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 = (1R)-1-[(1R)-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 [ZnMe2] in a 1:1 molar ratio in toluene afforded the mononuclear and monoalkyl zinc complexes [Zn(Me)(κ3-NNO)] (1–3), respectively. However, when the same reaction was carried out with [ZnEt2], [Zn t Bu2], or [Zn(CH2SiMe3)2], new dinuclear complexes of the type [Zn(R)(κ-NNμ-O)]2 (κ-NNμ-O = bpzbe, R = Et 4, t Bu 5, CH2SiMe3 6; bpzte, R = Et 7, t Bu 8, CH2SiMe3 9; (R,R)-bpzmm, R = Et 10, t Bu 11, CH2SiMe3 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 κ3 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. 1H 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 1H 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.

The previously described reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(κ(3)-pbpamd)(THF)] and [Li(κ(3)-tbpamd)(THF)] with a series of commercially available Grignard reagents RMgCl in an equimolecular ratio yielded the magnesium monoalkyls [Mg(R)(κ(3)-NNN)] (NNN = pbpamd, R = CH2SiMe3, Et (1), Bn (2); NNN = tbpamd, R = CH2SiMe3, Et (3), Bn (4)). However, subsequent reaction of these monoalkyls [Mg(R)(κ(3)-NNN)] with two additional equivalents of the same RMgCl in tetrahydrofuran gave rise to dinuclear dialkyls of the type [RMg(κ(3)-N,N,N;κ(2)-C,N)MgR(thf)] (κ(3)-N,N,N;κ(2)-C,N = pbpamd(-), R = CH2SiMe3 (5), Et (6); κ(3)-N,N,N;κ(2)-C,N = tbpamd(-), R = CH2SiMe3 (7), Et (8)). Furthermore, when the reaction was carried out in a mixture of tetrahydrofuran/dioxane with the same stoichiometry, a new family of tetranuclear tetraalkyl magnesium complexes [{RMg(κ(3)-N,N,N;κ(2)-C,N)MgR}2{μ-O,O-(C4H8)}] (κ(3)-N,N,N;κ(2)-C,N = pbpamd(-), R = CH2SiMe3 (9), Et (10), Bn (11); κ(3)-N,N,N;κ(2)-C,N = tbpamd(-), R = CH2SiMe3 (12), Et (13), Bn (14)) was obtained. In both families, an apical methine C-H activation process on the heteroscorpionate takes place. The single-crystal X-ray structures of 4, 8, 9, and 12 confirm the nuclearity of each family, with 4-coordinative arrangements for all magnesium atoms. More importantly, the presence in the di- and tetranuclear complexes of unprecedented apical carbanions with a direct σ-C(sp(3))-Mg covalent bond, and as a result, the existence of stereogenic magnesium centers, have been unambiguously confirmed. Interestingly, the dinuclear dialkyls 5 and 7, as well as the tetranuclear tetraalkyls 9, 10, and 12, can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides. Lactide (LA) polymerizations afforded polylactide (PLA) materials with medium molecular weights in only a few minutes even at 20 °C for L-LA and in a few hours at 50 °C for rac-LA propagations. More importantly, microstructural analysis of the poly(rac-lactide) materials revealed that the tetranuclear tetra-alkyl 12 exerts enhanced levels of heteroselectivity on the PLAs under mild conditions, with Ps values up to 0.78.

The work described here represents the first example of an enantiopure hybrid scorpionate/cyclopentadiene ligand. The ligand was obtained in a one-pot synthetic procedure by an efficient and highly diastereoselective nucleophilic addition of an organolithium reagent to an electrophilically activated olefin in a new fulvene with a chiral substrate to control the stereochemistry of a newly created asymmetric center. We verified the potential utility of this ligand as a valuable scaffold that is able to induce chirality in organometallic/coordination chemistry. This was achieved through the preparation of a new enantiomerically pure zinc complex in which the ligand behaves in a tridentate manner with a κ2 NN-η1(π)-Cp coordination mode. This alkylzinc complex constitutes the first example of an organozinc derivative which behaves as an efficient initiator for the ROP of rac-LA in the production of isotactic-enriched poly(lactides) with P i = 0.77.

