The nonadentate podand tris[3-(6-diethylcarbamoylpyridine-2-carboxamido)propyl]amine (L14) is prepared according to a multistep strategy using the flexible tris(3-(N-methylamino)propyl)amine (Me-TRPN) covalent tripod. L14 exists as a statistical mixture of four conformers in solution whose distribution is slightly affected by protonation of the apical nitrogen atom in [L14 + H]+. The pKa value depends on the length of the spacer separating the apical nitrogen and the appended electron-withdrawing tertiary amide groups, and increases by three orders of magnitude when going from the Me-TREN tripod in [L13 + H]+ (ethylene spacer) to Me-TRPN in [L14 + H]+ (trimethylene spacer). Reactions of L14 and [L14 + H]+ with Ln(ClO4)3 (Ln = La–Lu) produce flexible and poorly stable 1∶1 podates [Ln(L14)]3+ and [Ln(L14 + H)]4+ in which the terdentate chelating binding units exhibit partial dynamic on–off complexation equilibria. Comparisons of structural and thermodynamic data for [Ln(Ln)]3+ (n = 13 or 14) in solution point to a drastic decrease of the molecular organisation of the podand when the constrained Me-TREN tripod is replaced by the elongated Me-TRPN tripod in nine-co-ordinate lanthanide podates, a crucial limiting factor for the design of supramolecular lanthanide complexes with predetermined properties.
非腭合的podand类化合物tris[3-(6-
二乙基氨基甲酰基
吡啶-2-羧酰胺)丙基]胺(L14)是通过使用柔性的三(3-(N-甲基
氨基)丙基)胺(Me-TR
PN)共价三脚架的多步骤策略制备的。L14在溶液中以四种构象异构体的统计混合形式存在,其分布略受[L14 + H]+中顶端氮原子的质子化影响。pKa值取决于将顶端氮原子与附加的吸电子叔酰胺基团分隔开的间隔基的长度,从[L13 + H]+中的Me-TREN三脚架(
乙烯间隔基)过渡到[L14 + H]+中的Me-TR
PN三脚架(亚丙基间隔基)时,pKa值增加了三个数量级。L14和[L14 + H]+与Ln(
ClO4)3(Ln = La–Lu)的反应产生了柔性且稳定性较差的1:1腭合物[Ln(L14)]3+和[Ln(L14 + H)]4+,其中的三配位螯合
配体单元展现出部分动态的“开–关”配位平衡。溶液中[Ln(Ln)]3+(n = 13或14)的结构和热力学数据的比较表明,当九配位
镧系腭合物中的受限Me-TREN三脚架被延长的Me-TR
PN三脚架取代时,腭合
配体的分子组织性急剧下降,这是设计具有预定性质的超分子
镧系配合物时的一个关键限制因素。