Backbone Dehydrogenation in Pyrrole-​Based Pincer Ligands

By Krishnan, V. Mahesh; Davis, Ian; Baker, Tessa M.; Curran, Daniel J.; Arman, Hadi D.; Neidig, Michael L.; Liu, Aimin; Tonzetich, Zachary J. From Inorganic Chemistry (2018), 57(15), 9544-9553

Treatment of both [CoCl(tBuPNP)​] and [NiCl(tBuPNP)​] (tBuPNP = anion of 2,​5-​bis((di-​tert-​butylphosphino)​methyl)​pyrrole) with one equiv. of benzoquinone affords the corresponding chloride complexes contg. a dehydrogenated PNP ligand, tBudPNP (tBudPNP = 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(tBudPNP)​] and [CoEt(tBudPNP)​]​, which are more challenging (hydride) or not possible (ethyl) to prep. with the parent PNP ligand.  Electrochem. 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, oxidn. to trivalent Co was reversible, and subsequent reaction with AgSbF6 afforded a rare example of a square-​planar Co(III) species.  Corresponding redn. of [CoCl(tBudPNP)​] with KC8 produced the diamagnetic Co(I) species, [Co(N2)​(tBudPNP)​]​.  Further redn. of the Co(I) complex was found to generate a pincer-​based π-​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 N2.  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 calcns. on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.