于 謙,林 毅,聶 鵬,溫庭斌

(廈門大學(xué)化學(xué)化工學(xué)院,福建廈門361005)

氮雜環(huán)卡賓(N-heterocyclic carbene,NHC)現(xiàn)已成為繼有機(jī)膦配體之后的又一類重要配體[1-5].與傳統(tǒng)的有機(jī)膦配體相比,NHC具有更強(qiáng)的σ-供電子能力,可與過渡金屬形成更穩(wěn)定的金屬-卡賓σ鍵,所生成的配合物熱穩(wěn)定性高，對水和空氣穩(wěn)定;同時,NHC易于修飾,使得其空間和電子性質(zhì)易于調(diào)節(jié),結(jié)構(gòu)更加多元化[1-2,5-7].這些獨(dú)特的優(yōu)勢使得NHC在過渡金屬催化和金屬有機(jī)化學(xué)中得到了廣泛的應(yīng)用[1-4,8-10].而且在許多催化反應(yīng)中,NHC配合物表現(xiàn)出比相應(yīng)的膦配合物更高的催化活性,如鈀催化的偶聯(lián)反應(yīng)[11-12]、釕催化的烯烴復(fù)分解反應(yīng)[13]、銥催化的氫轉(zhuǎn)移反應(yīng)[14]等.不僅如此,NHC配體還可發(fā)生N-側(cè)臂上取代基的C—H鍵活化而生成環(huán)金屬化產(chǎn)物[15-17],這進(jìn)一步豐富了NHC的反應(yīng)性質(zhì)．近年來,隨著一些NHC環(huán)金屬化配合物在催化[17-20]及發(fā)光材料[16,21-22]等領(lǐng)域的應(yīng)用,NHC配體的環(huán)金屬化引起人們越來越大的研究興趣,特別是釕配合物中NHC環(huán)金屬化方面的研究[18-20,23-25].

圖1 已報(bào)道的同時包含Tp和NHC配體的兩個釕配合物的合成[35-36] Fig.1The reported two ruthenium complexes including both Tp and NHC ligands[35-36]

鑒于TpRuCl(PPh3)2配合物在金屬有機(jī)合成和催化反應(yīng)中的重要作用以及NHC配體所表現(xiàn)出的優(yōu)勢,本課題組試圖以TpRuCl(PPh3)2(5)和H2IMes(H)(OBut)(6)為原料,合成一個與多功能用途TpRuCl(PPh3)2類似的NHC配合物TpRuCl(NHC)(PPh3),以期在后續(xù)研究中以其為前體,開展TpRu(NHC)骨架配合物的金屬有機(jī)化學(xué)研究.因此,本研究考察了TpRuCl(PPh3)2和NHC配體H2IMes的反應(yīng),但發(fā)現(xiàn)該反應(yīng)未能分離得到預(yù)期產(chǎn)物TpRuCl(H2IMes)(PPh3) (A),而是生成了H2IMes配體環(huán)金屬化的產(chǎn)物TpRu(H2IMes-H)(PPh3)(7) (其中H2IMes-H表示H2IMes的一個N-側(cè)臂上鄰位的甲基脫除了1個質(zhì)子).分離得到的化合物7在二氯甲烷溶液中加熱可解離PPh3配體,促成H2IMes-H配體進(jìn)一步發(fā)生環(huán)金屬化側(cè)臂上芐基的C—H活化,經(jīng)α-H消除轉(zhuǎn)化為螯合的雙卡賓配體H2IMes-2H,得到類似于化合物4的產(chǎn)物TpRuCl(H2IMes-2H) (8).

1 實(shí)驗(yàn)部分

1.1 儀器與試劑

除另有說明外,所有實(shí)驗(yàn)均在室溫下、氮?dú)?或氬氣)氣氛中采用標(biāo)準(zhǔn)的Schlenk技術(shù)操作.四氫呋喃、乙醚、正己烷、甲苯均為分析純,使用前在氮?dú)鈿夥罩薪?jīng)鈉-二苯甲酮除水后重蒸.二氯甲烷,分析純,用前經(jīng)氫化鈣除水后重蒸.起始原料KTp[37]、TpRuCl(PPh3)2(5)[38]、H2IMes(H)(BF4)[24]和H2IMes(H)(OBut)(6)[24]根據(jù)文獻(xiàn)中的方法合成,其他試劑均從Sigma-Aldrich、ACROS和Alfa-Aesar公司購買.

