Examinando por Autor "Otero, Antonio"

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Copolymerization of Cyclic Esters Controlled by Chiral NNO-Scorpionate Zinc Initiators
(ACS Publications, 2016) Otero, Antonio; Fernández-Baeza, Juan; Sánchez-Barba, Luis F.; Honrado, Manuel; Garcés, Andrés; Lara-Sánchez, Agustín; Rodríguez, Ana M.
Reaction of chiral alcohol-scorpionate compoundbpzteH [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)(k2-NN μ-O)Zn(R)2] (1–2) [k2-NN μ-O= bpzte, R = Me 1, CH2SiMe32]. Using these trialkyl complexes in an alcoholysis reaction with ArOH (1 equiv; Ar = 2,6-C6H3Me2) yielded the chiral dinuclear zinc complexes [(ZnR)2(kN:kN- μ-O)( μ-EAr)] (3– 4) [kN:kN- μ-O= bpzte, R = Me 3, CH2SiMe34]. The structures of the different compounds were determined by spectroscopic methods and, in addition, 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. Trisalkyl and alkyl-aryloxide-containing zinc complexes 1–4 can act as single-component initiators for the ring-opening homopolymerization of e-caprolactone and L-/rac-lactide, affording materials with low molecular weights and narrow monomodal molecular weight distributions under mild conditions in only a few hours. Microstructural analysis of poly(rac-lactide) revealed that the alkyl-aryloxide 3 exerts a moderate level of heteroselectivity (Ps= 0.68). More interestingly, this initiators 3 and 4 also allowed the well-controlled random copolymerization of ε-CL and LLA as indicated by both the values of the reactivity ratios of the two comonomers (rCL = 1.15 and 0.92, and rLA = 1.37 and 1.05, for 3 and 4, respectively), and the average lengths of the caproyl and lactidyl sequences (LCL ~ 2.0; LLA ~ 2.5, for both initiators). The copolymers also showed to possess monomer contents in close agreement with their composition in the initial monomer feed. Inspection by differential scanning calorimetry on the thermal properties such as Tg of various copolymers produced by 3 revealed a strong dependence on monomer content, as these values vary linearly with the molar percentage of L-lactide unit in the copolymer over a temperature range of -30 to 42 ºC.
Design of Novel Group 4 ansa-Metallocene Compounds and their Role as Catalysts in the Polymerization of Olefins
(2014-01-28) Prashar, Sanjiv; Gómez-Ruiz, Santiago; Fajardo, Mariano; Antiñolo, Antonio; Fernández-Galán, Rafael; Otero, Antonio; Rivilla, Iván
Since the discovery by Sinn and Kaminsky that zirconocene complexes in the presence of MAO were able to polymerize olefins there has been a dramatic expansion in this field. One of the main focuses of this research is centred on the tailoring of metallocene complexes that can direct the catalytic reaction in producing polymers with desired physical properties. Ligand design is therefore of utmost importance in the development of "made to measure" single-site olefin polymerization catalysts. In addition the introduction of functional groups in the ligand system can be later exploited in immobilizing the catalyst on different substrates. In this communication we describe the synthetic strategies used in the design of group four ansa-metallocene complexes and the application of these compounds as catalysts in the polymerization of ethylene and propylene. The principal design aims addressed include: 1. Variable substitution at the ansa-carbon bridging atom of the metallocene catalysts 2. Synthesis of hydride silylene ansa-bridged complexes that act as hydrosilylation reagents. 3. Functionalization of ansa-metallocene complexes 4. Synthesis of C1 symmetric ansa-metallocene catalysts which are isospecfic in the polymerization of propylene
Enantiopure N,N,O-Scorpionate Zinc Amide and Chloride Complexes as Efficient Initiators for the Heteroselective ROP of Cyclic Esters
(Royal Society of Chemistry, 2014) Honrado, Manuel; Otero, Antonio; Fernández-Baeza, Juan; Sánchez-Barba, Luis F.; Garcés, Andrés; Lara-Sánchez, Agustín; Rodríguez, Ana M.
