Influence Of Schiff Base And Lanthanide Metals On The Synthesis, Stability, And Reactivity Of Monoamido Lanthanide Complexes And Divalent Lanthanide Complexes Bearing Two Schiff Bases

Using Schiff base as ancillary ligands, 35 lanthanide complexes were synthesized. Among them, the molecular structures of 32 complexes were determined by X-ray single crystal diffraction. The influence of Schiff base and lanthanide metals on the synthesis, stability, and reactivity of monoamido lanthanide complexes and divalent lanthanide complexes bearing two Schiff bases were studied. Furthermore, catalytic behavior of some of these complexes for the guanylation of amines and polymerization of caprolactone/lactide were studied. The main contents were listed below.1. Reactions of tridentate Schiff base 3,5-Bu₂^t-2-（OH）-C₆H₂CH=N-8-C₉H₆N （HL¹） with Ln[N（TMS）₂]₃ afforded 5 monoamido lanthanide complexes L₂¹LnN（TMS）₂ （Ln = Yb（1）, Y（2）, Eu （3）, Nd （4）, La （5））. The reaction conditions, such as the molar ratio of the starting materials, reaction temperature, solvent and the method of adding schiff base, have great influence on the synthesis of these complexes. These complexes were characterized by elemental analysis, IR. The solid structures of these complexes were studied by the X-ray diffraction. single crystal structure analysis and showed that the distances between the N （5） atom of -N（TMS）₂ group and the H （15） atom of one quinoline ring in complexes 1-5 become smaller with decrease of the size of lanthanide metals, in complexes 1 and 2, the distances are shorter than the sum of van der Waals radius of N and H atoms. These indicate that there might be the four-members-ring structure of Ln-C-H-N. The solution behavior of diamagnetic complexes 2 and 5 were studied by NMR. It was found that the structures of these complexes in solution were in accordance with their solid structures.2. Reactions of bidentate Schiff base 3,5-Bu₂^t-2-（OH）-C₆H₂CH=N-2,6-Pr₂ⁱ-C₆H₃ （HL²） with Ln[N（TMS）₂]₃ afforded 4 monoamido lanthanide complexes L₂²LnN（TMS）₂ （Ln = Yb （6）, Y （7）, Nd （8）, La （9））. The molar ratio of the starting materials also have great influence on the synthesis of these complexes. These complexes were characterized by elemental analysis, IR, X-ray diffraction and the NMR for the complexes 7 and 9. Single crystal structure analysis showed that these complexes are monomeric with no coordination solvents. The coordination number of metal center is 5. There are few examples for amido lanthanide complexes bearing Schiff base with low coordination number in the literature. Further study revealed that complex 7 could also be prepared by the salt metathesis reaction of L₂²Y（THF）Cl （10） formed via the reaction of YCl₃ with two equiv of NaL′, with NaN（TMS）₂ in THF.3. Reactions of the less bulky bidentate Schiff base 3,5-Bu₂^t-2-（OH）-C₆H₂CH=N-2,6-Me₂-C₆H₃ （HL³） with Ln[N（TMS）₂]₃ afforded the desired monoamido lanthanide complexes L₂³LnN（TMS）₂ （Ln = Yb （11）, Y （12）） only for the smaller ionic radius of Yb and Y. Instead, the homoleptic tris-Schiff base lanthanide complexes L₃³Ln (Ln =Nd （13）, La （14） were formed for the larger ionic radius of La and Nd. It was found that not only the molar ratio of the starting materials, but also the central metals have crucial influence on the synthesis of amido lanthanide complexes.4. Reactions of the other two tridentate Schiff bases C₄H₄NCH=N-2-MeO-C₆H₄ （HL⁴） and C₄H₄NCH=N-2-PPh₂-C₆H₄ （HL⁵） with Y[N（SiMe3）₂]₃ and Yb[N（SiMe3）₂]₃ afforded the corresponding homoleptic tris-Schiff base lanthanide complexes L₃⁴Y （15） and L₃⁵Yb （16）. Theβ-ketoiminato ligands, as one kind of bidentate Schiff bases, were also tried in the amine-elimination reaction. When the ligand of CH₃COCH=C（Me）NH-2,6-Pr₂ⁱ-C₆H₃ （HL⁶） was used, the final products had not been obtained as it’s good solubility. Whilst, using the less bulky ligand CH₃COCH=C（Me）NHPh （HL⁷） to decrease the solubility of the final products, the dimeric lanthanide complexes [L₂⁷Ln（μ-η²-L⁷）]₂ （Ln = Y （17）, Nd （18）, La （19）） were isolated. Each of the two metals is coordinated by threeβ-ketoiminato ligands and bridged through the oxygen atom from one of theβ-ketoiminato ligands. The salt metathesis reaction of bis-（β-ketoiminato） lanthanide chloride with NaN（TMS）₂ were designed for the synthesis of amido lanthanide complexes, but it was failed only the complexes 17-19 were formed. This may be because the lanthanide chloride via the reaction of LnCl₃ with two equiv of NaL⁷ is not easy to synthesize. However, the mixture of complex 17 and the lanthanide chloride [L⁷Y（μ-η²-L⁷）Cl]₂ （20） might be obtained for the smaller ionic radius of Y. 5. The stability of the bis-（Schiff base） monoamido lanthanide complexes 1-9, 11-12 was also studied. It was found that the Schiff base ligands and the size of the lanthanide metals have a significant impact on the stability of these monoamido lanthanide complexes. Complexes 1 and 2 are easy to decompose to the new complexes 21 and 22 via the intramolecular C-H bond activation and subsequent functionalization reaction. Such transformation for complex 4 to complex 23 could be observed at 70℃for 5 days; but complex 5 is quite stable, no such a conversion was observed even at 70℃for 10 days. This is the first example of C-H bond activation mediated by amido lanthanide complex. While, the bis-（bidentate Schiff base） monoamido lanthanide complexes 6-9, 11-12 are stable, no migration of amido group to C=N bond or C-H bond activation reaction was occurred. The intermolecular C-H bond activation was tried by the reaction of complexes 2 and 5 with o-methylpyridine, and the complexes 24 and 25 were obtained, respectively.6. The catalytic behavior of bis-（Schiff base） monoamido lanthanide complexes 1-5, 7, 12 in the guanidine-forming reaction was examined. It is noted that almost no differences in activity among the amido lanthanide complexes with various Schiff base ligands; however, the activity is largely influenced by the central metals. The possible mechanism of this reaction was also studied. Complex 2 was also found to be an efficient catalyst for the ring-opening polymerization ofε-caprolactone and L-lactide, with higher catalytic reactivity and narrow PDI of the polymer.7. Reactions of tridentate Schiff base HL¹ with Ln[N（SiMe₃）₂]₂（THF）₂ afforded the divalent Schiff base lanthanide complex L₂¹Eu（THF）₂ （26） for the less active Eu, while the complexes 21 and 27 were formed for Yb and Sm. The possible mechanisms for the formation of 21 and 27 were also presented.8. Reactions of tridentate Schiff base 3,5-Bu₂^t-2-（OH）-C₆H₂CH=N-2-C₅H₄N （HL⁹） with Ln[N（SiMe₃）₂]₂（THF）₂ were tested. It was found that the corresponding divalent lanthanide complexes could not be stabilized by this ligand, and would transform to the corresponding compexes 28 and 29 for Yb and Sm via the ligand reductive coupling. The possible mechanisms for the formation of both complexes were also included.9. Reactions of bidentate Schiff base HL² with Ln[N（SiMe₃）₂]₂（THF）₂ afforded the corresponding divalent lanthanide complexes L₂²Ln（THF）₂ （Ln = Eu （30）, Yb （31））; while the amido lanthanide complex L₂²Sm[N（TMS）₂] （32） was formed for Sm, and the plausible reasons for the formation of 32 was given. 10. Reactions of bidentate Schiff base HL³ with Ln[N（SiMe₃）₂]₂（THF）₂ afforded the divalent lanthanide complexes L₂³Ln（THF）₂ （Ln = Eu （33）, Yb （34）） for Eu and Yb, although the complex 34 was not characterized by X-Ray diffraction; while no divalent lanthanide complex was formed judging from the color of reaction system for Sm, and tried to isolation of detectable complex was failed. The reactivity of complex 33 with O₂ was tested, and the peroxide bridged lanthanide complex [L₂³Eu（THF）]₂（μ-η²:η²-O₂） （35） was isolated.