Synthesis and Characterization of Palladium(II) and Nickel(II) Alcoholate-Functionalized NHC Complexes and of Mixed Nickel(II)?Lithium(I) Complexes

摘要

The synthesis of Pd(II) and Ni(II) alcohol-functionalized N-heterocyclic carbene (NHC) complexes was explored to examine the possible influence of the functional arm attached to the NHC backbone on their structure and reactivity and, in the case of a Ni(II) complex, on its catalytic properties in ethylene oligomerization. Starting from the alcohol-functionalized imidazolium salt [ImDiPP(C_2OH)]Cl (2), the new functionalized NHC palladium(II) complex [PdCl(acac){ImDiPP(C_2OH)-C_(NHC)}] (3) was synthesized and fully characterized. Two byproducts, [PdCl{??-ImDiPP(C_2O)-C_(NHC),O}]_2 (4) and trans-[PdCl_2{ImDiPP(C_2OH)-C_(NHC)}_2] (5), formed during the synthesis of 3, were also fully characterized. Acids promoted the transformation of 3 into the new CNHC-bound complex [PdCl(??-Cl){ImDiPP-(C_2OH)-C_(NHC)}]_2 (6), unveiling the lability of the acac ligand and the resistance of the Pd?NHC bond to acids. Complex 6 reacted with a base to afford complex 4, in which alkoxide coordination to Pd(II) has occurred to generate a C_(NHC),O chelate. The stability of 3 was also assessed under basic conditions, and the new complex [Pd(acac){ImDiPP(C_2O)-C_(NHC),O}] (7) was characterized. The new nickel(II) alcoholate-functionalized NHC complex [NiCl{??-ImDiPP(C_2O)-C_(NHC),O}]_2 (8) was synthesized by the reaction of the imidazolium salt 2 with n-BuLi and [NiCl_2(dme)]. The reaction of 8 with HCl regenerates the imidazolium and alcohol functions to give [ImDiPP(C_2OH)]_2[NiCl_4] (9). The mixed-metal Ni(II)?Li(I) complexes [Ni_2{??-ImDiPP(C_2O)-C_(NHC),??-O}_4Li]BF_4 (10), [Ni_2{??-ImDiPP(C_2O)-C_(NHC),??-O}_4Li]Cl (11), and [Ni{ImDiPP(C_2O)-C_(NHC),??-O}_2LiBr] (12) were isolated and characterized. However, it was not possible to synthesize a Ni(II) alcohol-functionalized NHC complex in high yield. Small amounts of the square-planar complex [NiCl_2{ImDiPP-(C_2OH)-C_(NHC)}_2] (13) could be isolated, and this complex was characterized by single-crystal X-ray diffraction. In 13, only the CNHC atom of the alcohol-functionalized NHC ligand is bound to the metal. The structures of the imidazolium salt 2??2H_2O and of the complexes 3, 4, 4-polymorph, 5, 6??CH_2Cl_2, and 8?13 were established by single-crystal X-ray diffraction.