Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

Treatment of both [Cocl((PNP)-P-tBu)] and [NiCl((PNP)-P-tBu)] [(PNP)-P-tBu = anion of 2,5-bis((di-tert-butylphosphino)methyl)pyrrole) with one equivalent of benzoquinone affords the corresponding chloride complexes containing a dehydrogenated PNP ligand, (tBu)dPNP ((tBu)dPNP = anion of 2,5-bis((di-tert-butylphosphino)methylene)-2,5-dihydropyrrole). Dehydrogenation of PNP to dPNP results in minimal change to steric profile of the ligand but has important consequences for the resulting redox potentials of the metal complexes, resulting in the ability to isolate both [CoH((tBu)dPNP)] and [CoEt((tBu)dPNP)], which are more challenging (hydride) or not possible (ethyl) to prepare with the parent PNP ligand. Electrochemical measurements with both the Co and Ni dPNP species demonstrate a substantial shift in redox potentials for both the M(II/III) and M(II/I) couples. In the case of the former, oxidation to trivalent Co was found to be reversible, and subsequent reaction with AgSbF6 afforded a rare example of a square-planar Co(III) species. Corresponding reduction of [CoCl((tBu)dPNP)] with KC8 produced the diamagnetic Co(I) species, [Co(N-2)((tBu)dPNP)]. Further reduction of the Co(I) complex was found to generate a pincer-based pi-radical anion that demonstrated well-resolved EPR features to the four hydrogen atoms and lone nitrogen atom of the ligand with minor contributions from cobalt and coordinated N-2. Changes in the electronic character of the PNP ligand upon dehydrogenation are proposed to result from loss of aromaticity in the pyrrole ligand, resulting in a more reducing central amido donor. DFT calculations on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.