Rh(I) coordination chemistry of chiral α-aminophosphine(η6-arene)chromium tricarbonyl ligands

The diastereoselective addition of Ph2PH to the chiral ortho-substituted eta(6)-benzaldimine complexes (eta(6)-o-X-C6H4-CH = NAr)Cr(CO)(3) (1, X = MeO, Ar = p-C6H4OMe; 2, X = Cl, Ar = Ph) leads to the formation of the corresponding chiral aminophosphines (alpha-P,N) Ph2P-CH(Ar-1)-NHAr2 (3, Ar-1 = o-C6H4(OCH3)[Cr(CO)(3)], Ar-2 = p-C6H4OCH3; 4, Ar-1 = o-C6H4Cl[Cr(CO)(3)], Ar-2 = Ph) in equilibrium with the starting materials. The uncomplexed benzaldimine (o-ClC6H4CH = NPh), 2', analogously produces an equilibrium amount of the corresponding aminophosphine Ph2P-CH(Ar-1)-NHAr2 (4', Art = o-C6H4Cl, Ar-2 = Ph). Depending on the equilibrium constant, the subsequent addition Of 1/2 equiv of [RhCl(COD)](2) (COD = 1,5-cyclooctadiene) leads to either Ph2PH oxidative addition in the case of 3 or to the corresponding [RhCl(COD)(alpha-P,N)] complexes [RhCl(COD)(Ph2P-CH{o-C6H4Cl[Cr(CO)(3)]}-NHPh)] (5) and [RhCl(COD)(Ph2P-CH(o-C6H4Cl)-NHPh)] (5') in the cases of the aminophosphines 4 and 4'. The addition of the latter ligands, as racemic mixtures, to 1/4 equiv of [Rh(CO)(2)Cl](2) leads to the [RhCl(CO)(alpha-P,N)(2)] complexes [RhCO(Ph2P-CH{o-C6H4Cl[Cr(CO)(3)]}-NHPh)(2)Cl] (7) or [RhCO(Ph2P-CH(o-C6H4Cl)-NHPh)(2)Cl] (7') as mixtures of (R-C,S-C)/(S-C,R-C) and (R-C,R-C)/(S-C,S-C) diastereomers. The rhodium complexes 5 and 7' have been fully characterized by IR and P-31 NMR spectroscopies and X-ray crystallography. These compounds exhibit intramolecular Rh-Cl...H-N interactions in the solid state and in solution. The stability of the new rhodium complexes has been studied under different CO pressures. Under I atm of CO, 5 is converted to an unstable complex [RhCl(CO)(2)(alpha-P,N)], 6, which undergoes ligand redistribution leading to 7 plus an unidentified complex. This reaction is inhibited under higher CO or syngas pressure, as confirmed by the observation of the same catalytic activity in hydroformylation when styrene was added to a catalytic mixture that was either freshly prepared or left standing for 20 h under high CO pressure.