Multisite Ligand Noninnocence of (Cp^N3)Fe(CO)₃⁺ with Exogenous Hydride Donors: Kinetics and Mechanism

We show that amine-rich CpN3 ligands coordinated to iron display chemically noninnocent behavior in the presence of exogenous hydride donors (CpN3 = 1,4-dimethyl-5,7-diphenyl-6-(pyrrolidin-1-yl)-2,3,4,6-tetrahydro-1H-cyclopenta[b]pyrazin-6-yl). The reaction of piano-stool iron complex [(CpN3)Fe(CO)3]+ with three different hydride reagents furnishes three different iron complexes, two of which result in net H– transfer to the CpN3 ligand. Exposure of [(CpN3)Fe(CO)3]+ to NaHBEt3 results in endo hydride addition to the CpN3 ligand, reaction with NaBH4 yields an exo-CpN3H product, and an iron hydride complex forms in the presence of [nBu4N][HCOO]. While state-of-the-art DFT computations indicate that NaBH4 addition to [(CpN3)Fe(CO)3]+ happens via direct H– transfer to the CpN3 ligand, reactions with HBEt3– or HCOO– initially generate a key iron formyl intermediate which has been experimentally observed. Herein, we present a blend of experimental and computational data to unravel some elementary steps that lead to the formation of these chemically distinct hydride addition products.