曾 慧,陳秀梅,呂 鑫

(廈門大學(xué)化學(xué)化工學(xué)院,固體表面物理化學(xué)國家重點實驗室,福建省理論與計算化學(xué)重點實驗室,福建 廈門361005)

S4N4和Se4N4中的跨空同共軛與同芳香性

曾 慧,陳秀梅,呂 鑫*

(廈門大學(xué)化學(xué)化工學(xué)院,固體表面物理化學(xué)國家重點實驗室,福建省理論與計算化學(xué)重點實驗室,福建 廈門361005)

通過量子化學(xué)計算從理論上證明了具有奇特雙楔形籠狀結(jié)構(gòu)的S4N4和Se4N4分子具有同芳香性,其芳香性源于分立的NXN(X=S,Se)結(jié)構(gòu)組元間極強(qiáng)的跨空同共軛效應(yīng).

雙葉同芳香性;跨空同共軛效應(yīng);電子離域;虛核化學(xué)位移

圖1　具2e同共軛的3,5-雙脫氫環(huán)己烷基正離子1和具6e同共軛的半瞬烯2柯普重排過渡態(tài)2cFig.1 2e-homoconjugation in tris-homocyclopropenyl cation 1 and 6e-homoconjugation in the transition state 2c for cope rearrangement of semibullvalene 2

圖2　1,5-二膦-二硫化四氮分子及其2個V形NSN碎片上pπ軌道所形成的6c-10e跨空同共軛效應(yīng)Fig.2 1,5-diphosphadithiatetrazocines and the 6c-10e through-space homoconjugation formed by the pπorbitals of its V-shaped NSN subunits

1959年,Winstein首次提出同芳香性(homoaromaticity)概念[1],用于描述某些化合物其共軛性被破壞后但仍呈現(xiàn)芳香性的特征.以3,5-雙脫氫環(huán)己烷基正離子1[2]為例(圖1),被3個飽和亞甲基間隔開的3個sp2雜化碳原子的pπ軌道仍能構(gòu)成三中心-雙電子(3c-2e)的跨空同共軛(through-space homoconjugation),滿足(4N+2)休克爾規(guī)則,這種由非連續(xù)環(huán)狀同共軛效應(yīng)產(chǎn)生的芳香性即稱同芳香性,又因該同共軛由3個分立不飽和基團(tuán)構(gòu)成而稱為三葉同芳香性(trishomoaromaticity).此后,人們發(fā)現(xiàn)為數(shù)甚多的有機(jī)物(陽、陰)離子具有同芳香性[3-4];一些有機(jī)周環(huán)反應(yīng)(如狄爾斯-阿爾德反應(yīng)、1,3-偶極環(huán)加成反應(yīng)、柯普重排等)的活化能較低,也可由其過渡態(tài)具有同芳香性來解釋[5-6],例如,半瞬烯2柯普重排過渡態(tài)2c因其六中心-六電子(6c-6e)跨空同共軛而具有雙葉同芳香性(圖1).

值得注意的是,中性穩(wěn)定化合物具有同芳香性的實例極為罕見[3-4,7],理論設(shè)計中性同芳香性化合物則偶見報道[8-13].本課題組之前將同芳香性概念拓展到一系列無機(jī)簇合物離子(如[SeI2+]2、S82+和[I2+]2等)和一些P、N、S雜環(huán)分子[14-16],其中1,5-二膦-二硫化四氮(1,5-diphosphadithiatetrazocine)環(huán)狀分子中2個V形NSN亞單元均具三中心-五電子(3c-5e)π共軛,二者跨空形成六中心-十電子(6c-10e)同共軛體系,滿足(4N+2)休克爾規(guī)則,因而具雙葉同芳香性(圖2)[15-16].注意到具有雙重折疊環(huán)結(jié)構(gòu)(亦稱為雙楔形籠狀結(jié)構(gòu))的X4N4(X=S,Se)分子3和4中均具有類似的V形NXN(X=S,Se)亞單元及其間距[17-18](圖3),由此推測這類分子中也可能存在跨空同共軛和同芳香性,本文中將通過量子化學(xué)計算加以證明.

