Amino pyridine iron(II) complexes: Characterization and catalytic application for atom transfer radical polymerization and catalytic chain transfer

Abstract

The amino-pyridine ligand scaffold has achieved widespread use for base metal catalysis. Atom Transfer Radical Polymerization (ATRP) is one realm where base metals have achieved success in catalysis, in particular copper and iron. Herein, the synthesis and characterization of two amino-pyridine iron(II) complexes is described where the amino carbon substitution is the point of differentiation. We hypothesized that a sterically hindered, electron rich t-butyl substituent in this position might improve the propensity of said complex to achieve ATRP since inductive electron donation from the t-butyl group may improve catalyst activity and shift the ATRP equilibrium towards the active polymer species and corresponding Fe(III) complex. Dimeric 1 and 2 ([2-[(2,6-Me2-C6H3)NHCH(R)]C5H4N]FeCl2)2 (R = t-butyl or ethyl, respectively) were identified by single crystal X-ray diffraction. Both complexes favor a high-spin iron(II) state, as evidenced by Evans NMR magnetic susceptibility measurements and suggested by gas-phase computations at the M06-L level of theory. Complexes 1 and 2 catalyze styrene polymerization at elevated temperatures (120 °C) and polymerization data suggests that ATRP operates and catalytic chain transfer (CCT) competes at extended reaction times. Complex 1 with its t-butyl substituted amino carbon displays a slightly higher ATRP activity as compared to 2 [kobs(1) = 0.31 h−1; kobs(2) = 0.10 h−1], suggesting the importance of ligand optimization for future iron ATRP catalyst development.