Syntheses, Characterization And Catalytic Behavior Of Bimetallic And Monometallic Lanthanide And Aluminum Complexes Stabilized By Phenolato Ligands

In this thesis, the controlled synthesis and catalytic behavior of bimetallic complexes,the cooperative effect in bimetallic aluminum systems and the catalytic mechanism ofaluminum complexes for-caprolactone polymerization were elucidated. Usingpiperazidine-bridged bis（phenolato） and mono（phenolato） as ancillary ligands,21lanthanide complexes and18aluminum complexes were synthesized, in which34complexes were structural characterized by X-ray single crystal diffraction. Theinfluence of reaction conditions on the synthesis, and stability of these bimetallic andmonometallic complexes was studied. Furthermore, the catalytic behavior of some ofthese complexes for the ring-opening polymerization （ROP） of-caprolactone andlactides was explored to elucidate whether there is cooperative effect in the bimetallicsystems. The main contents were listed below.1. The influence of reaction temperature and the order of adding starting materialson the synthesis of bimetallic complexes were studied. Reactions ofLn[N（TMS）₂]₃（μ-Cl）Li（THF）₃with C₄H₈N₂[1,4-（2-O-3,5-di-t^Bu₂-C₆H₂CH₂）₂（H₂[ONNO]₁） in a2:1molar ratio at60°C, after workup, the desired bimetallicpiperazidine-bridged bis（phenolato） lanthanide complex[ONNO]¹{Ln[N（TMS）₂]₂（μ-Cl）Li（THF）}2（Ln=Y（1）） was isolated. Whereas the samereactions conducted at room temperature gave the neutral monometallic complexes[ONNO]¹LnN（TMS）₂（THF）（Ln=Y（2）, Sm（3））. The reaction of Y[N（TMS）₂]₃withH₂[ONNO]₁in a2:1molar ratio at60°C afforded also the neutral bimetallic lanthanideamido complex [ONNO]¹{Y[N（TMS）₂]₂}（4）. These complexes were characterized byelemental analysis, and IR spectroscopy. The solid state structures of these complexeswere determined by the single crystal X-ray diffraction. The solution behavior of the diamagnetic complexes1,2, and4were studied by NMR spectroscopy.2. The influence of the ionic radii of the lanthanide metals on the synthesis ofbimetallic complexes was studied. It was found that, for the small lanthanide metals （Y,Er, Eu and Sm）, the reactions of Ln[N（TMS）₂]₃（μ-Cl）Li（THF）3with H₂[ONNO]₁in a2:1molar ratio at60°C gave the desired anionic bimetallic piperazidine-bridgedbis（phenolato） lanthanide amido complexes [ONNO]¹{Ln[N（TMS）₂]₂（μ-Cl）Li（THF）}2（Ln=Er （5）, Eu （6）, Sm（7））; whereas the neutral monometallic bis（phenolato） lanthanideamido complexes [ONNO]¹LnN（TMS）₂（THF）（Ln=Nd（8）, Pr（9）） were isolated for thelarger metals （Nd and Pr） under the same reaction conditions. Complexes7-9werecharacterized by elemental analysis, IR and the single crystal X-ray diffraction for7and8.3. The influence of the steric bulkiness of the amido group on the synthesis ofbimetallic complexes was studied. It was found that, for the less bulky amido complexes,only the monometallic piperazidine-bridged bis（phenolato） lanthanide complexes[ONNO]Ln[N（SiMe₂H）₂]（THF）（Ln=Y （10）, Yb （11）, Nd （12）） and[ONNO]¹Y[N（SiMe₃）（C₆H₅）]（THF）（Ln=Y （13）, Yb （14）, Sm （15）, Nd （16）） as neatproducts can be isolated by the reactions of H₂[ONNO]¹with Ln[N（SiMe2H）2]3（THF）2,and Ln[N（SiMe₃）（C₆H₅）]₃（THF） at60°C, respectively, even when the reactions wereconducted in a1:2molar ratio. These results indicated that less bulky amido groupscannot stabilize the bimetallic lanthanide amido complexes. All of these complexeswere characterized by elemental analysis, IR, single crystal X-ray diffraction and NMRspectroscopy for diamagnetic complex10and13.4. Complexes1,4-7can initiate the polymerization of L-lactide with high activityunder mild conditions. In comparison with the anionic bimetallic yttrium