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

The first imidazolyl “upper-rim” derivatives of 1,3,5-triaza-7-phoshaadamantane (PTA), namely, 1-methylimidazolyl-(1,3,5-triaza-7-phosphatricyclo­[3.3.1.1^3,7]­dec-6-yl)­methanol (2, PTA-CH­(1-MeIm)­OH) and bis­(1-methylimidazolyl)­(1,3,5-triaza-7-phosphatricyclo­[3.3.1.1^3,7]­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[3.3.1.1^3,7]­dec-6-yl)­methanol (PZA) and 4′-dimethylaminophenyl­(1,3,5-triaza-7-phosphatricyclo­[3.3.1.1^3,7]­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.