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

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 Pr-i; additionally, the groups decorating phosphorus (R') are varied between Bu-t or Pr-i. The addition of each ligand to RuHCl(PPr3i)(2)(CO) in the presence of KOBut generates four enamido-phosphine complexes RuH{(R)(NP)(R)'(PPr3i)(CO) that were characterized by NMR spectroscopy, elemental analyses, and, in the case of R = Pr-i and R' = Bu-t or Pr-i, X-ray crystallography. Depending on R', the reaction of RuH{(R)(NP)(R)'}(PPr3i)(CO) with H-2 generates varying amounts of the iminephosphine complex RuH2{(R)(NP)(R)'H}(PPr3i)(CO). Insights into the mechanism of H-2 activation by these enamido derivatives were explored using RuH{(Pr)(NP)(Pri)(PPr3i)(CO), for which an intermediate was identified as the dihydrogen-dihydride complex, RuH2(H-2){(Pri)(NP)H-Pri}(PPr3i)(CO), on the basis of the T-1,T-min value of 22 ms for the H-1 NMR resonance at delta -7.2 at 238 K (measured at 400 MHz). The N donor of the enamine tautomeric form of the ligand is protonated by H-2 or D-2 and dissociates from Ru. Tautomerization of the enamine to the imine form of the dissociated arm is involved in formation of the final product.