Multisite Ligand Noninnocence of (CpN3)Fe(CO)3+ with Exogenous Hydride Donors: Kinetics and Mechanism

We show that amine-rich Cp^N3 ligands coordinated to iron display chemically noninnocent behavior in the presence of exogenous hydride donors (Cp^N3 = 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 [(Cp^N3)Fe(CO)₃]⁺ with three different hydride reagents furnishes three different iron complexes, two of which result in net H^– transfer to the Cp^N3 ligand. Exposure of [(Cp^N3)Fe(CO)₃]⁺ to NaHBEt₃ results in endo hydride addition to the Cp^N3 ligand, reaction with NaBH₄ yields an exo-Cp^N3H 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 [(Cp^N3)Fe(CO)₃]⁺ happens via direct H^– transfer to the Cp^N3 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.