Binding affinity of alkynes and alkenes to low-coordinate iron

We report the synthesis, spectroscopy, and structural characterization of iron−alkyne and −alkene complexes of the type LMeFe(ligand) [LMe = bulky β-diketiminate, ligand = HCCPh, EtCCEt, CH2CHPh, EtCHCHEt, HCC(p-C6H4OCH3), HCC(p-C6H4CF3)]. The neutral ligand exchanges rapidly at room temperature, and the equilibrium constants have been measured or estimated. The binding affinity toward the low-coordinate Fe follows the trend HCCPh > EtCCEt > CH2CHPh > EtCHCHEt ∼ PPh3 > benzene ≫ N2. This trend is consistent with a model in which π back-bonding from the formally FeI center is the dominant interaction in determining the relative binding affinities. In nitrogenase, alkynes are reduced while alkenes are unreactive, and this work suggests that the different binding affinities to low-coordinate Fe might explain the differential activity of the enzyme toward these two substrates.