核磁共振(NMR)譜1H、31P{1H}、13C{1H}在Bruker AV400 (400 MHz)核磁共振儀上測定,1H-NMR和13C{1H}-NMR采用四甲基硅烷(TMS)為內(nèi)標(biāo),31P{1H}-NMR采用85%(質(zhì)量分?jǐn)?shù))H3PO4為內(nèi)標(biāo).如無特別說明,操作溫度為298 K.

X-射線單晶衍射在Oxford CCD衍射儀上收集數(shù)據(jù),采用石墨單色化的Mo Kα射線 (λ=0.071 073 nm),電壓50 kV,電流30 mA.

1.2 化合物的合成

1.2.1 TpRu(H2IMes-H)(PPh3)(7)

將10 mL甲苯加入到RuTpCl(PPh3)2(5)(0.20 g,0.23 mmol)和H2IMes(H)(OBut)(6)(0.30 g,1.00 mmol)的混合物中,80 ℃下攪拌4 h,冷卻后過濾,將濾液減壓濃縮至約1 mL,加入10 mL正已烷,析出黃色固體,用砂芯過濾,得到的固體用3×3 mL正已烷洗滌,真空下抽干,產(chǎn)量0.14 g,產(chǎn)率69%.1H-NMR (CDCl3,400.1 MHz):δ0.95 (s,3H,Me),1.35 (s,3H,Me),1.79 (d(unresolved),2JHH=13 Hz,1H,RuCH2),2.02 (s,3H,Me),2.09 (s,3H,Me),2.58 (s,3H,Me),2.64 (d(unresolved),2JHH=13 Hz,1H,RuCH2),3.46～3.53 (m,2H,NCH2),3.68 (m,1H,NCH2),4.57 (m,1H,NCH2),5.20 (t,3JHH=1.8 Hz,1H,Tp),5.89 (t,3JHH=1.8 Hz,1H,Tp),6.07 (s,1H,m-Ar),6.17 (t,3JHH=1.8 Hz,1H,Tp),6.21 (s,1H,m-Ar),6.32 (s,1H,m-Ar),6.40 (s,1H,m-Ar),7.12～6.72 (m,15H,PPh3),7.36 (s,br,1H,Tp),7.40 (s,br,1H,Tp),7.48 (d,3JHH=1.8 Hz,1H,Tp),7.60 (s,br,1H,Tp),7.61 (d,3JHH=1.8 Hz,1H,Tp),7.71 (s,1H,Tp);31P {1H}-NMR (CDCl3,162.0 MHz)：δ57.4 (s,PPh3);13C{1H}-NMR (CDCl3,100.6 MHz)：δ15.8,18.4 (s,Me),19.0 (d,JPC=7.8 Hz,RuCH2),21.0(s,Me×2),21.3(s,Me),50.5,53.3 (s,NCH2),104.3,104.8,106.1 (s,CH,Tp),126.7～151.3 (m,PPh3and Ar),234.6 (d,JPC=13 Hz,RuCN).元素分析得分子式C48H50BN8PRu,計(jì)算值(%):C 65.38，H 5.72，N 12.71;測定值(%):C 65.08,H 5.64,N 12.62.

將正己烷緩慢注入到化合物7的二氯甲烷溶液的上層,通過緩慢擴(kuò)散得到黃色塊狀晶體.

1.2.2 TpRuCl(H2IMes-2H)(8)

將正己烷緩慢注入到化合物8的二氯甲烷溶液的上層,通過緩慢擴(kuò)散得到綠色塊狀晶體.

1.3 晶體結(jié)構(gòu)的解析

挑選適當(dāng)?shù)幕衔?和8的晶體,在173 K下進(jìn)行X-射線單晶衍射表征.全部強(qiáng)度數(shù)據(jù)均經(jīng)過SADABS吸收校正．晶體結(jié)構(gòu)采用SHELXS-97程序包解析,晶體結(jié)構(gòu)由直接法解出．對全部非氫原子坐標(biāo)及其各向異性熱參數(shù)進(jìn)行全矩陣最小二乘法修正(SHELXS-97程序包)．化合物7晶體結(jié)構(gòu)的不對稱單元中包含1個二氯甲烷溶劑分子．化合物7·CH2Cl2和8的晶體學(xué)數(shù)據(jù)見表1,主要的鍵長及鍵角數(shù)據(jù)列于表2和3.晶體結(jié)構(gòu)數(shù)據(jù)存于英國劍橋數(shù)據(jù)中心(Cambridge Crystallographic Data Centre,CCDC)，編號分別為1543564 (7)和1543565 (8).

