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 KB = 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.