徐時清，張 沛，呂延超，李燕麗，楊 華，劉子陽

(中國計量大學(xué) 材料科學(xué)與工程學(xué)院，浙江 杭州 310018)

大碳籠內(nèi)嵌釓氧化物團簇金屬富勒烯Gd2O@C88:實驗和理論研究

徐時清，張 沛，呂延超，李燕麗，楊 華，劉子陽

(中國計量大學(xué) 材料科學(xué)與工程學(xué)院，浙江 杭州 310018)

利用改進的電弧放電技術(shù)合成及多步高效液相色譜(HPLC)分離方法，得到了大碳籠含釓氧化物團簇的金屬富勒烯Gd2O@C88.激光解吸電離碰撞誘導(dǎo)解離質(zhì)譜/質(zhì)譜的研究結(jié)果表明，氧化釓團簇位于碳籠內(nèi)部.密度泛函理論計算結(jié)果表明，金屬富勒烯結(jié)構(gòu)為Gd2O@D2(35)-C88.內(nèi)嵌團簇向碳籠轉(zhuǎn)移4個電子，價態(tài)+4，碳籠-4價，金屬富勒烯的電子結(jié)構(gòu)表示為[Gd2O]4+@[D2(35)-C88]4-.

金屬富勒烯；氧化物團簇；大碳籠；密度泛函理論

1 實驗部分

1.1 金屬富勒烯的電弧放電制備

金屬富勒烯的制備采用自制的電弧放電裝置進行.放電裝置主要由真空水冷腔體、20 mm直徑金屬鎢陰極、5根φ8 mm石墨棒陽極以及可調(diào)節(jié)進料速度步進電機等部分組成.為了得到氧化釓團簇內(nèi)嵌金屬富勒烯，向鉆有6 mm孔的石墨棒中充填Gd2O3、Co3O4和石墨粉混合物，其中Gd/C/Co摩爾比為1∶40∶0.1.在真空條件下，首先對以上石墨陽極進行200 A電流活化5 min.然后，在0.027 MPa的氦氣和氧氣(He和O2體積比9∶1)壓力下，80 A電流條件下放電，制得電弧放電煙炱.

1.2 金屬富勒烯的提取與分離

原始煙炱用鄰二氯苯在超聲波震蕩條件下，于40 ℃重復(fù)提取3次.提取物以氯苯為流動相，經(jīng)過三輪高效液相色譜(HPLC)分離，得到Gd2O@C88.第一輪分離使用Buckyprep-M柱，取8～12 min流出組分，該組分用Buckyprep柱分離，取19.5～22.6 min組分，第三輪使用5PBB柱，取40.1～44.6 min色譜柱，得到純Gd2O@C88.分離使用的色譜柱均為φ10 mm×250 mm規(guī)格，氯苯流動相流速4.5 mL/min，色譜檢測波長450 nm.

1.3 光譜與質(zhì)譜表征

紫外-可見-近紅外光譜由日本島津公司UV-3600，測定波長范圍400～1 600 nm.

質(zhì)譜表征在AB Sciex公司3700 TOF/TOF質(zhì)譜系統(tǒng)上進行.采用激光解吸電離(LDI)方式.在串聯(lián)質(zhì)譜(MS/MS)實驗中，選擇碰撞能量3 keV，碰撞氣體為Ar氣.

1.4 密度泛函理論計算

所有密度泛函理論計算均使用Gaussian09軟件.符合獨立五邊形規(guī)則的C88全部初始構(gòu)型由螺旋算法生成[14].該構(gòu)型經(jīng)過半經(jīng)驗理論優(yōu)化后，作為密度泛函水平優(yōu)化的輸入構(gòu)型.使用B3LYP混合密度泛函對空心富勒烯和金屬富勒烯進行計算，碳采用B3LYP/6-31G(d)基組，Gd采用CEP-31贗勢模型式和贗勢基組.

2 結(jié)果與討論

金屬富勒烯提取物以氯苯為流動相，使用三種不同保留行為的色譜柱，經(jīng)過三輪HPLC分離，得到純度較高的Gd2O@C88異構(gòu)體.圖1顯示了該異構(gòu)體的色譜和質(zhì)譜分析圖.色譜分析中，采用Buckyprep-M色譜柱，甲苯流動相.由圖1可見，色譜流出曲線在21.3～23.6 min呈高斯峰形狀，表明所分離的Gd2O@C88異構(gòu)體純度可能較高.質(zhì)譜分析檢驗中，激光解吸電離飛行時間質(zhì)譜圖上僅出現(xiàn)強度較大的Gd2O@C88分子離子峰，其它雜質(zhì)峰非常小.由質(zhì)譜峰的強度計算可知，分離到的Gd2O@C88純度大于99%.