silylamidocomplex1, the neutral bimetallic yttrium amido complex4showed higher catalyticactivity. Complexes2,3,8,9and10-12are also highly efficient initiators for thering-opening polymerization of L-lactide. It was found that the activities of the lessbulky monometallic lanthanide amido complexes10-12were higher than those of thecomplexes2,3,8,9. In addition, the catalytic behavior of complexes1,2,4,6and10for the ring-opening polymerization of rac-lactide was also tested. It can be seen that these complexes are efficient initiators for the polymerization of rac-lactide. Thepolymerizations proceed smoothly in THF at room temperature and gave polymers withhigh molecular-weights and relatively narrow molecular-wieght distributions. Howeverall of the complexes showed moderate stereoselectivity for rac-lactide polymerization.5. The attempts to synthesize mono（phenolate） lanthanide diamido complexeswere unsuccessful to date. Amine-elimination reactions of mono（phenolato） ligand2-(CH₂NC₅H¹⁰)-4,6-^tBu₂-C₆H₃OH （H[NO]¹） with Ln[N（TMS）₂]₃（-Cl）Li（THF）₃in a1:1or2:1molar ratio in THF gave only the bis（phenolato） lanthanide monoamidocomplexes [NO]₂¹LnN（TMS）₂（Ln=Yb（17）, Y（18）, Gd（19）, Sm（20）, Nd（21））. All ofthese complexes were characterized by elemental analysis, NMR spectroscopy fordiamagnetic complex18and single crystal X-ray diffraction for17-20. Complexes17-21can initiate the polymerization of L-lactide with high activity under mildconditions. The ionic radii of the central metals have a significant effect on the catalyticactivity, and the observed activity increasing orders are in agreement with the order ofionic radii. Complexes17-21can also initiate effectively the polymerization ofrac-lactide in THF at room temperature to give heterotactic-rich polylactides.6. It was found that the mono（phenolate） aluminum complexes can be preparedunder suitable conditions. Treatment of the ligand precursor, H[NO]1, with AlEt3in a1:1or2:1molar ratio in THF gave the bis（phenolato） aluminum monoethyl complex[ON]₂¹AlEt （22） in moderate isolated yield. However, the reaction of H[NO]¹,2-(CH₂NC₅H₁₀)-4-Me-6-^tBu₂-C₆H₃OH （H[NO]²） with AlEt3and AlMe3in a1:2molarratio in THF, respectively, afforded the desired mono（phenolato） aluminum biethyl andbimethyl complexes [ON]1AlEt2（23）,[ON]2AlEt2（24）[ON]¹AlMe₂（AlMe₃）（25）, and,[ON]²AlMe₂（AlMe₃）（26） in good isolated yields. Complex22-26were characterized byelemental analyses, NMR spectroscopy and the single crystal X-ray diffraction forcomplexes22,23,25, and26.7. The mono（phenolato） aluminum bi-ethyl complex23and the dinuclearaluminum methyl complex complex25were screened for the ring-openingpolymerization of-caprolactone. It was found that both complex23and complex25can initiate the ring-opening polymerization of-caprolactone with high activity, and the polymerization accelerated dramatically in the presence of BnOH. In comparisonwith the monometallic complex23, the bimetallic complex25showed higher catalyticactivity under the same conditions.8. To elucidate the cooperative effect, the bimetallic aluminum alkyl complexesstablized by the piperazidine-bridged bis（phenolato） ligands were prepared. Reactionsof H₂[ONNO]¹, C₄H₈N₂[1,4-（2-O-3-tBu-5-Me-C6H2CH2）2]（H₂[ONNO]²） andC₄H₈N₂[1,4-（2-O-3,5-di-Me-C₆H₂CH₂）₂]（H₂[ONNO]³） with AlMe₃, and AlEt₃in a1:2.5molar ratio in THF, respectively, gave the dinuclear aluminum methyl complexes[ONNO]¹（AlMe₂）₂（27）,[ONNO]²（AlMe₂）₂（28）,[ONNO]³（AlMe₂）₂（29） and ethylcomplexes [ONNO]¹（AlEt₂）₂（30）,[ONNO]²（AlEt₂）₂（31） and [ONNO]³（AlEt₂）₂（32） inhigh yields. The influence of the steric hindrance of the ancillary ligands on thesynthesis of bimetallic complexes was studied. It was found that variations