Synthesis and Coordination Chemistry of Pentadienyl Ligands Derived from (1R)-(−)-Myrtenal

Synthesis and characterisation of an enantiomerically pure scandium pentadienyl complex and its application in the polymerisation of rac-lactide

The alkyl-functionalised scandium complex [(pdl*SiMe₂N^tBu)Sc(thf)(CH₂SiMe₃)] (2) was synthesised in enantiomerically pure form and characterised by NMR spectroscopy and X-ray diffraction analysis. Complex 2 features a chiral constrained geometry ligand derived from the natural compound (1R)-(-)-myrtenal, in which the pentadienyl (pdl*) fragment coordinates in η³:η²-allyl-en fashion to the scandium atom. Compound 2 catalyses the polymerisation of rac-lactide at 30 °C and 50 °C yielding amorphous poly(lactide) with slightly heterotactic enchainment (P_m = 0.36 and 0.37). In agreement with the data obtained from GPC and DSC measurements, a chain-end control mechanism is proposed with fast chain propagation relative to the initiation, which leads to broad molecular weight distributions (Đ ≈ 1.80) and higher than expected molecular weights. Furthermore, chain transfer processes are observed, but only small amounts of transesterification and racemisation occur. Kinetic studies reveal a second-order dependence in rac-lactide (monomer) concentration and a first-order dependence in the concentration of catalyst 2.

Enantiomerically Pure Constrained Geometry Complexes of the Rare-Earth Metals Featuring a Dianionic N-Donor Functionalised Pentadienyl Ligand: Synthesis and Characterisation

We report the preparation of enantiomerically pure constrained geometry complexes (cgc) of the rare-earth metals bearing a pentadienyl moiety (pdl) derived from the natural product (1R)-(-)-myrtenal. The potassium salt 1, [Kpdl*], was treated with ClSiMe₂ NHtBu, and the resulting pentadiene 2 was deprotonated with the Schlosser-type base KOtPen/nBuLi (tPen=CMe₂ (CH₂ Me)) to yield the dipotassium salt [K₂ (pdl*SiMe₂ NtBu)] (3). However, 3 rearranges in THF solution to its isomer 3' by a 1,3-H shift, which elongates the bridge between the pdl and SiMe₂ NtBu moieties by one CH₂ unit. This is crucial for the successful formation of various monomeric C₁ - or dimeric C₂ -symmetric rare-earth cgc complexes with additional halide, tetraborohydride, amido and alkyl functionalities. All compounds have been extensively characterised by solid-state X-ray diffraction analysis, solution NMR spectroscopy and elemental analyses.

Synthesis and molecular structure of enantiomerically pure pentadienyl complexes of the rare-earth metals

The enantiomerically pure pentadienyl (Pdl*) ligand derived from the natural product (1R)-(-)-myrtenal forms with MCl₃ (M = La, Ce, Pr, and Nd) the corresponding homoleptic [(η⁵-U-Pdl*)₃M compounds (1-M). These complexes were fully characterised by ¹H NMR spectroscopy, elemental analyses and X-ray diffraction. They exhibit in solution and solid state idealized C₃ symmetry, and their molecular structures also reveal that the Pdl* ligand adopts a U-conformation and coordinates exclusively with its less sterically encumbered face to the rare-earth metal atom. For the paramagnetic representatives an assignment of the ¹H NMR resonances based on a simple dipolar model gave satisfactory results.

Preparation of enantiomerically pure open calcocene and strontocene complexes and their application in ring opening polymerizations of rac-lactide

The synthesis of C2 symmetric enantiomerically pure open Ca and Sr metallocenes, [(η(5)-pdl*)2Ca(thf)] (1) and [(η(5)-pdl*)2Sr(thf)2] (2) (pdl* = dimethylnopadienyl) is described and these complexes were fully characterized. The solid state structures confirm that the pdl* ligands coordinate exclusively with the less sterically demanding site to the Ca and Sr atoms. These complexes are active catalysts for the controlled ring opening polymerization (ROP) of rac-lactide to give heterotactically enriched polylactides (PL) with narrow polydispersities (PDI = 1.29-1.31) and without adding further activators.

Molecular and electronic structures of donor-functionalized dysprosium pentadienyl complexes

The molecular structures of two dysprosium complexes of donor-functionalized pentadienyl ligands are described. Electronic structure calculations reveal that the W-shaped pentadienyl ligand strongly influences the orientation of the main magnetic axis of the ground Kramers doublets in both complexes.