Discrete, Solvent-Free Alkaline-Earth Metal Cations: Metal···Fluorine Interactions and ROP Catalytic Activity

Abstract

Efficient protocols for the syntheses of well-defined, solvent-free cations of the large alkaline-earth (Ae) metals (Ca, Sr, Ba) and their smaller Zn and Mg analogues have been designed. The reaction of 2,4-di-tert-butyl-6-(morpholinomethyl)phenol ({LO(1)}H), 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenol ({LO(2)}H), 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO(3)}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol ({RO(3)}H) with [H(OEt(2))(2)](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) readily afforded the doubly acidic pro-ligands [{LO(1)}HH](+)[X](-) (1), [{LO(2)}HH](+)[X](-) (2), [{LO(3)}HH](+)[X](-) (3), and [{RO(3)}HH](+)[X](-) (4) ([X](-) = [H(2)N{B(C(6)F(5))(3)}(2)](-)). The addition of 2 to Ca[N(SiMe(3))(2)](2)(THF)(2) and Sr[N(SiMe(3))(2)](2)(THF)(2) yielded [{LO(2)}Ca(THF)(0.5)](+)[X](-) (5) and [{LO(2)}Sr(THF)](+)[X](-) (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO(2)}CaN(SiMe(3))(2) (7) with [H(OEt(2))(2)](+)[X](-). Complexes [{LO(3)}M](+)[X](-) (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO(3)}M](+)[X](-) (M = Zn, 13; Mg, 14; Ca, 15; Sr, 16; Ba, 17) were synthesized in high yields (70-90%) by reaction of 3 or 4 with the neutral precursors M[N(SiMe(3))(2)](2)(THF)(x) (M = Zn, Mg, x = 0; M = Ca, Sr, Ba, x = 2). All compounds were fully characterized by spectroscopic methods, and the solid-sate structures of compounds 1, 3, 7, 8, 13, 14, {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2) were determined by X-ray diffraction crystallography. Whereas the complexes are monomeric in the case of Zn and Mg, they form bimetallic cations in the case of Ca, Sr and Ba; there is no contact between the metal and the weakly coordinating anion. In all metal complexes, the multidentate ligand is κ(6)-coordinated to the metal. Strong intramolecular M···F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2). VT (19)F{(1)H} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae···F energies of stabilization of 25-26 (Ca, Ba) and 40 kcal·mol(-1) (Sr) were calculated by NBO analysis on DFT-optimized structures. The identity and integrity of the cationic complexes are preserved in solution in the presence of an excess of alcohol (BnOH, (i)PrOH) or L-lactide (L-LA). Efficient binary catalytic systems for the immortal ring-opening polymerization of L-LA (up to 3,000 equiv) are produced upon addition of an excess (5-50 equiv) of external protic nucleophilic agents (BnOH, (i)PrOH) to 8-12 or 13-17. PLLAs with M(n) up to 35,000 g·mol(-1) were produced in a very controlled fashion (M(w)/M(n) ≈ 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn < Ca < Sr ≈ Ba; also, Ae complexes supported by the aryloxide ...

Heteroscorpionate Rare‐Earth Metal Zwitterionic Complexes: Syntheses, Characterization, and Heteroselective Catalysis on the Ring‐Opening Polymerization of rac‐Lactide

“…However, stereocontrol in polymerization is very difficult; it requires the concerted influence of the metal center and the chelating ligands. Most efforts have been dedicated to metal catalysts based on main-group and d-block metals [14,[17][18][19][20][21] such as aluminum, zinc, and so on, because these small-radius metal ions allow one to easily tune the spatial environment around the active metal centers by varying the coordinating ancillary that is believed to be responsible for the stereoselectivity. Be that as it may, few ligands such as bisphenolate or b-diiminate match these metal ions in achieving high stereoselectivity.…”

K+···Cπ and K+···F Non-Covalent Interactions in π-Functionalized Potassium Fluoroalkoxides

“…The fluoroalcohols {RO 1 }H-{RO 4 }H bearing two strongly electron-withdrawing CF 3 groups in α position to the hydroxyl can be prepared in high yield by treatment of 2,2-bis(trifluoromethyl)oxirane with the appropriate amine in Et 2 O [16,20] β-Si-H···Ae 2+ agostic distortions when using the N(SiMe2H) − amido co-ligand [15][16][17][18]. More recently, we have prepared Ae-olefin and Ae-alkyne fluoroalkoxo complexes that both exhibit strong intramolecular Ae 2+ ···Cπ in the solid state and in solution [19,20].…”

“…The fluoroalcohols {RO 1 }H-{RO 4 }H bearing two strongly electron-withdrawing CF3 groups in α position to the hydroxyl can be prepared in high yield by treatment of 2,2-bis(trifluoromethyl)oxirane with the appropriate amine in Et2O [16,20] . They all crystallized as tetranuclear complexes in a K4O4 cubane arrangement (vide infra).…”

“…The proteo-ligands {RO 1 }H-{RO 3 }H were obtained as described earlier [16,20]. The new {RO 4 }H was obtained following the same protocols, using HNEt 2 as starting material; see the Supplementary Materials (Sections S1-S9) for detail.…”

“…As part of our program aimed at implementing the large alkaline earths (Ae = Ca, Sr, Ba) in molecular catalysis, we prepared some time ago several heteroleptic amido-Ae aryloxides and fluoroalkoxides stabilized by secondary interactions, particularly intramolecular Ae 2+ ···F contacts and β-Si-H···Ae 2+ agostic distortions when using the N(SiMe 2 H) − amido co-ligand [15][16][17][18]. More recently, Inorganics 2017, 5, 13 2 of 13 we have prepared Ae-olefin and Ae-alkyne fluoroalkoxo complexes that both exhibit strong intramolecular Ae 2+ ···C π in the solid state and in solution [19,20].…”