Coupled molecular switches: a redox-responsive ligand and the redox-switched complexation of metal ions.
1,1'-(OC2H4OTos)2-ferrocene was treated with various diaza-[n]-crown-m (n/m = 12/4, 15/5, 18/6) to give three ferrocene cryptands (n/m = 12/4 (FcCrypt), 15/5, 18/6). The complexation of Group 1 and 2 metal ions by FcCrypt leads to large shifts in the redox potentials (up to +500 mV relative to FcCrypt) and consequently to a drastic decrease in the binding strength (up to 10(8)) in the ferrocene cryptands. The redox potential of Fcpda (1,1'-N,N'-bis-(dipicol-2-ylamino)-3.3',4,4'-tetraphenylferrocene) can be modified reversibly by complexation of Zn2 (E(Fcdpa) = -0.13 V, E(Fcdpa-2Zn+) = +0.66 V and E(Fcdpa-Zn2+) = +0.72 V). The X-ray crystal structure of FcCrypt-Ca-(ClO4)2 . H2O was determined; Ca2+ is coordinated by six oxygen (Ca2+-O 238.7, 239.1, 239.5, 242.6, 243.6, 247.7 pm) and two nitrogen donors (Ca2+-N 256.1, 259.2 pm) and displays a short Fe-Ca2+ contact (402.7 pm). The stability constants of FcCrypt-Na+ (IgK=8.32 in CH3CN) and FcCrypt-K+ (IgK = 4.54 in CH3CN) were determined. The precise adjustment of complex stability and redox potentials of Fcdpa, Fcdpa-Zn2+, FcCrypt (+0.12 V), and FcCrypt-Na+ (+0.395 V) allows coupling of the redox-switchable ferrocene cryptand and the redox-responsive aminoferrocene. In a cyclic process starting from a mixture of Fcdpa+PF6- and FcCrypt-Na+ the addition of Zn(CF3SO3)2 raises the redox potential of Fcdpa+ to that of Fcdpa+-Zn2+. This complex oxidizes FcCrypt-Na+, while the oxidized crypt- and displays a drastically reduced affinity towards Na+, so that a mixture containing FcCrypt+, Fcdpa-Zn2+, and uncoordinated Na+ results. The alkali metal ion is recomplexed after cyclam-assisted removal of Zn2+ from the Fcdpa-Zn2+ complex, since Fcdpa is oxidized by FcCrypt+ with reformation of FcCrypt-Na+. Thus two independent chemical processes--the complexation/ decomplexation of Zn2+ and of Na+--are linked indirectly with mediation by electron-transfer reactions.
Plenio
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Institut für Anorganische Chemie, Darmstadt, Germany. plenio@tu-darmstadt.de
Aberle
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