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Ligand Displacement Reaction Paths in a Diiron Hydrogenase Active Site Model Complex
Author(s) -
Blank Jan H.,
Moncho Salvador,
Lunsford Allen M.,
Brothers Edward N.,
Darensbourg Marcetta Y.,
Bengali Ashfaq A.
Publication year - 2016
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.201601677
Subject(s) - alkyne , chemistry , ligand (biochemistry) , associative substitution , alkene , photochemistry , reaction mechanism , crystallography , computational chemistry , stereochemistry , medicinal chemistry , catalysis , organic chemistry , biochemistry , receptor
The mechanism and energetics of CO, 1‐hexene, and 1‐hexyne substitution from the complexes (SBenz) 2 [Fe 2 (CO) 6 ] (SBenz=SCH 2 Ph) ( 1 ‐CO), (SBenz) 2 [Fe 2 (CO) 5 (η 2 ‐1‐hexene)] ( 1 ‐(η 2 ‐1‐hexene)), and (SBenz) 2 [Fe 2 (CO) 5 (η 2 ‐1‐hexyne)] ( 1 ‐(η 2 ‐1‐hexyne)) were studied by using time‐resolved infrared spectroscopy. Exchange of both CO and 1‐hexyne by P(OEt) 3 and pyridine, respectively, proceeds by a bimolecular mechanism. As similar activation enthalpies are obtained for both reactions, the rate‐determining step in both cases is assumed to be the rotation of the Fe(CO) 2 L (L=CO or 1‐hexyne) unit to accommodate the incoming ligand. The kinetic profile for the displacement of 1‐hexene is quite different than that for the alkyne and, in this case, both reaction channels, that is, dissociative (S N 1) and associative (S N 2), were found to be competitive. Because DFT calculations predict similar binding enthalpies of alkene and alkyne to the iron center, the results indicate that the bimolecular pathway in the case of the alkyne is lower in free energy than that of the alkene. In complexes of this type, subtle changes in the departing ligand characteristics and the nature of the mercapto bridge can influence the exchange mechanism, such that more than one reaction pathway is available for ligand substitution. The difference between this and the analogous study of (μ‐pdt)[Fe(CO) 3 ] 2 (pdt=S(CH 2 ) 3 S) underscores the unique characteristics of a three‐atom S−S linker in the active site of diiron hydrogenases.