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

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.