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)arnino]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)center dot 3CD(2)Cl(2), {16}(4)center dot 3CD(2)Cl(2), and {{17}(4)center dot EtOH}center dot 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 kappa(6)-coordinated to the metal. Strong intramolecular M center dot center dot center dot F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}(4)center dot 3CD(2)Cl(2), {16}(4)center dot 3CD(2)Cl(2), and {{17}(4)center dot EtOH}center dot 3CD(2)Cl(2). VT F-19{H-1} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae center dot center dot center dot 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, (PrOH)-Pr-i) 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 nucleophili agents (BnOH, (PrOH)-Pr-i) 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 approximate to 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg << Zn < Ca < Sr approximate to 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 center dot center dot center dot 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 Delta H-double dagger = 14.8(5) kcal.mol(-1) and Delta S-double dagger = -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.