The reaction of the enantiopure NNO-scorpionate ligand (R,R)-bpzmmH with [ZnR2] (R = Me, Et, CH2SiMe3) in a 1:2 molar ratio afforded the dinuclear tris-alkyls [Zn(R)(κ2-NNμ-O)Zn(R)2] (R = Me (1), Et (2), CH2SiMe3 (3)). The reaction of the bimetallic tris-alkyls 1–3 with an aromatic alcohol and thioalcohol yielded the dinuclear zinc complexes containing aryloxide/arylthioxide ligands [(ZnR)2(κN:κN-μ-O)(μ-EAr)] (Ar = 2,6-C6H3Me2; E = O, R = Me (4), Et (5), CH2SiMe3 (6); E = S, R = Me (7), Et (8), CH2SiMe3 (9)) by an alcoholysis or thioalcoholysis reaction. The structures of the different compounds were determined by spectroscopic methods and, in addition, the X-ray crystal structures of 2 and 5 were also unambiguously established. Alkyl- and aryloxide/thioaryloxide-containing enantiopure zinc complexes 3–6 and 9 can act as single-component living initiators for the ring-opening polymerization of lactides, affording materials with low molecular weights under mild conditions in only a few hours. Interestingly, microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed that the most sterically hindered initiator, [(ZnCH2SiMe3)2(R,R-bpzmm)(μ-OAr)] (6), is one of the first examples of a zinc complex capable of exerting a significant influence on the degree of enantioselectivity, with enriched-isotactic PLAs (P i = 0.74) produced through an enantiomorphic site control mechanism.

Reaction of the chiral alcohol-scorpionate compound bpzteH [bpzteH = 2,2-bis­(3,5-dimethylpyrazol-1-yl)-1-para-tolylethanol] with [ZnR2] (R = Me, CH2SiMe3) in a 1:2 molar ratio afforded the dinuclear chiral alkyl zinc complexes [Zn­(R)­(κ2-NNμ-O)­Zn­(R)2] (1 and 2) [κ2-NNμ-O = bpzte, R = Me 1, CH2SiMe3 2]. Subsequent alcoholysis reaction with ArOH (1 equiv; Ar = 2,6-C6H3Me2) yielded the chiral dinuclear zinc complexes [(ZnR)2(κN:κN-μ-O)­(μ-OAr)] (3 and 4) [κN:κN-μ-O = bpzte, R = Me 3, CH2SiMe3 4]. The X-ray crystal structures of 1 and 3 confirmed a dinuclear structure in all complexes, with the alkoxide of the heteroscorpionate ligands in a μ-bridging mode between the two Zn­(II) centers. The zinc complexes 1–4, bearing trisalkyl and alkyl-aryloxide ligands, are shown to function as highly efficient single-component initiators for the ring-opening homopolymerization of ε-caprolactone and l-/rac-lactide. These initiators enable rapid polymerization at ambient temperatures, affording materials with low molecular weights that exhibit narrow, monomodal molecular weight distributions. Microstructural analysis of poly­(rac-lactide) revealed that the alkyl-aryloxide 3 exerts a moderate level of heteroselectivity (P s = 0.68). More interestingly, initiators 3 and 4 enabled random copolymerization of ε-CL and l-LA with near-equal reactivity ratios for the two comonomers (r CL = 1.15 and 0.92, and r LA = 1.37 and 1.05, for 3 and 4, respectively), resulting in closely similar average sequence lengths for the caproyl and lactidyl segments (L CL ≈ 2.0; L LA ≈ 2.5, for both initiators). It was also found that the copolymers had monomer contents in close agreement with the composition of the initial monomer feed. Analysis by differential scanning calorimetry of the thermal properties such as T g of various copolymers produced by 3 revealed a strong dependence on the monomer content, as these values vary in a linear fashion with the molar percentage of l-lactide units in the copolymer over a temperature range from −30 to 42 °C.