Organoaluminum Compounds And Oxo-Bridged Heterometallic Oxides: Synthesis, Characterization And Catalytic Properties

Oxo-bridged heterometallic oxides are of significant interest ranging from advanced materials to valuable catalysts, owing to the cooperative feature among different metals in an appropriate proximity. The present dissertation aims to develop the novel organometallic synthetic methodology to prepare the organoaluminum hydroxide precursors stabilized byβ-diketiminato ligands, and then to synthesize the well-defined molecular heterometallic compounds containing a M-O-M' or M-O-M'-O-M core. Furthermore, the catalytic properties of selected heterometallic oxides are preliminarily evaluated.Threeβ-diketiminato (L¹ = HC[C(Me)N(Ar)]₂, Ar = 2,4,6-Me₃C₆H₂; L² = HC[C(Me)N(Ar)]₂, Ar = 2,6-iPr₂C₆H₃; L³ = HC[C(tBu)N(Ar)]₂, Ar = 2,6-iPr₂C₆H₃) with different steric demanding were employed to synthesize a series of organoaluminum chlorides LAlR(Cl) with varied R substituents (R = Me, Et, Ph, OEt, OSiMe₃). Subsequently, the controlled hydrolysis of these chlorides in the presence of carbene (:C) as a HCl scavenger afforded the corresponding organoaluminum hydroxides LAlR(OH). It was shown by NMR, IR and single crystal X-Ray analysis that the hydroxides supported by the less bulky ligand L¹ was prone to dimerizing in solid state under formation of a planar Al₂O₂ ring, while in solution these compounds featured a monomer-dimer equilibrium evidenced by the ¹H NMR spectra data. During the preparation of LAlR(Cl), a prereaction strategy was developed to introduce a bulky substituent to the heavily shielded Al center.A series of organoaluminum chloride amides L²Al(Cl)NR₂ (R = Me, iPr, SiMe₃) were synthesized. The alkylation and fluorination of L²Al(Cl)NR₂ led to the formation of L²Al(Me)NR₂ (R =Me, iPr) and L²Al(F)NR₂ (R = Me, iPr, SiMe₃). The controlled hydrolysis of L²Al(Cl)NMe₂ and L²Al(Me)NMe₂ afforded known compounds [L²AlCl(μ-OH)]₂ and L²Al(Me)OH, respectively, accompanied by the intermolecular elimination of the secondary amines. The hydrolysis of organoaluminum chloride amides provided an alternative access to the organoaluminum hydroxides.The reaction of LAlR(OH) with one equivalent of Cp₂ZrMe₂ and homoleptic Cp₃M under the intermolecular elimination of methane or HCp led to the formation of the expected oxo-bridged heterobimetallic oxides LAlR(μ-O)ZrMeCp₂(L = L¹, L², L³; R = Me, Et, Ph, OEt, OSiMe₃) and L²AlEt(μ-O)M(THF)Cp₂ (M = Yb, Er, Dy, Y), respectively. The result of single crystal X-ray analysis suggested that the modification of R on Al offered a possibility for tuning the Al-O-Zr angle and the Al-Zr separation.The OH functionality of different organometallic hydroxides LMR(OH) or L'M'R'(OH) for the assembly of the same M-O-M' linkage was examined. In the Al-O-Ge system, the reaction of L AlMe(OH) with one equivalent of Ge[N(SiMe₃)₂]₂ resulted in the condensation of organoaluminum hydroxide to give L Al(Me)-L . In contrast, the oxo-bridged heterobimetallic oxide L²Ge(μ-O)AlMe₂ was expectedly obtained when employing the organogermanium hydroxide L²GeOH as a precursor to react with an equimolar AlMe₃. The similar reaction of L²GeOH with Cp₃M also exhibited a good reactivity to give the well-defined L²Ge(μ-O)M(THF)Cp₂ (M = Yb, Y). As for the Zn-O-Al system, the treatment of [L²Zn(μ-OH)]₂ with AlMe₃ led to the known compound L²ZnMe due to the less Bronsted acidity of hydroxide, while L²AlMe(μ-O)ZnMe containing the Al-O-Zn core was accessible by the reaction of L²AlMe(OH) with ZnMe₂ in toluene under reflux.Among the compounds with a M-O-M' core (M = Al, Ge; M' = Ti, Zr, Hf, Y, Ln), it was found that the M-O bond length became shorter when M' owned larger atomic radii or its cation exhibited stronger Lewis acidity.The oxo-bridged heterotrimetallic oxide [L AlMe(μ-O)]₂Sn was obtained smoothly by the 2:1 reaction of L²AlMe(OH) and Sn[N(SiMe₃)₂]₂. Single crystal X-ray analysis confirmed the presence of an open chain of Al-O-Sn-O-Al core with a planar triangular geometry around the two-coordinate Sn(â…¡) center. The reaction of L²GeOH with AlMe₃ in a ratio of 2:1 afforded another oxo-bridged heterotrimetallic oxide [L²Ge(μ-O)]₂AlMe with an open Ge-O-Al-O-Ge linkage.Complexes L²AlEt(μ-O)M(THF)Cp₂ (M = Dy, Yb) for the polymerization ofε-caprolactone showed living catalytic activity with high yield (94-97%). L²AlEt(μ-O)Dy(THF)Cp₂ gave a polymeric product with higher molecular weight (M_n = 65861) and lower polydispersity (M_w/M_n= 1.39), when compared with its methyl analogue at room temperature.MAO activated L²AlEt(μ-O)ZrMeCp₂ exhibited obvious catalytic activity in polymerization of ethylene at room temperature. The MAO/catalyst ratios dependence activity was revealed by a gradual increase in the activity with the MAO/catalyst till to 400, followed by a slow decrease as the MAO/catalyst ratio was raised further. The ¹³C NMR data showed resonance (8 29.81 ppm) mode corresponding to the linear polyethylene. MAO/L²AlPh(μ-O)ZrMeCp₂ catalyst system showed lower activity compared to that of MAO/L²AlR(μ-O)ZrMeCp₂ (R = Me, Et). It was observed that the catalytic activity of compounds LAlR(μ-O)ZrMeCp₂ varied depending on the substituent R group at the aluminum center. The activity was indicated in the order Me > Et > Ph.