| 15530098 |
Daida EJ, Peters JC: Considering Fe (II/IV) redox processes as mechanistically relevant to the catalytic hydrogenation of olefins by [PhBP iPr 3] Fe-H x species. Inorg Chem. 2004 Nov 15;43(23):7474-85.
Several coordinatively unsaturated pseudotetrahedral iron (II) precursors, [PhBP (iPr)(3)] Fe-R ([PhBP (iPr)(3)] = [PhB (CH (2) P (i) Pr (2))(3)](-); R = Me (2), R = CH (2) Ph (3), R = CH (2) CMe (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); PR (3) = 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 (3) BH 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)] Fe (II)(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 (3) Fe-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 (2) CH (3)) (13) and an equivalent of ethane. Mechanistic issues concerning the overall reaction are described. |
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