Prodigiosin Analogue Designed for Metal Coordination: Stable Zinc and Copper Pyrrolyldipyrrins

The
pyrrolyldipyrrin motif is found in several naturally occurring prodigiosin
pigments. The potential roles of the interactions of prodigiosins
with transition metals and the properties of metal-bound pyrrolyldipyrrins,
however, have been difficult to assess because of the very limited
number of well-characterized stable complexes. Here, we show that
the introduction of a meso-aryl substituent and an
ethyl ester group during the sequential assembly of the three heterocycles
affords a pyrrolyldipyrrin of enhanced coordinating abilities when
compared to that of natural prodigiosins. UV–visible absorption
studies indicate that this ligand promptly binds Zn­(II) ions with
2:1 ligand-to-metal stoichiometry and Cu­(II) ions with 1:1 stoichiometry.
Notably, no addition of base is required for the formation of the
resulting stable complexes. The crystal structures reveal that whereas
the tetrahedral zinc center engages two nitrogen donors on each ligand,
the pseudosquare planar copper complex features coordination of all
three pyrrolic nitrogen atoms and employs the ester group as a neutral
ligand. This first example of coordination of a redox-active transition
metal within a fully conjugated pyrrolyldipyrrin framework was investigated
spectroscopically by electron paramagnetic resonance to show that
the 1:1 metal-to-ligand ratio found in the crystal structure is also
maintained in solution.