Manganese PCNHCP Pincer Complex on a Non-Cooperative Ligand Platform: Synthesis and Reactivity
Rohit Kamte
Catalysis plays a very important role in modern organic synthesis. The most commonly used homogenous catalysts are based on using precious metals such as Ru, Rh, Pd, and Ir. However, their limited availability, high cost and negative environmental footprints resulted in recent efforts to replace these noble metals with more abundant and environmentally friendly first row transition metals. However, the development of catalysts based on earth-abundant metals remains challenging and often requires different ligand design strategies to stabilize the different oxidation and/or spin-state of the resulting metal complexes. In particular, for manganese, catalysis can be hampered by unfavourable spin-states (e.g., high-spin Mn(II) d5) that diminish the reactivity of the metal center. As a result, many methodologies have been developed to overcome some of the limitations through bespoke ligand design. In particular metal-ligand cooperativity has utilized to overcome the unfavourable one-electron chemistry.
Our group’s approach focuses on using the strong field ligands in order to access the low-spin manifold which allows us to mimic the chemistry of noble metals with first row transition metals. In this seminar, I will discuss the design and synthesis of a series of Mn(I) PCNHCP pincer complexes that will be used for various organic transformations such as C–C and C–N bond forming reactions. In particular, we have demonstrated that Mn(I)PCNHCP pincer complexes are highly active catalysts for the α -methylation of ketones and indoles using methanol as a C1 source.1 In contrast to previously reported mechanisms, the herein reported mechanism does not depend on metal-ligand co-operativity that will be discussed in this seminar. Furthermore, this Mn(I) PCNHCP pincer complex has been explored as an efficient catalyst to functionalize a variety of electronically and sterically differentiated α,β-unsaturated ketones, in particular for the chemoselective hydrogenation of C=C double bonds using H2 and through reductive α-methylation methodologies with methanol as both hydrogen and C1-source.2
In the final part of my seminar, I will discuss how the reactivity of Mn(I) PCNHCP pincer complexes can be harnessed to facilitate the hydration of variety of aliphatic and aromatic nitriles.3 Detailed mechanistic studies were performed to elucidate the reaction mechanisms, which includes reaction kinetics, deuterium labeling studies and the isolation of reactive intermediates. All three protocols exhibit a large substrate scope and are compatible with a variety of functional groups.
References
1) R. Thenarukandiyil†, R. Kamte†, S.Garhwal, P. Effnert, N. Fridman, and G. de Ruiter. α‑Methylation of Ketones and Indoles Catalyzed by a Manganese(I) PCNHCP Pincer Complex with Methanol as a C1 Source. Organometallics 2023, 42, 62−71.
2) K. Dey, R. Kamte, and G. de Ruiter. Reductive α‑Methylation of α,β-Unsaturated Ketones Catalyzed by a Mn(I) PCNHCP Pincer Complex with Methanol as Both H2 and C1−Source Organometallics 2025, 44, 18, 2035–2041.
3) R. Kamte, R. Thenarukandiyil, K. Dey, N. Fridman and G. de Ruiter. Nitrile hydration and α-deuteration of amides catalyzed by a PCNHCP Mn(I) pincer complex. Chem. Commun., 2025, 61, 17420-17423.