Synthesis, Characterization, and Electrochemical Properties of Benzyloxy-Functionalized Diiron 1,3-Propanedithiolate Complexes Relevant to the Active Site of [FeFe]-Hydrogenases

A series of new benzyloxy-functionalized 1,3-propanedithiolate (PDT)-type model complexes (A and 1–7) have been synthesized and structurally characterized. The benzyloxy-functionalized all-carbonyl complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)6 (A) can be prepared by condensation reaction of 2-benzyloxy-1,3-dibromopropane with the in situ generated (μ-LiS)2Fe2(CO)6, whereas it reacts with the in situ formed N-heterocyclic carbene 1-mesityl-3-methylimidazol-2-ylidene (IMes/Me) to give the corresponding carbene monosubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(IMes/Me) (1). The PMe3-monosubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(PMe3) (2) can be prepared by substitution of the CO ligand in parent complex A with 1 equiv of PMe3 in the presence of Me3NO·2H2O, whereas PPh3-monosubstituted and PPh3-disubstituted complexes [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5(PPh3) (3) and [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)2 (4) are prepared by reaction of A with 2 equiv of PPh3 under similar conditions. While the PPh3-disubstituted complex 4 can also be prepared by treatment of 3 in MeCN with 2 equiv of PPh3 in the presence of Me3NO·2H2O, treatment 4 with 2 equiv of PMe3 in refluxing toluene afforded unexpected PPh3/PMe3-disubstituted complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(PPh3)(PMe3) (5). Particularly interesting is that although the reaction of A with 1 equiv of diphosphine dppe in refluxing toluene affords the dppe-chelated single-butterfly complex [(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)4(dppe) (6), treatment of A in MeCN with 1 equiv of dppe in the presence of Me3NO·2H2O results in formation of the dppe-bridged double-butterfly complex {[(μ-SCH2)2CH(OCH2Ph)]Fe2(CO)5}2(dppe) (7). All new model complexes have been chatacterized by elemental analysis, spectroscopy, and particularly for 1 and 4–7 X-ray crystallography. Furthermore, complexes A, 3, and 4 have been found to be catalysts for HOAc proton reduction to H2 under electrochemical conditions.