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.
The reaction of RuHCl(PPh3)3 with the tetradentate ligand [PPh2((ortho-C6H4)CH2NHCH2
-
)]2 {ethP2(NH)2} in THF produces the new complex trans-RuHCl{ethP2(NH)2} (1) as a mixture of two isomers.
The complex RuHCl{ethP2(NH)2} (1) when activated with KOtBu/KH is a very active catalyst for the
hydrogenation of benzonitrile to benzylamine in toluene, more active than the known catalyst Ru(H2)2H2(PCy3)2 (2). A mixture of 1 and 2 and base also results in efficient conversion of benzonitrile to
benzylamine. The complex RuHCl{tmeP2(NH)2} (3) where tmeP2(NH)2 is [PPh2((ortho-C6H4)CH2NHCMe2
-
)]2 is a less active catalyst for this reaction. These catalyst systems are air sensitive and extremely
moisture sensitive. Experimental and theoretical (DFT) evidence is presented for a new mechanism for
nitrile hydrogenation: the successive hydrogenation of the CN triple bond and then the CN double bond
of the intermediate imine by H+/H- transfer from a trans dihydride active catalyst. The amido complex
RuH{tmeP2N(NH)} (4) has similar activity to 3/base for the base-free hydrogenation of benzonitrile and
is moderately active for the catalytic hydration of benzonitrile. The new complex RuH(BH4){ethP2(NH)2} (7) was prepared by reacting 1 with NaBH4 but is found to be a poor catalyst for nitrile hydration.
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