Over the course of the titration of macrocycle 2 with H2P2O7 2-, two distinct complexes were also identified (Figure ). Exchange was slow on the NMR chemical shift scale and distinct signals were observed for several protons which could be easily assigned (Figure ). The ratio between the different complexes could be determined by integration of the signal corresponding to the azomethine Hd or the NHc (Figure ). Crucially, this revealed that, during the first few additions, the amount of new complex formed corresponded to twice the amount of guest added, implying the formation of a 2:1 host to guest complex. Furthermore, the fact that almost complete formation of the second complex takes place once 1 eq. of H2P2O7 2-is added implies that the second binding event results in the formation of a strong 1:1 complex.