The reaction of the heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N,N‘-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equiv of RMgCl proceeds to give very high yields of the neutral heteroscorpionate alkyl magnesium complexes [Mg(R)(NNN)] (NNN = tbpamd, R = C3H5 1, t Bu 2, CH2SiMe3 3; NNN = pbpamd, R = C3H5 4, t Bu 5, CH2SiMe3 6). On heating toluene solutions of complexes 1 − 3, 5, and 6, a ligand redistribution process occurs to give the corresponding 6-coordinated sandwich complexes [Mg(tbpamd)2] (7) and [Mg(pbpamd)2] (8). Interestingly, the allyl derivative 4 can be easily transformed to 8 at room temperature. In addition, the cationic sandwich complex [Mg(tbpamdH)2]Cl2 (9) [tbpamdH = N-ethyl-N‘-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidine] was obtained from 7 by means of a protonation process. Finally, alkyl-containing complexes 1 − 6 can act as highly effective single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides over a wide range of temperatures. ε-Caprolactone is polymerized within seconds to give high molecular weight polymers with narrow polydispersities. Lactide afforded PLA materials with medium molecular weights and polydispersities as narrow as M w/M n = 1.05. Additionally, polymerization of l-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(l-lactides) with high melting temperatures (T m = 160 °C). Polymer end-group analysis shows that the polymerization process is initiated by alkyl transfer to the monomer.

The reaction of bis(3,5-di-tert-butylpyrazol-1-yl)methane (bdtbpzm) with BunLi and carbodiimide derivatives, namely, N,N′-diisopropyl and 1-tert-butyl-3-ethyl carbodiimides, gives rise to the new sterically hindered lithium acetamidinate [Li(tbptamd)(THF)] (1) [tbptamd = N-ethyl-N′-tert-butylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate] and [Li(pbptamd)(THF)] (2) [pbptamd = N,N′-diisopropylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate]. Subsequent hydrolysis of 1 and 2, and the recently reported heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N′-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N,N′-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with NH4Cl/H2O in ether cleanly affords the corresponding amidine ligands Htbpamd (3), Hpbpamd (4), Htbptamd (5), and Hpbptamd (6) in very good yields. The X-ray diffraction molecular structure of 3 was obtained. Reaction of the amidine-heteroscorpionate ligands 3–6 with 1 equiv of ZnR′2 proceeds in very high yields to give the neutral heteroscorpionate alkyl zinc complexes [Zn(R′)(NNN)] (NNN = tbpamd, R′ = Me 7, Et 8, CH2SiMe3 9; NNN = pbpamd, R′ = Me 10, Et 11, CH2SiMe3 12; NNN = tbptamd, R′ = Me 13, Et 14; NNN = pbptamd, R′ = Me 15, Et 16). The single-crystal X-ray structures of the derivatives 8, 12, 15, and 16 confirm a four-coordinative structure with the zinc metal center in a distorted tetrahedral geometry and the heteroscorpionate ligands arranged in κ3-coordination mode. The new lithium salts 1 and 2 and the alkyls 7–9, 13, and 14 can act as efficient single-component initiators for the ring-opening polymerization of ϵ-caprolactone and lactides over a wide range of temperatures. ϵ-Caprolactone is polymerized within minutes to give high-medium molecular weight polymers with medium broad values of polydispersities. Lactide afforded PLA materials with medium molecular weights and polydispersities as narrow as M w/M n = 1.05. Additionally, polymerization of l-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(l-lactides) with high melting temperatures (T m = 165 °C). rac-Lactide polymerization also produces enriched levels of heterotactic poly(lactide). Polymer end group analysis shows that the polymerization mediated by alkyl zinc complexes is initiated by alkyl transfer to monomer.

