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Issue 11, 2004
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Ligand self-recognition in the stereoselective assembly of [2 + 2] metallomacrocycles from racemic chiral bisbipyridyl molecular clefts and zinc(ii)

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Abstract

The synthesis of racemic and optically pure ligand L, in which two 6,6′-disubstituted bipyridines are connected by methyleneoxy linkers to the molecular cleft dibenzobicyclo[b,f][3.3.1]nona-5a,6a-diene-6,12-dione, is reported. In the presence of 2 equivalents of zinc(II) trifluoromethansulfonate (±)-L undergoes slow reversible coordination over 24 h to form a pair of enantiomeric [2 + 2] metallomacrocycles, [Zn2(+)L2](OTf)4 and [Zn2(−)L2](OTf)4 respectively, that contain either two (+)-L ligands or two (−)-L ligands. This assignment was confirmed by independent studies with either (+)-L or (−)-L which formed the same complexes but at a significantly faster rate (3 h), and circular dichroism spectra of [Zn2(+)L2](OTf)4 and [Zn2(−)L2](OTf)4 which gave signals of the same intensity with the opposite sign. Treatment of (±)-L or optically pure L with copper(I) showed rapid formation of a mixture of oligomers as well as the [2 + 2] metallomacrocycle. The complex Zn2L2(OTf)4 exhibits slow exchange between two species on the NMR time scale at room temperature. The results are consistent with the formation of a library of metal complexes in which the zinc(II) binds initially to the most accessible bipyridyl binding sites in (±)-L. Equilibration over several hours results in self-recognition of enantiomeric ligands to form a pair of enantiomeric metallomacrocycles, which have been tentatively assigned as having the helical configuration. Slow exchange is attributed to the preference for both metal centres to adopt 6-coordinate geometries involving the linker oxygens, but are limited to exchanging 5-coordinate complexes due to the shape of the cleft and the short linker.

Graphical abstract: Ligand self-recognition in the stereoselective assembly of [2 + 2] metallomacrocycles from racemic chiral bisbipyridyl molecular clefts and zinc(ii)

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Publication details

The article was received on 17 Mar 2004, accepted on 21 Apr 2004 and first published on 07 May 2004


Article type: Paper
DOI: 10.1039/B404054F
Dalton Trans., 2004, 1708-1714

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    Ligand self-recognition in the stereoselective assembly of [2 + 2] metallomacrocycles from racemic chiral bisbipyridyl molecular clefts and zinc(II)

    P. I. Anderberg, J. J. Turner, K. J. Evans, L. M. Hutchins and M. M. Harding, Dalton Trans., 2004, 1708
    DOI: 10.1039/B404054F

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