpeng Xie,† Peng Jin,*,§ Hongyun Fang,*,† Fang-Fang Li,*,† and Xing Lu*,† ǁ

Department of Materials Physics, School of Physics and Mechanical& Electronical Engineering, Hubei University of Education 129 Gaoxin 2nd Rd, Wuhan Hi-Tech Zone, Wuhan 430205 (P.R. China) † 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 (P.R. China) E-mail: [email protected], [email protected], [email protected] §

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China E-mail: [email protected]

Abstract: We herein demonstrate for the first time the unexpected oxygen-involving reaction between M3N@Ih(7)-C80 (M = Sc, Lu) and 1,3-bis(diisopropylphenyl)imidazol-2-ylene (1). By introducing a tiny amount of oxygen into the reaction, unprecedented products (2a for Sc3N@C80 and 3a for Lu3N@C80) with the normal carbene center C2 singly bonded to a triple hexagonal junction (THJ) cage carbon together with an oxygen atom bridging the same THJ carbon atom and a neighboring carbon atom forming an epoxy structure are obtained. In situ mechanism study, in combination with theoretical calculations, reveals that the bond-breaking peroxidation facilitates the formation of the unexpected products 2a and 3a, providing new insights into fullerene chemistry.

INTRODUCTION Lewis acids are a collection of organic species that accept pairs of electrons to form chemical bonds.1 In this regard, fullerenes such as C60 and C70 are potential Lewis acids, in view of their low-lying degenerated lowest-unoccupied molecular orbitals (LUMOs) and strong electron-accepting abilities.2-4 In 2011, Bazan and co-workers 1

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confirmed that C60 and C70 can react with a typical Lewis base, i.e., a bulky N-heterocyclic carbene (NHC), to form the corresponding Lewis acid-base pairs.5,6 Theoretical investigations reveal that the substantial dispersion attraction between the bulky diisopropylphenyl groups of NHC and the fullerene cages play a critical role in the adduct formation.7 These zwitterionic adducts are expected to be useful in the recently emerged “Frustrated Lewis Pair (FLP) chemistry”.8 However, to be used as metal-free catalysts to active small molecules, unquenched acidity and basicity for both Lewis partners have to be achieved to interact with the substrates.9 NICS calculations of the reported Lewis acid-based pairs of C60/C70 suggested that their acidity may not be strong enough to act as the catalysts.5,6 Accordingly, searching for better Lewis acids with stronger electron-accepting ability is important for the construction of FLP with high catalytic activities. As an alternative, endohedral metallofullerenes (EMFs), that is, fullerenes with metallic species encapsulated inside the hollow cavity,10-16 present vast resources as the candidates of carbon-based Lewis acids. However, unlike the electron-deficient empty fullerenes such as C60 and C70,2,3 EMFs possess electron-rich carbon cages ascribed to the electron transfer from the encapsulated metallic species to the fullerene cage.10,11 Therefore, whether EMFs can act as Lewis acids was still unknown until our recent report revealing that the prototypical Sc3N@Ih-C80 can behave as a good Lewis acid to react with a Lewis base, N-heterocyclic carbene (NHC), namely 1,3-bis(diisopropylphenyl)-imidazol-2-ylene (1) to form the corresponding Lewis acid-base pair.17 Interestingly, the abnormal carbene center C5 instead of the conventional carbene center C2 in NHC is singly bonded to a triple-hexagon-junction (THJ) carbon atom on the C80 cage, which is different from the cases of C60/C70 because of the steric hindrance between the bulky NHC moiety and the relatively large C80 cage.5 Herein, we demonstrate that introducing a small amount of oxygen in the reaction mixture induces a totally unexpected addition and facilitates the regioselective formation of the normal C2-bound M3N@Ih-C80 adduct with an additional oxygen atom breaking a cage bond next to the site of Lewis acid-base 2


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complexation. The rather different formation mechanism compared with the previous oxygen-free reaction was discussed based on both experimental and theoretical evidences, revealing an unexpected way for steric hindrance release in fullerene chemistry. RESULTS AND DISCUSSION The reaction between 1 and M3N@Ih-C80 (M = Sc, Lu). Scheme 1. The reaction between 1 and M3N@Ih(7)-C80 (M = Sc, Lu). H N

