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

Abstract

The reaction of 2 equiv of the new pyrrole-substituted cyclopentadienyl ligand cyclo- C 4 H 4 NSiMe 2 C 5 H 5 ( 2a, Cp py H) with Zn[N(SiMe 3 ) 2 ] 2 gives the bis(cyclopentadienyl)zinc complex Zn(Cp py ) 2 ( 3a ). The reaction of 2a and the related derivatives cyclo- 2,5-Me 2 C 4 H 2 NSiMe 2 C 5 H 5 ( 2b, Cp pyme H) and 3,5-Me 2 C 6 H 3 CH 2 CMe 2 C 5 H 5 ( 2c, Cp mes H) with dibutylmagnesium gives the corresponding magnesocenes MgCp R 2 ( 3a − c ). Prolonged heating of the appropriate cyclopentadiene with zinc dialkyls in toluene, or alternatively stirring with [XZnN(SiMe 3 ) 2 ] 2 in toluene at 40 °C for 1 h, gives the monocyclopentadienyl zinc alkyl complexes Cp R ZnX ( 6a, Cp R = Cp py, X = Me; 6c, Cp R = Cp mes, X = Me; 7a, Cp R = Cp py, X = Et; 7c, Cp R = Cp mes, X = Et), which provide spectroscopic evidence for a Zn···(hetero)arene interaction. Addition of tetramethylethylenediamine (TMEDA) gives Cp R ZnX(TMEDA) ( 8a, b, X = Me; 9a − c, X = Et). The crystal structures of 8b and 9c show η 2 -bound cyclopentadienyl ligands, with one long and one short Zn−C interaction. The reaction of 7c with B(C 6 F 5 ) 3 in toluene proceeds with alkyl/C 6 F 5 exchange to give Cp mes Zn(C 6 F 5 ) ( 10 ). Treatment of 9c with B(C 6 F 5 ) 3 in toluene results in an ionic product, [Cp mes Zn(TMEDA)] + [EtB(C 6 F 5 ) 3 ] - ( 11 ). On the other hand, the reaction of 9a with B(C 6 F 5 ) 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(C 6 F 5 ) 3 mixtures catalyze the polymerization of cyclohexene oxide and ε-caprolactone.