Nickelalactones with an allyl subunit - the effect of penta-coordination on structures and stability

A series of allyl modified nickelalactone derivatives of the general formula [(L)nNi{CH2C(CH3)C(CH3)CH2COO}] was synthesized via ligand exchange reactions in order to investigate the influence of the neutral ligand L on the structure and stability of these complexes. While the square planar 1,8-diazabicyclo[5,4,0]undec-7-ene ligated derivative 1 is stable in the solid state and in solution at ambient temperature, the use of the chelating ligands 6-diphenylphosphino-1,8-diazabicyclo[5.4.0]undec-7-ene, 1,1-bis(dicyclohexylphosphino)methane and 1,2-bis(dicyclohexylphosphino)ethane (dcpe) led to square pyramidal derivatives 2, 4, and 5 with a novel ligand arrangement. In solution, these derivatives are fluxional and show increasing tendencies in the order 2 < 4 < 5 to decouple the nickelalactone moiety to 2,3-dimethylbutadiene (dmbd), CO2 and zero-valent nickel complexes. During the investigation of 5, the tetrahedral complex [(dcpe)2Ni] (6) and the trigonal planar derivative [(dcpe)Ni(dmbd)] (7) were observed as predominant decomposition products. In the case of the application of 1,1-bis(diphenylphosphino)methane as a neutral ligand, a dinuclear nickelalactone species [(μ-dppm)(Ni{CH2C(CH3)C(CH3)CH2COO})2] (3) was isolated in which the two nickel atoms realize different coordination geometries (SP-4 and SPY-5) in the solid state. The complexes were characterized by NMR techniques, single crystal X-ray diffraction measurements and infrared spectroscopy.