Chiral N,N,O-Scorpionate Zinc Alkyls as Effective and Stereoselective Initiators for the Living ROP of Lactides

The chiral and the enantiopure bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the alcohol compounds (bpzbeH) [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide], (bpzteH) [bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1- para- tolylethoxide], and ( R, R )-bpzmmH {( R, R )-bpzmm = (1 R )-1-[(1 R )-6,6-dimethylbicyclo[3.1.1]-2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide} have been utilized to obtain new NNO-scorpionate zinc alkyl complexes. The reaction of bpzbeH, bpzteH (racemic mixture), or ( R, R )-bpzmmH (enantiopure) with [ZnMe 2 ] in a 1:1 molar ratio in toluene afforded the mononuclear and monoalkyl zinc complexes [Zn(Me)(κ 3 -NNO)] ( 1 – 3 ), respectively. However, when the same reaction was carried out with [ZnEt 2 ], [Zn t Bu 2 ], or [Zn(CH 2 SiMe 3 ) 2 ], new dinuclear complexes of the type [Zn(R)(κ-NNμ-O)] 2 (κ-NNμ-O = bpzbe, R = Et 4, t Bu 5, CH 2 SiMe 3 6; bpzte, R = Et 7, t Bu 8, CH 2 SiMe 3 9; ( R, R )-bpzmm, R = Et 10, t Bu 11, CH 2 SiMe 3 12 ) were obtained. The single-crystal X-ray structure of derivative 1 confirms a monomeric 4-coordinative structure in which the metal center is in a distorted tetrahedral geometry with the heteroscorpionate ligands in a κ 3 coordination mode, whereas 4·2C 7 H 8, 6, and 7 reveal an asymmetric κ-NNμ-O arrangement in a dimeric molecular disposition. Interestingly, alkyl-containing zinc complexes 1, 4, 5, 6, 9, and 12 can act as single-component initiators for the ring-opening polymerization of lactides at 50 °C. Thus, lactides were polymerized to afford PLA materials with low molecular weights in a few hours. The polymerizations are living, as evidenced by the narrow polydispersities ( M w / M n = 1.03) of the isolated polymers in conjunction with the linear nature of the number-average molecular weight versus conversion plot. Inspection of the kinetic parameters for l -LA showed that propagations present the usual pseudo-first-order dependence on monomer and catalyst concentration. 1 H NMR and MALDI-TOF mass spectra confirmed that the initiation occurs through nucleophilic attack of alkyl on the lactide monomer. Microstructural analysis of poly( rac -lactide) by 1 H NMR spectroscopy revealed that the myrtenal substituent on the alkoxide fragment has a moderate influence on the degree of stereoselectivity, producing at mild temperatures enriched-heterotactic PLAs with a P r value of up to 0.77.