Multi-electron redox reactivity of a samarium(ii) hydrido complex

Well-defined low-valent molecular rare-earth metal hydrides are rare, and limited to Yb²⁺ and Eu²⁺ centers. Here, we report the first example of the divalent samarium(ii) hydrido complex [(Cp^Ar5)Sm^II(μ-H)(DABCO)]₂ (4) (Cp^Ar5 = C₅Ar₅, Ar = 3,5-ⁱPr₂-C₆H₃; DABCO = 1,4-diazabicyclooctane) supported by a super-bulky penta-arylcyclopentadienyl ligand, resulting from the hydrogenolysis of the samarium(ii) alkyl complex [(Cp^Ar5)Sm^II{CH(SiMe₃)₂}(DABCO)] (3). Complex 4 exhibits multi-electron redox reactivity toward a variety of substrates. Exposure of complex 4 to CO₂ results in the formation of the trivalent samarium(iii) mixed-bis-formate/carbonate complex [(Cp^Ar5)Sm^III(μ-η²:η¹-O₂CH)(μ-η²:η²-CO₃)(μ-η¹:η¹-O₂CH)Sm^III(Cp^Ar5)(DABCO)] (8), mediated by hydride insertion and reductive disproportionation reactions. Complex 4 shows four-electron reduction toward four equivalents of CS₂ to afford the trivalent samarium(iii) bis-trithiocarbonate complex [(Cp^Ar5)Sm^III(μ-η²:η²-CS₃)(DABCO)]₂ (9). A mechanistic study of the formation of complex 8 was carried out using DFT calculations.