Synthesis, Structures, and Ring-Opening Polymerization Reactions of Substituted Cyclopentadienyl Complexes of Zinc

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Abstract

The reaction of 2 equiv of the new pyrrole-substituted cyclopentadienyl ligand cyclo-C4H4NSiMe2C5H5 (2a, CppyH) with Zn[N(SiMe3)2]2 gives the bis(cyclopentadienyl)zinc complex Zn(Cppy)2 (3a). The reaction of 2a and the related derivatives cyclo-2,5-Me2C4H2NSiMe2C5H5 (2b, CppymeH) and 3,5-Me2C6H3CH2CMe2C5H5 (2c, CpmesH) with dibutylmagnesium gives the corresponding magnesocenes MgCpR2 (3a−c). Prolonged heating of the appropriate cyclopentadiene with zinc dialkyls in toluene, or alternatively stirring with [XZnN(SiMe3)2]2 in toluene at 40 °C for 1 h, gives the monocyclopentadienyl zinc alkyl complexes CpRZnX (6a, CpR = Cppy, X = Me; 6c, CpR = Cpmes, X = Me; 7a, CpR = Cppy, X = Et; 7c, CpR = Cpmes, X = Et), which provide spectroscopic evidence for a Zn···(hetero)arene interaction. Addition of tetramethylethylenediamine (TMEDA) gives CpRZnX(TMEDA) (8a, b, X = Me; 9a−c, X = Et). The crystal structures of 8b and 9c show η2-bound cyclopentadienyl ligands, with one long and one short Zn−C interaction. The reaction of 7c with B(C6F5)3 in toluene proceeds with alkyl/C6F5 exchange to give CpmesZn(C6F5) (10). Treatment of 9c with B(C6F5)3 in toluene results in an ionic product, [CpmesZn(TMEDA)]+[EtB(C6F5)3]- (11). On the other hand, the reaction of 9a with B(C6F5)3 in toluene or dichloromethane proceeds with β-H abstraction, with concomitant loss of ethene, as well as both ethyl and cyclopentadienyl abstraction. Both 11 and 9a/B(C6F5)3 mixtures catalyze the polymerization of cyclohexene oxide and ε-caprolactone.

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