We also briefly explored the coordination of divalent first row transition metal ions to 5H. Addition of Cu(ClO4)2 to a solution of 5H initially gave a pale-yellow solution that on standing (24 h) yielded a dark red solution and precipitate. Aqueous work up gave a mixture of [Cu a (5H)] + and 5H. Conversely, we were unable to detect coordination of Ni II , Co II or Zn II to 5H by ESI-MS or, in the case of Zn II , 1 H NMR. In the latter case we observed changes consistent with partial protonation of 5H. Based on these results, 5H is well suited to the coordination of monovalent d 10 metal ions but, presumably due to exclusion of additional anionic ligands from the primary coordination sphere, coordination of divalent metal ions is unfavorable.