The Selectivity of Water-based Pyrophosphate Recognition is Tuned by Metal Substitution in Dimetallic Receptors

The three dimetallic compounds [Ga ₂(bpbp)(OH) ₂(H ₂O) ₂](ClO ₄) ₃, [In ₂(bpbp)(CH ₃CO ₂) ₂](ClO ₄) ₃ and [Zn ₂(bpbp)(HCO ₂) ₂](ClO ₄) (bpbp ^- = 2,6-bis((N,N′-bis(2-picolyl)amino)methyl)-4-tertbutylphenolate) were evaluated as stable solid state precursors for reactive solution state receptors to use for the recognition of the biologically important anion pyrophosphate in water at neutral pH. Indicator displacement assays using in situ generated complex-pyrocatechol violet adducts, {M ₂(bpbp)(H _xPV)} ⁿ⁺ M = Ga ³⁺, In ³⁺, Zn ²⁺, were tested for selectivity in their reactions with a series of common anions: pyrophosphate, phosphate, ATP, arsenate, nitrate, perchlorate, chloride, sulfate, formate, carbonate and acetate. The receptor employing Ga ³⁺ showed a slow but visually detectable response (blue to yellow) in the presence of one equivalent of pyrophosphate but no response to any other anion, even when they were present in much higher concentrations. The systems based on In ³⁺ or Zn ²⁺ show less selectivity in accord with visibly discernible responses to several of the anions. These results demonstrate a facile method for increasing anion selectivity without modification of an organic dinucleating ligand scaffold. The comfortable supramolecular recognition of pyrophosphate by the dimetallic complexes is demonstrated by the single crystal X-ray structure of [Ga ₂(bpbp)(HP ₂O ₇)](ClO ₄) ₂ in which the pyrophosphate is coordinated to the two gallium ions via four of its oxygen atoms.