Mononuclear, helical binuclear palladium and lithium complexes bearing a new pyrrole-based NNN-pincer ligand: fluxional property

Abstract

A new pyrrole based NNN-pincer ligand, 2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrole 2, was readily synthesized in two steps from pyrrole in 56% yield. The lithiation of the pincer ligand 2 using n-BuLi led to isolation of the dimeric lithium complex, [Li{μ-C₄H₂N-2,5-(CH₂Me₂pz)₂-N,N,N}]₂4, in 23% crystalline yield. The transmetalation reaction of 4 with [Pd(PhCN)₂Cl₂] afforded the mononuclear Pd(II) complex, [PdCl{C₄H₂N-2,5-(CH₂Me₂pz)₂-N,N,N}] 5, containing one chloride ion in 45% yield. Alternatively 5 was obtained in an excellent yield of 87% by the reaction of 2 with [Pd(COD)Cl₂] in the presence of triethylamine. On the contrary, a 20-membered macrometalacyclic molecule, [Pd₂Cl₄{μ-C₄H₃N-2,5-(CH₂Me₂pz)₂-N,N}₂] 6, in which two PdCl₂ units are bridged by two molecules of 2 to give a helical structure, was synthesized by the reaction of 2 with [Pd(COD)Cl₂] in the absence of base. The acetate analogue of complex 5, [Pd(OAc){C₄H₂N-2,5-(CH₂Me₂pz)₂-N,N,N}] 3, was obtained by the treatment of 2 with [Pd(OAc)₂]. The pyrrole twist angle of 5 is higher than that of 3. Complexes 3 and 5 show an AB pattern for their methylene protons at room temperature in CDCl₃ as well as in DMSO-d₆. The variable temperature NMR studies showed that the acetate and chloride complexes exhibit slightly different coalescence temperatures, which is a solvent dependent phenomenon, and twist angles.