Structural variation, dynamics, and catalytic application of palladium(ii) complexes of di-N-heterocyclic carbene–amine ligands

Abstract

A series of palladium(II) complexes incorporating di-NHC–amine ligands has been prepared and their structural, dynamic and catalytic behaviour investigated. The complexes [trans-(κ²-^tBuCN(Bn)C^tBu)PdCl₂] (12) and [trans-(κ²-^MesCN(H)C^Mes)PdCl₂] (13) do not exhibit interaction between the amine nitrogen and palladium atom respectively. NMR spectroscopy between −40 and 25 °C shows that the di-NHC–amine ligand is flexible expressing C_s symmetry and for 13 rotation of the mesityl groups is prevented. In the related C₁ complex [(κ³-^tBuCN(H)C^tBu)PdCl][Cl] (14) coordination of NHC moieties and amine nitrogen atom is observed between −40 and 25 °C. Reaction between 12–14 and two equivalents of AgBF₄ in acetonitrile gives the analogous complexes [trans-(κ²-^tBuCN(Bn)C^tBu)Pd(MeCN)₂][BF₄]₂ (15), [trans-(κ²-^MesCN(H)C^Mes)Pd(MeCN)₂][BF₄]₂ (16) and [(κ³-^tBuCN(H)C^tBu)Pd(MeCN)][BF₄]₂ (17) indicating that ligand structure determines amine coordination. The single crystal X-ray structures of 12, 17 and two ligand imidazolium salt precursors ^tBuC(H)N(Bn)C(H)^tBu][Cl]₂ (2) and [^tBuC(H)N(H)C(H)^tBu][BPh₄]₂ (4) have been determined. Complexes 12–14 and 15–17 have been shown to be active precatalysts for Heck and hydroamination reactions respectively.