Discrete, solvent-free alkaline-earth metal cations: metal***fluorine interactions and ROP catalytic activity.
Résumé
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 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]/dt = k(p)*[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.