Our group has had a long-standing interest in exploring the reactivity of polypyridine ligated nickel catalysts which we have demonstrated have tremendous potential in the construction of molecular complexity. However, the nuanced mechanistic details of these reactions were unclear and undoubtably complex, making further innovation in the field challenging. In previous studies, our group has shown the comproportionation reactions to form off-cycle species was a significant deactivation pathway with redox innocent phosphine ligands and we believed these types of electron transfer events were broadly involved in nickel catalyzed reactions.1-2 Comproportionation and disproportionation reactions were particularly exciting topics to me since I’ve found they are often glossed over in the literature and have an “enigmatic air” to them. I remember trying to learn more about nickel-catalysis before starting my PhD and reading a review by Jamison3 where the possibility of these reactions occurring was mentioned but not much more information was provided. As such, in this work we explored the reactions of comproportionation, disproportionation and reduction from well-defined species and then extend our findings to demonstrate their relevance to catalysis (Figure 1. - https://www.nature.com/articles/s41929-023-00925-4).
Figure 1. Implications of electron transfer events in comproportionation, disproportionation and reduction on catalyst turnover.
Initially we approached this question of how these reactions occur by determining a model ligand that meets our needs of being a well-defined single species that is soluble and relevant to nickel-catalysis. Evaluating various ligands we found bathrocuprine; a 2,6-dimethyl substituted variant of phenanthroline that also has phenyl groups on the ligand backbone fit these criteria. Settled on this ligand, we synthesized a series of Ni(II) complexes with halide and pseudohalide ligands (-OCOtBu, -OCOPh, -OPh) and evaluated their reactivity in comproportionation reactions. As previously shown in the literature, the Ni-halide complexes underwent comproportionation but the unstudied Ni-pseudohalide contrasted this outcome and were completely unreactive. We suspected that if electron transfer events had a low kinetic barrier, which is often the case, then the disproportionation of Ni(I)-pseudohalides should occur spontaneously as it is the reverse reaction of comproportionation. Indeed this turned out to be true, where we were able to generate a Ni(I) carboxylate NiI-OCOR in situ which spontaneously formed the corresponding Ni(II)-OCOR and Ni(0) species. This finding was further validated and rationalized by electrochemical and computational studies that supported the notion that the halide ligands decreased the spin density by back donation to Ni(I) which stabilized the complex to disproportionation while pseudohalides were less effective back donating ligands. Extending the importance of these findings to catalysis, we show that in reductive coupling reactions that disproportionation of on-cycle Ni(I) species is detrimental to catalysis since commonly used reductants such as Zn or Mn are ineffective at reducing the Ni(II)-pseudohalide complexes that result from disproportionation to catalytically active Ni(I) and Ni(0) species. Importantly, catalytic activity could be regenerated by the addition of exogeneous halide salts to reform the active Ni-halide catalysts. We believe these investigations set the foundation to understanding the roles of comproportionation, disproportionation and reduction events much broader than the scenarios reported here and will act as blueprints for those studying these reactions in other first-row transition metals. Check out more at: https://www.nature.com/articles/s41929-023-00925-4.
Figure 2. Disproportionation and comproportionation involved in nickel-catalyzed reactions.
- Somerville, R. J.; Hale, L. V. A.; Gómez-Bengoa, E.; Burés, J.; Martin, R. Intermediacy of Ni–Ni Species in sp2 C–O Bond Cleavage of Aryl Esters: Relevance in Catalytic C–Si Bond Formation. J. Am. Chem. Soc. 2018, 140 (28), 8771-8780.
- Day, C. S.; Somerville, R. J.; Martin, R. Deciphering the dichotomy exerted by Zn(II) in the catalytic sp2 C–O bond functionalization of aryl esters at the molecular level. Nat. Catal. 2021, 4 (2), 124-133.
- Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Recent advances in homogeneous nickel catalysis. Nature 2014, 509 (7500), 299-309.