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.

Pairing Suzuki–Miyaura cross-coupling and catalyst transfer polymerization

“…8) are highly effective for alkyl-and ester-functionalized thiophenes. 34,41,46 We have also noted that diphosphine catalysts can also be highly effective in 3-alkylthiophene polymerizations (Green/Light Blue Squares, Fig. 8) with fast reaction times and high molecular weights.…”

“…34,41,46 In our work, we discovered how critical additional water was for fast turnover in Ni-catalysed SM CTP, and how water can lead to hydrolysis and deactivation of dihalide precatalysts. 41 Fortunately, the hydrolysis event was serendipitous, as it revealed the importance of added ligand in the chain-growth polymerization of thiophenes with diphosphine ligands. With both Ni(dppp) and Ni(sepp) catalysts, additional ligand in the reaction mixture was necessary to achieve good end-group fidelity and narrow dispersities in the polymerization of 3-alkylthiophene.…”

“…In our own work, we have also noted the importance of additional equivalents of phosphine and diphosphine ligand in chain-growth polymerizations (with both Pd and Ni). 33,34,41 Choi also noted that dialkylbiarylphosphines result in superior control as compared to P t Bu 3 in the polymerization of 3-alkylthiophenes.…”

“…1). 41 Importantly, the ratedetermining step may change depending on the choice of electrophile in the reaction (e.g. Ar-Cl).…”

Chain-Growth Polymerization of Benzotriazole Using Suzuki–Miyaura Cross-Coupling and Dialkylbiarylphosphine Palladium Catalysts

“…Building from these efforts, we envisioned using Suzuki cross-coupling to polymerize benzotriazole (Scheme ). ,− The mild reaction conditions and exceptional functional group tolerance of this cross-coupling strategy should make the method applicable to a broad range of other monomers. Since dialkylbiarylphosphine palladium catalysts display outstanding catalytic activity in Suzuki–Miyaura reactions, they were natural candidates to explore with benzotriazole.…”