Bimetallic lanthanide amido complexes as highly active initiators for the ring-opening polymerization of lactides

Abstract

A series of neutral bimetallic lanthanide amido complexes supported by rigid phenylene bridged bis(β-diketiminate) ligands were synthesized, and their catalytic behavior for the polymerization of L-lactide and rac-lactide was explored. The amine elimination reaction of Ln[N(TMS)(2)](3)(μ-Cl)Li(THF)(3) with PARA-H(2), [PARA-H(2) = 2[2,6-(i)Pr(2)C(6)H(3)NHC(Me)C(H)C(Me)N]-(para-phenylene)] in a 2:1 molar ratio in THF at 25 °C afforded the corresponding bimetallic lanthanide amido complexes PARA-{Ln[N(SiMe(3))(2)](2)}(2) [Ln = Nd(1), Sm(2), Y(3)] in high isolated yields. Similar reaction of Nd[N(TMS)(2)](3)(μ-Cl)Li(THF)(3) with META-H(2), [META-H(2) = 2[2,6-(i)Pr(2)C(6)H(3)NHC(Me)C(H)C(Me)N]-(meta-phenylene)] at 90 °C in toluene for about 48 h gave META-{Nd[N(SiMe(3))(2)](2)}(2) (4). Complexes 1-4 were well characterized by elemental analysis, IR spectroscopy, and their definitive structures were confirmed by an X-ray crystal structure analysis. The coordination environment and coordination geometry around the metal atoms are similar in these complexes. Each of the metal atoms is four-coordinated with two nitrogen atoms from the N,N-chelating β-diketiminate unit, and two nitrogen atoms from two (Me(3)Si)(2)N- groups to form a distorted tetrahedron. These complexes can serve as highly active initiators for L-lactide polymerization in toluene. In addition, they also showed high activity towards rac-lactide polymerization in THF at room temperature, giving heterotactic-enriched polymers (P(r) ≈ 0.70), and complex 4 displays obviously higher activity in comparison with complex 1.

Synthesis and structural characterization of amido scorpionate rare earth metals complexes

“…This distortion is manifested in the angles N(3)-Y(1)-N( 1 5) Å and correlate well with the corresponding distances found in other yttrium amide complexes. 13 These bond lengths are shorter than the Y-N bond distances from the pyrazole ring [2.548(5)-2.512( 4) Å], thus confirming that the N atoms of the silylenediamide ligand are attached to the yttrium centre in an anionic fashion. In these complexes, the C 5 H 4 ring is symmetrically bonded to the metal centre with Y-C bond distances in the range 2.610(6)-2.669(4) Å.…”

“…A pseudo five-coordinate environment for these complexes can be proposed and a plane of symmetry exists: i.e., a pseudo square pyramidal geometry (see Scheme 1). However, the room temperature 1 H and 13 C { 1 H} NMR spectra of complexes 3 and 4 (chiral compounds) show two singlets for the H 4 , Me 3 and Me 5 pyrazole protons and carbons, four multiplets for the cyclopentadienyl protons and one set of signals for the silylenediamide moiety.…”

“…1 H NMR (C 6 D 6 , 297 K): δ = 6.30 (s, 1 H, CH), 5.32, 5.30 (s, 2 H, H 4,4′ ), 2.45, 2.36 (s, 6 H, Me 3,3′ ), 1.78, 1.66 (s, 6 H, Me 5,5′ ), 2.57 (s, 1 H, CH a ), 0.79 [s, 9 H, C(CH 3 ) 3 ], 7.03, 6.90 (m, 2 H, H d,d′ -Cp), 5.81, 5.75 (m, 2 H, H c,c′ -Cp), 5.17 (m, 2 H, NSiHMe 2 ), 0.42 (d, 3 J HH = 2.4 Hz, 12 H, NSiHMe 2 ), 0.67 (s, 6 H, SiMe 2 ). 13 C{ 1 H} NMR (C 6 D 6 , 297 K): δ = 66.6 (CH), 151.6, 151.5, 142.0, 139.5 (C 3,3′ or 5,5′ ), 107.7, 106.7 (C 4,4′ ), 14.4, 14.3 (Me 3,3′ ), 11.8, 10.9 (Me 5,5′ ), 57.9 (C a ), 35. 29 Si NMR (C 6 D 6 , 297 K): −22.9 (m, SiMe 2 ), −26.8 (ds, 1 J SiH = -79.9 Hz, 2 J SiH = 5.8 Hz, NSiHMe 2 ).…”

Zinc bimetallics supported by a xanthene-bridged dinucleating ligand: synthesis, characterization, and lactide polymerization studies

“…6,7 The structure of the metal catalyst affects the properties of the resulting poly(lactic acid), including tacticity, molecular weight, and molecular weight distribution. Several recent reports demonstrated advantageous reactivity of predesigned bimetallics [8][9][10][11][12][13][14][15][16] that may exhibit higher catalytic activity, higher molecular weights, or improved tacticity, in comparison with related mononuclear species. The observed synergism of predesigned bimetallics is corroborated by the interesting observation that bimetallic alkoxide-bridged active sites may be preferential even for precursors lacking dinucleating ligands.…”

Magnesium and Zinc Complexes Supported by N,N,O Tridentate Ligands: Synthesis and Catalysis in the Ring‐Opening Polymerization of rac‐Lactide and α‐Methyltrimethylene Carbonate

“…A large number of metal complexes based predominantly (but not exclusively) on alkali metals, aluminum, iron,, titanium, zirconium,, magnesium, zinc, calcium, and trivalent lanthanides have been reported to be effective catalysts/initiators for the ROP of lactides. Among them, aluminum complexes, especially those supported by salen‐type ligands and their derivatives, exhibit satisfactory isoselectivities toward rac ‐lactide ( rac ‐LA) polymerization, which is highly desired for obtaining PLAs with enhanced thermal properties .…”