The reaction of bpzbeH, bpzteH (racemic mixture) or (R,R)-bpzmmH (enantiopure) with the amide complexes Zn{N(SiMe3)2}2 or Zn{N(SiHMe2)2}2 in 1 : 1 molar ratio in toluene afforded the mononuclear amide zinc complexes [Zn(NR2)(κ3-NNO)] (1–6) [κ3-NNO = bpzbe, R = SiMe31, SiHMe22; bpzte, R = SiMe33, SiHMe24; (R,R)-bpzmm, SiMe35, SiHMe26]. These complexes were employed in a protonolysis reaction with HCl–Et2O in 2 : 1 molar ratio to yield the dinuclear amide/chloride zinc complexes [Zn(κ2-NN-μ-O)2{ZnCl(NR2)}] (7–12) [κ2-NN-μ-O = bpzbe, R = SiMe37, SiHMe28; bpzte, R = SiMe39, SiHMe210; (R,R)-bpzmm, SiMe311, SiHMe212]. The mononuclear complexes 5 and 6 and dinuclear complexes 11 and 12 are the first enantiopure-scorpionate zinc amide complexes to be synthesized. The single-crystal X-ray structure analysis of derivatives 1 and 3 confirmed a monomeric 4-coordinative structure in which the heteroscorpionate ligands are in a κ3 coordination mode, while 8 had a dimeric molecular disposition with two μ-bridging alkoxides of the heteroscorpionate ligands between the two six- and four-coordinate Zn(II) centers. Interestingly, the chiral amide-containing zinc complexes 1–5 and 11 can act as single-component initiators for the ring-opening polymerization of ε-caprolactone and lactides under mild conditions, affording, in a few hours, medium/low molecular weight polymers with low polydispersity indices. MALDI-ToF mass spectra confirmed that the initiation occurred through a nucleophilic attack by the amide on the lactide monomer, and inspection of the kinetic parameters showed that propagations present the usual pseudo-first order dependence on monomer and catalyst concentrations. In addition, microstructural analysis of poly(rac-lactide)s revealed that the myrtenal substituent on the alkoxide fragment has a significant influence on the degree of stereoselectivity, producing enriched-heterotactic PLAs with a Ps value of up to 0.79 under mild conditions.
Highly Thermally Stable and Robust Enantiopure NNNScorpionate Zirconium Initiators for the Controlled Ring- Opening Polymerization of rac-Lactide
(Royal Society of Chemistry, 2017) Otero, Antonio; Fernández-Baeza, Juan; Garcés, Andrés; Sánchez-Barba, Luis F.; Lara- Sánchez, Agustín; Martínez-Ferrer, Jaime; Carrión, María P.; Rodríguez, Ana M.
A series of enantiopure alkoxide and thioalkoxide zirconium derivatives [Zr(ER)3(κ3-R,R-fbpza)] (1–6) (E = O, R = CHMe21, CHMeEt 2, CH2SiMe33, 2,6-C6H3Me24, 4-tBuPh 5; E = S, R = 4-tBuPh 6) has been prepared for use as thermally stable and robust initiators in the ROP of rac-lactide. The compounds were prepared by alcoholysis or thioalcoholysis of the tris(amide) precursor [Zr(NMe2)3(κ3-R,R-fbpza)] [R,R-fbpzaH = N-p-fluorophenyl-(1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]-2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethylamine] with ROH and ArEH (E = O, S) in a 1 : 3 molar ratio. The structures of the different compounds were determined by spectroscopic methods and, in addition, the X-ray crystal structure of 6 was also established. Interestingly, the tris(amide) precursor and complexes 2, 5, and 6 act as single-site living initiators for the well-controlled ring-opening polymerization of rac-lactide both in solution and in the melt. These processes produce polymers with medium molecular weights in good agreement with theoretical values and with narrow dispersity ranges. The activity of all initiators increased markedly with temperature and, more importantly, complex 2 exhibits the highest activity reported to date for a group 4-based initiator in the ROP of rac-LA under the industrially preferred melt and solvent-free conditions. Surprisingly, complex 2 is still highly active in the melt when using an unpurified monomer and it shows an unprecedented tolerance to water and impurities (49% conv., 15 min, 130 °C). Microstructural analysis of the poly(rac-lactide)s revealed a moderate heteroactivity in solution, with a Ps value of up to 0.70.
