Substituent-dependent structures and catalysis of benzimidazole-tethered N-heterocyclic carbene complexes of Ag(I), Ni(II) and Pd(II)

Homoleptic cationic benzimidazole-imidazolin-2-ylidene N-heterocyclic carbene (NHC = L) complexes of Ni-II and Pd-II have been prepared directly from the ligand precursor in salt form [H.L]Cl and from the transmetallation route via Ag-I. 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 d(8) products. Use of a benzyl-substituted NHC gives [Ag-4(L-Bn)(2)Cl-4], 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(L-Bn)(2)](2+)[Ag4Cl8](4-) (M = Ni (3a) and Pd (4a)). The nuclearity of the Ag-4 aggregate is maintained in the transmetallation process. Their Ag-free forms [M(L-Bn)(2)]Cl-2 (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 sigma-benzimidazole nitrogen donor to be trans to them. A sterically demanding mesityl pendant however gives the dinuclear dissymmetic [Ag-2(L-Mes)(2)Cl-2], 2b (Mes = mesityl) that shows a 12-membered metallomacrocyclic ring with a 2-coordinated [Ag-I(NHC)(2)] and 4-coordinated [Ag-I(Imd)(2)Cl-2] (Imd = imidazole). Transmetallation of the latter, or direct metallation from [HLMes]Cl, 1b, gives [M(L-Mes)(2)]Cl-2 (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.