Water soluble diphosphine ligands based on 1,3,5-triaza-7-phosphaadamantane (PTA-PR2): Synthesis, coordination chemistry, and ruthenium catalyzed nitrile hydration

Abstract

Two chiral chelating 1,3,5-triaza-7-phosphaadamantane (PTA) derivatives were synthesized, in racemic form, by addition of ClPiPr2 or ClP(NiPr)2(CH2)2 to lithiated PTA. PTA-PiPr2 (1) and PTA-P(NiPr)2(CH2)2 (2) were isolated in good yield, 73% and 56% respectively, and fully characterized by multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. PTA-PiPr2 is highly air sensitive, but stable and somewhat soluble in degassed water. PTA-P(NiPr)2(CH2)2 (2) is air-stable, but decomposes in water over the course of hours. Two tungsten tetracarbonyl complexes were prepared by addition of the PTA-PR2 to [W(CO)4(pip)2] and characterized by NMR and IR spectroscopies and X-ray crystallography in the case of [W(CO)4(PTA-PiPr2)]. Based on IR spectroscopy PTA-PiPr2 (1) is more electron donating than PTA-P(NiPr)2(CH2)2 (2) or the previously published PTA-PPh2. Products isolated from the reaction of [(η6-toluene)RuCl2]2 with two equivalents of 1 were found to contain monodentate (κ1), bidentate (κ2), and possibly bridging coordination modes of PTA-PiPr2. These ruthenium complexes were explored as catalysts for aqueous phase nitrile hydration. Of the ruthenium complexes explored [(η6-toluene)RuCl2(κ1-PTA-PiPr2)] (7) was the most active towards nitrile hydration. In the presence of air at 100°C 7 converted various nitriles to the respective amides with 43–99% conversions in 7h.