Piperazine-Based N4-Type 16-Membered Macroheterocycles and Their Nickel(II) Complexes

Abstract

Square-planar diamagnetic nickel(II) complexes S. Z. Vatsadze et al. конформация сохраняется и в растворе, однако макроцикл оказался конформационно гибким -он подверга-ется вырожденной интерконверсии с активационным барьером 41.9±0.8 кДж/моль.Ключевые слова: 16-Членные макрогетероциклы, диамагнитные комплексы никеля(II), позитронно-эмиссионная томография, рентгеновская дифракция, ЯМР спектроскопия, конформационный анализ. one down), even for the four-coordinate complexes with no additional axial donor ligand(s). We envisaged that such a conformation might, in the long term, allow for asymmetric synthetic applications for metal-ligand multiply-bonded complexes containing such ligands. Finally, we noted that the coordination chemistry of piperazines [10,11] and their open-chain [8,[12][13][14][15][16] and macrocyclic [17][18][19][20][21][22][23] derivatives has only been developed for late transition metals.The coordination and supramolecular, as well as synthetic, chemistry of piperazine-based molecules is a currently emerging area. This activity is defined, first of all, by the conformational features of the piperazine ring. For example, in a recent paper Stucchi et al. reported the application of piperazine-based peptidomimetics.[24] Although Thirunarayanan et al. reported the complexation properties of piperazinophanes, [25] they did not adequately address the conformational behavior of the ring. The other direction of the application of piperazine-based ligands is the use of titanium complexes designed for the ring-opening polymerization of rac-lactide.[26] Piperazine-based macrocycles are known to form supramolecular tubular structures [27] and promising receptor-mimic phanes.[28]In the present work we have synthesized and studied the crystal and solution structures of two Cu(II) and four Ni(II) complexes with N 4 -type ligands (Figure 2).For 1a and 1b it has been shown that anilinic protons could be easily changed for SiMe 3 groups by the action of TMSCl/DABCO (trimethylsilyl chloride / 1,4-diazabicyclo-[2.2.2]octane).[9] The already-mentioned interesting feature of the ligands 1a and 1b in their Ni, Cu, Pd and Ti complexes studied so far, [8,9] is the non-planar, pseudo-C 2 symmetric (transoid) conformation that they adopt.Complexes 5a and 5b were included in this study because they contain the imine nitrogen that favors strong Ni-N bonding due to back-donation from Ni to the π* N=C orbital. Another feature of cyclic diimine ligands 4a and 4b is the rigid xylylidene bridge between imine nitrogens, which creates an additional reason for the cisoid conformation. This conformation is of particular interest for catalysis due to one