Ligand vs. Metal Protonation of an Iron Hydrogenase Active Site Mimic

The protonation behavior of the iron hydrogenase active-site mimic [Fe ₂(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(PMe ₃)₂]⁺ ([1H]⁺), the Fe-Fe bond to give [Fe₂-(μ-adt)(μ-H)(CO)₄(PMe₃) ₂]⁺ ([1Hy]⁺), or both sites simultaneously to give [Fe₂(μ-Hadt)(μ-H)(CO)₄(PMe₃] ²⁺ ([1HHy]²⁺). Complex 1 and its protonation products have been characterized in acetonitrile solution by 1R, 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 calcula ions have been performed on all protonation states and a comparison between calculated and experimental spectra confirms the structural assignments. The ligand protonated complex [1H]⁺ (pK_a = 12) is the initial, metastable protonation product while the hydride [1Hy] ⁺ (pK_a=15) is the thermodynamically stable singly protonated form. Tautomerization of cation [1H]⁺ to [1Hy]⁺ does not occur spontaneously. However, it can be catalyzed by HCl (k = 2.2 M^-1S^-1), which results in the selective formation of cation [1Hy]⁺. The protonations of the two basic sites have strong mutual effects on their basicities such that the hydride (pK_a = 8) and the ammonium proton (pK_a = 5) of the doubly protonated cationic complex [1HHy]²⁺ are considerably more acidic than in the singly protonated analogues. The formation of dication [1HHy]²⁺ from cation [1H]⁺ is exceptionally slow with perchloric or trifluoromethanesulfonic acid (k = 0.15 M^-1S^-1), while the dication is formed substantially faster (k> 10²M ^-1S^-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 [1H]⁺, [1Hy]¹, and [1HHy]²⁺, respectively, upon protonation. The doubly protonated form [1HHy]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. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.