A 3-Fluoropyridine-Substituted Grubbs III Catalyst as a 19F NMR Probe for Chain Chelation in ROMP

Abstract

Propagation rates in ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) can be attenuated by the transient coordination of the growing chain to the metal center. Direct spectroscopic observation of such chelation under catalytically relevant conditions remains challenging, owing to the fast exchange dynamics of propagating intermediates and spectral overlap in conventional ¹H NMR. Here, we report a third-generation Grubbs catalyst (GIII) in which the ancillary pyridine is substituted with 3-fluoropyridine (F-GIII), such that chain chelation may be observed using variable-temperature ¹⁹F nuclear magnetic resonance (VT-NMR) spectroscopy. F-GIII is readily prepared analogously to GIII from GII, giving a solid-state structure closely related to its 3-bromopyridine analogue, and exhibits comparable reactivity. A selection of chelation-prone or nonchelating norbornene- and oxanorbornene-based monomers is investigated using ¹⁹F VT NMR and displays either well-defined temperature-dependent equilibria between coordinated and dissociated pyridine or persistent dissociation consistent with chelating behavior. These results establish ¹⁹F NMR as a convenient, noninvasive, and direct probe for studying growing polymer chain-catalyst interactions during ROMP.