Selective and adaptable access to N,N'-asymmetrically substituted imidazol-2-ylidene bis-NHC ligands: Pd(II) complexes featuring wide variation in N-alkyl and aryl steric bulk

N,N'-Asymmetrically substituted, methylene-linked bis(imidazol-2-ylidene) complexes have been prepared subsequent to a selective synthesis of the bis(imidazolium) salt precursors involving the quarternisation of N-alkyl and -aryl imidazoles with N-halomethyl imidazolium salts. The adaptability of the ligand precursor synthesis is illustrated through access to the N-Me/N'-Mes and N-Mes/N'-2,6-(i-Pr)₂Ph systems, leading to the Pd(ii) complexes [{(MeIm)(MesIm)CH₂}Pd(L)₂]ⁿ⁺, L = Cl/I (n = 0) and NCMe (n = 2), and [{(MesIm)[2,6-(i-Pr)₂PhIm]CH₂}Pd(L)₂], L = Cl/I. The dicationic hybrid N,N'-alkyl/aryl complex was inactive in the copolymerisation of ethylene/carbon monoxide, displaying reactivity akin to N,N'-dialkyl analogues.