N-heterocyclic carbenes. 23. High yield syntheses of sterically demanding bis(N-heterocyclic carbene) complexes of palladium

The synthesis of bis(N-heterocyclic carbene) chelates of formula [cis-E{N(H )C=C(H)N(R)C}(2)PdX2] (E = spacer linkage; R = alkyl, aryl; X = halide) is presented via improved high-yielding, air-stable procedures (for E = CH2). A detailed study of the intermediates involved in the preparation of [cis-C H2{N(H)C=C(H)N(t-Bu)C}(2)PdX2] (X = Br, I) provide evidence for the difficu lty in extending our synthetic procedures to include chelates of different ring sizes and also in preparing the analogous nickel complexes.