Modelling the polymerisation mechanism of -diimine iron catalysts

The -diimine iron complexes have been proposed as the first example outside the realm of biomolecules where the spin-state of the transition metal involved dictates the catalytic behaviour [1]. Here, we study a typical complex of this family, R”[N,N]FeCl2, with [N,N] = Cy– N=CR”–CR”=N–Cy (Cy=cyclohexyl), R”=PhF (para-fluorophenyl), PhOMe (para-methoxyphenyl), PhNMe2 (para-dimethylaminophenyl). With R”=PhF, PhOMe, polymerisation proceeds as a catalytic chain transfer (CCT) mechanism; with R”=PhNMe2, the polymerisation follows an atom transfer radical polymerisation (ATRP) pathway. Contrary to previous suggestions, we show that the spin states of the complexes involved are not affected by the R” group, however. Instead, the different behaviour arises from a subtle interplay between the electronwithdrawing or donating character of the phenyl substituent and the iron centre, located more than 8 Å away. Small but crucial differences in the reorganisation energies effected during the reactions are also noted and discussed.