圖3　S4N4中的2套6c-10e跨空同共軛效應(yīng)Fig.3 Two equivalent sets of 6c-10e throughspace homoconjugation within S4N4

1 結(jié)果與討論

1.1雙楔形籠狀分子X4N4(X=S,Se)的幾何結(jié)構(gòu)與芳香性

X射線晶體結(jié)構(gòu)衍射及電子衍射實驗[17-18]明確了X4N4(X=S,Se)分子具有雙楔形籠狀結(jié)構(gòu)(圖3),分子對稱性屬D2d點群,可看成是八元環(huán)在S原子處對折后形成雙重折疊結(jié)構(gòu).其奇特性在于:1)N原子均為二配位,S—N鍵長均為0.162 nm(Se—N鍵長0.179 nm),且比常規(guī)S—N單鍵(約0.177 nm)短得多(常規(guī)Se—N單鍵長約0.185 nm),說明X—N間存在一定強(qiáng)度的π鍵;2)跨環(huán)S…S間距為0.259 nm (Se…Se間距為0.275 nm),明顯短于相應(yīng)原子間范德華半徑之和,又明顯長于S—S(或Se—Se)單鍵,說明跨環(huán)X…X間并非簡單X—Xσ鍵.因此吸引了大量理論研究[19-23],但未見關(guān)于此類結(jié)構(gòu)芳香性的討論.

借鑒1,5-二膦-二硫化四氮體系中的跨空同共軛模型(圖2),僅考慮S4N4中π類型價層原子軌道和電子,如圖3所示,該分子上半部2個分立的NSN亞單元各含1套3c-5e的π共軛,通過跨空軌道相互作用,組成了1套6c-10e的跨空同共軛體系3a,滿足(4N+2)休克爾規(guī)則,因而可能具有雙葉同芳香性;類似地,該分子下半部也可形成1套6c-10e的跨空同共軛體系3b.因此,S4N4(或Se4N4)可能有兩重雙葉同芳香性.

B3LYP/6-311+G(3df)[24-26]計算優(yōu)化的X4N4(X =S,Se)分子結(jié)構(gòu)如圖4所示,跨環(huán)X…X間距的理論計算預(yù)測值(S…S 0.275 nm,Se…Se 0.289 nm)略長于實驗觀測值,其他結(jié)構(gòu)參數(shù)的理論預(yù)測值與實驗觀測值一致,說明該方法能合理預(yù)測分子內(nèi)跨環(huán)相互作用.結(jié)合GIAO(gauge-independent atomic orbital)方法[27]計算得到分子體心位置(即N4正方形中心)處虛核化學(xué)位移(nucleus-independent chemical shift, NICS)[28-29]值分別為-31.3(S4N4)和-31.5 (Se4N4),證明該類分子確因跨空同共軛效應(yīng)而具有極強(qiáng)芳香性.更為有趣的是,NICS極值點并不在體心位置,而是在距離N4正方形平面中心之上、下0.05 nm處(圖4),這是因為2套交錯的6c-10e跨空同共軛體系正好是以N4平面為分界線.ACID(anisotropy of the current-induced density)分析[30]也表明,該類分子在外磁場作用下可產(chǎn)生極強(qiáng)的抗磁環(huán)流(圖5).

圖4　B3LYP/6-311+G(3df)計算預(yù)測的S4N4、Se4N4、S4N5-和(CH)2S2N42-結(jié)構(gòu)和GIAO-NICS值Fig.4 Geometries and GIAO-NICS values of S4N4, Se4N4,S4N5-and(CH)2S2N42-predicted at the B3LYP/6-311+G(3df)level of theory

圖5　S4N4(D2d)中抗磁環(huán)流的ACID分析Fig.5 Diatropic ring currents within S4N4(D2d)revealed by ACID analysis

本文中還考察了2個模型化合物S4N5-(5)[31]和(CH)2S2N42-(6),它們與1,5-二膦-二硫化四氮分子相似,均包含1套具6c-10e跨空同共軛的(NSN)2結(jié)構(gòu)組件,NICS(0)值分別為-12.2和-19.9,具同芳香性.X4N4分子的電子離域效應(yīng)和芳香性更強(qiáng),導(dǎo)致其N原子相當(dāng)高的負(fù)電荷(0.58左右)以及更短的跨環(huán)X…X(X=S,Se)間距(圖4).