M3N@Ih -C 80

N

N

N

N O

N

O2 toluene/90oC

M 3N@Ih-C80

M 3N@Ih-C80

1 2a

M = Sc

3a

M = Lu

2b 3b

As shown in Scheme 1, procedures for the synthesis of the Lewis acid-base pairs 2a (M = Sc) and 3a (M = Lu) are similar to those reported previously17 except that the reaction was conducted under ambient atmosphere instead of argon. Typically, a toluene solution containing both M3N@Ih(7)-C80 (M = Sc or Lu) and an excess amount (c.a. 30-fold) of 1 was heated to 90oC (Scheme 1). For clarity, we here just take Sc3N@Ih-C80 as an example and the results relating to Lu3N@Ih-C80 are put in the Supporting Information (Figure S1). The reaction process was monitored with HPLC (Figure. 1). After one hour, two new peaks appeared at 9.3 min and 16.6 min which are ascribed to the products 2a and 2b, respectively. The reaction was completed after 6hs when the peak of Sc3N@Ih(7)-C80 at 35.0 min disappeared completely. 2a and 2b were isolated and purified by preparative HPLC. The retention time of 2b is identical to that of the previously reported NHC-Sc3N@Ih-C80 Lewis acid-base adduct.17 The mass spectrum and UV-Vis-NIR spectrum of 2b further confirmed that it is the same product as the reported one17 (Figure. 2 and Figure. 3). 2a is a new product. No peaks corresponding to bis- or multi-adducts are observed, indicating the high regioselectivity for mono-adduct formation in this reaction.

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Figure 1. HPLC profiles for the reaction mixture containing 1 and Sc3N@Ih(7)-C80 probed at different times. Conditions: Buckyprep column (ø 4.6 mm×250 mm), 1.0 mL min-1 toluene ﬂow and 330 nm detection wavelength.

Figure 2. MALDI-TOF mass spectra of 2a and 2b.

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra of 2a and 2b are shown in Figure. 2. 2a exhibits a clear peak at m/z 1513.4 revealing the addition of an NHC group and an oxygen atom on the Sc3N@Ih(7)-C80 cage, namely, NHC-Sc3N@C80O. The peak at m/z 1124.6 originates from the loss of the NHC moiety to form Sc3N@C80O, and the peak at m/z 1108.6 corresponds to Sc3N@C80 because of the complete detachment of the functional groups. The mass value of 2b at m/z 1498.1 indicates the attachment of the NHC moiety on the

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Sc3N@C80 forming the mono-adduct NHC-Sc3N@C80. The peak at m/z 1108.5 is assigned Sc3N@C80, corresponding to detachment of the NHC moiety from 2a and the NHC moiety and an oxygen atom from 2b. Mass spectra of 3a and 3b also confirmed the formation of NHC-Lu3N@C80O. The UV-Vis-NIR spectra of 2a, 2b, 3a and 3b along with those of pristine Sc3N@Ih(7)-C80 and Lu3N@Ih(7)-C80 are shown in Figure. 3. Sc3N@Ih(7)-C80 shows a weak absorption at 370 nm and Lu3N@Ih(7)-C80 shows two absorption peaks at 408 and 684 nm, respectively. The oxygen-containing products 2a and 3a display featureless curves but the oxygen-absent products 2b and 3b show distinct absorption bands at 450 nm and 497 nm, respectively. These differences indicate that the electronic structures of the pristine EMFs have been disturbed by addition of the electron-donating NHC moiety and the oxygen atom as well.

Figure 3. UV-Vis-NIR spectra of 2a, 2b, 3a, 3b, Sc3N@Ih-C80 and Lu3N@Ih-C80.

