New types of bi- and tri-dentate pyrrole-piperazine ligands and related zinc compounds: Synthesis, characterization, reaction study, and ring-opening polymerization of ε-caprolactone

• New types of bi- and tri-dentate pyrrole-piperazine ligands.
• Zinc compounds and ring-opening polymerization of ε-caprolactone.
• Hydroxo-bridged tri-zinc cluster.

Reactions of one or two equivalents of formaldehyde and phenylpiperazine with pyrrole in methanol or ethanol generated substituted pyrrole ligands C4H3NH-[2-CH2N(CH2CH2)2NPh] (1) and C4H2NH-{2,5-[CH2N(CH2CH2)2NPh]2} (2), respectively. Reacting 1 with one equivalent of ZnMe2 in toluene generated a di-zinc compound 3, {ZnMe{C4H3N-[2-CH2N(CH2CH2)2NPh]}}2, in moderate yield. Further reacting 3 with two equivalents of p-cresol in toluene overnight afforded pheoxide bridged di-zinc compound 4, {Zn(μ-O-C6H4-4-Me){C4H3N-[2-CH2N(CH2CH2)2NPh]}}2. Similarly, the reactions of one and two equivalents of tridentate substituted pyrrole ligand 2 with ZnMe2 afforded Zn compounds ZnMe{C4H2N-{2,5-[CH2N(CH2CH2)2NPh]2}} (5) and Zn{C4H2N-{2,5-[CH2N(CH2CH2)2NPh]2}}2 (6), respectively, in moderate yield. All of these compounds were characterized using 1H and 13C NMR spectroscopy. Compound 5 is relatively air and moisture sensitive and decomposes during the recrystallization process to yield a tri-Zn cluster, {Zn(μ2-OH){C4H2N-{2,5-[CH2N(CH2CH2)2NPh]2}}}3 (7). The molecular geometries of compounds 1, 3, 4, 6, and 7 were also determined using single crystal X-ray diffractometric analysis. Compounds 3, 4, and 6 were used as initiators for the ring opening polymerization of ε-caprolactone, and all of the zinc compounds showed high conversion with broad or bimodal molecular weight distributions.

A series of new bi- and tri-dentate pyrrole-piperazine ligands and related zinc compounds were synthesized and related reactions were studied.Figure optionsDownload as PowerPoint slide