Magnesium cyclopentadienyl complexes

The tetramethylethanediyl-bridged, f-butyl-substituted bis-cyclopentadienyl complex Me4C2(3-f-BuC5H3)2Mg(THF) (129) is present in the solid state as a me o-diastere-oisomer. It has a structure in which both cyclopentadienyl groups are /7 -bonded to magnesium while only one THF molecule is coordinated to magnesium (Figure 62) . 

Hybrid scorpionate/cyclopentadienyl-Mg (63) and -Zn (64,65) complexes were structurally characterized and reported to catalyze the formation of PLAs with medium molecular weights and narrow polydispersities [85]. Among them, the magnesium complex 63 is much more active than the others, giving a polymerization of L-lactide in toluene at 90 °C with 97% conversion in 2.5 h. However, it takes 30 h for zinc complexes 64 and 65 to reach similar results under the same conditions. Some representative structures of magnesium and zinc complexes are summarized in Table 2 as they display closely related ROP activity of lactide, and often stmcrnrally similar ligand systems are employed to construct these initiators. 

In a similar way Cp(Me)Mg(OEt2) is capable of deprotonating iV,Ai -bis(2,4,6-tri-methylphenyl)(ierr-butyl)amidine to form the corresponding cyclopentadienylmagnesium amidinate complex (226) (equation 17). An X-ray crystal-structure determination of 226, of which the structure is shown schematically (equation 17), showed that this compound also exists as a monomer in the solid state. Like in 225 the cyclopentadienyl group is /7 -bonded to magnesium while the amidinate anion is Ai,A -chelate bonded with almost equal Mg—N bond distances [Mg-N 2.090(2) and 2.097(2) A]. Furthermore, an additional... 

The metallocene complexes are only used in catalytic amounts and may be recovered from the reaction mixture. Bis() 5-cyclopentadienyl)zirconium(TV) chloride (Cp2ZrCI2) and fluoride (Cp2ZrF2) in the presence of magnesium/mercury(II) chloride are equally effective. 

Bis(2,4-pentanedionato)chromium, 3320 Bis(benzene)chromium dichromate, 3845 Bis(benzeneiron)—fulvalenediyl complex, 3828 Bis(cycloctatetraene)uranium(0), 3701 Bis(cyclopentadienyl)bis(pentafluorophenyl)zirconium, 3824 Bis(cyclopentadienyldinitrosylchromium), 3269 Bis(cyclopentadienyl)hexafluoro-2-butynechromium, 3629 Bis(cyclopentadienyl)lead, 3288 Bis(cyclopentadienyl)magnesium, 3271 Bis(cyclopentadienyl)manganese, 3272 Bis(cyclopentadienyl)niobium tetrahydroborate, 3318... 

Mono-n-cyclopentadienylmagnesium compounds have so far only been isolated in the form of adducts with oxygen or nitrogen bases. The bis-(tetrahydrofuran) complexes of cyclopentadienyl(phenyl)- and -(methyl)-magnesium have been synthesized by Whitesides and co-workers (96) according to Eq. (4). 

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