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

Monomeric and dimeric nickelalactones with N-heterocyclic carbene ligands [(py)(im(mes))Ni(CH2CH2COO)] (1), [(im(mes))Ni(CH2CH2-COO)](2) (2), [(im(t-) (but))Ni(C(Et)=C(Et)-COO)](2) (3), [(im(mes))Ni(C(Et)=C(Et)-COO)](2) (4), and [(im(mes))Ni(CH2-C(CH3)-C(CH3)-CH2-COO)] (5) (im(mes): 1,3-dimesityimidazol-2-ylidene; im(t-but): 1,3-di-t-butylimidazol-2-ylidene) were synthesized and investigated by H-1, C-13 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 dinters 2 and 3 are bridged by two endocyclic oxygen donor atoms of the carboxylate groups resulting in Ni2O2-four-membered rings. Reaction of [(im(2)(mes))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 eta(3)-allyl group, a monodentate carboxylate group and an im(mes) ligand. (c) 2006 Elsevier B.V. All rights reserved.