2 結(jié)果與討論

2.1 產(chǎn)物的合成及結(jié)構(gòu)表征

NHC配體H2IMes的叔丁氧基咪唑啉前體H2IMes(H)(OBut) (6)在80 ℃下加熱可消除一分子叔丁醇得到NHC配體H2IMes[24],因此,通過TpRuCl(PPh3)3(5)與化合物6以摩爾比1∶4在甲苯溶液中于80 ℃下攪拌反應(yīng)4 h后,經(jīng)處理分離得到黃色固體產(chǎn)物TpRu(H2IMes-H)PPh3(7),產(chǎn)率69% (圖2).如果反應(yīng)中減少化合物6的投量,反應(yīng)進(jìn)行得緩慢且不完全.在考察化合物7的性質(zhì)過程中,發(fā)現(xiàn)分離得到的化合物7在二氯甲烷中回流可轉(zhuǎn)化為新的產(chǎn)物,反應(yīng)10 h后,通過檢測反應(yīng)液的原位31P和1H-NMR譜,表明化合物7已完全反應(yīng),主產(chǎn)物為新生成的金屬卡賓物種,同時有少量未知的副產(chǎn)物生成.經(jīng)過硅膠柱層析分離,得到主產(chǎn)物TpRuCl(H2IMes-2H)(8),產(chǎn)率54% (圖2).

化合物7經(jīng)過X-射線單晶衍射表征,其晶體結(jié)構(gòu)如圖3所示.可以看出,化合物7中H2IMes配體取代了TpRuCl(PPh3)2的一個PPh3配體,并且其中一側(cè)的N-側(cè)臂上的均三甲基苯基中鄰位的甲基發(fā)生環(huán)金屬化形成了一個螯合的NHC配體H2IMes-H,并消除了一分子HCl.化合物7中的Ru金屬中心的配位構(gòu)型為一個扭曲的八面體.Ru1—C10的鍵長為0.200 9(3) nm,是典型的Ru—CNHC的鍵長,Ru1—C19的鍵長為0.212 9(3) nm,是典型的Ru—C單鍵的鍵長,與已報(bào)道的類似結(jié)構(gòu)的化合物的相應(yīng)鍵長十分接近.例如：化合物RuH(IMes-H)(CO)(PPh3)2(IMes=1,3-N,N-雙(2,4,6-三甲基苯基)-咪唑-2-卡賓,IMes-H表示IMes脫除了1個質(zhì)子)中的Ru—CNHC和Ru—Calkyl的鍵長分別為0.207 46(12)和0.214 29(12) nm[24];RuCl(H2IMes-H)(CO)(PCy3)中相應(yīng)的兩個鍵長分別為0.203 7(3)和0.209 7(2) nm[24];RuH(H2IMes-H)(CO)(PPh3)2中分別為0.204 5(2)和0.216 2(2) nm[39].

表1 化合物7·CH2Cl2和8的晶體學(xué)數(shù)據(jù)Tab.1 Crystal data and structure refinement of compounds 7·CH2Cl2 and 8

注：表中()內(nèi)的數(shù)字為最后一位(或兩位)有效數(shù)值的誤差值，下同.

表2 化合物7·CH2Cl2和8的主要鍵長Tab.2 Selected bond lengths of compounds 7·CH2Cl2 and 8

表3 化合物7·CH2Cl2和8的主要鍵角Tab.3 Selected bond angles of compounds 7·CH2Cl2 and 8

圖2 化合物7和8的合成 Fig.2Synthesis of compounds 7 and 8

圖3 化合物7的晶體結(jié)構(gòu) Fig.3Molecular structure of compound 7

化合物7的NMR數(shù)據(jù)與其晶體結(jié)構(gòu)一致,其31P{1H}-NMR譜圖在δ57.4處顯示1個單峰信號.在1H-NMR譜圖中,只觀察到5組甲基的質(zhì)子信號,發(fā)生環(huán)金屬化的亞甲基上的2個質(zhì)子由于所處化學(xué)環(huán)境不一樣而表現(xiàn)出不同的化學(xué)位移,分別在δ1.79和2.64.NHC環(huán)上的2個亞甲基也存在類似情況,4個質(zhì)子信號分別位于δ3.46～3.53 (m,2H,2個質(zhì)子信號重疊),3.68 (m,1H),4.57 (m,1H).在13C{1H}-NMR譜圖中,環(huán)金屬化的亞甲基的碳信號位于δ19.0,NHC配體上的卡賓碳信號位于δ234.6.

