Considering Fe^(II/IV) Redox Processes as Mechanistically Relevant to the Catalytic Hydrogenation of Olefins by [PhBP^(iPr)_3]Fe-H_x Species

Several coordinatively unsaturated pseudotetrahedral iron(II) precursors, [PhBP^(iPr)_3]Fe−R ([PhBP^(iPr)_3] = [PhB(CH2P^(iPr)_2)_3]^-; R = Me (2), R = CH_2Ph (3), R = CH_2CMe_3 (4)) have been prepared from [PhBP^(iPr)_3]FeCl (1) that serve as precatalysts for the room-temperature hydrogenation of unsaturated hydrocarbons (e.g., ethylene, styrene, 2-pentyne) under atmospheric H_2 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^(iPr)_3]-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^(iPr)_3]Fe(H)_3(PR_3) (PR_3 = PMe_3 (5); PR_3 = PEt_3 (6); PR3 = PMePh_2 (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^(iPr)_3]Fe(H)(PMePh_2) (9). Attempts to prepare 9 by addition of LiEt_3BH to 1 instead lead to the Fe(I) reduction product [PhBP^(iPr)_3]Fe(PMePh_2) (10). The independent preparations of the Fe(II) monohydride complex [PhBP^(iPr)_3]FeII(H)(PMe_3) (11) and the Fe(I) phosphine adduct [PhBP^(iPr)_3]Fe(PMe_3) (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 P_3Fe−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 H_2, the Fe(I) complex 8 (S = 3/2) is inert toward atmospheric H_2. Complex 10 is likewise inert toward H_2. 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^(iPr)_3]Fe(CH_2CH_3) (13) and an equivalent of ethane. Mechanistic issues concerning the overall reaction are described.