Spin-Switching Transmetalation at Ni Diimine Catalysts

Transmetalation is a ubiquitous transformation used for synthesizing organic molecules. In catalyst-transfer polymerization (CTP), conjugated monomers are polymerized using transmetalation of Grignard reagents to make versatile organic semiconductors such as poly­(3-hexylthiophene). This study presents the complete mechanistic viewpoint for this transmetalation reaction, taking into consideration the catalyst electronic states, steric environment, and realistic models of each reagent. These quantum chemical results reveal that singlet–triplet crossing is routine in these transmetalation reactions, and switching between low-spin square-planar and high-spin tetrahedral geometries naturally occurs during the catalytic cycle. In this cycle transmetalation preferentially occurs from a triplet state but forces the metal center back into a singlet state after monomer addition. Furthermore, the relative preference of singlet vs triplet state can be modulated by the ancillary ligand. This model therefore captures reactive and ancillary ligand interactions and demonstrates how the relative distortion of the tetrahedral and square-planar geometries can be used to quantify these sensitive ligand effects on the electronic state. Additionally, the activation barriers for transmetalation follow trends dictated by steric environment and the lateness of the transition state, measured in terms of the Ni–C bond distance. Together, these models provide predictive insight into ancillary ligand-based reactivity trends.