Hydrosilylation in the Design and Functionalization of ansa-Metallocene Olefin Polymerization Catalysts
(2014-01-29) Gómez-Ruiz, Santiago; Prashar, Sanjiv; Fajardo, Mariano; Antiñolo, Antonio; Otero, Antonio
The group 4 ansa-metallocene complexes [M{Me(H)Si(¿5-C5Me4)(¿5-C5H4)}Cl2] (M = Ti, Zr, Hf),that contain a Si-H bond, have been used as hydrosilylation reagents with the silane substrates, Si(CH=CH2)4 and Me2Si(CH=CH2)2. Our results show that the hydrosilylation reaction takes place at only one of the vinyl groups to give [M{(CH2=CH)3SiCH2CH2(Me)Si(¿5-C5Me4)(¿5-C5H4)}Cl2] (M = Ti, Zr, Hf) and [M{(CH2=CH)Me2SiCH2CH2(Me)Si(¿5-C5Me4)(¿5-C5H4)}Cl2] (M = Ti, Zr, Hf). Hydrosilylation of the remaining vinyl groups can be achieved by using simple silane reagents, for example, HSiEt3 to give [M{(Et3SiCH2CH2)3SiCH2CH2(Me)Si(¿5-C5Me4)(¿5-C5H4)}Cl2] (M = Ti, Zr) and [M{(Et3SiCH2CH2)Me2SiCH2CH2(Me)Si(¿5-C5Me4)(¿5-C5H4)}Cl2] (M = Ti, Zr). All the compounds have been tested as homogeneous catalysts in the polymerization of ethylene and propylene
Mono- and Binuclear Chiral N,N,O-Scorpionate Zinc Alkyls as Efficient Initiators for the ROP of rac- Lactide
(Royal Society of Chemistry, 2017) Otero, Antonio; Fernández-Baeza, Juan; Sánchez-Barba, Luis F.; Sobrino, Sonia; Garcés, Andrés; Lara-Sánchez, Agustín; Rodríguez, Ana M.
The preparation of new chiral bis(pyrazol-1-yl)methane-based N,N,O-donor scorpionate ligands in the form of the alcohol compounds bpzampeH (1) {2,2-bis(3,5-dimethylpyrazol-1-yl)-1-[4-(dimethylamino)phenyl]ethanol}, bpzaepeH (2) {2,2-bis(3,5-dimethylpyrazol-1-yl)-1-[4-(diethylamino)phenyl]ethanol}, and bpzimeH (3) {2,2-bis(3,5-dimethylpyrazol-1-yl)-1-[1-methyl-1H-imidazol-2-yl]ethanol} has been carried out by the 1,2-addition reactions of a series of aldehydes. These new chiral heteroscorpionate ligands reacted with [ZnR2] (R = Me, Et, CH2SiMe3) in a 1 : 1 molar ratio in toluene to give the mononuclear monoalkyl zinc complexes [Zn(R)(κ3-NNO)] (4–12). When these reactions were carried out in a 1 : 2 molar ratio the binuclear trisalkyls [Zn(R)(κ2-NNμ-O)Zn(R)2] (13–18) were obtained. The structures of these complexes were elucidated by 1H and 13C{1H} NMR spectroscopy and the X-ray crystal structures of 4 and 5 were also established. Interestingly, alkyl-containing zinc complexes 4–13, 15 and 17 act as efficient single-component initiators for the ring-opening polymerization of rac-lactide at 20 °C to afford PLA materials with low molecular weights in a few hours. The dinuclear trisalkyls showed higher activity in comparison with the mononuclear zinc counterparts, suggesting a cooperative effect of the two remote metals. The narrow dispersity ranges (Mw/Mn = 1.05) of the isolated polymers in conjunction with the linear nature of the number average molecular weight versus conversion plot provided evidence for living behavior. Inspection of the kinetic parameters showed that the propagations have the usual pseudo-first-order dependence on rac-lactide and catalyst concentration. End-group analysis and MALDI-TOF mass spectrometry confirmed that the initiation occurs through nucleophilic attack of the alkyl on the lactide monomer. Microstructural analysis of poly(rac-lactide)s revealed that the most sterically hindered ligand on the alkoxide fragment exerts a moderate influence on the degree of stereoselectivity, with heteroenriched-PLAs (Ps = 0.68) produced at room temperature.
New Racemic and Single Enantiopure Hybrid Scorpionate/Cyclopentadienyl Magnesium and Zinc Initiators for the Stereoselective ROP of Lactides
(ACS, 2015) Honrado, Manuel; Otero, Antonio; Fernández-Baeza, Juan; Sánchez-Barba, Luis F.; Garcés, Andrés; Lara-Sánchez, Agustín; Martínez-Ferrer, Jaime; Sobrino, Sonia; Rodríguez, Ana M.
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.
