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

摘要

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-(morphohnomethyl)phenol ({LO~1｝H), 2-{[bis(2-methosyemyl)amino]methyl}-4,6-di-tert-butylphenol ({LO~2｝H), 2-[(l, 4,7,10-tetraoxa-13-azacydopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO~3}H), and 2-[(l,4,7,10-tetraoxa-13-azacydo-pentadecan-13-yl)methyi]-l,l,1,3,3,3-hexafluoropropan-2-ol ({RO~3}H) with [H(OEt_2)_2]~+[H_2N{B(C_6F_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_2N{B(C_6F_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, IS; 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_2C1_2, {16}_4-3CD_2C1_2, and {{17}_4-EtOH}·3CD_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 k~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_2C1_2, {16}4·3CD_2C1_2, and {{17}_4·EtOH} ·3CD_2Cl_2· VT ~(19)F{~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 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 ligand are more active than their parents supported by the fluorinated alkoxide ancillary, possibly owing to the presence of Ae • • • F interactions in the latter case. The rate law -d[L-LA]/df = kp· [L-LA]~1.0· [16]~1.0· [BnOH]~1.0 was established by NMR kinetic investigations, with the corresponding activation parameters AH* = 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.