Expanding the PCET Thermochemistry of CpN3: N–H Bond Strengths of Metal-Free CpN3 Molecules and the Influence of Fe(CO)3 Coordination

Amine rich cyclopentadienyl (CpN3) ligands are electronically distinct from classical Cp ligand architectures, as they exhibit fascinating proton-coupled electron transfer (PCET) chemistry under acidic conditions in the presence and absence of transition metals. We now report that the exocyclic N–H bond of the isopropylamine moiety in [RCpN3]+ can be deprotonated under basic conditions (R = iPr, tBu), enabling the experimental determination of N–H bond strengths via pKa and redox potential measurements in acetonitrile. From these data, the experimentally determined N–H acidity and bond dissociation free energy of [iPrCpN3]+ are pKa = 22.6 and BDFE = 82.6 kcal/mol in acetonitrile. These results are linked with the known C–H bond thermochemistry data of [iPrCpN3]+, and computational (DFT) data sets are in excellent agreement with experiment. The exocyclic N–H bond acidity was also investigated with [iPrCpN3]+ coordinated to Fe(CO)3, and surprisingly, its pKa is largely unperturbed in the presence of a transition metal (pKa = 20–24 (experiment); 23.7 (DFT)). This prompted us to investigate the formal oxidation state of the iron center in our “FeII” starting material, with 57Fe Mössbauer spectroscopy, XANES spectroscopy, charge distribution via reporter (CDVR) analysis, and computations, revealing that the metal center is best interpreted as having a +2 formal oxidation state. These data suggest that the electronic perturbations at the electron-rich CpN3 ligand upon coordination to FeII do not significantly influence the exocyclic N–H bond acidity.