Ligand transfer reactions of mixed-metal lanthanide/magnesium allyl complexes with β-diketimines:: Synthesis, structures, and ring-opening polymerization catalysis

The reaction of the mixed-metal lanthanide/magnesiurn allyl complexes [Ln(eta(3)-C3H5)(3)(mu-C4H8O2)center dot Mg(eta(1)-C3H5)(2)(mu-C4H8O2)(1.5)](n) (Ln = La, 1; Y, 2) with the diketimine 2-(2,6-diisopropylphenyl)amino4-(2,6-diisopropylphenyl)imino-2-pentene (BDI-H) in THF at 60 degrees C leads to the isolation of the magnesium beta-diketiminato complex Mg{HC(MeCNC6H3Pr2i-2,6)(2)}(eta(1)-C3H5)(THF) (3). The same product was obtained in the reaction of BDI-H with the ionic allyl lanthanate compounds [Mg(THF)(6)][Nd(eta(3)-C2H5)(4)](2)center dot 2THF (4) and [Mg(THF)(6)] [Sm(eta(3)-C3H5)(4)](2)-center dot 2THF (5). No lanthanide-containing product could be isolated. The X-ray structure of 3 shows that the complex is a monomeric neutral species with the magnesium center in a distorted tetrahedral arrangement. Complex 3 was found to hydrolyze after repeated crystallization attempts at -26 degrees C to give the hydroxy complex [Mg{HC(MeCNC6H3Pr2i-2,6)(2)}(mu-OH)(THF)](2)-center dot 4THF (6). The X-ray structure of 6 consists of a hydroxide-bridged dimer with the magnesium center in a distorted square pyramid. Lanthanide BDI compounds were, however, obtained by reacting LnCl(3)(THF)(x) (Ln = La, x = 4; Ln = Y, x = 3) with the lithium salt [Li(BDI)(THF)](2) to give the dimeric complexes [LnCl(2){HC(MeCNC6H3Pr2i-2,6)(2)}](2)(THF)center dot 2toluene (Ln = La, 7; Y, 8). According to its X-ray structure, 8 possesses distorted octahedral yttrium centers, one terminal chloride, and three bridging chloride ligands. The reaction of 7 or 8 using 4 equiv of allylMgCl did not give the expected mixed-ligand allyl lanthanide complex but led again to the magnesium complex 3 via BDI ligand exchange. Complexes 1-5 are highly effective single-component catalysts for the ring-opening polymerization of c-caprolactone. Complex 3 also polymerizes rac-lactide efficiently and under mild conditions but with low stereospecificity. c-Caprolactone is polymerized in minutes to high molecular weight materials by an allyl end-group transfer mechanism. The catalysts are active over a wide temperature range.