Synthesis, Crystal Structure Determination and Electrochemistry of Homoleptic Pd(0) Complexes Supported by Normal and Abnormal N-Heterocyclic Carbene Ligands

Abnormal carbene complex of Pd(L1)₂, where L1 = 1,3-Bis(2,6-diisopropylphenyl)-2,4-diphenylimidazole and normal carbene complex of Pd(L2)₂, where L2 = 1,3-dimesityl-4,9-dioxonaphtho-2,3-diimidazole were prepared from a Pd(0) precursor Pd(COD)(CH₂TMS)₂. Both the complexes crystalize in the triclinic space group P-1 and cell parameters of Pd(L1)₂ and Pd(L2)₂ are a = 12.0477 Å, b = 17.5269 Å, c = 18.9786 Å, α = 114.4855°, β = 92.5295°, γ = 107.9472° and a = 11.5007 Å, b = 15.2316 Å, c = 17.9007 Å, α = 67.061°, β = 72.143°, γ = 72.183° respectively. In both case N-heterocyclic ligands are distorted towards palladium direction. Further, crystal structures reveal that the palladium atom is covalently bonded to the two aNHC and nNHC ligands respectively. The electrochemical properties of these complexes were determined by cyclic voltammetry analysis in THF solution under argon atmosphere. The enunciated reversibility of the complexes can be explained by sequential one electron oxidations of the palladium centre from Pd(0) to Pd(I) and then Pd(II).

Graphical Abstract

Abnormal carbene complex of Pd(L1)₂, where L1 = 1,3-Bis(2,6-diisopropylphenyl)-2,4-diphenylimidazole and normal carbene complex of Pd(L2)₂, where L2 = 1,3-dimesityl-4,9-dioxonaphtho-2,3-diimidazole were prepared from a Pd(0) precursor of Pd(COD)(CH₂TMS)₂. The crystal structures were determined by single crystal X-ray diffraction data and electrochemical properties were also determined by cyclic voltammetric analysis.