Trispyrazolylborate ligands as ancillary ligands in the development of single-site metal alkoxide catalysts for ring-opening polymerization of cyclic esters

The development of a new class of single-site metal alkoxide catalysts employing trispyrazolyl ligands is described where the metal ions are Mg(2+), Zn(2+) and Ca(2+). A particularly promising ligand for the kinetically labile Ca(2+) ion is tris[3(-2-methoxy-1,1-dimethylpyrazolyl)] hydroborate, TpC∗. This ligand is capable of being extremely flexible in its coordination modes and its coordination with various group 1 and 2 metal ions is described. The complexes TpC∗MI exist as salts [TpC∗M]+I−, where M = Mg and Ca, but TpC∗ZnI contains a four coordinate Zn(2+) center. The complexes TpC∗CaN(SiMe3)2 and TpC∗CaOC6H4-p-Me contain 5 and 7 coordinate Ca(2+) ions and serve as initiators for the ring-opening polymerization of lactide and ε-caprolactone. The compounds [TpC∗M]+[TpC∗]−, where M = Mg and Ca, exist as salts in the solid-state and in solution show exchange between coordinated and free TpC∗ ligands as determined by NMR spectroscopy.

Graphical abstractThe kinetic lability of the group metal II ions presents a challenge in the preparation of single site catalysts of the form LMOR for the ring-opening polymerization of cyclic esters such as lactide. This article describes the use of bulky tridentate trispyrazolylborate ligands, Tp∗, which for calcium prove labile toward the Schlenk equilibrium and thus unsatisfactory. However, a modification of the Tp ligand to incorporate hemilabile ether appendages and thus convert a tridentate ligand to a hexadentate one proves more satisfactory.Figure optionsDownload full-size imageDownload as PowerPoint slide