Ruthenium Complexes Stabilized by Bidentate Enamido-Phosphine Ligands: Aspects of Cooperative H₂ Activation

Abstract

Four bidentate, hybrid ligands (^R(NP)^R′H) featuring imine-nitrogen and alkyl-phosphine donors linked by a cyclopentyl ring were synthesized. The ortho position of the aryl group attached to nitrogen is varied such that R is Me or Prⁱ; additionally, the groups decorating phosphorus (R′) are varied between Bu^t or Prⁱ. The addition of each ligand to RuHCl­(PPrⁱ₃)₂(CO) in the presence of KOBu^t generates four enamido-phosphine complexes RuH­{^R(NP)^R′}­(PPrⁱ₃)­(CO) that were characterized by NMR spectroscopy, elemental analyses, and, in the case of R = Prⁱ and R′ = Bu^t or Prⁱ, X-ray crystallography. Depending on R′, the reaction of RuH­{^R(NP)^R′}­(PPrⁱ₃)­(CO) with H₂ generates varying amounts of the imine-phosphine complex RuH₂{^R(NP)^R′H}­(PPrⁱ₃)­(CO). Insights into the mechanism of H₂ activation by these enamido derivatives were explored using RuH­{^Pr(NP)^Pri}­(PPrⁱ₃)­(CO), for which an intermediate was identified as the dihydrogen–dihydride complex, RuH₂(H₂)­{^Pri(NP)^PriH}­(PPrⁱ₃)­(CO), on the basis of the <i>T</i>_1,min value of 22 ms for the ¹H NMR resonance at δ −7.2 at 238 K (measured at 400 MHz). The N donor of the enamine tautomeric form of the ligand is protonated by H₂ or D₂ and dissociates from Ru. Tautomerization of the enamine to the imine form of the dissociated arm is involved in formation of the final product