Studies of bicarbonate binding by dinuclear and mononuclear Ni(II) complexes

Bicarbonate ion reacts with the dinuclear nickel(II) complex containing the taec ligand (taec = N,N′,N″,N′″-tetrakis-(2-aminoethyl) -1,4,8,11-tetraazacyclotetradecane) in buffered aqueous solution to form the μ-η²,η²-carbonate complex with a large effective binding constant for bicarbonate ion, log K_B = 4.39 at pH = 7.4. In contrast, the dinuclear nickel(II) complex containing the o-xyl-DMC₂ ligand (o-xyl-DMC₂ = α,α′- bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene) does not react with bicarbonate or carbonate ion in aqueous solution. In propylene carbonate, the reaction of [Ni₂(o-xyl-DMC₂)]⁴⁺ with bicarbonate proceeds rapidly to form the μ-η¹, η¹-carbonate complex. The structure of this carbonate complex has been determined by an X-ray diffraction study that confirms the μ-η¹,η¹-carbonate binding mode. A mononuclear analogue of [Ni₂(taec)]⁴⁺, [Ni(2,3,2-tetraamine)] ²⁺ does not form a detectable mononuclear or dinuclear product with bicarbonate ion in aqueous solution, but [NiDMC]²⁺ (DMC = 5,7-dimethyl-1,4,8,11-tetraazacyclotetradecane) reacts slowly with carbonate ion in aqueous solution to form a 2:1 complex.