Synthesis, Structures, and Some Properties of Diiron Oxadiselenolate (ODSe) and Thiodiselenolate (TDSe) Complexes as Models for the Active Site of [FeFe]-Hydrogenases

Parent complexes (μ-ODSe)­Fe₂(CO)₆ (A, ODSe = SeCH₂OCH₂Se) and (μ-TDSe)­Fe₂(CO)₆ (B, TDSe = SeCH₂SCH₂Se) could be prepared by oxidative addition of (HSeCH₂)₂X (X = O, S) with Fe₃(CO)₁₂. While reactions of A with 1 equiv of monophosphines in the presence of the decarbonylating agent Me₃NO afforded the corresponding phosphine-monosubstituted complexes (μ-ODSe)­Fe₂(CO)₅(L) (1, L = Ph₃P; 2, L = Ph₂POMe), the N-heterocyclic carbene (NHC)-monosubstituted complexes (μ-ODSe)­Fe₂(CO)₅(L) (3, L = I_Mes; 4, L = I_Mes/Me) were prepared by reactions of the 1,3-bis­(mesityl)­imidazolium salt I_Mes·HCl and 1-mesityl-3-methylimidazolium salt I_Mes/Me·HI with n-BuLi, followed by treatment of the corresponding NHC intermediates with A. The phosphine-containing imidazolium salt I_{Mes/CH2CH2PPh2}·HCl reacted with A in the presence of Me₃NO to give the imidazolium/phosphine-monosubstituted complex (μ-ODSe)­Fe₂(CO)₅(I_{Mes/CH2CH2PPh2}·HCl) (5), whereas it reacted with t-BuOK or n-BuLi, followed by treatment of A with the resulting intermediate NHC/phosphine or both the resulting NHC/phosphine and phosphine Ph₂PCHCH₂, to afford the corresponding NHC/phosphine-disubstituted complex [(μ-ODSe)­Fe₂(CO)₅]₂(I_{Mes/CH2CH2PPh2}) (6) and complex 6 along with (μ-ODSe)­Fe₂(CO)₅(Ph₂PCHCH₂) (7), respectively. In addition, 7 could also be produced simply by reaction of 6 with n-BuLi. The phosphine-monosubstituted complexes (μ-TDSe)­Fe₂(CO)₅(L) (8, L = Ph₃P; 9, L = Ph₂PH) were similarly prepared by reactions of B with 1 equiv of the corresponding monophosphines in the presence of Me₃NO, whereas reaction of B with m-chloroperoxybenzoic acid afforded the corresponding bridgehead S atom-oxidized complex (μ-TDSeO)­Fe₂(CO)₆ (10, TDSeO = SeCH₂S­(O)­CH₂Se). While complexes A/B and 1–10 were structurally characterized, a comparative study on H₂ production from HOAc catalyzed by parent complexes A/B and their sulfur analogues (μ-ODT)­Fe₂(CO)₆/(μ-TDT)­Fe₂(CO)₆ was carried out.