Expanding the PCET Thermochemistry of Cp^N3: N–H Bond Strengths of Metal-Free Cp^N3 Molecules and the Influence of Fe(CO)₃ Coordination

Amine rich cyclopentadienyl (Cp^N3) 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 [^RCp^N3]⁺ can be deprotonated under basic conditions (R = ⁱPr, ^<i>t</i> Bu), enabling the experimental determination of N-H bond strengths via p<i>K</i> _a and redox potential measurements in acetonitrile. From these data, the experimentally determined N-H acidity and bond dissociation free energy of [^iPrCp^N3]⁺ are p<i>K</i> _a = 22.6 and BDFE = 82.6 kcal/mol in acetonitrile. These results are linked with the known C-H bond thermochemistry data of [^iPrCp^N3]⁺, and computational (DFT) data sets are in excellent agreement with experiment. The exocyclic N-H bond acidity was also investigated with [^iPrCp^N3]⁺ coordinated to Fe-(CO)₃, and surprisingly, its p<i>K</i> _a is largely unperturbed in the presence of a transition metal (p<i>K</i> _a = 20-24 (experiment); 23.7 (DFT)). This prompted us to investigate the formal oxidation state of the iron center in our "Fe^II" 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 Cp^N3 ligand upon coordination to Fe^II do not significantly influence the exocyclic N-H bond acidity.