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

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(C2OH)]Cl (2), the
new functionalized NHC palladium­(II) complex [PdCl­(acac)­{ImDiPP­(C2OH)-CNHC}] (3) was
synthesized and fully characterized. Two byproducts, [PdCl­{μ-ImDiPP­(C2O)-CNHC,O}]2 (4) and trans-[PdCl2{ImDiPP­(C2OH)-CNHC}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(μ-Cl)­{ImDiPP­(C2OH)-CNHC}]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 CNHC,O chelate. The
stability of 3 was also assessed under basic conditions,
and the new complex [Pd­(acac)­{ImDiPP­(C2O)-CNHC,O}] (7) was characterized.
The new nickel­(II) alcoholate-functionalized NHC complex [NiCl­{μ-ImDiPP­(C2O)-CNHC,O}]2 (8) was synthesized by the reaction of the imidazolium
salt 2 with n-BuLi and [NiCl2(dme)]. The reaction of 8 with HCl regenerates the imidazolium
and alcohol functions to give [ImDiPP­(C2OH)]2[NiCl4] (9). The mixed-metal Ni­(II)–Li­(I)
complexes [Ni2{μ-ImDiPP­(C2O)-CNHC,μ-O}4Li]­BF4 (10), [Ni2{μ-ImDiPP­(C2O)-CNHC,μ-O}4Li]­Cl
(11), and [Ni­{ImDiPP­(C2O)-CNHC,μ-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 [NiCl2{ImDiPP­(C2OH)-CNHC}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 2·2H2O and of the complexes 3, 4, 4-polymorph, 5, 6·CH2Cl2, and 8–13 were established by single-crystal X-ray diffraction.