Reduction of the Allylic Substituents in NiI(1,8-dipropenyl-1,4,8,11-tetraazacyclotetradecane)+ by the Central Ni(I) in Aqueous Solutions

The complex NiII(1,8,-di-2-propenyl-1,4,8,11-tetraazacyclotetradecane)2+, (NiL1)2+, was synthesized. X-ray crystallography demonstrates that the complex obtained is the trans-III isomer. The allylic substituents shift the redox couples (NiL1)3+/2+ and (NiL1) 2+/+ anodically relative to the corresponding couples for NiII(1,4,8,11-tetraazacyclotetradecane)2+, (NiL2)2+, as expected. Surprisingly, the lifetime of (NiL1)+ in neutral aqueous solutions is shorter than that of (NiL2)+. Pulse radiolysis experiments reveal that the allylic substituents are reduced by the central Ni(I) ion. The first step in this reduction is a general acid catalyzed process. The results suggest that this step involves schematically the reaction Ni(I)-NCH2CH=CH2+ + H+ (right arrow)Ni(III)-NCH2CH2CH22+. The latter transient decomposes slowly with a half-life time of several minutes. Preliminary results support the suggestion that (NiL2)+, or other Ni(I)L complexes of this family, might reduce many alkenes present in the solution.