HPLC條件：色譜柱Buckyprep-M 10×250 mm，甲苯流動相流速4.5 mL/min，檢測波長450 nm.質(zhì)譜條件：激光解吸電離，插圖上圖為展寬的分子離子峰，下圖為Gd2O@C88峰同位素的理論分布圖1 Gd2O@C88異構(gòu)體的HPLC譜圖和激光解吸 電離飛行時間質(zhì)譜圖Figure 1 HPLC profile and LDI-TOF-MS of the purified Gd2O@C88

圖2 Gd2O@C88的激光解吸電離串聯(lián)質(zhì)譜圖Figure 2 Spectrum LDI-MS/MS of Gd2O@C88

Gd2O@C88的紫外-可見-近紅外光譜圖示于圖3.由圖可見，在400～1 600 nm光譜范圍內(nèi)，光譜圖顯示了了一系列特征的譜帶或譜峰.在1 095 nm出現(xiàn)一個非常強的吸收峰，在555 nm處有一弱峰.而在623 nm和671 nm處有兩個中等強度的肩帶，此外，在800和920 nm附近有兩個強度較弱的寬帶.值得指出的是，Gd2O@C88的光譜特征與已報道的基于C88碳籠形成的金屬富勒烯M3N@C88(M=Gd, Tb, Nd等)[17-19],M2@C88(M=Dy, Er)[20-21]and M@C88(M=Ca, Yb)[22-23]沒有任何相似之處，然而與M2C90(M=Er, Dy, Gd)[20-21]有一定的相似性.從圖3的光譜上可以看出，吸收轉(zhuǎn)折點在1 382 nm處，對應(yīng)于光譜帶隙0.89 eV，與區(qū)分富勒烯帶隙大小的1.0 eV的標(biāo)準(zhǔn)相比，Gd2O@C88屬于小帶隙富勒烯，這正是金屬富勒烯的一個特點.然而，其帶隙比具有特殊電化學(xué)可逆性，且?guī)斗浅Ｐ〉腉d3N@C88稍大.

圖3 Gd2O@C88在CS2溶液中的紫外-可見-近紅外光譜圖Figure 3 UV-vis-NIR absorption spectrum of Gd2O@C88 in CS2

表1 最穩(wěn)定的10個C88和的相對能量 (ΔE, 4.186 8×103J/mol)和HOMO-LUMO帶隙(eV)Table 1 Relative energies (ΔE, 4.186 8×103J/mol) and HOMO-LUMO gaps (gap, eV) of the ten most stable C88, computed at the DFT Level

注：1kcal=4.186 8×103J

表2 密度泛函理論計算得到的最穩(wěn)定的5個Gd2O@C88異構(gòu)體的相對能量(ΔE, 4.186 8×103J/mol) 和HOMO-LUMO帶隙(eV)Table 2 Relative energies (ΔE, 4.186 8×103J/mol) andHOMO-LUMO gaps (eV) of Gd2O cluster encapsulatedinside the five most stable C88 tetra-anionscomputed at the DFT Level

注：1 kcal=4.1868×103J

圖4 D2(35)-C88碳籠的中性、負(fù)4價和6價離子的能級 和HOMO-LUMO帶隙圖，計算在B3LYP/6-31G(d)水平Figur 4 Orbital levels and HOMO-LUMO gap scheme of neutral, tetra-, and hexa-anionicD2(35)-C88calculated at DFT B3LYP/6-31G(d) level

密度泛函理論計算結(jié)果顯示，金屬富勒烯Gd2O@D2(35)-C88的整體的對稱性屬于D2點群，與碳籠的D2對稱性保持一致，這種一致性在金屬富勒烯領(lǐng)域尚屬首次.金屬富勒烯的D2對稱性使得Gd2O為線形結(jié)構(gòu)，即Gd-O-Gd鍵角為180°，2個Gd原子位于三個C2軸中的一個軸上，如圖5所見.這種線形結(jié)構(gòu)與已鑒定結(jié)構(gòu)的Sc2O團簇金屬富勒烯完全不同，所有Sc2O單元都形成彎曲結(jié)構(gòu)[7-10].此外，文獻(xiàn)報道的硫化鈧團簇金屬富勒烯Sc2S@Cs(10528)-C72中，Sc2S也是彎曲結(jié)構(gòu).在Gd2O@D2(35)-C88中，Gd位于六元環(huán)下方，到該環(huán)上碳原子的距離分別為2.62、2.67和2.68 ?，比Gd3N@C80中相應(yīng)的距離稍大39.Gd-O鍵長2.07 ?，稍小于常見Gd的有機化合物中Gd-O鍵的長度，這可能由于受到碳籠空間大小的限制所引起的.通過電荷布居分析可知，Gd為正3價，O為負(fù)2價，這樣，金屬富勒烯Gd2O@D2(35)-C88的電子結(jié)構(gòu)可以確定表示為[Gd2O]4+@[D2(35)-C88]4-.

