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Issue 5, 2004
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Complexes of 5,5′-aminoacido-substituted 2,2′-bipyridyl ligands: control of diastereoselectivity with a pH switch and a chloride-responsive combinatorial library

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Abstract

The synthesis and coordination chemistry of a new chiral ligand, 2,2′-bipyridine substituted at the 5 and 5′ positions by N-methyl-L-valine methyl ester (5), is presented. The ligand readily forms complexes [M(5)3]2+ where M = Co(II) and Fe(II) in CH3CN, and the complexation reaction is slightly diastereoselective (d.e. = ca. 20%) in favour of the Δ diastereomer. The addition of six equivalents of HCl to these complexes [MII(5)3]2+ leads to formation of Δ-[MII(5H2)3]8+ with a d.e. of 100%. This high diastereoselectivity can be reversed by the addition of base i.e. the diastereoselectivity can be controlled by the pH. Δ-[Fe(5H2)3]8+ was found to bind chloride ions in CD3OD–CD3CN (6 ∶ 1) with a binding constant of 260 M−1. [CoII(5)3]2+ can be oxidised to Δ-[CoIII(5H2)3]9+. Formation constants for both [CoII(5)3]2+ and [CoII(5H2)3]8+ in acetonitrile were obtained by spectrophotometric titrations. In the former case, the stability constant, log β3 = 19.5(8), is very similar to that measured for [CoII(bipy)3]2+ (log β3 = 19.3(7)) but this drops significantly when the amine groups of 5 are protonated (log β3 = 16.5(2)). A dynamic combinatorial library was prepared by mixing three equivalents of 5, three equivalents of bipy, and two equivalents of Co(II) in CD3CN. The presence of all possible Δ- and Λ-[CoII(5)x(bipy)3−x]2+ complexes was inferred from 1H NMR and ES-MS spectra. Addition of protons to this library reduced the number of components by inducing diastereoselectivity, and presence of chloride further simplified the 1H NMR spectrum, indicating that {Cl2⊂Δ-[CoII(5H2)3]}6+ and [CoII(bipy)3]2+ were the dominant products.

Graphical abstract: Complexes of 5,5′-aminoacido-substituted 2,2′-bipyridyl ligands: control of diastereoselectivity with a pH switch and a chloride-responsive combinatorial library

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Publication details

The article was received on 24 Nov 2003, accepted on 15 Jan 2004 and first published on 06 Feb 2004


Article type: Paper
DOI: 10.1039/B315232D
Citation: Dalton Trans., 2004,0, 699-705
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    Complexes of 5,5′-aminoacido-substituted 2,2′-bipyridyl ligands: control of diastereoselectivity with a pH switch and a chloride-responsive combinatorial library

    S. G. Telfer, X. Yang and A. F. Williams, Dalton Trans., 2004, 0, 699
    DOI: 10.1039/B315232D

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