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

Tridentate amine-imine-phosphine ligands, R2PC5H7NC2H4NEt2 [(PNN)-P-R(H)], where R = Pr-i or Bu-t 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(PPr3i)(2)(CO) forms the desired coordination compounds, RuHCl[(PNN)-P-R(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[(PNN)-P-R(H)](CO) precursors with potassium tert-butoxide allows for the synthesis of two new ruthenium enamido-phosphine complexes, RuH[(PNN)-P-R](CO), which were fully characterized. The structure of one of the derivatives was confirmed by X-ray crystallography (R = Pr-i). The reactivity of the enamido-phosphine complexes with H-2 and benzyl alcohol is also reported. For the enamido phosphine complex where R = Pr-i, the reaction with H-2 is reversible and forms (RuH(CO)[(PNN)-P-Pri(H)])(2)(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.