Synthesis and solution complexation behaviour of tetradentate diamines with hard phosphinate donors

Seven acyclic tetradentate compounds of varying lipophilicity incorporating two nitrogen and two alkyl or aryl phosphinate donors have been prepared and their complexation of divalent ions has been studied in aqueous solution by pH-metric, NMR and electrospray mass spectrometric methods of analysis. The hard phosphinate oxygen donor favours binding to the charge-dense Mg²⁺ ion but binds copper(II) only very weakly in solution. Nickel and zinc ions do form 1:1 complexes involving significant phosphinate ligation and this donor-atom preference leads to an inversion of the Irving-Williams stability sequence.