The molecular structures of 2a, 2b, 3a and 3b are unambiguously established by single crystal X-ray crystallographic measurements (Figure. 4). The structure of 2b is identical to that reported previously, where the NHC group uses its abnormal carbene center C5 to attach a THJ carbon (C6) of Sc3N@Ih-C80 with a bond length of 1.515 Å.17 The crystal of 2a falls into the orthorhombic Pnma space group which contains one molecule of 2a and two CS2 molecules. It is evident that a NHC moiety together with an oxygen atom attaches to the C80 cage. Significantly, the normal 5



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carbene center C2 instead of the abnormal C5 of NHC as observed in 2b is singly bonded to the cage on a THJ carbon (C6), and an oxygen atom unexpectedly attaches to C6 and a neighboring [5,6,6]-junction cage carbon atom (C7) by breaking the C6–C7 bond on the cage. The C6–C7 distance is 2.344 Å, suggestive of an open epoxy structure. This [6,6]-addition of the oxygen is different from the [5,6]-addition in the reported oxide benzyne derivative of Sc3N@Ih(7)-C80.18 The bond length of C6 (cage carbon)–C2 (carbene carbon) is 1.536 Å which is slightly longer than that in 2b (1.515 Å),17 showing the single bond character and Lewis acid-base pair formation. The molecular structures and addition patterns of 3a and 3b (Figure. 4) are similar to those of 2a and 2b, respectively. In the oxygen-containing product 3a, the normal C2 site of NHC is singly bonded to the Lu3N@Ih-C80 cage at a THJ carbon atom (C6) accompanied with an oxygen atom bridging the same C6 and an another cage carbon atom. However, the abnormal C5-center of NHC is singly bonded to the Lu3N@Ih-C80 in the oxygen-absent product 3b. As can be seen from these structures, although the presence of the oxygen atom in 2a and 3a is unexpected, it is vital for the formation of these normal C2-bound Lewis acid-base adducts that have been anticipated.

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Figure 4. ORTEP drawings of 2a, 2b, 3a and 3b with thermal ellipsoids shown at the 25% probability level. The solvent molecules and the minor disordered sites are omitted for clarity.19

Accordingly, theoretical calculations were conducted to rationalize the formation of the oxygen-bridging adduct 2a. Its optimized geometry which is denoted as the C2-[6,6,6] isomer (Figure. 5) agrees very well with the single crystal structure of 2a. The formation of a single bond between the NHC moiety and Sc3N@C80O was confirmed by the calculated C6-C2 bond length of 1.533 Å (experimental value: 1.536 Å). Natural bond order (NBO) analysis reveals that the corresponding Wiberg bond index and electron occupancy are 0.92 and 1.96 e, respectively. The C6-C2 single bond has a hybrid composition of 0.696 C6 (sp3.32) + 0.718 C2(sp1.52). The total natural population analysis (NPA) charges of the NHC and Sc3N@C80O cage are +0.85 e and -0.85 e, respectively, which are similar with corresponding C60 adduct with +0.84e located on the NHC framework and -0.84e delocalized on the cage,[5] confirming the zwitterionic nature of the adduct. The dipole moment, which points in a direction from the C80 cage to the NHC moiety, is as large as 17.9 D. In sharp contrast to that in 2b, where C6 (NPA charge: -0.10 e) accepts electrons from the NHC moiety, both C6 and C2 in 2a bear positive charges of +0.27 e and +0.54 e, respectively, whereas the charge located on the oxygen atom is -0.55 e. Clearly, the presence of the oxygen atom in the cage framework of 2a has considerably altered the electron distribution.

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Figure 5. Optimized structures of different NHC-Sc3N@Ih-C80O isomers. Their relative energies are shown in parentheses (kcal/mol, M06-2X/6-31G*~ LanL2DZ level).

To further understand the formation of 2a, we calculated the normal C2- and abnormal C5-bonding NHC adducts for each of the two types of addition sites adjacent to the oxygen atom, namely the [5,6,6]-C7 and [6,6,6]-C6 sites, leading to four isomers in total. As shown in Fig. 5, both of the C5-bonding isomers are more than 6 kcal/mol higher in energy than the corresponding C2-ones at the same site. In addition, the two [6,6,6]-adducts are energetically more favorable than the two [5,6,6]-ones by more than 19 kcal/mol. Therefore, the formation of the oxygen-involving C2-[6,6,6] adduct 2a is a thermodynamically favored process.[17] It is thus significant that the inclusion of oxygen in the reaction dramatically changes the addition pattern of the NHC to the carbon cage and leads to the preferential formation of the thermodynamic product 2a.

