New Racemic and Single Enantiopure Hybrid Scorpionate/Cyclopentadienyl Magnesium and Zinc Initiators for the Stereoselective ROP of Lactides

The
preparation of the first racemic bis­(pyrazol-1-yl)­methane-based NNCp-donor
hybrid scorpionate/cyclopentadienyl magnesium and zinc complexes [Mg­(R)­(κ2-η5-NNCp)] (1–6) [NNCp = bpztcp, R = Me (1), Et (2), CH2SiMe3 (3); bpzpcp, R = Me (4), Et (5), CH2SiMe3 (6)] and [Zn­(R)­(κ2-η1-NNCp)] (7–12) [NNCp = bpztcp, R = Me (7), Et (8), CH2SiMe3 (9); bpzpcp, R = Me (10), Et (11), CH2SiMe3 (12)] has been carried out by
the reaction of the corresponding racemic NNCp-H-donor scorpionate
pro-ligands with Grignard reagents RMgCl, after deprotonation with nBuLi, or directly with ZnR2. The resulting alkyl
magnesium complexes (1–6) have been
employed in a protonolysis reaction with 2,6-dimethylphenol to yield
the aryloxide magnesium complexes [Mg­(OAr)­(κ2-η5-NNCp)] (13 and 14) [NNCp = bpztcp
(13), bpzpcp (14)]. Subsequently, the enantiopure
scorpionate pro-ligand (R,R)-bpmycpH
(16a) {bpmycpH = 1-{2,2-bis­(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2R)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]­ethyl}­cyclopentadiene}
and (R,R)-bpmycp′H (16b) {bpmycp′H = 2-{2,2-bis­(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2R)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]­ethyl}­cyclopentadiene}
reacted with RMgCl (after addition of nBuLi) or with ZnR2 to give the single enantiopure complexes [Mg­(R)­(κ2-η5-R,S-bpmycp)] (17–19) (R = Me (17), Et (18), CH2SiMe3 (19)) {R,S-bpmycp = 2-{2,2-bis­(3,5-dimethyl-1H-pyrazol-1-yl)-1-(2S)-2-[(1R)-6,6-[3.1.1]-2-hepten-2-yl]­ethyl}­cyclopentadienyl} and [Zn­(R)­(κ2-η1-R,S-bpmycp)] (20–22) (R = Me (20), Et (21), CH2SiMe3 (22)). The structures of these complexes were elucidated by 1H and 13C­{1H} NMR spectroscopy, and
the X-ray crystal structures of 2, 3, 7 and 21 were also established. These racemic
and single enantiopure complexes 1–3, 5–9, 11–14, 19, and 21 can act as single-component
initiators for the ring-opening polymerization of rac-lactide under mild conditions. Microstructural analysis of poly­(rac-lactide)­s revealed that, whereas the myrtenal substituent
on the single enantiomerically pure initiator 21 exerts
an appreciable influence on the degree of stereoselectivity to produce
isotactic-enriched poly­(lactide)­s with a Pi value up to 0.77, the racemic mixtures of initiators impart more
discrete levels of heteroselectivity in the production of PLAs, with
a Ps value of 0.72.