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 reaction of the sterically hindered heteroscorpionate lithium acetamidinate [Li(κ3-pbptamd)(THF)] (pbptamd = N,N′-diisopropylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate) and [Li(κ3-tbptamd)(THF)] (tbptamd = N–ethyl-N′-tert-butylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate) with 1 equiv of RMgCl proceeds cleanly to give very high yields of the corresponding nonchiral monoalkyl magnesium complexes [Mg(R)(κ3-NNN)] (NNN = pbptamd, R = Me (1), Et (2), CH2SiMe3 (3), CH2Ph (4), C3H5 (5); NNN = tbptamd, R = Me (6), Et (7), CH2SiMe3 (8), CH2Ph (9), C3H5 (10)). Alternatively, these monoalkylmagnesium complexes can be obtained by protonolysis of the corresponding dialkylmagnesium reagents, R2Mg, with the proligands Hpbptamd and Htbptamd in very good yields. Interestingly, alkyls 6–10 are obtained as a mixture of structural isomers (a + b) in a ratio that depends on the level of steric demand of the alkyl group. The single-crystal X-ray structures of derivatives 4, 6a·Et2O, and 7a confirm a four-coordinate structure in which the metal center is in a distorted-tetrahedral geometry and the bulky heteroscorpionate ligands have a κ3 coordination mode. Interestingly, the alkyl-containing magnesium complexes 1–3 and 8a can act as highly efficient single-component initiators for the very rapid ring-opening polymerization of ε-caprolactone and lactides even at −10 °C. While ε-caprolactone was polymerized within seconds to give high-molecular-weight polymers with narrow polydispersities (M n > 105, M w/M n = 1.17), lactide afforded PLA materials with medium molecular weights in only a few minutes. The polymerizations are living, as evidenced by the narrow polydispersities (M w/M n = 1.01) of the isolated polymers and the linear nature of the number average molecular weight versus conversion plot. Inspection of the kinetic parameters for rac-LA showed that propagations present the usual first-order dependence on monomer and catalyst concentration and that the polymerization rates observed compare favorably with the highest rates reported to date for the ROP of lactides. Microstructural analysis of poly(rac-lactide) by 1H NMR spectroscopy revealed that the substituents on the amidinate fragment have a significant influence on both the degree of stereoselectivity and the rate of polymerization. Thus, propagations occur with enhanced levels of heteroselectivity, producing at low temperatures enriched-heterotactic PLAs with a P r value of up to 0.79. ROP experiments with [Mg(κ3-pbptamd)(CH2SiMe3)] (3) revealed that this complex is extremely fast for rac-lactide, with 420 equiv polymerized in 93% yield in 2 min at 20 °C.

The reaction of the heteroscorpionate lithium salts [Li(pbpamd)(THF)] [pbpamd = N,N'-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N'-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equivalent of ZnCl2 in THF affords very high yields of the neutral heteroscorpionate chloride zinc complexes [ZnCl(NNN)] (NNN = pbpamd 1 and tbpamd 2). Compound 1 was used as a convenient starting material for the synthesis of aromatic amide zinc compounds [Zn(NHAr)(pbpamd)], where NHAr = 4-methylphenylamide (NH-4-MeC6H4) 3, 2,4,6-trimethylphenylamide (NH-2,4,6-Me3C6H2) 4 and 2,6-diethylphenylamide (NH-2,6-Et2C6H3) 5, by the reaction of the corresponding aromatic primary amide lithium salts. Alternatively, aliphatic amide derivatives [Zn(NR2)(pbpamd)] (R = SiMe3 6, SiHMe2 7 and iPr 8) were cleanly prepared by reacting the amidine-heteroscorpionate compound Hpbpamd with the corresponding bis(amide) zinc complexes [Zn(NR2)2] (R = SiMe3, SiHMe2 and iPr). The single-crystal X-ray structures of complexes 2, 3 and 6 confirm a 4-coordinate arrangement in all cases, with the zinc metal surrounded in a distorted tetrahedral geometry and the heteroscorpionate ligands arranged in a kappa3 coordination mode. Whereas aliphatic amide heteroscorpionates 6-8 can act as efficient single-component initiators for the ring-opening polymerization of epsilon-caprolactone at room temperature, aromatic amide derivatives were not capable of yielding polymers even at high temperature. Epsilon-caprolactone is polymerized within minutes to give medium-high molecular weight polymers under mild conditions and with narrow polydispersities (M(w)/M(n) = 1.26). Polymer end group analysis shows that the polymerization mediated by aliphatic amide zinc complexes is initiated by amide transfer to the monomer.

