New Diphosphine Ligands Containing Ethyleneglycol and Amino Alcohol Spacers for the Rhodium-Catalyzed Carbonylation of Methanol

Abstract

The new diphosphine ligands Ph2PC6H4C(O)X(CH2)2OC(O)C6H4PPh2 (1: X=NH; 2: X=NPh; 3: X=O) and Ph2PC6H4C(O)O(CH2)2O(CH2)2OC(O)C6H4PPh2 (5) as well as the monophosphine ligand Ph2PC6H4C(O)X(CH2)2OH (4) have been prepared from 2‐diphenylphosphinobenzoic acid and the corresponding amino alcohols or diols. Coordination of the diphosphine ligands to rhodium, iridium, and platinum resulted in the formation of the square‐planar complexes [(PP)Rh(CO)Cl] (6: PP=1; 7: PP=2; 8: PP=3), [(PP)Rh(CO)Cl]2 (9: PP=5), [(P‐P)Ir(cod)Cl] (10: PP=1; 11: PP=2; 12: PP=3), [(PP)Ir(CO)Cl] (13: PP=1; 14: PP=2; 15: PP=3), and [(PP)PtI2] (18: PP=2). In all complexes, the diphosphine ligands are trans coordinated to the metal center, thanks to the large spacer groups, which allow the two phosphorus atoms to occupy opposite positions in the square‐planar coordination geometry. The trans coordination is demonstrated unambiguously by the single‐crystal X‐ray structure analysis of complex 18. In the case of the diphosphine ligand 5, the spacer group is so large that dinuclear complexes with ligand 5 in bridging positions are formed, maintaining the trans coordination of the P atoms on each metal center, as shown by the crystal structure analysis of 9. The monophosphine ligand 4 reacts with [{Ir(cod)Cl}2] (cod=cyclooctadiene) to give the simple derivative [(4)Ir(cod)Cl] (16) which is converted into the carbonyl complex [(4)Ir(CO)2Cl] (17) with carbon monoxide. The crystal structure analysis of 16 also reveals a square‐planar coordination geometry in which the phosphine ligand occupies a position cis with respect to the chloro ligand. The diphosphine ligands 1, 2, 3, and 5 have been tested as cocatalysts in combination with the catalyst precursors [{Rh(CO)2Cl}2] and [{Ir(cod)Cl}2] or [H2IrCl6] for the carbonylation of methanol at 170 °C and 22 bar CO. The best results (TON 800 after 15 min) are obtained for the combination 2/[{Rh(CO)2Cl}2]. After the catalytic reaction, complex 7 is identified in the reaction mixture and can be isolated; it is active for further runs without loss of catalytic activity.

Square‐Planar Carbonylchlororhodium(I) Complexes Containing trans‐Spanning Diphosphine Ligands as Catalysts for the Carbonylation of Methanol

“…As shown by their single-crystal X-ray structure analyses, the isomeric 6a and 6b differ in the orientation of the two acetyl ligands and the coordination sites of the bridging diphos ligand [12]. The formation of these two isomers is presumably due to the rotational freedom of the CÀO (acyl) bond with respect to the more restricted rotation of the CÀN (amide) bond of pbpbz in 1, which can be explained by an increased steric angle of the N-atom carrying an additional substituent.…”

“…Precursor [12] [16] and N,N'-naphthalene-1,8-diylbis[2-(diphenylphosphino)benzamide] (nbpb) [17] as well as the well known trans-configurated racemic ligand…”

“…The stability of such Rh-complexes employed as catalysts under the harsh carbonylation conditions is a crucial point. Therefore, we decided to develop trans-spanning P,P-linked bis [ [12]: after generation of the catalytically active iodo derivative 2, the oxidative addition of MeI generates complex 3, which rearranges to give complex 4. Uptake of CO converts 4 into 5, which reductively eliminates AcI to regenerate 2.…”

“…Uptake of CO converts 4 into 5, which reductively eliminates AcI to regenerate 2. From the catalytic mixture, we have isolated the dinuclear complexes 6a and 6b, which arise from the mononuclear complex 4 by loss of a bis[phosphine] ligand; 6a and 6b may be considered a resting state of the key species 4 [12]. Scheme …”

New dinuclear arene ruthenium complexes with P,S- or P,O-chelating ligands

“…Diphosphine ligands have received particular attention, because in general they form more stable complexes than their non-chelating phosphine analogues under the harsh reaction conditions required for catalysis. Diphosphine ligands have been used in particular for the synthesis of catalysts for allylic alkylation [2] and for methanol carbonylation [3]. Tetradentate C 2 -symmetric ligands, possessing two phosphine groups and two other heteroatoms which increase the electron density at the metal centre, have been intensively studied [3][4][5][6][7].…”