Ligand versus Metal Protonation of an Iron Hydrogenase Active Site Mimic

The protonation behavior of the iron hydrogenase active-site mimic Fe2(mu-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(mu-Hadt)(CO)4(PMe3)2+ (1 H+), the Fe-Fe bond to give Fe2(mu-adt)(mu-H)(CO)4(PMe3)2+ (1 Hy+), or both sites simultaneously to give Fe2(mu-Hadt)(mu-H)(CO)4(PMe3)22+ (1 HHy2 +). 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 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 (pK(a)=8) and the ammonium proton (pK(a)=5) of the doubly protonated cationic complex 1 HHy2+ are considerably more acidic than in the singly protonated analogues. The formation of dication 1 HHy2+ 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>10(2) 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 HHy2+, respectively, upon protonation. The doubly protonated form 1 HHy2+ is reduced at less negative potential than all previously reported hydrogenase models, although catalytic proton reduction at this potential is characterized by slow turnover.