A One-Step Preparation of Tetradentate Ligands with Nitrogen and Phosphorus Donors by Reductive Amination and Representative Iron Complexes.

The synthesis and use of the first examples of unsymmetrical, mixed phosphine donor tripodal NPP2' ligands N(CH2CH2PR2)2(CH2CH2PPh2) are presented. The ligands are synthesized via a convenient, one pot reductive amination using 2-(diphenylphosphino)ethylamine and various substituted phosphonium dimers in order to introduce mixed phosphine donors substituted with P/P', those being Ph/Cy (2), Ph/iPr (3), Ph/iBu (4), Ph/o-Tol (5), and Ph/p-Tol (6). Additionally, we have developed the first known synthesis of a symmetrical tripodal NP3 ligand N(CH2CH2PiBu2)3 using bench safe ammonium acetate as the lone nitrogen source (7). This new protocol eliminates the use of extremely dangerous nitrogen mustard reagents typically required to synthesize NP3 ligands. Some of these tetradentate ligands and also P2NN' ligands N(CH2-o-C5H4N)(CH2CH2PR2)2 (P2NN'-Cy, R = Cy; P2NN'-Ph, R = Ph) prepared by reductive amination using 2-picolylamine are used in the synthesis and reactions of iron complexes. FeCl2(P2NN'-Cy) (8) undergoes single halide abstraction with NaBPh4 to give the trigonal bipyramidal complex [FeCl(P2NN'-Cy)][BPh4] (9). Upon exposure to CO(g), complex 9 readily coordinates CO giving [FeCl(P2NN'-Cy)(CO)][BPh4] (10), and further treatment with an excess of NaBH4 results in formation of the hydride complex [Fe(H)(P2NN'-Cy)(CO)][BPh4] (11). Our previously reported complex FeCl2(P2NN'-Ph) undergoes double halide abstraction with NaBPh4 in the presence of the coordinating solvent to give [Fe(NCMe)2(P2NN'-Ph)][BPh4]2 (12). Ligand 3 can be coordinated to FeCl2, and upon sequential halide abstraction, treatment with NaBH4, and exposure to an atmosphere of dinitrogen, the dinitrogen hydride complex [Fe(H)(NPP2'-iPr)(N2)][BPh4] (13) is isolated. Our symmetrical NP3 ligand 7 can also be coordinated to FeCl2 and, upon exposure to an atmosphere of CO(g), selectively forms [FeCl(NP3)(CO)][BPh4] (14) after salt metathesis with NaBPh4. Complex 14 can be treated with an excess of NaBH4 to give the hydride complex [Fe(H)(NP3)(CO)][BPh4] (15), which can further be deprotonated/reduced to the Fe(0) complex Fe(NP3)(CO) (16) upon treatment with an excess of KH.

Enantioselective Hydrogenation of Activated Aryl Imines Catalyzed by an Iron(II) P-NH-P' Complex.

Chiral amines are key building blocks in synthetic chemistry with numerous applications in the agricultural and pharmaceutical industries. Asymmetric imine hydrogenation, particularly with iridium catalysts, is well developed. However, imine reduction still remains challenging in the context of replacing such a precious metal with a cheap, nontoxic, and environmentally friendly substitute such as iron. Here, we report that an unsymmetrical iron P-NH-P' catalyst that was previously shown to be effective for the asymmetric hydrogenation of aryl ketones is also a very effective catalyst for the asymmetric hydrogenation of prochiral aryl imines activated with N-diphenylphosphinoyl or N-tosyl groups. The P-NH-P' abbreviation stands for (S,S)-PPh2CHPhCHPhNHCH2CH2PiPr2. Density functional theory results suggest that, surprisingly, the NH group on the catalyst activates and orients the imine to hydride attack by hydrogen bonding to the PO or SO group on the imine nitrogen, as opposed to the imine nitrogen itself. This may explain why N-Ph and N-Bu imines are not hydrogenated.

Unsymmetrical Iron P-NH-P' Catalysts for the Asymmetric Pressure Hydrogenation of Aryl Ketones.

The reductive amination of α-dialkylphosphine acetaldehydes with enantiopure ß-aminophosphines is a new, versatile route to unsymmetrical tridentate (pincer) ligands P-NH-P'. Four new ligands PR2 CH2 CH2 NHCHR'CHR''PPh2 (R=iPr, Cy, R'=Ph, CH(CH3 )2 , R''=Ph, H) prepared in this way are used to make the iron(II) complexes mer-FeCl2 (CO)(P-NH-P') and mer-FeCl(H)(CO)(P-NH-P'). The hydride complex with the rigid ligand with R'=R''=Ph is an efficient and highly enantioselective homogeneous asymmetric pressure hydrogenation (APH) catalyst. Prochiral aryl ketones are reduced under mild conditions (THF, 0.1 mol % catalyst, 1 mol % KOtBu, 5-10 bar, 50 °C) to the (S)-alcohols, usually in enantiomeric excess (ee) greater than 90 %. DFT calculations provided transition-state structures for the enantiodetermining hydride-transfer step.