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

Abstract

Tridentate amine–imine–phosphine ligands, R₂PC₅H₇NC₂H₄NEt₂ [^RPNN(H)], where R = Prⁱ 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(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(μ-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.