Solvent Dependent Assembly of Lanthanide Metallacrowns Using Building Blocks with Incompatible Symmetry Preferences

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• 作者：Joseph Jankolovits ; Jeff. W. Kampf ; Vincent L. Pecoraro
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：July 21, 2014
• 年：2014
• 卷：53
• 期：14
• 页码：7534-7546
• 全文大小：623K
• ISSN：1520-510X

文摘

Solvent dependence in the assembly of coordination driven macrocycles is a poorly understood phenomenon. This work presents the solvent dependent assembly of 8 lanthanide metallacrowns (LnMCs) in solution using picoline hydroxamic acid (picHA), Zn(II), and Ln(III) ions. ESI-MS and single-crystal X-ray crystallography reveal the selective assembly of LnZn₄(picHA)₄³⁺, LnZn₅(picHA)₅³⁺, LnZn₈(picHA)₈³⁺, LnZn₁₂(picHA)₁₂³⁺, LnZn₁₆(picHA)₁₆³⁺, Ln₂Zn₃(picHA)₄⁴⁺, Ln₂Zn_7鈥?(picHA)_8鈥?0, and Ln₄Zn_4鈥?(picHA)_8鈥? complexes in five different solvents. The coordination preferences of the hard Ln(III) ion and relatively soft Zn(II) ion dictate the solvent selectivity in this system. The LnMCs assemble with open or closed Zn(II) and/or Ln(III) coordination sites based on the behavior of the solvent as an ancillary ligand. This structural promiscuity is attributed to the symmetry incompatible building blocks, which generate assemblies with substantial geometric strain such that no clear thermodynamic minimum exists between the different LnMCs. These LnMCs assemble from a Zn₅(picHA)₄²⁺ intermediate, which is monitored using ¹H NMR and ESI-MS to assess the stability of the complexes and possible assembly pathways based on kinetic considerations. LnMC assemblies that can be generated through central metal substitution reactions such as the LnZn₄(picHA)₄³⁺, LnZn₅(picHA)₅³⁺, and LnZn₈(picHA)₈³⁺ effectively reach equilibrium after 24 h at room temperature. In contrast, LnMCs that must disrupt the Zn₅L₄²⁺ structure to assemble, such as the LnZn₁₆L₁₆³⁺, reach equilibrium after heating for 24 h at 65 掳C. A pathway for LnMC assembly is presented where the Zn₅L₄²⁺ is the key intermediate based on these reaction data and shared structural motifs in the complexes. These results correlate solvent dependent assembly to the building block geometry, highlighting synthetic approaches for generating novel complexes.