the stericbulkiness of substituents at ortho-and para-positions have no obvious effect on theoutcome of these reactions. Both the bulky t-Bu and the less bulky Me group at ortho-and para-positions of phenolato can stabilize the bimetallic aluminum complexes.Complexes27-32were characterized by elemental analyses, NMR spectroscopy and thesingle crystal X-ray diffraction, except complex31.9. The attempts to synthesize bimetallic aluminum alkoxo complexes wereunsuccessful by similar alkane elimination reactions. The reactions of H₂[ONNO]¹, andH₂[ONNO]²with PhCH₂OAlEt₂oriPrOAlEt₂in a1:1molar ratio, respectively, gave themonometallic aluminum alkoxo complexes [ONNO]¹Al（OCH₂Ph）（33）,[ONNO]¹Al（OiPr）（35）,[ONNO]²Al（OCH₂Ph）（36）. When the molar ratio wasincreased to1:2, a mixture of monometallic aluminum ethyl complexes [ONNO]¹AlEt₂（34）,[ONNO]²AlEt₂（37） and monometallic aluminum alkoxo complexes33and35were isolated, respectively. No bimetallic aluminum alkoxide complexes were formed.We postulated that the reactions of these ligands with ethyl aluminum alkoxo complexesin a1:1molar ratio at first, and then the ligand redistribution reaction occurred in thepresence of excess ethyl aluminum alkoxo complexes to give the aluminum ethylcomplexes. Complexes33-37were characterized by elemental analyses, NMRspectroscopy and the single crystal X-ray diffraction. 10. The monometallic aluminum alkyl complexes stablized by thepiperazidine-bridged bis（phenolato） ligands were prepared conveniently. Reactions ofH₂[ONNO]¹, and H₂[ONNO]³with AlMe3and AlEt3in a1:1.2molar ratio in THF,respectively, afforded the monometallic aluminum methyl complex [ONNO]¹AlMe₂（38）and ethyl complex [ONNO]³AlEt₂（39）. However, the reactions of H₂[ONNO]¹,H₂[ONNO]²with AlEt3in a1:1molar ratio were conducted under different reactionconditions, but after workup, colorless precipitate was obtained. The precipitate givescomplicated NMR spectrum, and no resonance of the corresponding monometallicspecies is observed. Complex38and39were characterized by elemental analyses,NMR spectroscopy and the single crystal X-ray diffraction.11. Dinuclear aluminum alkyl complexes27-32can initiate the ring-openingpolymerization of ε-caprolactone under mild polymerization conditions with highactivity. It was found that both the steric hindrance of the ancillary ligands and thestructures of the alkyl groups have obvious effect on the catalytic activity. Thering-opening polymerization of ε-caprolactone initiated by complexes27-32in thepresence of BnOH accelerated dramatically, but the molecular weights of the polymersobtained are apparently smaller than those of the polymers obtained in the absence ofbenzyl alcohol. In order to elucidate the initiation mechanism, the end-group analysis ofthe oligomers was conducted and the polymerization reaction was monitored by1HNMR. It was found that the aryloxo groups are superior to alkyl groups for initiating thepolymerization when the aluminum alkyl complexes were utilized as the initiators;whereas both the aryloxo group and the alkoxo group formed in situ can initiate thepolymerization when aluminum complexes combined benzyl alcohol were used as theinitiators. Furthmore, multiple active centers in the latter resulted in the apparentlydecreased in the molecular weights of the resultant polymers.12. In order to assess the potential cooperative effect from metal centers in a closeproximity, the activity of the bimetallic aluminum complex30and the monometallicaluminum complex23was compared, and cooperative effect was not observed in thebimetallic aluminum system of complex30, which can be attributed to that the distancebetween two metals in complex30is too far.