Studies On Synthesis, Characterization, Catalytic Properties Of Metal Complexes Bearing Anilido-Aldimine Ligands

Salen ligands and their transition metal complexes have been extensively explored as catalysts in the last few decades. Many of these complexes have been found to be excellent catalysts for a number of reactions. A catalyst with proper ligand and proper metal center can catalyze a proper reaction. From structural point of view, there are two substituents at the ortho-position of the coordinating O atoms in the salen ligand. The steric effect of the substituents on the coordinating atoms in the ligands should be larger than those in the salen ligands. With such a consideration in mind, we have designed three classes of tetra-azane chelating ligands and their transition metal complexes in which there are two substituents on the coordinating N atoms. The steric and electronic effect of the two substituents in the new ligands should be better than those in the salen ligands and the PPNN ligands that modified the catalytic activity and selectivity.Ligands [ortho-C₆H₄(NHAr)CH=N]₂CH₂CH₂ (Ar = 2,6-Me₂-C₆H₃, L_A1H₂; Ar = 2,6-ⁱPr₂C₆H₃, LA₂H₂) were prepared by condensation between ethylenediamine with two equivalents of 2- fluorobenzaldehyde in methanol, followed by reaction with two equivalents of lithium salt of substituted aniline in THF. Treatment of L_A1H₂ with one equiv of AlMe₃ gives the tetracoordinated monometallic complex L_A1HAlMe₂. The hetrobimetallic complex L_A1ZnEtAlMe₂ was prepared by reaction of one equiv of ZnEt₂ with the complex L_A1HAlMe₂ in toluene. Treatment of the ligand L_A1H₂ or L_A2H₂ with two equiv of AlMe₃ gives the bimetallic complex L_A1(AlMe₂)₂ or L_A2(AlMe₂)₂, and reaction of L_A1H₂ or L_A2H₂ with two equiv of ZnEt2 leads to the formation of bimetallic complex L_A1(ZnEt)₂ or L_A2(ZnEt)₂ , respectively. The complexes were all characterized by elemental analyses and 1H and 13C NMR spectroscopy. Crystal structures of complexes L_A1HAlMe2, L_A1ZnEtAlMe2, L_A1(AlMe2)2, L_A2(AlMe2)2, and L_A1(ZnEt)2 were determined by single-crystal X-ray diffraction. All of the complexes are efficient catalysts for ring-opening polymerization ofε-caprolactone in the presence of benzyl alcohol and catalyze the polymerization ofε-caprolactone in living fashion yielding polymers with a narrow polydispersity index.Anilido-imine ligands ortho-C₆H₄(CH=NAr1)(NHAr²) [Ar1 = C₆H₅, Ar² = C₆H₅ (LB1H); Ar1 = 2,6-Me2C₆H₃, Ar² = 2,6-Me2C₆H₃ (LB2H); Ar1 = 2,6-Et2C₆H₃, Ar²= 2,6-Et2C₆H₃ (LB3H); Ar1 = 2,6-iPr2C₆H₃, Ar² = 2,6-Me2C₆H₃ (LB4H); Ar1 = 2,6-iPr2C₆H₃, Ar²= 2,6-Et2C₆H₃ (LB5H); Ar1 = 2,6-iPr₂C₆H₃, Ar²=2,6-iPr2C₆H₃ (LB₆H)] were synthesized according to the literature procedure. Treatment of the ligands with AlMe₃ in hexane gives the desired anilido-imine Al complexes. The anilido-imine Al complexes were all characterized by elemental analyses and 1H NMR spectroscopy. All of these anilido-imine Al complexes are efficient catalysts for ring-opening polymerization ofε-caprolactone in the presence of benzyl alcohol and catalyze the polymerization ofε-caprolactone in living fashion.Ligands [ortho-C₆H₄(NHAr)CH=N]₂C₆H₍10 [(1R,2R)-L_C1H₂ (Ar = 2,6-Me₂C₆H₃), (1R,2R)-LC₂H₂ (Ar = 2-MeC₆H₄), (1R,2R)-L_C3H₂ (Ar = 4-MeC₆H₄)]were prepared by condensation between chiral cylclohexane diamine with two equivalents of 2- fluorobenzaldehyde in methanol, followed by reaction with two equivalents of lithium salt of substituted aniline in THF. The transition metal complexes [L_C1ZrCl₂, L_C2ZrCl₂, L_C3ZrCl₂, L_C1CrCl, L_C2CrCl, L_C3CrCl, L_C1Co, L_C2Co, L_C3Co, L_C1Ni, L_C2Ni, L_C3Ni, L_C1Cu, L_C2Cu, L_C3Cu] were synthesized in good yields via metathesis of MCl₂ (M=ZrCl₂(THF)2, Co, Ni(DME), Cu) with the lithium salt of the corresponding ligands, respectively. Molecular structure of (1R,2R)-L¹H₂, L_C1ZrCl₂, L_C1CrCl, L_C1Co, L_C1Ni, L_C3Ni and L_C1Cu were determined by X-ray crystallography. All of the Ni complexes are efficient catalysts for the coupling reaction of propylene oxide with CO2 in the presence of a quaternary ammonium salt.The new ligand [ortho-C₆H₄(CH=NC₆H₃iPr2-2,6)NH]2C₆H₄ (LDH₂) was synthesized via a nucleophilic displacement of fluorine in ortho-C₆H₄F- (CH=NC₆H₃iPr2-2,6) by dilithium salt of 1,2-benzendiamine in THF. Reaction of the dilithium salt of LDH₂ with CrCl₂(THF) in THF gives the six-coordinated Cr(IV) complex. Molecular structure of the Cr(IV) complex was determined by X-ray crystallography. Upon activation with AliBu3 and Ph₃CB(C₆F₅)₄ as cocatalysts, the Cr(IV)complex was found to be active towards ethylene polymerization to give solid polyethylenes. The activity is moderate in room temperature though the molecular weight of the polyethylene is very high (Mv: 1650000).