Geometric isomerization and geometry controlled catalytic alcohol aminations of ruthenium hydride compounds containing bidentate pyrrolyl-imines

A series of ruthenium hydride compounds containing bidentate pyrrole-imine ligands were synthesized and characterized and their properties were fully studied. Reacting [C₄H₃NH-(2-CH=N-R) (L₁H, R = CH₂-2-furanyl; L₂H, R = CH₂-morpholine; L₃H, R = CH₂-2-pyridyl) with one equivalent of n-BuLi afforded the corresponding lithium reagents (LiL₁∼LiL₃). Combining RuHCl(CO)(PPh₃)₂ with LiL₁∼LiL₃ in THF at 0 °C with stirring at room temperature for 12 h generated pure trans-Pyr-Ru-H 1–3 in moderate yields after repeated fractional recrystallization. When the reactions were performed at reflux temperature, pure cis-Pyr-Ru-H 1–3 were obtained by fractional recrystallization in relatively high yields. A thermal isomerization study showed that pure cis-Pyr-Ru-H 1–3 and trans-Pyr-Ru-H 1–3 can be interconverted in THF after refluxing and equilibrium was attained. Reacting a bulkier ligand [C₄H₃NLi-(2-CH=N-^tBu)] (LiL₄) with one equivalent RuHCl(CO)(PPh₃)₃ at room temperature afforded only cis-Pyr-Ru-H 4 after purification. No trans-Pyr-Ru-H 4 was observed, indicating the bulkier t-butyl group of the L₄ ligand altered the stability of cis- and trans-Pyr-Ru-H 4. A catalytic alcohol amination study showed the cis forms of Pyr-Ru-H were the active catalysts for converting benzyl alcohol to N-benzylaniline.