Elucidating the Role of Diphosphine Ligand in Nickel-Mediated Suzuki–Miyaura Polycondensation

Abstract

Synthesizing conjugated polymers using controlled polymerization techniques offers tremendous flexibility in the preparation of well-defined semiconducting materials. Suzuki–Miyaura coupling has emerged as a powerful method for these polymerizations since it can be conducted under mild conditions, a range of boron reagents can be employed, and it is exceptionally functional group tolerant. We have analyzed the Ni(dppp)Cl2 mediated Suzuki–Miyaura chain-growth polycondensation of 3-hexylthiophene and discovered that a portion of the precatalyst hydrolyzes at the outset of the reaction. Surprisingly, this loss of catalyst is critical to achieving a controlled polymerization as it releases free ligand into the reaction mixture, suppressing chain transfer. The identity of the ligand is also critical, as PPh3 and dppe do not produce the same behavior. Overall, the study offers an improved understanding of conjugated polymer synthesis when using weaker nucleophiles such as boronate esters or boronic acids in Suzuki–Miyaura cross-coupling.