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

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 ({LO1}H), 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenol ({LO2}H), 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO3}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol ({RO3}H) with [H(OEt2)2]+[H2N{B(C6F5)3}2]− readily afforded the doubly acidic pro-ligands [{LO1}HH]+[X]− (1), [{LO2}HH]+[X]− (2), [{LO3}HH]+[X]− (3), and [{RO3}HH]+[X]− (4) ([X]− = [H2N{B(C6F5)3}2]−). The addition of 2 to Ca[N(SiMe3)2]2(THF)2 and Sr[N(SiMe3)2]2(THF)2 yielded [{LO2}Ca(THF)0.5]+[X]− (5) and [{LO2}Sr(THF)]+[X]− (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO2}CaN(SiMe3)2 (7) with [H(OEt2)2]+[X]−. Complexes [{LO3}M]+[X]− (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO3}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(SiMe3)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}4·3CD2Cl2, {16}4·3CD2Cl2, and {{17}4·EtOH}·3CD2Cl2 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 κ6-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}4·3CD2Cl2, {16}4·3CD2Cl2, and {{17}4·EtOH}·3CD2Cl2. VT 19F{1H} 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, iPrOH) 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, iPrOH) to 8–12 or 13–17. PLLAs with Mn up to 35 000 g·mol–1 were produced in a very controlled fashion (Mw/Mn ≈ 1.10–1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn t = kp·[L-LA]1.0·[16]1.0·[BnOH]1.0 was established by NMR kinetic investigations, with the corresponding activation parameters ΔH⧧ = 14.8(5) kcal·mol–1 and ΔS⧧ = −7.6(2.0) cal·K–1·mol–1. DFT calculations indicated that the observed order of catalytic activity matches an increase of the L-LA coordination energy onto the cationic metal centers with parallel decrease of the positive metal charge.