Substituent-dependent structures and catalysis of benzimidazole-tethered N-heterocyclic carbene complexes of Ag(i), Ni(ii) and Pd(ii)
作者:Fuwei Li、Jian Jin Hu、Lip Lin Koh、T. S. Andy Hor
DOI:10.1039/c000722f
日期:——
Homoleptic cationic benzimidazole-imidazolin-2-ylidene N-heterocyclic carbene (NHC = L) complexes of NiII and PdII have been prepared directly from the ligand precursor in salt form [H.L]Cl and from the transmetallation route via AgI. The N-tether of the imidazolinylidene ring imposes a significant influence on the nuclearity of the intermediate Ag(I)-NHC complexes and the geometric isomer outcome of the d8 products. Use of a benzyl-substituted NHC gives [Ag4(LBn)2Cl4], 2a (from [HLBn]Cl, 1a, and Ag2O) (Bn = benzyl), which shows an alignment of four silver atoms bridged by the difunctional C–N ligands and chlorides. Its transmetallation with NiCl2(PPh3)4 and PdCl2(MeCN)2 results in double-metal salts 2[M(LBn)2]2+[Ag4Cl8]4− (M = Ni (3a) and Pd (4a)). The nuclearity of the Ag4 aggregate is maintained in the transmetallation process. Their Ag-free forms [M(LBn)2]Cl2 (M = Ni (5) and Pd (6)) were prepared by direct deprotonation of 1a with M(OAc)2. The two carbenic carbon donor are cis- to each other in both 3a and 4a, thus imposing the weaker σ-benzimidazole nitrogen donor to be trans to them. A sterically demanding mesityl pendant however gives the dinuclear dissymmetic [Ag2(LMes)2Cl2], 2b (Mes = mesityl) that shows a 12-membered metallomacrocyclic ring with a 2-coordinated [AgI(NHC)2] and 4-coordinated [AgI(Imd)2Cl2] (Imd = imidazole). Transmetallation of the latter, or direct metallation from [HLMes]Cl, 1b, gives [M(LMes)2]Cl2 (M = Ni (3b) and Pd (4b)) with the two carbonic carbon trans to each other. The catalytic potential of 3b and 4b, which are more effective than 5 and 6, has been demonstrated by their high activities in Ni-catalyzed Kumada at room temperature and Pd-catalyzed Heck couplings of aryl and/or heteroaryl halides, respectively.
均配正离子苯并咪唑-咪唑啉亚基N-杂环卡宾(NHC = L)NiII和PdII配合物可以通过盐形式的配体前驱体[H.L]Cl以及通过AgI的转金属路线来直接制备。咪唑啉亚基环的N-连接对中间体Ag(I)-NHC配合物的核数和d8产物的几何异构体结果产生了显著影响。使用苯甲基取代的NHC得到[Ag4(LBn)2Cl4],2a(由[HLBn]Cl,1a,和Ag2O制备)(Bn = 苯甲基),显示出四个银原子通过双功能的C-N配体和氯化物桥接的对齐。其与NiCl2(PPh3)4和PdCl2(MeCN)2的转金属反应得到了双金属盐2[M(LBn)2]2+[Ag4Cl8]4-(M = Ni (3a) 和 Pd (4a))。Ag4聚集体的核数在转金属过程中得以保持。它们的Ag-free形式[M(LBn)2]Cl2(M = Ni (5) 和 Pd (6))是通过1a与M(OAc)2的直接去质子化制备的。两个卡宾碳供体在3a和4a中都是cis相对的,因此使较弱的σ-苯并咪唑氮供体与其相对。然而,空间位阻较大的甲基苯基悬挂物给出了二核不对称的[Ag2(LMes)2Cl2],2b(Mes = 甲基苯基),显示出具有2配位的[AgI(NHC)2]和4配位的[AgI(Imd)2Cl2](Imd = 咪唑)的12元金属大环。后者的转金属反应,或直接从[HLMes]Cl,1b,金属化得到了[M(LMes)2]Cl2(M = Ni (3b) 和 Pd (4b)),其中两个碳碳彼此相对。3b和4b的催化潜力,它们比5和6更有效,已经通过它们在室温下Ni催化的Kumada和Pd催化的芳基和/或杂芳基卤化物的Heck偶联反应中的高活性得到了证明。