ABSTRACT
The mechanism and energetics of CO, 1-hexene, and 1-hexyne substitution from the complexes (SBenz)2 [Fe2 (CO)6 ] (SBenz=SCH2 Ph) (1-CO), (SBenz)2 [Fe2 (CO)5 (η(2) -1-hexene)] (1-(η(2) -1-hexene)), and (SBenz)2 [Fe2 (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 (SN 1) and associative (SN 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(CH2 )3 S) underscores the unique characteristics of a three-atom S-S linker in the active site of diiron hydrogenases.
Subject(s)
Alkenes/chemistry , Hydrogenase/chemistry , Iron-Sulfur Proteins/chemistry , Binding Sites , Catalysis , Crystallography, X-Ray , Iron-Sulfur Proteins/metabolism , Kinetics , Ligands , Models, Molecular , Spectrophotometry, Infrared , ThermodynamicsABSTRACT
Photolysis of an iron tricarbonyl complex in the presence of 1-hexyne results in the activation of the terminal C-H bond to yield an iron-alkynyl species. The reaction proceeds through a single transition state with an activation enthalpy of 13.5 kcal mol(-1). The resulting molecule may have potential as a C-C bond formation reagent.
ABSTRACT
Studies focused on the dehydrogenation of amine-borane by diiron complexes that serve as well-characterized rudimentary models of the diiron subsite in [FeFe]-hydrogenase are reported. Complexes of formulation (µ-SCH2XCH2S)[Fe(CO)3]2, with X = CH2, CMe2, CEt2, NMe, NtBu, and NPh, 1-CO through 6-CO, respectively, were determined to be photocatalysts for release of H2 gas from a solution of H3B â NHMe2 (B:A(s)), dissolved in THF. The thermal displacement of the tertiary amine-borane, H3B â NEt3 (B:A(t)) from photochemically generated (µ-SCH2XCH2S)[Fe(CO)3][Fe(CO)2(µ-H)(BH2-NEt3)], 1-B:A(t) through 6-B:A(t), by P(OEt)3 was monitored by time-resolved FTIR spectroscopy. Rates and activation barriers for this substitution reaction were consistent with a dissociative mechanism for the alkylated bridgehead species 2-CO through 6-CO, and associative or interchange for 1-CO. DFT calculations supported an intermediate [I] for the dissociative process featuring a coordinatively unsaturated diiron complex stabilized by an agostic interaction between the metal center and the C-H bond of an alkyl group on the central bridgehead atom of the SRS linker. The rate of H2 production from the initially formed 1-B:A(s) through 6-B:A(s) complexes was inversely correlated with the lifetime of the analogous 1-B:A(t) through 6-B:A(t) adducts. Possible mechanisms are presented which feature involvement of the pendent nitrogen base as well as a separate mechanism for the all carbon bridgeheads.