圖4 化合物8的晶體結(jié)構(gòu) Fig.4Molecular structure of compound 8

2.2 反應(yīng)機(jī)理

圖5 生成化合物7和8的可能機(jī)理 Fig.5Proposed mechanism for the formation of compounds 7 and 8

上述兩個反應(yīng)中,生成TpRu(H2IMes-H)(PPh3) (7)和TpRuCl(H2IMes-2H) (8)的可能機(jī)理如圖5所示.H2IMes(H)(OBut)(6)在加熱條件下消除叔丁醇，首先產(chǎn)生的H2IMes與TpRuCl(PPh3)2(5)反應(yīng),取代一個PPh3配體,生成中間體TpRuCl(H2IMes)(PPh3) (A).由于反應(yīng)體系中過量的H2IMes具有較強(qiáng)的堿性,可通過堿輔助的金屬化-去質(zhì)子化過程促進(jìn)N-側(cè)臂鄰位上甲基的C—H鍵活化,并消除一分子HCl得到產(chǎn)物7.Hong等[40]報(bào)道了類似的通過體系中解離的PCy3作為堿促進(jìn)的NHC配體N-側(cè)臂芳基在釕中心的C—H鍵活化,而且與PCy3相比，H2IMes具有更強(qiáng)的堿性.本研究的實(shí)驗(yàn)結(jié)果也表明,如果反應(yīng)中減少化合物6的投量,反應(yīng)進(jìn)行得緩慢且不完全.分離得到的化合物7在溶液中加熱發(fā)生可逆地解離PPh3配體產(chǎn)生中間體B,16-電子的中間體B可發(fā)生芐基上可逆的α-H消除,轉(zhuǎn)化為含Ru—H的芐卡賓中間體C,中間體C上的氫配體再和二氯甲烷溶劑發(fā)生H/Cl交換得到最終產(chǎn)物8.已有不少報(bào)道表明一些Ru—H化合物易和二氯甲烷發(fā)生H/Cl交換而轉(zhuǎn)化為Ru—Cl化合物[36,41-42].本研究的實(shí)驗(yàn)過程中也發(fā)現(xiàn),化合物5和6在溶液中加熱反應(yīng)時,即使在二氯甲烷存在下,也可使反應(yīng)停留在產(chǎn)物7而不繼續(xù)轉(zhuǎn)化.不難理解,這是由于反應(yīng)中解離下來的第一個PPh3配體存在于體系中,抑制了化合物7中PPh3配體的繼續(xù)解離,使其難以發(fā)生后續(xù)反應(yīng).另外,分離得到的化合物7在C6D6中即使加熱24 h也未見有明顯反應(yīng),如果往溶液中加入二氯甲烷繼續(xù)加熱,經(jīng)過10 h后即可完全反應(yīng),因此,后續(xù)發(fā)生的H/Cl交換可使溶液中產(chǎn)生的中間體C不斷繼續(xù)反應(yīng)生成化合物8,也促使化合物7不斷地轉(zhuǎn)化為B,再生成C,最后推動反應(yīng)得以完全．

3 結(jié) 論

鑒于TpRuCl(PPh3)2配合物在金屬有機(jī)合成和催化反應(yīng)中的多種用途以及NHC配體所表現(xiàn)出的優(yōu)勢,本研究考察了TpRuCl(PPh3)2與NHC配體H2IMes的反應(yīng),試圖合成一個和TpRuCl(PPh3)2類似的NHC配合物前體TpRuCl(H2IMes)(PPh3) (A).該反應(yīng)未能分離得到預(yù)期產(chǎn)物A,而是發(fā)生了H2IMes配體一個N-側(cè)臂上鄰位甲基的C—H鍵活化,生成了H2IMes配體環(huán)金屬化的產(chǎn)物TpRu(H2IMes-H)(PPh3) (7).分離得到的化合物7在二氯甲烷溶液中加熱可解離PPh3配體,促使H2IMes-H配體進(jìn)一步發(fā)生環(huán)金屬化側(cè)臂上芐基的C—H活化,經(jīng)α-H消除轉(zhuǎn)化為螯合的雙卡賓配體H2IMes-2H配位的產(chǎn)物TpRuCl(H2IMes-2H) (8)．雖然釕配合物中NHC配體因N-側(cè)臂上取代基的C—H鍵活化而環(huán)金屬化已有廣泛的研究,但是環(huán)金屬化后進(jìn)一步發(fā)生α-H消除而生成螯合的雙卡賓配位產(chǎn)物的例子卻很罕見.在后續(xù)工作中將繼續(xù)考察TpRuCl(PPh3)2與其他NHC配體的反應(yīng),并研究環(huán)金屬化產(chǎn)物7和雙卡賓螯合的釕配合物8的反應(yīng)性質(zhì).

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