Organo-Aluminum and Zinc Acetamidinates: Preparation, Coordination Ability, and Ring-Opening Polymerization Processes of Cyclic Esters
(ACS, 2018) Garcés, Andrés; Sánchez-Barba, Luis F.; Fernández-Baeza, Juan; Otero, Antonio; Fernández, Israel; Lara-Sánchez, Agustín; Rodríguez, Ana M.
The reaction of the highly sterically demanding NNN′-heteroscorpionate protioligands pbptamd-H, tbptamd-H, and phbptamd-H (a) and the low sterically hindered analogs pbpamd-H, tbpamd-H, and phbpamd-H (b), with 1 equiv of AlR3 (R = Me, Et) proceed in high yields to give two families of complexes: the mononuclear dialkyl aluminum bidentate-acetamidinates [AlR2(κ2-N′N′)] (κ2-N′N′ = pbptamd, R = Me 1, Et 2; tbptamd, R = Me 3, Et 4; phbptamd, R = Me 5, Et 6) and the monodentate-acetamidinates [AlR2(κ2-NN′)] (κ2-NN′ = tbpamd, R = Me 7; phbpamd, R = Me 8, Et 9). In complexes 7–9, the presence of two possible CH–NH tautomers as low extended π-N–C–N′(sp2)–Al and high extended π-HN–C2–N′(sp2)–Al complexes, respectively, could be identified. Moreover, the reaction of aluminum dimethyls 7 and 8 with ZnMe2 afforded the isolation of the more stable scorpionate zinc monoalkyls [Zn(Me)(κ3-NNN′)] (NNN′ = tbpamd 10 and phbpamd 11), through a very unusual ligand exchange process, involving a zinc-to-aluminum transmetalation of an alkyl group. The X-ray crystal structures of 1, 3, 7, and 8, as well as that of 11, confirmed unambiguously the different κ2-arrangements proposed for bi- or monodentate acetamidinate dialkyls 1–6 and 7–9, respectively, the presence of NH tautomer in 7 and 8, and a κ3-NNN′ coordination in monoalkyl 11. Density functional theory calculations were used to explore the three different favored κ2-arrangements found in acetamidinate aluminum dialkyls 1–9, the relative stability of both CH–NH tautomers, and the ligand transfer reaction leading to the formation of κ3-NNN′ zinc monoalkyls 10 and 11. Interestingly, dialkyls 1, 5, 7, and 8 can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and rac-lactide in mild conditions after hours. These initiators efficiently mediated the immortal polymerization in the presence of excess of benzyl alcohol (up to 20 equiv), as evidenced by the narrow dispersity values and the good agreement between the experimental Mn values and monomer/benzyl alcohol ratios. In addition, the most sterically hindered initiator, 5, exhibits enhanced levels of heteroselectivity on the produced PLAs, reaching Ps values up to 0.70.
Síntesis y estudio electroquímico de complejos de niobio incorporando ligandos ansa-ciclopentadienilo asimétricos
(2014-01-29) Antiñolo, Antonio; Expósito, Teresa; Hierro, Isabel del; Lucas, Dominique; Mugnier, Yves; Orive, Irache; Otero, Antonio; Prashar, Sanjiv
Se han preparado los complejos asimétricos bicloruro de ansa-nioboceno(IV): [Nb{Me2Si(eta5-C5Me4)(eta5-C5H3R)}Cl2] (R = H (1a), Me (2a), SiMe3 (3a), Pri (4a), PPh2 (5a)). 1a-5a se han caracterizado mediante los correspondientes espectros de RSE, IR y análisis elemental. Los estudios electroquímicos de estos derivados nos han permitido evaluar los potenciales de oxidación y reducción que se han comparados con complejos análogos no ansa. También han sido sintetizados nuevos complejos niobocenos [Nb(eta5-C5H4R)2}Cl2] (R = Pri (6a), PPh2 (7a)) y estudiados electroquímicamente.
Stereoselective ROP of rac-Lactide Mediated by Enantiopure NNO-Scorpionate Zinc Initiators
(American Chemical Society, 2014) Honrado, Manuel; Otero, Antonio; Fernández-Baeza, Juan; Sánchez-Barba, Luis F.; Garcés, Andrés; Lara-Sánchez, Agustín; Rodríguez, Ana M.
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 (Pi = 0.74) produced through an enantiomorphic site control mechanism.
Studies on Multinuclear Magnesium tert-Butyl Heteroscorpionates: Synthesis, Coordination Ability and Heteroselective ROP of rac-Lactide
(American Chemical Society, 2017) Garcés, Andrés; Sánchez-Barba, Luis F.; Fernández-Baeza, Juan; Otero, Antonio; Honrado, Manuel; Lara-Sánchez, Agustín; Rodríguez, Ana M.
The reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(κ3-NNN)(thf)] {NNN = pbpamd (1) [pbpamd = N,N′-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate], tbpamd (2) [tbpamd = N-ethyl-N′-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate], ttbpamd (3) [ttbpamd = N,N′-di-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate], phbpamd (4) [phbpamd = N,N′-di-p-tolylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and the highly sterically demanding [Li(κ3-phbptamd)(thf)] (5) [phbptamd = N,N′-di-p-tolylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate] with the commercially available Grignard reagent tBuMgCl in an equimolecular ratio yielded the magnesium tert-butyl heteroscorpionates [Mg(tBu)(κ3-NNN)] [NNN = pbpamd (6), tbpamd (7), ttbpamd (8), and phbptamd (9)]. In addition, subsequent reaction of monoalkyls 6–8 with two additional equivalents of tBuMgCl gave rise to two different classes of chiral tetraalkyls, which contain apical methine C–H activated heteroscorpionates that show two possible coordination modes, namely, the tetranuclear complexes [{(tBu)Mg(κ3-N,N,N;κ2-C,N)Mg(tBu)}2{μ-O,O-(OC4H8O)}] (κ3-N,N,N;κ2-C,N = pbpamd–10, tbpamd–11a) and [(tBu)Mg(κ2-N,N;κ2-N,N)Mg(tBu){μ-O,O-(OC4H8O)}]2 (κ2-N,N;κ2-N,N = tbpamd–11b). This reaction for 8 was unsuccessful, but an analog such as dinuclear complex [(thf)(tBu)Mg(κ2-N,N;κ2-N,N)Mg(tBu)(thf)] (κ2-N,N;κ2-N,N = phbpamd–12) could be obtained from its lithium salt. Contrary to complexes 10–12, the C–H activation process was not observed for complex 9, but interestingly, the coordination of one additional equivalent of MgtBu2 afforded dinuclear complex [Mg(tBu)(κ3-phbptamd){Mg(tBu)2}] (13). Similarly, coordination of MgtBu2 to 10 produces chiral trinuclear [{(tBu)Mg(pbpamd–)Mg(tBu)}{μ-O,O-(OC4H8O)}{Mg(tBu)2(thf)}] (14). The X-ray crystal structures of complexes 4, 6, 8, 9, 11b, 12, 13, and 14 confirm the nuclearity of each family, and the existence of apical σ-C(sp3)–Mg (14) and extended π-C2N2(sp2)–Mg2 (11b and 12) covalent bonds have unambiguously been also confirmed. Furthermore, mononuclear compounds 8 and 9, tetranuclear compound 10, and dinuclear compounds 12 and 13 can act as highly efficient single-component living initiators for the ring-opening polymerization of rac-lactide. More interestingly, the most sterically hindered initiators exhibit enhanced levels of heteroselectivity on the PLAs, with 9 reaching Ps values up to 0.85.
Synthesis of cationic niobocene imido derivatives
(2014-01-29) Hierro, Isabel del; Fajardo, Mariano; Garcés, Andrés; Gómez-Ruiz, Santiago; Pérez, Yolanda; Prashar, Sanjiv; Antiñolo, Antonio; Otero, Antonio; López-Mardomingo, Carmen
Following from our previous work in the synthesis and reactivity of niobium imido complexes,1 we have studied the preparation of cationic derivatives. The reduction of [Nb(=NPh)(eta5-C5H4SiMe3)2Cl] gives the short-lived niobocene(IV) radical [Nb(=NPh)(eta5-C5H4SiMe3)2. which was rapidly oxidized to the niobocene(V) cationic species [Nb(=NPh)(eta5-C5H4SiMe3)2(L)]+ (L = CO, ButNC) by the reaction with [Fe(eta5-C5H5)2][BPh4]. In a similar manner, the ansa-niobocene compound [Nb(=NBut){Me2Si(eta5-C5Me4)(eta5-C5H4)}(ButNC)][BPh4] was synthesized. However, in the attempted reduction of [Nb(=NBut){Me2Si(eta5-C5H4)2}Cl], the expected radical species was not observed. Instead the niobocene (V) imido dimer product was identified. The reaction with H2 and I2 gave [Nb(=NBut){Me2Si(eta5-C5H4)2}H] and [Nb(=NBut){Me2Si(eta5-C5H4)2}I], respectively.