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

<i>N,N'</i>-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 <i>N</i>-alkyl and -aryl imidazoles with <i>N</i>-halomethyl imidazolium salts. The adaptability of the ligand precursor synthesis is illustrated through access to the <i>N</i>-Me/<i>N'</i>-Mes and <i>N</i>-Mes/N'-2,6-(i-Pr)₂Ph systems, leading to the Pd(ii) complexes [{(MeIm)(MesIm)CH₂}Pd(L)₂]ⁿ⁺, L = Cl/I (<i>n</i> = 0) and NCMe (<i>n</i> = 2), and [{(MesIm)[2,6-(i-Pr)₂PhIm]CH₂}Pd(L)₂], L = Cl/I. The dicationic hybrid <i>N,N'</i>-alkyl/aryl complex was inactive in the copolymerisation of ethylene/carbon monoxide, displaying reactivity akin to <i>N,N'</i>-dialkyl analogues.