Considering Fe^II/IV Redox Processes as Mechanistically Relevant to the Catalytic Hydrogenation of Olefins by [PhBP^iPr₃]Fe−H_x Species

Several coordinatively unsaturated pseudotetrahedral iron(II) precursors, [PhBP^iPr₃]Fe−R ([PhBP^iPr₃] = [PhB(CH₂PⁱPr₂)₃]^-; R = Me (2), R = CH₂Ph (3), R = CH₂CMe₃ (4)) have been prepared from [PhBP^iPr₃]FeCl (1) that serve as precatalysts for the room-temperature hydrogenation of unsaturated hydrocarbons (e.g., ethylene, styrene, 2-pentyne) under atmospheric H₂ pressure. The solid-state crystal structures of 2 and 3 are presented. To gain mechanistic insight into the nature of these hydrogenation reactions, a number of [PhBP^iPr₃]-supported iron hydrides were prepared and studied. Room-temperature hydrogenation of alkyls 2−4 in the presence of a trapping phosphine ligand affords the iron(IV) trihydride species [PhBP^iPr₃]Fe(H)₃(PR₃) (PR₃ = PMe₃ (5); PR₃ = PEt₃ (6); PR₃ = PMePh₂ (7)). These spectroscopically well-defined trihydrides undergo hydrogen loss to varying degrees in solution, and for the case of 7, this process leads to the structurally identified Fe(II) hydride product [PhBP^iPr₃]Fe(H)(PMePh₂) (9). Attempts to prepare 9 by addition of LiEt₃BH to 1 instead lead to the Fe(I) reduction product [PhBP^iPr₃]Fe(PMePh₂) (10). The independent preparations of the Fe(II) monohydride complex [PhBP^iPr₃]Fe^II(H)(PMe₃) (11) and the Fe(I) phosphine adduct [PhBP^iPr₃]Fe(PMe₃) (8) are described. The solid-state crystal structures of trihydride 5, monohydride 11, and 8 are compared and demonstrate relatively little structural reorganization with respect to the P₃Fe−P‘ core motif as a function of the iron center's formal oxidation state. Although paramagnetic 11 (S = 1) is quantitatively converted to the diamagnetic trihydride 5 under H₂, the Fe(I) complex 8 (S = ³/₂) is inert toward atmospheric H₂. Complex 10 is likewise inert toward H₂. Trihydrides 5 and 6 also serve as hydrogenation precatalysts, albeit at slower rates than that for the benzyl complex 3 because of a rate-contributing phosphine dependence. That these hydrogenations appear to proceed via well-defined olefin insertion steps into an Fe−H linkage is indicated by the reaction between trihydride 5 and ethylene, which cleanly produces the ethyl complex [PhBP^iPr₃]Fe(CH₂CH₃) (13) and an equivalent of ethane. Mechanistic issues concerning the overall reaction are described.