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

Abstract

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 d⁸ products. Use of a benzyl-substituted NHC gives [Ag₄(L^Bn)₂Cl₄], 2a (from [HL^Bn]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 Ag₄ aggregate is maintained in the transmetallation process. Their Ag-free forms [M(L^Bn)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(L^Mes)₂Cl₂], 2b (Mes = mesityl) that shows a 12-membered metallomacrocyclic ring with a 2-coordinated [Ag^I(NHC)₂] and 4-coordinated [Ag^I(Imd)₂Cl₂] (Imd = imidazole). Transmetallation of the latter, or direct metallation from [HL^Mes]Cl, 1b, gives [M(L^Mes)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.