Half-sandwich rhodium and iridium complexes containing homochiral imidazolyl-imine ligands: synthesis, characterization and catalytic applications

Abstract

Condensation of homochiral primary amines with 1-methyl-1H-imidazole-2-carbaldehyde affords the corresponding imidazolyl-imine compounds (L₁1–L₃3) which have been employed as ligands for the preparation of half-sandwich rhodium and iridium complexes of the formula [(η⁵-C₅Me₅)MClL_nnn][SbF₆]. Treatment of these chloride compounds with AgSbF₆ renders dicationic aqua-complexes [(η⁵-C₅Me₅)ML_nnn(H₂O)] [SbF₆]₂ which act as catalysts for the Diels–Alder reaction between methacrolein and cyclopentadiene. Catalysis occurs with good exo : endo selectivity and poor enantioselectivity. All the compounds have been completely characterized by analytical and spectroscopic methods. Characterization includes the molecular structure determination of the complexes [(η⁵-C₅Me₅)MClL_nnn][SbF₆] (L_nnn = L₁1111, M = Rh, (1) Ir (4); L_nnn = L₃333, M = Ir (6)) and [(η⁵-C₅Me₅)ML₁1111(H₂O)][SbF₆]₂ (M = Rh (7), Ir (10)) using X-ray diffraction. From the stereochemical properties of the organometallic precursors the catalytic outcome is discussed.