Saturated and unsaturated nickelalactones with N-heterocyclic carbene ligands: Synthesis and structures

Monomeric and dimeric nickelalactones with N-heterocyclic carbene ligands [(py)(immes)Ni(CH2CH2COO)] (1), [(immes)Ni(CH2CH2COO)]2 (2), [(imt-but)Ni(C(Et)double bond; length as m-dashC(Et)-COO)]2 (3), [(immes)Ni(C(Et)double bond; length as m-dashC(Et)-COO)]2 (4), and [(immes)Ni(CH2partial double bondC(CH3)partial double bondC(CH3)-CH2-COO)] (5) (immes: 1,3-dimesityimidazol-2-ylidene; imt-but: 1,3-di-t -butylimidazol-2-ylidene) were synthesized and investigated by 1H, 13C NMR and IR spectroscopic measurements. The solid-state structures of 1–3 and 5 were also elucidated by X-ray diffraction analyses of single crystals. In the compounds the Ni(II) ion has square-planar geometry. The metal centers in the dimers 2 and 3 are bridged by two endocyclic oxygen donor atoms of the carboxylate groups resulting in Ni2O2-four-membered rings. Reaction of View the MathML source[(im2mes)Ni with 2,3-dimethylbutadiene and CO2 resulted in an oxidative coupling of the two dienes followed by insertion of CO2 to form the dimeric complex 6. The X-ray structure of 6 shows two substituted heptenoic acid dianions which connect the two Ni(II) centers. Each nickel atom is surrounded by a η3-allyl group, a monodentate carboxylate group and an immes ligand.