Ligand versus metal protonation of an iron hydrogenase active site mimic

The protonation behavior of the iron hydrogenase active-site mimic [Fe(2)(mu-adt)(CO)(4)(PMe(3))(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 [Fe(2)(mu-Hadt)(CO)(4)(PMe(3))(2)](+) ([1H](+)), the Fe-Fe bond to give [Fe(2)-(mu-adt)(mu-H)(CO)(4)(PMe(3))(2)](+) ([1][Hy](+)), or both sites simultaneously to give [Fe(2)(mu-Hadt)(mu-H)(CO)(4)(PMe(3))(2)](2+) ([1HHy](2+)). Complex 1 and its protonation products have been characterized in acetonitrile solution by IR, (1)H, and (31)P 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 I in the solid state. Densitv 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 [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 HCI (k=2.2M(-1)S(-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(-1)S(-1)), while the dication is formed substantially faster (k > 102 M(-1) S(-1)) with hydrobromic acid. Electrochemically, I 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](+), 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.