Synthesis, Characterization And Catalytic Behavior Of Lanthanide Amides Supported By Dianionic β-diketiminate Ligand

A series of lanthanide amides supported by dianionic β-diketiminate ligand L1（L1={（2,6-iPr₂C₆H₃）NC（CH₂）CHC（CH₃）N（2,6-iPr₂C₆H₃）}（²-）） were synthesized by stericallyinduced deprotonation of β-diketiminate ligand L (L={[（2,6-iPr₂C₆H₃）NC（CH₃）₂CH}^1-) and were well characterized. The catalytic activity ofdianionic β-diketiminate lanthanide amides in addition of amines to carbodiimidesaffording the corresponding guanidine compounds were tested. Furthermore, catalyticbehavior of dianionic β-diketiminate lanthanide amides in polymeraztion of cyclicesters were studied. Reactions of dianionic β-diketiminate lanthanide amides withdifferent unsaturated organic moleculars, such as carbodiimides, p-CH₃C₆H₄N=C=O,nitriles and ketenimine were studied and afforded some novel lanthanide amidessupported by modifided β-diketiminate ligand. Furthermore, reactions of dianionicβ-diketiminate lanthanide amides with different organic compounds containing activehydrogen, such as Ph2CHCN and [HNEt3][BPh4] were studied. The main achievementsare as follows:1. A series of dianionic β-diketiminate lanthanide monoamido complexesL’LnN（SiMe₃）₂（THF）（Ln=Yb （1）, Y （2）, Gd （3）, Sm （4）） were prepared via stericallyinduced deprotonation of β-diketiminate ligand L (L={[（2,6-iPr₂C₆H₃）NC（CH₂）CHC（CH₃）₂CH}^1-) by reaction of lanthanide dichlorides LLnCl2（THF）2with two equivalents of NaN（SiMe₃）₂, which represented the first example of lanthanideamides supported by a dianionic β-diketiminate ligand. The size of the lanthanidemetals has a profound influence on the deprotonation reaction of L, indicating that anover-crowded coordination environment around the central metal is necessary fordeprotonation of L. However, replacing the above dichlorides by the large Nd metal complex LNdCl₂（THF）₂did not afford the analogous complex L1NdN（SiMe₃）₂, but thenormal diamide LNd[N（SiMe₃）₂]₂（5） was isolated. Thus, the deprotonation of L in thepresent case proceeds by the following steps. The first step is monosubstitution of oneof the chlorine atoms in LLnCl2（THF）2to form a monoamide A. The intermediate Areacts with another NaN（SiMe₃）₂immediately by C-H bond-breaking due to the stericcongestion around the metal center which favors the base-assisted dehydrochlorinationvia monodeprotonation of a methyl group on the backbone of L instead of anucleophilic Cl/N（SiMe₃）₂substitution. These complexes were characterized byelemental analysis, IR, X-ray diffraction and the NMR for the complex2.2. Complexes1-4were found to be efficient catalysts for the ring-openingpolymerization of ε-caprolactone, with higher catalytic reactivity and narrow PDI of thepolymer. It is noted that the activity is largely influenced by the central metals. Theactivity decreased with the decrease of the size of lanthanide metals and the activitysequence is Sm> Gd> Y≈Yb. Complex4was also found to be an efficient catalyst forthe ring-opening polymerization of L-lactide, with higher catalytic reactivity andrelatively narrow PDI of the polymer.3. The catalytic behavior of complexes1-4in the guanidine-forming reaction wasexamined. It is noted that the activity is largely influenced by the central metals. Theactivity decreased with the decrease of the size of lanthanide