Organometallic calcium and strontium borohydrides as initiators for the polymerization of ε-caprolactone and l-lactide: Combined experimental and computational investigations

摘要

Ca(BH_4)_2(THF)_2] (1a), a known compound, was easily prepared following a convenient new procedure from [Ca(OMe)_2] and BH_3·THF in THF. Reaction of 1a with KCp~* (Cp~* = (η~5-C_5Me_5)) and K{(Me_3SiNPPh_2)_2CH} in a 1:1 ratio in THF resulted in the corresponding dimeric heteroleptic mono-borohydride derivatives [Cp~*Ca(BH_4)(THF)_n]_2 (2a) and [{(Me 3_SiNPPh_2)_2CH}Ca(BH_4)(THF)_2] (3a), respectively. Both compounds were fully characterized and the solid-state structure of 3a was established by single crystal X-ray diffraction. Compounds 1a, 2a, and 3a, together with the earlier reported compounds [Sr(BH_4)_2(THF)_2] (1b), [Cp~*Sr(BH_4)(THF)_2]_2 (2b), and [{(Me_3SiNPPh_2)_2CH}Sr(BH_4)(THF)_2] (3b), were used as initiators for the ROP of polar monomers. The general performances of the complexes in the ROP of ε-caprolactone and l-lactide demonstrate a relatively good control of the polymerization under the operating conditions established. α,ω-Dihydroxytelechelic poly(ε-caprolactone)s (PCLs) and poly(lactide)s (PLAs) were thus synthesized. DFT calculations on the initiation step of the ROP of ε-CL were carried out. Gibbs free energy profiles were determined for the three calcium complexes highlighting slightly more active calcium complexes as compared to strontium analogues, in agreement with experimental findings.