As expected for complexes lacking protonated thioamides, 3-Na and 4 do not have any IR bands above 3050 cm -1 that correspond to N-H stretching vibrations (Figure , blue and green traces). Proton NMR spectra of ferric 4 contain very broad peaks between -20 and +12 ppm (Figure ), and the 31 P NMR spectrum is featureless, as expected for nuclei directly bound to a paramagnetic metal. The relative integrations of the evident peaks in the 1 H NMR spectra of 3-M in THF-d8 are consistent with C2v symmetry. Taken together, the IR and NMR data support double deprotonation of the thioamide groups of the supporting ligand in 3-Na and 4. Scheme 2. Conversion of 2 to 4 using stepwise and concerted H atom removal. Yields were determined by 1 H NMR using an internal standard. Cyclic voltammetry (CV) studies of 4 showed a reversible Fe 3+/2+ couple at E1/2 = -1.42 V vs. Fc + /Fc in THF (Figure ). The peak currents displayed a linear dependence on the square root of the scan rate, indicating that the process is diffusion controlled (Figure ). Next, we tested the ability of different organic bases to deprotonate 2, using changes in the 31 P NMR chemical shifts to determine the ratio of 2 and deprotonated 2 since the conjugate base and acid are in rapid equilibrium. We assumed that the chemical shifts from the 31 P NMR spectrum of 3-K represent the shifts of fully-deprotonated 2. Addition of neither Et3N (pKaH = 12.5 in THF) nor DBU (pKaH = 16.9 in THF) gave changes in the NMR spectra of 2. An initial 1.10 equiv addition of triazabicyclodecene (TBD, pKaH = 21.0 in THF) equilibrated with 2, on the other hand, gave a 31 P NMR spectrum with peaks at 12.1 ppm and 10.8 ppm. These shifts are 4.8 ppm upfield from the 31 P resonances of 3-K but 1.4 ppm downfield of 31 P resonances of 2 (Figure ). This result indicates that the pKa of 2 is 22 ± 1 in neat THF-d8. An equivalent NMR experiment in 0.3 M [N n Bu4][PF6] in THF-d8 (the electrolytic solution used for CV) indicated a pKa of 21 ± 1. (Figure ) These data enable us to define the bond dissociation free energy (BDFE) of the N-H bond using the Bordwell equation, with CG(THF) = 60.4 ± 2 kcal/mol. This analysis gave a BDFE(N-H) of 56 ± 2 kcal/mol (Scheme 2).