Imidazolyl-PTA Derivatives as Water-Soluble (P,N) Ligands for Ruthenium-Catalyzed Hydrogenations

Abstract

The first imidazolyl “upper-rim” derivatives of 1,3,5-triaza-7-phoshaadamantane (PTA), namely, 1-methylimidazolyl-(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (2, PTA-CH(1-MeIm)OH) and bis(1-methylimidazolyl)(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (3, PTA-C(1-MeIm)2OH), were synthesized in fair yields by reaction of PTA-Li with 1-methyl-2-imidazole carboxyaldehyde and bis(N-methylimidazole-2-yl) ketone, respectively. Compounds 2 and 3 exhibit higher water solubility than most upper-rim derivatives of PTA. The two ligands reacted cleanly with [Ru(η6-p-cymene)Cl2]2 in refluxing CHCl3 to form κ2-P,N-[(η6-p-cymene)Ru{PTA-CH(1-MeIm)OH}Cl]Cl (8) and κ2-P,N-[(η6-p-cymene)Ru{PTA-C(1-MeIm)2OH}Cl]Cl (9). Ligands phenyl(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (PZA) and 4′-dimethylaminophenyl(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (PZA-NMe2) were also used to coordinate to Ru, and the corresponding κ1-P-[(η6-p-cymene)Ru(PZA)Cl2] (6) and κ1-P-[(η6-p-cymene)Ru(PZA-NMe2)Cl2] (7) were isolated and characterized. The ability of the ligands to coordinate in κ1-P vs κ2-P,E (E = O, N) modes was established by NMR experiments and complemented by DFT calculations. The X-ray crystal structure of the iodide analogue of 7 was obtained. Complexes 6–9 were tested as catalysts for acetophenone reduction using different hydrogen sources under mild conditions, and preliminary results are here described.