圖5 優(yōu)化的Gd2O@D2(35)-C88兩個垂直方向的結(jié)構(gòu)圖Figure 5 Two orthogonal views of the optimized Gd2O @D2(35)-C88

3 結(jié) 語

通過向石墨陽極引入強氧化劑Co3O4以及在氣相中引入氧氣成分，成功合成并分離得到了大碳籠含釓氧化物團簇的金屬富勒烯.對該樣品的激光解吸電離碰撞誘導(dǎo)解離質(zhì)譜/質(zhì)譜的研究表明，氧化釓團簇位于碳籠內(nèi)部，紫外可見近紅外光譜表明Gd2O@C88具有典型的金屬富勒烯特征，帶隙小于空心富勒烯.密度泛函理論計算結(jié)果顯示，金屬富勒烯采用D2點群對稱性的D2(35)-C88碳籠，金屬富勒烯Gd2O@D2(35)-C88中，Gd2O單元呈線性結(jié)構(gòu)，位于碳籠的一個C2軸上，整個分子也呈D2點群對稱性.內(nèi)嵌團簇向碳籠轉(zhuǎn)移4個電子，價態(tài)+4，金屬富勒烯的電子結(jié)構(gòu)表示為[Gd2O]4+@[D2(35)-C88]4-.

[1] AKASAKA T, NAGASE S. Endofullerenes: A New Family of Carbon Clusters[M]. London: Kluwer Academic Publishers,2005:50-70.

[2] POPOV A A, YANG S, DUNSCH L. Endohedral fullerenes[J]. Chemical Reviews,2013,113(8):5989-6113.

[3] AKASAKA T, WAKAHARA T, NAGASE S, et al. La@C82anion. An unusually stable metallofullerene[J]. Journal of the American Chemical Society,2000,122:9316.

[4] PAVANELLO M, JALBOUT A F, TRZASKOWSKI B. et al. Fullerene as an electron buffer: charge transfer in Li@C60[J]. Chemical Physics Letters,2007,442(4-6):339-343.

[5] CAO B P, SUENAGA K, OKAZAKI T, et al. Production, isolation, and EELS characterization of Ti2@C84dititanium metallofullerenes[J]. Journal of Physical Chemistry,2002,106(36):9295-9298.

[6] STEVENSON S, MACKEY M A, STUART M A, et al. A Distorted tetrahedral metal oxide cluster inside an icosahedral carbon cage. synthesis, isolation, and structural characterization of Sc4((3-O)2@Ih-C80[J]. Journal of the American Chemical Society,2008,130(36):11844-11845.

[7] TANG Q Q, ABELLA L, HAO Y J, et al. Sc2O@C2v(5)-C80: dimetallic oxide cluster inside a C80fullerene cage[J]. Inorganic Chemistry,2015,54(20):9845-9852.

[8] MERCADO B Q, STUART M A, MACKEY M A, et al. Balch, Sc2(μ2-O) trapped in a fullerene cage: the isolation and structural characterization of Sc2(μ2-O)@Cs(6)-C82and the relevance of the thermal and entropic effects in fullerene isomer selection[J]. Journal of the American Chemical Society,2010,132(34):12098-12105.

[9] TANG Q Q, ABELLA L, HAO Y J, et al. Sc2O@C3v(8)-C82: a missing isomer of Sc2O@C82[J]. Inorganic Chemistry,2016,55(4):1926-1933.

[10] YANG T, Y. HAO J, ABELLA L, et al. Sc2O@Td(19151)-C76: hindered cluster motion inside a tetrahedral carbon cage probed by crystallographic and computational studies[J]. Chemistry-A European Journal,2015,21(31):11110-11117.

[11] CHEN N, C. BEAVERS M, MULET-GAS M. Echegoyen, et al. Sc2S@Cs(10528)-C72: a dimetallic sulfide endohedral fullerene with a non isolated pentagon rule cage[J]. Journal of the American Chemical Society,2012,134(18):7851-7860.

[12] ZHAO P, LI M Y, GUO Y J, et al. Single step stone-wales transformation linking two thermodynamically stable Sc2O@C78isomers[J]. Inorganic Chemistry,2016,55(5):2220-2226.

[13] GUO Y J, ZHAO X, ZHAO P, et al. Theoretical insight into Sc2O@C84: interplay between small cluster and large carbon cage[J]. Journal of Physical Chemistry,2015,119(41):10428-10439.

