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Ligand versus Metal Protonation of an Iron Hydrogenase Active Site Mimic
Author(s) -
Eilers Gerriet,
Schwartz Lennart,
Stein Matthias,
Zampella Giuseppe,
de Gioia Luca,
Ott Sascha,
Lomoth Reiner
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700019
Subject(s) - protonation , chemistry , hydride , phosphine , tautomer , ligand (biochemistry) , hydrogenase , crystallography , stereochemistry , metal , medicinal chemistry , catalysis , organic chemistry , ion , biochemistry , receptor
The protonation behavior of the iron hydrogenase active‐site mimic [Fe 2 (μ‐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 (μ‐Hadt)(CO) 4 (PMe 3 ) 2 ] + ( [1 H] + ), the FeFe bond to give [Fe 2 (μ‐adt)(μ‐H)(CO) 4 (PMe 3 ) 2 ] + ( [1 Hy] + ), or both sites simultaneously to give [Fe 2 (μ‐Hadt)(μ‐H)(CO) 4 (PMe 3 ) 2 ] 2+ ( [1 HHy] 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 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] + (p K a =12) is the initial, metastable protonation product while the hydride [1 Hy] + (p K a =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 (p K a =8) and the ammonium proton (p K a =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 >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 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.