Synthesis and Coordination Chemistry of an Enantiomerically Pure Pentadienyl Ligand

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.

The importance of the counter-cation in reductive rare-earth metal chemistry: 18-crown-6 instead of 2,2,2-cryptand allows isolation of [YII(NR2)3]1- and ynediolate and enediolate complexes from CO reactions

The use of 18-crown-6 (18-c-6) in place of 2.2.2-cryptand (crypt) in rare earth amide reduction reactions involving potassium has proven to be crucial in the synthesis of Ln(ii) complexes and isolation of their CO reduction products. The faster speed of crystallization with 18-c-6 appears to be important. Previous studies have shown that reduction of the trivalent amide complexes Ln(NR2)3 (R = SiMe3) with potassium in the presence of 2.2.2-cryptand (crypt) forms the divalent [K(crypt)][LnII(NR2)3] complexes for Ln = Gd, Tb, Dy, and Tm. However, for Ho and Er, the [Ln(NR2)3]1- anions were only isolable with [Rb(crypt)]1+ counter-cations and isolation of the [YII(NR2)3]1- anion was not possible under any of these conditions. We now report that by changing the potassium chelator from crypt to 18-crown-6 (18-c-6), the [Ln(NR2)3]1- anions can be isolated not only for Ln = Gd, Tb, Dy, and Tm, but also for Ho, Er, and Y. Specifically, these anions are isolated as salts of a 1 : 2 potassium : crown sandwich cation, [K(18-c-6)2]1+, i.e. [K(18-c-6)2][Ln(NR2)3]. The [K(18-c-6)2]1+ counter-cation was superior not only in the synthesis, but it also allowed the isolation of crystallographically-characterizable products from reactions of CO with the [Ln(NR2)3]1- anions that were not obtainable from the [K(crypt)]1+ analogs. Reaction of CO with [K(18-c-6)2][Ln(NR2)3], generated in situ, yielded crystals of the ynediolate products, {[(R2N)3Ln]2(μ-OC[triple bond, length as m-dash]CO)}2-, which crystallized with counter-cations possessing 2 : 3 potassium : crown ratios, i.e.{[K2(18-c-6)3]}2+, for Gd, Dy, Ho. In contrast, reaction of CO with a solution of isolated [K(18-c-6)2][Gd(NR2)3], produced crystals of an enediolate complex isolated with a counter-cation with a 2 : 2 potassium : crown ratio namely [K(18-c-6)]2 2+ in the complex [K(18-c-6)]2{[(R2N)2Gd2(μ-OCH[double bond, length as m-dash]CHO)2]}.

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.

Ligand conformations and spin states in open metallocenes of the first row transition metals having U-shaped 2,4-dimethylpentadienyl ligands

The lowest energy (2,4-Me₂C₅H₅)₂M structures (M = Ti, V, Cr, Mn, Fe) have the metal sandwiched between two U-shaped pentahapto pentadienyl ligands. However, those for the later transition metals Co and Ni contain one and two trihapto ligands, respectively.

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.