Probing the Steric and Electronic Characteristics of a New Bis-Pyrrolide Pincer Ligand

A new
pincer ligand is synthesized to be dianionic, with the potential
to be redox active. It has pyrrrole rings attached to both ortho sites
of a pyridine, as the linking element. This H2L can be
doubly deprotonated and then used to replace two chloride ligands
in MCl2(NCPh)2, to form LM­(NCPh) for M = Pd,
Pt. The acid form H2L reacts with ZnEt2 with
elimination of only 1 mol of ethane to yield (HL)­ZnEt, a three-coordinate
species with one pendant pyrrole NH functionality. This molecule binds
the Lewis base p-dimethylaminopyridine (DMAP) to
give first a simple 1:1 adduct that eliminates ethane on heating to
form four-coordinate LZn­(DMAP), which has an unusual structure due
to the strong preference of the pincer ligand to bind in a mer (planar) geometry. A molecule with two HL– ligands each bonded in a bidentate manner to FeCl2 is
synthesized and shown to contain four-coordinate iron with a flattened-tetrahedral
structure. The electrochemistry of LM­(NCPh) and (L)­Zn­(DMAP) shows
three oxidation processes, which is interpreted to involve at least
two oxidations of the pyrrolide arms.