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Coupled Molecular Switches: A Redox‐Responsive Ligand and the Redox‐Switched Complexation of Metal Ions
Author(s) -
Plenio Herbert,
Aberle Clemens
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20011015)7:20<4438::aid-chem4438>3.0.co;2-k
Subject(s) - cryptand , redox , chemistry , ferrocene , metal , ligand (biochemistry) , crystallography , metal ions in aqueous solution , inorganic chemistry , electrochemistry , ion , stereochemistry , medicinal chemistry , organic chemistry , receptor , electrode , biochemistry
1,1′‐(OC 2 H 4 OTos) 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 Zn 2+ ( E (Fcdpa)=−0.13 V, E (Fcdpa‐2 Zn + )=+0.66 V and E (Fcdpa‐Zn 2+ )=+0.72 V). The X‐ray crystal structure of FcCrypt‐Ca(ClO 4 ) 2 ⋅H 2 O was determined; Ca 2+ is coordinated by six oxygen (Ca 2+ −O 238.7, 239.1, 239.5, 242.6, 243.6, 247.7 pm) and two nitrogen donors (Ca 2+ −N 256.1, 259.2 pm) and displays a short Fe−Ca 2+ contact (402.7 pm). The stability constants of FcCrypt‐Na + (lg K =8.32 in CH 3 CN) and FcCrypt‐K + (lg K =4.54 in CH 3 CN) were determined. The precise adjustment of complex stability and redox potentials of Fcdpa, Fcdpa‐Zn 2+ , 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 + PF 6 − and FcCrypt‐Na + the addition of Zn(CF 3 SO 3 ) 2 raises the redox potential of Fcdpa + to that of Fcdpa + ‐Zn 2+ . This complex oxidizes FcCrypt‐Na + , while the oxidized cryptand displays a drastically reduced affinity towards Na + , so that a mixture containing FcCrypt + , Fcdpa‐Zn 2+ , and uncoordinated Na + results. The alkali metal ion is recomplexed after cyclam‐assisted removal of Zn 2+ from the Fcdpa‐Zn 2+ complex, since Fcdpa is oxidized by FcCrypt + with reformation of FcCrypt‐Na + . Thus two independent chemical processes—the complexation/decomplexation of Zn 2+ and of Na + —are linked indirectly with mediation by electron‐transfer reactions.