Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

Abstract

Treatment of both [CoCl(tBuPNP)] and [NiCl(tBuPNP)] (tBuPNP = 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, 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 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(tBudPNP)] with KC8 produced the diamagnetic Co(I) species, [Co(N2)(tBudPNP)]. Further reduction of the Co(I) complex was found to generate a rare 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 calculations on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.

An Iron‐Hydrogen Bond Resistant to Protonation and Oxidation

“…required), [22a,23] which has been employed to oxidize the pincer backbone in ( tBu PNP)MCl (M=Ni, Co) [24] . However, such a strategy is unsuccessful with the analogous iron complex ( tBu PNP)FeCl, which has been attributed to oxidation of the iron center [24] . Given this result, it was particularly interesting to find that mixing 1 with 1,4‐benzoquinone (Scheme 6) produced the same iron hydride obtained from the galvinoxyl reaction.…”

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…This led to the protonation of the methine bridges with concomitant formation of a neutral PNP ligand (in complex 79 ). In the case of the 78‐Co , reduction of the dehydrogenated pincer complex was shown to afford the cobalt dinitrogen complex 80 comprising a formally dianionic PNP ligand with a significant π ‐radical character found on the ligand backbone . Upon reduction of 77 with KC 8 , a similar π ‐radical species has been obtained for manganese …”

“…Yet another strategy to modify the ligand framework after initial complex formation, namely oxidative backbone dehydrogenation, has been uncovered for Pyr ( R PNP) pincer complexes by Tonzetich and co‐workers , . Starting from 75 , 76 and 21 , Tonzetich and co‐workers were able to use this approach for the synthesis of the manganese, cobalt and nickel complexes 77 and 78‐M (M = Co, Ni), respectively (see Scheme ).…”

“…In these reactions, one hydrogen atom is abstracted from each methylene sidearm leaving the overall charge of the pincer ligand unaltered , . In order to identify the preferred protonation site of these dehydrogenated species, the manganese dicarbonyl complex 77 was treated with the strong Brønsted acid [H(OEt 2 ) 2 ][BAr F 4 ] .…”

“…In the context of the aforementioned Pyr ( R PNP)‐coordinated manganese, cobalt and nickel complexes 75 , 21 and 76 (Scheme ) and the oxidative dehydrogenation of their pincer backbones (vide supra),, it was pointed out that the cobalt complex 80 bearing a formally dianionic PNP ligand was formed via reduction of 78‐Co with KC 8 (Scheme ) . For the neutral ligands Py ( R PNP), PyN ( R PNP) and TrzN ( R PNP), related dianionic PNP ligands have been prepared by deprotonating both sidearm NH‐ or methylene‐groups in each of these ligands.…”