Synthesis and coordination chemistry of tridentate (PNN) amine enamido phosphine ligands with ruthenium

Tridentate amine-imine-phosphine ligands{,} R2PC5H7NC2H4NEt2 [RPNN(H)]{,} where R = Pri or But are synthesized using a straightforward protocol of condensation{,} deprotonation{,} and addition of a chlorodialkylphosphine. Multinuclear NMR spectroscopy shows the ligands exist exclusively in the enamine tautomeric form in solution. Treating these ligands with RuHCl(PPri3)2(CO) forms the desired coordination compounds{,} RuHCl[RPNN(H)](CO){,} where the imine tautomeric form of the ligands coordinates to ruthenium. Deuterium labelling experiments show Ru-H/N-D scrambling occurs during ligand coordination. Treating the RuHCl[RPNN(H)](CO) precursors with potassium tert-butoxide allows for the synthesis of two new ruthenium enamido-phosphine complexes{,} RuH[RPNN](CO){,} which were fully characterized. The structure of one of the derivatives was confirmed by X-ray crystallography (R = Pri). The reactivity of the enamido-phosphine complexes with H2 and benzyl alcohol is also reported. For the enamido phosphine complex where R = Pri{,} the reaction with H2 is reversible and forms (RuH(CO)[PriPNN(H)])2([small mu ]-H)2{,} a hydride-bridged dimer that results from cooperative activation of H2. The reactivity of both amine-enamido-phosphine ruthenium compounds with benzyl alcohol establishes that the complexes are catalyst precursors for acceptorless dehydrogenation (AD){,} although the turnover frequencies measured using both catalyst precursors are modest.