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Copper(II)-assisted ligand fragmentation leading to three families of metallamacrocycle
Yu, Fei, kemik ...
We present an unprecedented copper(II)-assisted organic ligand fragmentation process under basic conditions leading to several ligands within three families of metallamacrocycle, ▫${\rm Cu_6}$▫, ...▫${\rm Cu_8}$▫, and ▫${\rm Cu_{16}}$▫. The sequential multistep reaction include (i) the deprotonation of the starting alcohol, 1,2-bis(3,5-dimethyl-pyrazol-1-yl)ethane-1,2-diol (▫$\mathbf{H_ 2 bdped}$▫) to its diolate bdped followed by complexation through six bonds (▫$\mu_6$▫) to three copper atoms in a ring, (ii) the breaking of the ethane-pyrazole C-N bonds by the different solvent alcohols to form 1-(3,5-dimethyl-pyrazol-1-yl)-2-methoxyethane-1,2-diolate ($\mathbf{dpmed}$) or 2-(3,5-dimethyl-pyrazol-1-yl)-2-hydroxyacetate (▫$\mathbf{dpet}$▫), while retaining coordination to the copper centers and (iii) the final step to ethane-1,1,2,2-tetraolate, ▫${\rm C_2H_2O_4^{4-}}$▫ (▫$\mathbf{et}$▫). Importantly, the latter product, only observed on two previous occasions, occupies the core of ▫${\rm Cu_6}$▫ and ▫${\rm Cu_{16}}$▫ through exceptionally eight coordination bonds (▫$\mu_8$▫). Its alkyl esters, 2-alkoxyethane-1,1,2-triolate (▫$\mathbf{met}$▫, ▫$\mathbf{eet}$▫, and ▫$\mathbf{pet}$▫), also occupy the central parts of ▫${\rm Cu_8}$▫ but forming six bonds (▫$\mu_6$▫) instead. The other product, 3,5-dimethylpyrazolate (▫$\mathbf{dp}$▫), acts as peripheral bridges (▫$\mu_2$▫) but it is not involved in coordination if the starting salt is copper acetate, this may be a consequence of acetate being a better ▫$\mu_2$▫-chelating ligand. In the presence of an oxidizing agent, ▫${\rm K_2Cr_2O_7}$▫, ▫${\rm C_2H_2O_4^{4-}}$▫ (▫$\mathbf{et}$▫) is oxidized to oxalate, ▫${\rm C_2O_4^{2-}}$▫ (▫$\mathbf{ox}$▫). Thus, an additional ▫$\mu_3$▫-hydroxide and ▫$\mu_4$▫-oxalate in ▫${\rm Cu_{16}}$▫ widen the complexity of the structures, not to mention the range of coordination geometries of the copper centers, though in the present cases they can be classed in only two types: distorted square-planar and square-pyramid. In addition to single-crystal crystallography, the results from different techniques such as IR, ESI-MS, optical UV-vis, and SQUID magnetometry help in the characterization of these rare metallamacrocycles made from unexpected and in situ generated ligands. We believe the results of the organic transformations are highly relevant to von Liebig's benzil-benzilic acid rearrangement.
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