The synthesis and application of aryl-substituted pyridine(diimine) iron complexes (RPDI)FeCH3 to the catalytic borylation of heteroarenes under thermal conditions is described. Improvements in catalyst design and performance were guided by precatalyst activation studies, where investigations into stoichiometric reactivities of iron borohydride (4- t Bu- iPrPDI)Fe(H2BPin) and iron furyl (4- t Bu- iPrPDI)Fe(2-methylfuryl) complexes revealed facile C(sp2)-H activation and a slower and potentially turnover-limiting C(sp2)-B formation step. Formation of the flyover dimer, [(4- t Bu- iPrPDI)Fe]2 was identified as a catalyst deactivation pathway and formally iron(0) complexes were found to be inactive for borylation. The pyridine(diimine) iron borohydride, flyover dimer and furyl complexes were characterized by X-ray diffraction and their electronic structures determined by a combination of NMR, EPR, and Mössbauer spectroscopies corroborated by DFT calculations. The role of the redox-active pyridine(diimine) ligand in catalytic C-H borylation was also investigated.
Keywords: C–H activation; borylation; iron; mechanism; redox-active ligand.