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 [(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(CO)(6) (A) can be prepared by condensation reaction of 2-benzyloxy-1,3-dibromopropane with the in situ generated (mu-LiS)(2)Fe-2(CO)(6), whereas it reacts with the in situ formed N-heterocyclic carbene 1-mesity1-3-methylimidazol-2-ylidene (I-Mes/Me) to give the corresponding carbene monosubstituted complex [(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(CO)S(I-Mes/Me) (1). The PMe3-monosubstituted complex [(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(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 center dot 2H(2)O, whereas PPh3-monosubstituted and PPh3-disubstituted complexes [(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(CO)(5)(PPh3) (3) and [(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(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 center dot 2H(2)O, treatment 4 with 2 equiv of PMe3 in refluxing toluene afforded unexpected PPh3/PMe3-disubstituted complex [(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(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 [(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(CO)(4)(dppe) (6), treatment of A in MeCN with 1 equiv of dppe in the presence of Me3NO center dot 2H(2)O results in formation of the dppe-bridged double-butterfly complex {[(mu-SCH2)(2)CH(OCH2Ph)]Fe-2(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 H-2 under electrochemical conditions.