Ligand versus Metal Protonation of an Iron Hydrogenase Active Site Mimic

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Abstract
The protonation behavior of the iron hydrogenase active‐site mimic [Fe2(μ‐adt)(CO)4(PMe3)2] (1; adt=N‐benzyl‐azadithiolate) has been investigated by spectroscopic, electrochemical, and computational methods. The combination of an adt bridge and electron‐donating phosphine ligands allows protonation of either the adt nitrogen to give [Fe2(μ‐Hadt)(CO)4(PMe3)2]+ ([1 H]+), the FeFe bond to give [Fe2(μ‐adt)(μ‐H)(CO)4(PMe3)2]+ ([1 Hy]+), or both sites simultaneously to give [Fe2(μ‐Hadt)(μ‐H)(CO)4(PMe3)2]2+ ([1 HHy]2+). Complex 1 and its protonation products have been characterized in acetonitrile solution by IR, 1H, and 31P NMR spectroscopy. The solution structures of all protonation states feature a basal/basal orientation of the phosphine ligands, which contrasts with the basal/apical structure of 1 in the solid state. Density functional calculations have been performed on all protonation states and a comparison between calculated and experimental spectra confirms the structural assignments. The ligand protonated complex [1 H]+ (pKa=12) is the initial, metastable protonation product while the hydride [1 Hy]+ (pKa=15) is the thermodynamically stable singly protonated form. Tautomerization of cation [1 H]+ to [1 Hy]+ does not occur spontaneously. However, it can be catalyzed by HCl (k=2.2 m−1 s−1), which results in the selective formation of cation [1 Hy]+. The protonations of the two basic sites have strong mutual effects on their basicities such that the hydride (pKa=8) and the ammonium proton (pKa=5) of the doubly protonated cationic complex [1 HHy]2+ are considerably more acidic than in the singly protonated analogues. The formation of dication [1 HHy]2+ from cation [1 H]+ is exceptionally slow with perchloric or trifluoromethanesulfonic acid (k=0.15 m−1 s−1), while the dication is formed substantially faster (k>102 m−1 s−1) with hydrobromic acid. Electrochemically, 1 undergoes irreversible reduction at −2.2 V versus ferrocene, and this potential shifts to −1.6, −1.1, and −1.0 V for the cationic complexes [1 H]+, [1 Hy]+, and [1 HHy]2+, respectively, upon protonation. The doubly protonated form [1 HHy]2+ is reduced at less negative potential than all previously reported hydrogenase models, although catalytic proton reduction at this potential is characterized by slow turnover.