Synthesis and Characterization of Iron(II) Complexes with Tetradentate Diiminodiphosphine or Diaminodiphosphine Ligands as Precatalysts for the Hydrogenation of Acetophenone

Abstract

Six complexes of the type trans-[Fe(NCMe)2(P-N-N-P)]X2 (X = BF4(-), B{Ar(f)}4(-)) (Ar(f) = 3,5-(CF3)2C6H3) containing diiminodiphosphine ligands and the complexes trans-[Fe(NCMe)2(P-NH-NH-P)][BF4]2 with a diaminodiphosphine ligand were obtained by the reaction of Fe(II) salts with achiral and chiral P-N-N-P or P-NH-NH-P ligands, respectively, in acetonitrile at ambient temperature. The P-N-N-P ligands are derived from reaction of ortho-diphenylphosphinobenzaldehyde with the diamines 1,2-ethylenediamine, 1,3-propylenediamine, (S,S)-1,2-disopropyl-1,2-diaminoethane, and (R,R)-1,2-diphenyl-1,2-diaminoethane. Some complexes could also be obtained for the first time in a one-pot template synthesis under mild reaction conditions. Single crystal X-ray diffraction studies of the complexes revealed a trans distorted octahedral structure around the iron. The iPr or Ph substituents on the diamine were found to be axial in the five-membered Fe-N-CHR-CHR-N- ring of the chiral P-N-N-P ligands. A steric clash between the imine hydrogen and the substituent probably determines this stereochemistry. The diaminodiphosphine complex has longer Fe-N and Fe-P bonds than the analogous diiminodiphosphine complex. The new iron compounds were used as precatalysts for the hydrogenation of acetophenone. The complexes without axial substituents on the diamine had moderate catalytic activity while that with axial Ph substituents had low activity but fair (61%) enantioselectivity for the asymmetric hydrogenation of acetophenone. The fact that the diaminodiphosphine complex has a slightly higher activity than the corresponding diiminodiphosphine complex suggests that hydrogenation of the imine groups in the P-N-N-P ligand may be important for catalyst activation. Evidence is provided, including the first density-functional theory calculations on iron-catalyzed outer-sphere ketone hydrogenation, that the mechanism is similar to that of ruthenium analogues.

Photocatalytic Hydrogen Generation from Water with Iron Carbonyl Phosphine Complexes: Improved Water Reduction Catalysts and Mechanistic Insights

“…Reactions in the presence of iron (0) [31] Table 1, entry 10), bearing two phosphine ligands, or simple iron(0) powder were used. The results for the different iron(II) and ironA C H T U N G T R E N N U N G (III) sources listed above showed that activity is strictly limited to low-valent iron(0) complexes.…”

Microwave-assisted synthesis of cyclopentadienone iron tricarbonyl complexes: molecular structures of [{η4-C4R2C(O)C4H8}Fe(CO)3] (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) and attempts to prepare Fe(II) hydroxycyclopentadienyl–hydride complexes

“…Ruthenium catalysts are used in a number of asymmetric hydrogen transfer systems. For example, the most effective and efficient class of catalysts for the enantioselective hydrogenation of pro-chiral ketones to chiral alcohols are the ruthenium(II) diphosphine-diaminebased complexes developed by Noyori et al [18,19], and recent practical [20,21] and theoretical [22,23] work has suggested that related iron complexes may be viable alternatives. The dimeric ruthenium complex (Scheme 1), widely known as the Shvo catalyst [24][25][26][27][28][29][30][31], acts as a catalyst for hydrogen transfer reactions between alcohols and aldehydes/ ketones, a process resulting from the scission of the Shvo catalyst into saturated 18-electron and unsaturated 16-electron mononuclear fragments (Scheme 1).…”

Asymmetric catalysis using iron complexes – ‘Ruthenium Lite’?

“…Complexes 4 -8 were also prepared, using the in situ templating method, and tested in ketone hydrogenation reactions. 21 The mechanism of the reduction reaction is not yet fully understood, however Morris has speculated that the 80 imine group in the ligands may be reduced, in-situ, to give the saturated complexes, which act as the active catalyst precursors. 21,22 Evidence for this came from the observation that complexes 4 and 6 give very similar conversions of ketones to hydrogenation products under the same conditions.…”

“…21 The mechanism of the reduction reaction is not yet fully understood, however Morris has speculated that the 80 imine group in the ligands may be reduced, in-situ, to give the saturated complexes, which act as the active catalyst precursors. 21,22 Evidence for this came from the observation that complexes 4 and 6 give very similar conversions of ketones to hydrogenation products under the same conditions. Should this be the case, then the mechanism may resemble that commonly associated with the closely-related ruthenium 5 complexes (Figure 2), 16 in which hydrogen is transferred to substrate through a concerted, 6-membered ring mechanism, the well-defined nature of which contributes to the high level of enantiocontrol in the reduction.…”