Commercially available products such as ZnEt2 (1), MgBu2 (2), and n-BuLi (3) can act as excellent catalytic precursors for the catalytic addition of amines to carbodiimides. Aromatic primary amines bearing different substituents, secondary amines, and heterocyclic amines can undergo this reaction. The synthesis and structural characterization of the zinc guanidinate complex [Zn(Et){(4-t-BuC6H4)NC(N-i-Pr)(NH-i-Pr)}]2 (15) and the synthesis and characterization of the lithium guanidinate complex [Li{(2,4,6-Me3C6H4)NC(N-i-Pr)(NH-i-Pr)}(THF)] (16), both of which act as catalysts in the guanylation reaction, allowed us to propose a mechanism involving the formation of amido intermediates. These amido compounds subsequently react with the carbodiimide in an insertion process.

The reaction of the hybrid scorpionate/cyclopentadienyl lithium salt [Li(bpzcp)(THF)] [bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl] with 1 equiv of RMgCl proceeds cleanly to give very high yields of the corresponding monoalkyl kappa(2)-NN-eta(5)-C(5)H(4) magnesium complexes [Mg(R)(kappa(2)-eta(5)-bpzcp)] (R = Me 1, Et 2, (n)Bu 3, (t)Bu 4, CH(2)SiMe(3) 5, CH(2)Ph 6). Hydrolysis of the hybrid lithium salt [Li(bpzcp)(THF)] with NH(4)Cl/H(2)O in ether cleanly affords the two previously described regioisomers: (bpzcpH) 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (a) and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene (b). Subsequent reaction of the bpzcpH hybrid ligand with ZnR(2) quantitatively yields the monoalkyl kappa(2)-NN-eta(1)(pi)-C(5)H(4) zinc complexes [Zn(R){kappa(2)-eta(1)(pi)-bpzcp}] (R = Me 7, Et 8, (t)Bu 9, CH(2)SiMe(3) 10). Additionally, magnesium alkyls 1, 2, 4, and 5 can act as excellent cyclopentadienyl and alkyl transfers to the zinc metal center and yield zinc alkyls 7-10 in good yields. The single-crystal X-ray structures of the derivatives 4, 5, 7, and 10 confirm a 4-coordinative structure with the metal center in a distorted tetrahedral geometry. Interestingly, whereas alkyl magnesium derivatives 4 and 5 present a eta(5) coordination mode for the cyclopentadienyl fragment, zinc derivatives 7 and 10 feature a peripheral eta(1)(pi) arrangement in the solid state. Furthermore, the reaction of the hybrid lithium salt [Li(bpzcp)(THF)] with 1 equiv of ZnCl(2) in tetrahydrofuran (THF) affords very high yields of the chloride complex [ZnCl{kappa(2)-eta(1)(pi)-bpzcp}] (11). Compound 11 was used as a convenient starting material for the synthesis of the aromatic amide zinc compound [Zn(NH-4-MeC(6)H(4)){kappa(2)-eta(1)(pi)-bpzcp}] (12), by reaction with the corresponding aromatic primary amide lithium salt. Alternatively, aliphatic amide and alkoxide derivatives were only accessible by protonolysis of the bis(amide) complexes [M{N(SiMe(3))(2)}(2)] (M = Mg, Zn) and the mixed ligand complex [EtZnOAr)] with the hybrid ligand bpzcpH to afford [Zn(R){kappa(2)-eta(1)(pi)-bpzcp}] (R = N(SiMe(3))(2) 13, R = 2,4,6-Me(3)C(6)H(2)O 14) and [Mg{N(SiMe(3))(2)}(kappa(2)-eta(5)-bpzcp)] (15). Finally, alkyl and alkoxide-containing complexes 1-10 and 14 can act as highly effective single-component living initiators for the ring-opening polymerization of epsilon-caprolactone and lactides over a wide range of temperatures. Epsilon-caprolactone is polymerized within minutes to give high molecular weight polymers with medium-broad polydispersities (M(n) > 10(5), M(w)/M(n) = 1.45). Lactide afforded poly(lactide) materials with medium molecular weights and polydispersities as narrow as M(w)/M(n) = 1.02. Additionally, polymerization of L-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(L-lactides) with very high melting temperatures (T(m) = 165 degrees C). Microstructural analy...

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.