[14] FOWLER P W, MANOLOPOULOS D E. An Atlas of Fullerenes[M]. Oxford: Clarendon Press,1995:30-50.

[15] HETTICH R, LAHAMER A, ZHOU L, et al. Investigation of the fragmentation and oxygen reactivity of endohedral metallofullerenes M@C60[J]. International Journal of Mass Spectrometry,1999,183:335.

[16] WEISS F D, ELKIND J L, O’BRIEN S C, et al. Photophysics of metal complexes of spheroidal carbon shells[J]. Journal of the American Chemical Society，1988,110:4464.

[17] CHAUR M N, MELIN F, ELLIOTT B, et al. Gd3N@C2n(n=40, 42, and 44): remarkably low HOMO-LUMO gap and unusual electrochemical reversibility of Gd3N@C88[J]. Journal of the American Chemical Society,2007,129:14826.

[18] ZUO T M, BEAVERS C M, DUCHAMP J C, et al. Isolation and structural characterization of a family of endohedral fullerenes including the large, chiral cage fullerenes Tb3N@C88and Tb3N@C86as well as the Ihand D5hisomers of Tb3N@C80[J]. Journal of the American Chemical Society,2007,129:2035.

[19] MELIN F, CHAUR M N, ENGMANN S, et al. The large Nd3N@C2n(40 ( n ( 49) cluster fullerene family: preferential templating of a C88cage by a trimetallic nitride cluster[J]. Angewandte Chemie-International Edition,2007,46:9032.

[20] TAGMATARCHIS N, SHINOHARA H. Production, separation, isolation, and spectroscopic study of dysprosium endohedral metallofullerenes[J]. Chemical Materials,2000,12:3222.

[21] TAGMATARCHIS N, ASLANIS E, PRASSIDES K, et al. Mono-, di- and trierbium endohedral metallofullerenes: Production, separation, isolation, and spectroscopic study[J]. Chemical Materials,2001,13:2374.

[22] ZHANG Y, XU J X, HAO C, et al. Synthesis, isolation, spectroscopic and electrochemical characterization of some calcium-containing metallofullerenes[J]. Carbon,2006,44:475.

[23] XU J X, WANG Z Y, SHI Z J, et al. Synthesis, isolation and spectroscopic characterization of Yb-containing high metallofullerenes[J]. Chemical Physics Letters,2005,409:192.

[24] LEE C, YANG W, PARR R G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density[J]. Physical Reviews B,1988.37:785.

[25] SUN G Y. Assigning the major isomers of fullerene C88by theoretical13C NMR spectra[J]. Chemical Physics Letters,2003,367(1-2):26-33.

[26] POPOV A A, DUNSCH L. Structure, stability, and cluster-cage interactions in nitride clusterfullerenes M3N@C2n(M = Sc, Y; 2n=68-98): a density functional theory study[J]. Journal of the American Chemical Society,2007,129(38):11835-11849.

[27] VALENCIA R, RODRIGUEZ-FORTEA A, POBLET J M. Large fullerenes stabilized by encapsulation of metallic clusters[J]. Chemical Communications,2007(40):4161-4163.

[28] DUNSCH L, YANG S. Metal nitride cluster fullerenes: Their current state and future prospects[J]. Small,2007,3(8):1298-1320.

High-carbon cage encasing gadolinium oxide cluster metallofulerene Gd2O@C88: an experimental and theoretical study

XU Shiqing, ZHANG Pei, LU Yanchao, LI Yanli, YANG Hua, LIU Ziyang

(College of Material Science and Technology, China Jiliang University, Hangzhou 310018, China)

The metallofullerene Gd2O@C88with Gd2O cluster encased in the high carbon cage C88was synthesized using arc discharging and isolated to isomer pure by the multi-step HPLC method. Laser desorption ionization collisional induced dissociation MS/MS indicated the Gd2O cluster was located inside the carbon cage. Density functional theory computation suggests that the whole structure can be depicted as Gd2O@D2(35)-C88and a further electronic population representation[Gd2O]4+@[D2(35)-C88]4-, with 4 electrons transferred from the encased cluster to the carbon cage. So the inside cluster shows +4 valent，while the carbon cage -4.

metallofullerene; oxide cluster; high-carbon cage; density functional theory

2096-2835(2016)04-0465-06

10.3969/j.issn.2096-2835.2016.04.019

2016-08-18 《中國計量大學(xué)學(xué)報》網(wǎng)址：zgjl.cbpt.cnki.net

國家自然科學(xué)基金資助項目(No.11274283,21271162),浙江省科技廳國際合作項目資助項目(No.2013C24017)，浙江省自然科學(xué)基金資助項目(No.LR1ZB01001).

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