metals and the activitysequence is Sm≈Gd> Y≈Yb. The results demonstrated that these complexes areefficient catalysts for the guanylation reaction. The systems have the advantage oftolerance to various substrates. The possible mechanism of this reaction was alsostudied. It is found that the catalyzed guanidine-forming reaction firstly took place at thedianionic β-diketiminate ligand.4. Reaction of complexes1-4with one equiv of DCC, gave the the novelγ-amidine-functionalized dianionic β-diketiminatolanthanide amide complexes,L2LnN（SiMe₃）₂(L₂={μ³-N,N,N-HC（C（CH₂）NAr）₂（C（NHC₆H₁1）NC₆H₁1）}^2-; Ln=Sm（8）, Gd（9）, Y（10）, Yb（11）), conversion of complex4with a mixture ofN,N’-diisopropylcarbodiimide （DIC） and Na（SiMe₃）₂and DCC and Na（SiMe₃）₂,respectively, furnishes the novel heterobimetallic complex of Sm/Na L3SmN（SiMe₃）₂ (L3={μ3-N,N,N-HC（C（CH₂）NAr）（C（CH₂）NAr）（C（NNa（DME）₂iPr）NiPr）}^2-)（12） and[L4SmN（SiMe₃）₂][Na（DME）₃]⁺（L4={μ3-N,N,N-HC（C（CH2）NAr）（C（CH2）NAr）（C（NCy）NCy）}3）（13）. Complex4showsstriking reactivity toward aromatic nitrile and ketenimine. Reaction of4with C6H5CNand p-MeOC6H4CN leads via nucleophilic addition followed by1,5-H shift to theentirely modified dianionic β-diketiminato samarium amides with two asymmetricalkylideneamido bridges [L6SmN（SiMe₃）₂（C₆H₅CN）]2（L6={μ-N=C（C6H5）CHC（CHC（CH3）NH（2,6-iPr2C6H3））N（2,6-iPr₂C₆H₃）}（²-））（15） and[L7SmN（SiMe₃）₂（p-MeOC6H4CN）]2（L7={μ-N=C（p-CH₃OC₆H₄）CHC（CHC（CH₃）NH（2,6-iPr₂C₆H₃））N（2,6-iPr₂C₆H₃）}（²-））（16）, while conversion of Ph₂C=C=NtBu affords thecycloadduct [L₈SmN（SiMe₃）₂（THF）](L8={（2,6-iPr2C6H3）NC（CHC（CH3）N（2,6-iPr₂C₆H₃））CHC（CH（C₆H₅）2）N（C（CH₃）₃）}^2-)（17） via nucleophilic addition and1,3-H shift. All the complexes were characterized by elemental analysis, IR and X-raydiffraction. Complexes8，10，11，16，17were found to be efficient catalysts for thering-opening polymerization of L-lactide, with higher catalytic reactivity and relativelynarrow PDI of the polymer. Notably, the binary8/BnOH system exhibited an“immortal” nature.5. Treatment of4with [HNEt₃][BPh₄] in THF at room temperature gave the novelcationic Sm β-diketiminate amide [LSmN（SiMe₃）₂（THF）2]+[BPh4]（18）, whichrepresented the second example of cationic lanthanide amides. Complex18wascharacterized by elemental analysis, IR and X-ray diffraction.6. Reaction of4with two equiv of Ph₂CHCN afforded the lanthanide–keteniminatecomplex （Ph₂C=C=N）₂SmN（SiMe₃）₂（DME）₂（19）, which represented the first exampleof lanthanide amides supported by a keteniminate ligand. Complex19was characterizedby elemental analysis, IR and X-ray diffraction.7. An example of dianionic β-diketiminate lanthanide monoaryloxo complexL1YbOAr’（THF）（OAr’=2,6-tBu-4-CH₃C₆H₂O）（21） was prepared via stericallyinduced deprotonation of β-diketiminate ligand L (L={[（2,6-iPr₂C₆H₃）NC（CH₃）]₂CH}^1-) by reaction of lanthanide monoaryloxo complexLYbOAr’（Cl）（THF） with one equivalent of NaN（SiMe₃）₂. Although the structure of20 could not be obtained by X-ray diffraction, it could be indirectly determined by reactionof20with one equiv of DCC, which afforded monoaryloxo complex supported by amodifided β-diketiminate ligand with an anionic sidearm L2YbOAr’(L₂={μ3-N,N,N-HC（C（CH₂）NAr）2（C（NHC₆H₁1）NC₆H₁1）}^2-, OAr’=2,6-tBu-4-CH₃C₆H₂O)（21） that has a similar molecular structure to11. Complex21was characterized byelemental analysis, IR and X-ray diffraction.