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 [HL(Bn)]Cl, 1a, and Ag(2)O) (Bn = benzyl), which shows an alignment of four silver atoms bridged by the difunctional C-N ligands and chlorides. Its transmetallation with NiCl(2)(PPh(3))(4) and PdCl(2)(MeCN)(2) results in double-metal salts 2[M(L(Bn))(2)](2+)[Ag(4)Cl(8)](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 [HL(Mes)]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.
Li
Fuwei
F
Department of Chemistry, National University of Singapore, 3, Science Drive 3, Singapore, 117543, Singapore.
Hu
Jian Jin
JJ
Koh
Lip Lin
LL
Hor
T S Andy
TS