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Scheme 2. Plausible formation mechanism of 2a O

N

Sc3 N@Ih-C80

N

N

O2

N

N

O

N

Sc 3N@I h-C 80

toluene

1 Sc3 N@I h-C80

O O

1

B

+ N

2a N

O

A

Finally, we propose a plausible mechanism to understand the formation process of 2a (Scheme 2). First, the oxygen in the solution was activated by the NHC carbene (1) to form a ketone product (A) and a dioxirane product (B). This oxidation process was confirmed by Gas Chromatography–Mass Spectrometry (GC–MS) where the presence of A and B were detected with B being the major product (Figure S3 in the Supporting Information). Second, B reacts with Sc3N@Ih-C80 to form the intermediate Sc3N@C80O. This oxidation process can be referred to that of the oxidation of C60 by a diepoxide which has been well documented.20 The oxide Sc3N@C80O further reacts with carbene 1 to form the Lewis acid-base pair NHC-Sc3N@C80O 2a (Scheme 2). The last step was theoretically supported by the fact that the THJ carbon (C6) has both large LUMO distribution and the lowest charge density among the cage carbon atoms of Sc3N@C80O (Figure S5), and is thus very favorable for the nucleophilic NHC attack. In addition, our calculations reveal that the above three reactions (1 + O2 → B; B + Sc3N@C80 → A + Sc3N@C80O; 1 + Sc3N@C80O → 2a) are all downhill in the energy profile by 33.7, 69.2 and 37.7 kcal/mol, respectively(Figure S6), suggesting the great feasibility of the formation pathway. CONCLUSIONS In

summary,

we

have

successfully

obtained

the

unprecedented,

oxygen-containing, C2-bound Lewis acid-base pairs 2a (M = Sc) and 3a (M = Lu) of M3N@Ih-C80 (M = Sc, Lu) by carrying out the reaction between a bulky NHC carbene 1 and the respective endohedrals under ambient atmosphere instead of under argon. Oxygen activation is evidenced by our experimental and theoretical results which further demonstrate that the addition of this special oxygen atom onto the M3N@Ih-C80 cage framework facilitates the regioselective formation of the normal 9



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carbene center C2-bound Lewis acid-base adducts which are not favorable thermodynamically. Addition of this oxygen atom unexpectedly releases the steric hindrance between the bulky NHC moiety and the fullerene cage, providing new insight into fullerene chemistry. EXPERIMENTAL SECTION General Instruments. High-performance liquid chromatography (HPLC) was conducted

with

toluene

as

the

mobile

phase.

Matrix-assisted

laser

desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was measured using 1,1,4,4-tetraphenyl-1,3-butadiene as a matrix. UV-Vis-NIR spectra were obtained in CS2. Theoretical Calculations. Density functional theory calculations were carried out with the Gaussian 09 software package21 at the M06-2X/6-31G*~LanL2DZ level of theory (i.e., we employed the 6-31G* basis set for non-metal elements and the LanL2DZ basis set and corresponding effective core potential for Sc).22-24 Synthesis and Isolation of 2a and 2b. M3N@Ih-C80 (M = Sc) were synthesized with a direct-current arc discharge method and were isolated with HPLC. A mixture of M3N@Ih-C80 (3mg, 2.7µmol for M = Sc)

and 1,3-bis(diisopropyl-phenyl)

imidazol-2-ylene (32.8 mg, 0.82 mmol) dissolved in 20 mL toluene was stirred at 90 °C under ambient atmosphere for 6h. The reaction mixtures were separated using preparative HPLC to obtain adducts 2a, 2b. Single-Crystal X-ray Diffraction Measurements. Single-crystal X-ray data of 2a and 2b were collected at 100 K. Data of 3a and 3b were collected at 173 K. A multi-scan method (SADABS) was used for absorption corrections. The structures were solved with direct method and were refined with SHELXL-2014.25 Single-crystal X-ray results of 2a and 3a. The solvent system for both 2a and 3a is CS2/hexane. The crystals fall into the orthorhombic Pnma space group. Thus, two cage orientations with equal occupancy of 0.50 are presented in the system and the NHC group is symmetrically related by the crystallographic mirror plane. In the final cycles of refinements, the two cage orientations were refined with anisotropic

