Ion-pair-mediated asymmetric synthesis of a configurationally stable mononuclear tris(diimine)-iron(II) complex.

Département de Chimie Organique Université de Genève quai Ernest Ansermet 30, 1211 Genève 4, Switzerland.
Angewandte Chemie International Edition (Impact Factor: 11.34). 08/2002; 41(13):2317-9. DOI: 10.1002/1521-3773(20020703)41:13<2317::AID-ANIE2317>3.0.CO;2-W
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    ABSTRACT: A series of ditopic ligands incorporating two 2,2′:6′,2″-terpyridine (tpy) metal-binding domains linked to a central naphthalenediyl core by ethyleneoxy chains of various lengths have been prepared and their iron(II) complexes prepared. The major thermodynamic products of complexation are ferracycles of [1 + 1], [2 + 2] or [3 + 3] stoichiometry. In the case of the [1 + 1] complexes, the linker between the two tpy domains is spatially restricted and the cations exhibit chirality associated with the conformation about the central N–Fe–N bonds. The dynamic processes by which the enantiomeric forms interconvert are investigated in the presence of chiral anions by NMR and circular dichroism spectroscopy making use of the Pfeiffer effect. We have shown that ditopic bis(2,2′:6′,2″-terpyridine) ligands give rise to conformationally restricted complexes with iron(II) centres. The absolute configuration of the cations in the diastereomeric ion pairs obtained through the Pfeiffer effect has been calculated using TDDFT methods.
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    ABSTRACT: A method is presented for the asymmetric synthesis of chiral ruthenium polypyridyl complexes that starts from racemic cis-[Ru(pp)(2)Cl(2)] (pp=2,2'-bipyridine or 1,10-phenanthroline ligands). The chiral bidentate ligands (R)-2-(isopropylsulfinyl)phenol, (R)-SO, and preferably the more electron-rich derivative (R)-2-(isopropylsulfinyl)-4-methoxyphenol, (R)-SO', serve as convenient chiral auxiliaries for the conversion of racemic starting complexes (1a: pp=2,2'-bipyridine; 1b: pp=5,5'-dimethyl-2,2'-bipyridine; c: pp=1,10-phenanthroline) into single diastereomers Λ-[Ru(pp)(2){(R)-SO}]PF(6) (Λ-(S)-2a-c) or Λ-[Ru(pp)(2){(R)-SO'}]PF(6) (Λ-(S)-2a') under a thermodynamically controlled dynamic transformation. The complexes Λ-(S)-2a-c and Λ-(S)-2a' themselves are direct precursors for the generation of optically active ruthenium-polypyridyl complexes by trifluoroacetic-acid-induced replacement of the sulfinylphenolate auxiliaries with bidentate pp ligands under retention of configuration, thereby affording Λ-[Ru(pp)(3)](PF(6))(2) (3a-c) complexes with high enantiomeric ratios of ≥98:2. In particular, by employing the methoxy-modified chiral auxiliary (R)-SO', enantiomeric ratios of >99:1 were reached. In the strategy introduced here, the high steric crowding of an octahedral coordination sphere was exploited by placing a sulfur-based stereocenter in direct proximity to the ruthenium stereocenter, thereby leading to a large difference in the stabilities of the intermediate Λ-S and Δ-S diastereomers and thus providing the opportunity to find suitable reaction conditions for conversion of the destabilized diastereomer into the thermodynamically more-stable one. This method should be of high practical value for the asymmetric synthesis of ruthenium-polypyridyl complexes because it allows one to use readily available racemic ruthenium complexes as starting materials.
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