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 <i>p</i>-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 <i>mer</i> (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