Synthesis And Characterization Of Organolanthanide Complexes Supported By Linked Bis(Amidinate)s And Their Catalytic Behavior

A series of lanthanide bimetallic, anionic, alkoxide, amido complexes and lanthanide divalent complexes supported by linked bis(amidinate)s (L = [Me₃SiNC(Ph)N(CH₂)₃NC(Ph)NSiMe₃] ) were synthesized and well characterized, including monometallic and bimetallic alkoxides, amides, cyclopentadieyl and amidinate, and anionic amidinates and amides, as well as a divalent ytterbium cluster and a bimetallic amidinate ytterbium iodide. The catalytic activity of these complexes in polymerization ofε-caprolactone and lactide, guanylation of amines with carbodiimides, amination of amines with nitriles and acylation of aldehydes with amines, respectively, was tested. The main results obtained are as follows:1 Reaction of dilithium bis(amidenate)s LLi₂ with LnCl₃ in 1:1 moral ratio afforded the three bis(amidinate)s chlorides LYb(μ2-Cl)₂Yb(THF) (3-1), LYbCl(THF)₂ (3-2) and LYCl(DME) (3-3). All these complexes were well characterized including X-ray diffraction, IR spectroscopy, elemental analysis and 1H NMR for 3-3.2 Reaction of bis(amidinate)s lanthanide chlorides with equimolar of HOiPr was examined. Complexes of [L′(H₂)]₂YbCl (3-4) and [L′(H₂)]₂NdCl (3-5) (L′(H₂) = [Me₃SiNHC(Ph)N(CH₂)₃NC(Ph)NHSiMe₃]) were isolated via the hydrolysis of one of amidinatos by HOiPr. The quantitative hydrolysis of a bridged bis(amidinate)s was proved to be a convenient route to an amidine-functionalized amidenate lanthanide complex. X-ray analysis showed the coordination geometries around the center metals in the two complexes were described as a distorted pentagonal bipyramid.3 Reaction of ytterbium chloride 3-2 with equimolar of NaCp afforded the corresponding monometallic cyclopentadienyl complex LYbCp(DME) (3-6), while reaction of bis(amidenate)s dilithium LLi₂ with two equiv of Cp₂YbCl yielded the bimetallic complex CpYbLYbCp (3-7). The results indicated that the ligand can act as a chelated ligand to coordinate to one metal or as a bridged ligand to bind to two metals depending on the reagents used.4 Reaction of ytterbium chloride 3-2 and 3-3 with 0.5 equiv of LLi₂, respectively, afforded the corresponding bimetallic complexes Yb(μ-L)₃Yb (3-8) and Y(μ-L)3Y (3-9) formed through the rearrangement reaction of L. Complexes 3-8 and 3-9 can also be synthesized by the reaction of LnCl₃ (Ln = Yb, Y) with 1.5 equiv of Li₂L. Reaction of LnCl₃ with 2 equiv of Li₂L gave the anionic complexes [Li(DME)3][L2Ln] (Ln = Yb 3-10, Y= 3-11, Nd 3-12, Eu 3-13). All the complexes were characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction.5 Bimetallic complexes 3-8 and 3-9 and the anionic complexes [Li(DME)₃][L₂Ln] (Ln = Yb 3-10, Y= 3-11, Nd 3-12, Eu 3-13) can catalyze the polymerization ofε-caprolactone, however, bimetallic complexes exhibit much higher reactivity than the anionic complexes. Center metal has a profound effect on the reactivity: the active sequence is Nd > Eu > Y > Yb. All systems showed poor controllability. Moreover, all complexes can catalyze guanylation of amines with carbodiimides and acylation of aldehydes with amines. The anionic complex showed much higher reactivity in comparison with the bimetallic complexes in both reactions.6 The molecular structures and reactivity of bis(amidinate)s ytterbium complexes with varied amides were investigated. Reaction of LYbCl(THF)₂ with equimolar NaNHAr′(Ar′NH = 2,6-Me₂C₆H₃NH) and NaNHAr (ArNH = 2,6-iPr₂C₆H₃NH), respectively, gave the corresponding monometallic amide complexes LYb(NHAr′)(DME) (4-1) and LYb(NHAr)(DME) (4-2), in which the linked bis(amidinate)s coordinate to the center metal as a chelating ligand. The same reaction with NaN(SiMe₃)₂ afforded the bimetallic amide complex (TMS)₂NYb(L)₂YbN(TMS)₂ (4-3) formed through the rearangement reaction of L induced by the bulky N(SiMe₃)₂ group. In complex 4-3 the two linked bis(amidinate)s act as bridging ligands to link two species of YbN(TMS)₂ in one molecule. All these amides are efficient initiators for polymerization ofε-caprolactone and L-lactide. The catalytic performance is substantially different between the two kinds of amides. The polymerizations initiated by complexes 4-1 and 4-2 proceeded in a living fashion as evidenced by the resulting polymers with narrow polydispersities, together with the linear natures of number average molecular weight versus conversion plots, while the polymerization system with complex 4-3 provided the polymers with rather broad molecular weight distributions. Moreover, these amides can catalyze the amination of amines with nitriles, complexes 4-1 and 4-2 show higher activity and better selectivity than 4-3.7 Reaction of 4-2 with equimolar LiNHAr (ArNH= 2,6-iPr₂C₆H₃NH) afforded the anionic amide [Li(DME)₃][LYb(NHAr)₂] (4-4), which was characterized by X-ray determination. Both complexes can catalyze guanylation of amines with carbodiimides, and the anionic complexes exhibit much higher reactivity than the corrsponding neutral amides.8 The influence of alkoxyl groups on molecular structures and reactivity of ytterbium alkoxide complexes supported by linked bis(amidinate)s L was investigated. Reaction of LYbCl(THF)₂ (3-2) with equimolar NaOAr (Ar = 2,6-iPr₂C₆H₃O) gave the monometallic aryloxide complex LYb(OAr)(DME) (5-1), while the same reaction with NaOiPr afforded the bimetallic alkoxide complex with two alkoxy-bridges Yb(L)₂(μ2-OiPr)₂Yb (5-2), which was prepared through the redistribution reaction of L occurred concomitantly during the metathesis reaction as the encapsulation of two OiPr bridges. In complex 5-2 the linked bis(amidinate)s acts as bridging ligands. The definite molecular structures of 5-1 and 5-2 were provided by a single-crystal X-ray analysis. Both of the complexes can initiate the polymerization of cyclic ester in a living fashion as evidenced by the resulting polymers with narrow molecular weight distributions, together with the linear natures of number average molecular weight versus conversion plots. Complexes 5-1 and 5-2 can catalyze guanylation of amines with carbodiimides, and 5-2 is more active.9 A novel divalent bis(amidinete)s ytterbium cluster 6-1 constructed by Li₂O and LiI was synthesized rationally via introduce of quantitative Li₂O formed by hydrolysis of n-LiBu with H₂O. The first divalent bimetallic ytterbium halide complex IYbLYbI (6-2) was synthesized by reaction of LLi2 with YbI2 in the moral ratio of 1:2. The further study indicated that both of the complexes can catalyze guanylation of amines with carbodiimides and amination of amines with nitriles, and cluster 6-1 exhibits much higher reactivity in comparison with the bimetallic complex 6-2.