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

The complex Ni ^II(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 (NiL ¹)^2+/+ anodically relative to the corresponding couples for Ni ^II(1,4,8,11-tetraazacyclotetradecane)²⁺, (NiL ²)²⁺, as expected. Surprisingly, the lifetime of (NiL ¹)⁺ 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⁺ → Ni(III) - NCH ₂CH₂CH₂²⁺. 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.