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thermal parameters. A SIMU 0.02 command was applied to smooth the displacement parameters in the structure of 2a. As for 3a, a EADP command was applied to the displacement parameters. Inside the cage of 2a, there are nine disordered positions of Sc atoms with occupancies varying from 0.19 to 0.50. In 3a, ten Lu positions are found with occupancies varying from 0.18 to 0.49. Single-crystal X-ray results of 2b and 3b. Both were crystallized from CS2/hexane falling in the triclinic P-1 space group. The functionalized cages and the CS2 molecules are fully ordered. There are no disordered positions of Sc atoms in 2b, but nine Lu positions with occupancies varying from 0.20 to 0.57 are found for 3b. In the final cycles of refinements, the two cages of 2b and 3b were refined with anisotropic thermal parameters. Crystal data for 2a: black blocks, 0.07 × 0.07 × 0.02 mm, orthorhombic, space group Pnma, a = 29.2512(13) Å, b = 19.830(3) Å, c = 11.1888(5) Å, V = 6377.1(5) Å3, Fw = 3384, λ = 0.77490 Å, Z = 4, Dcalc = 1.736Mgm-3, μ = 0.642 mm-1, T = 100 K; 115142 reflections, 11881 unique reflections; 8272 with I >2σ(I); R1 = 0.1267 [I >2s(I)], wR2 = 0.3318 (all data), GOF (on F2) = 1.134. The maximum residual electron density is 1.668 eÅ-3. Crystal data for 2b: black blocks, 0.04 × 0.02 × 0.02 mm, triclinic, space group P-1, a = 10.990(19) Å, b = 17.558(3) Å, c = 19.758(4) Å, α = 99.562(3)°, β = 90.938(3), γ = 107.217(3)°, V = 3376.2(10) Å3, Fw = 1714, λ = 0.71073Å, Z = 1, Dcalc = 1.661Mgm-3, μ = 0.514mm-1, T = 90K; 12355 reflections, 12355 unique reflections; 7675 with I >2σ(I); R1 = 0.0784 [I >2s(I)], wR2 = 0.2401 (all data), GOF (on F2) = 1.046. The maximum residual electron density is 1.225 eÅ-3. Crystal data for 3a: black blocks, 0.15 × 0.10 × 0.10 mm, orthorhombic, space group Pnma, a = 29.2512(13) Å, b = 19.830(3) Å, c =11.1888(5) Å, V = 6377.1(5) Å 3, Fw = 3832, λ = 0.70.73 Å, Z = 4, Dcalc = 2.063Mgm-3, μ = 4.743 mm-1, T = 173 K; 65746 reflections, 4676 unique reflections; 3694 with I >2σ(I); R1 = 0.0702 [I >2s(I)], wR2 = 0.1693 (all data), GOF (on F2) = 1.049. The maximum residual electron density is 2.032 eÅ-3.

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Crystal data for 3b: black blocks, 0.20 × 0.10 × 0.10 mm, triclinic, space group P-1, a = 11.028(5) Å, b = 17.708(5) Å, c = 19.684(5) Å, α = 99.143(5)°, β = 106.102(5)°, γ = 106.977(5)°, V = 3410(2) Å3, Fw = 2014, λ = 0.71069 Å, Z = 1, Dcalc = 2.025Mgm-3, μ = 4.527 mm-1, T = 100 K; 40568 reflections, 16488 unique reflections; 13848 with I >2σ(I); R1 = 0.0529 [I >2s(I)], wR2 = 0.1395 (all data), GOF (on F2) = 1.091. The maximum residual electron density is 2.461 eÅ-3. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website. HPLC profiles for the reaction mixture containing 1 and Lu3N@Ih(7)-C80, the mass spectra of 3a and 3b, GC-MS for the reaction between 1 and oxygen; LUMO distribution of Sc3N@C80O with the NHC addition site in 2a; the numerical documentation of

the theoretical calculations. Additional crystal data for 2a NHC-Sc3N@Ih-C80O•2(CS2) (CIF) Additional crystal data for 2b NHC-Sc3N@Ih-C80•3(CS2) (CIF) Additional crystal data for 3a NHC-Lu3N@Ih-C80O•(CS2) (CIF) Additional crystal data for 3b NHC-Lu3N@Ih-C80•2.5(CS2) (CIF)

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] *[email protected] Author Contributions ‡ Equal contribution. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS 12


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Professors M. M. Olmstead and A. L. Balch in UC Davis are gratefully acknowledged for their assistance in single-crystal X-ray measurements. Financial support from The National Thousand Talents Program of China, 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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