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Title: Hybrid scorpionate/cyclopentadienyl magnesium and zinc complexes: synthesis, coordination chemistry, and ring-opening polymerization studies on cyclic esters.
Author: Garcés A, Sánchez-Barba LF, Alonso-Moreno C, Fajardo M, Fernández-Baeza J, Otero A, Lara-Sánchez A, López-Solera I, Rodríguez AM.
Journal: Inorg Chem; 2010 Mar 15; 49(6):2859-71. PubMed ID: 20146427.
Abstract:
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 analysis of poly(rac-lactide) by (1)H NMR spectroscopy revealed that propagations occur without appreciable levels of stereoselectivity. Polymer end group analysis showed that the polymerization process is initiated by alkyl transfer to the monomer.

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