Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published October 14, 2004 | Supplemental Material
Journal Article Open

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

Abstract

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.

Additional Information

© 2004 American Chemical Society. Publication Date (Web): October 14, 2004. Received August 18, 2004. This work was supported by the NSF (CHE-01232216) and the DOE (PECASE). E.J.D. is grateful for the Laszlo Zechmeister Fellowship (Caltech). Lawrence Henling and Theodore A. Betley are acknowledged for crystallographic assistance. We thank Paul J. Chirik for sharing results prior to publication.

Attached Files

Supplemental Material - ic0488583si20040818_030525.pdf

Supplemental Material - ic0488583si20040818_030553.cif

Files

ic0488583si20040818_030525.pdf
Files (1.3 MB)
Name Size Download all
md5:4747ae875bff18699ab98d73555eab58
168.3 kB Download
md5:2674d7abc2da683f74641c02085c4c07
1.1 MB Preview Download

Additional details

Created:
August 19, 2023
Modified:
October 26, 2023