1.2X4N4(X=S,Se)的其他構(gòu)型與芳香性

X4N4分子還可能取船式構(gòu)型[19,32].計算表明,船式構(gòu)型也存在一定程度的跨環(huán)X…X(X=S,Se)相互作用(圖6);S4N4的船式構(gòu)型7與雙楔形籠式構(gòu)型能量接近,Se4N4的船式構(gòu)型8則比雙楔形籠式構(gòu)型能量高53.1 kJ/mol.實際上,已經(jīng)發(fā)現(xiàn)在金屬配合物中存在S4N4船式構(gòu)型[32],但尚無船式構(gòu)型Se4N4的實驗證據(jù).

圖6　B3LYP/6-311+G(3df)計算預(yù)測的X4N4(X=S,Se)船式構(gòu)型和NICS(0)值Fig.6 Geometries and NICS(0)values of the boat conformations of X4N4(X=S,Se) predicted at the B3LYP/6-311+G(3df)level of theory

兩化合物船式構(gòu)型中,對位的X…X間距和N…N間距均明顯長于對應(yīng)的原子范德華半徑和,因此不會有如3b所示的(NXN)2跨空同共軛效應(yīng);但是,船式結(jié)構(gòu)中除2個X—N鍵長接近X—N單鍵外,其余的X—N鍵長均明顯短于X—N單鍵,說明仍存在共軛效應(yīng),其最短的間位X…X間距(最短的間位S…S間距約0.269 nm、Se…Se間距約0.293 nm,圖6)也顯示間位X…X跨空作用的存在.兩化合物船式構(gòu)型的NICS(0)值分別為-6.3和-8.2,呈弱芳香性,可能源于其2個具3c-5eπ共軛的XNX結(jié)構(gòu)組件間較弱的跨空共軛效應(yīng).

X4N4的平面構(gòu)型因有12個π電子而具反芳香性,其單重態(tài)NICS(0)值為+42.6.

2 結(jié) 論

環(huán)狀分子X4N4(X=S,Se)雙重折疊結(jié)構(gòu)的電子離域本質(zhì)存在爭議,更未見關(guān)于其是否具有芳香性的文獻(xiàn)報道.本研究從理論上證明了此類分子中分立的NXN結(jié)構(gòu)組元具有3c-5eπ共軛,可以通過兩兩間的跨空軌道相互作用,形成2套交錯的6c-10e跨空同共軛體系,均滿足(4N+2)休克爾規(guī)則,因而具有極強(qiáng)同芳香性.

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Through-space Homoconjugation and Homoaromaticity in S4N4and Se4N4

ZENG Hui,CHEN Xiumei,LüXin*

(State Key Laboratory of Physical Chemistry of Solid Surface,Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,China)

We show,by means of quantum chemical calculations,that two neutral inorganic compounds S4N4and Se4N4,which have long been well-characterized to have peculiar double-wedge cage structures,are homoaromatic.Their aromaticity can be mainly ascribed to the through-space homoconjugation between the separated(NXN)-subunits(X=S,Se).

bishomoaromaticity;through-space homoconjugation;electron delocalization;nucleus-independent chemical shift

O 64

A

0438-0479(2016)06-0793-04

10.6043/j.issn.0438-0479.201604035

2016-04-18 錄用日期:2016-05-10

國家自然科學(xué)基金(21273177);高等學(xué)校博士學(xué)科點專項科研基金(20130121110004)

xinlu@xmu.edu.cn

曾慧,陳秀梅,呂鑫.S4N4和Se4N4中的跨空同共軛與同芳香性[J].廈門大學(xué)學(xué)報(自然科學(xué)版),2016,55(6): 793-796.

ZENG H,CHEN X M,LüX.Through-space homoconjugation and homoaromaticity in S4N4and Se4N4[J].Journal of Xiamen University(Natural Science),2016,55(6):793-796.(in Chinese)