ABSTRACT
Fluorescence resonance energy transfer (FRET) in appropriately tagged NHC-gold complexes can be utilized to in situ observe the formation of digold species, which are known to be catalytically relevant intermediates in various gold catalyzed reactions.
ABSTRACT
The reactions of [MCl(cod)]2 (M = Rh, Ir) with different NHC·HX (X = Cl, I), K2CO3 in technical grade acetone under air provide simple access to various [(NHC)MX(cod)] complexes; a facile one-pot synthesis of [(NHC)MCl(CO)2] (M = Rh, Ir) is also reported.
ABSTRACT
The double-bridged hemicarcerand [A,B-(CH2OH)2-cavitand]-(CH2NHCH2)2-[A,B-(CH2OH)2-cavitand] 23 (and several other related compounds) was synthesized by the condensation of the two complementary precursors A,B-(CH2NH2)2(CH2OH)2-cavitand and A,B-(CH2Br)2(CH2OAc)2-cavitand followed by hydrolysis of the acetate groups. This hemicarcerand has nitrogen and oxygen donor atoms located on the interior of the spherical cavity and thus allows endohedral coordination of metal ions. The cavity has a volume of approximately 0.12 nm3, a value obtained by calculating a Connolly-type contact surface and the molecular electrostatic potential. The Cu2+ complex of hemicarcerand 23 was studied in detail by EPR and DFT calculations at the UB3LYP/6-31G level to verify the anticipated endohedral nature of the metal complex. It could be shown that the copper ion is coordinated to four oxygen donor atoms and no deviation from axial symmetry at the copper site could be detected. No direct coordination to nitrogen atoms of the hemicarcerand could be observed; however, complexation with DMF solvent molecules was detected by ESEEM and HYSCORE experiments. The closed structure of the hemicarcerand was also confirmed by an evaluation of proton-copper distances. Results from DFT calculations are in accord with the EPR results, and further support suggested coordination of the Cu(II) within the hemicarcerand cavity by four oxygen donor atoms.
ABSTRACT
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.
ABSTRACT
The reaction of 1,1'-ferrocene-bis(methylenepyridinium) salt with 1,4,8,11-tetraazacyclotetradecane-5,12-dione, followed by LiAlH4 reduction results in the formation of FcCyclam. Metal complexes of FcCyclam with M2+ = Co2+, Ni2+, Cu2+, and Zn2+ were synthesized from FcCyclam and the respective metal triflates. The complexation of Cu2+ and FcCyclam in CH3CN is preceeded by a rapid electron transfer, followed by a slower complex formation reaction and a reverse electron transfer. The protonation constants of FcCyclam and the stability constants for the Cu2+ complex of FcCyclam (logK = 9.26(4) for the formation of the [Cu(FcCyclam)]2+ complex) were determined in 1,4-dioxane/water 70:30 v/v, 0.1 moldm(-3), KNO3, 25 degrees C. By using FcCyclam one can selectively sense the presence of Cu2+ ions in the presence of Ni2+, Zn2+, Cd2+, Hg2+, and Pb2+ with a very large deltaE approximately 200 mV, depending on pH. The X-ray crystal structures of FcCyclam and of complexes with Co2+, Ni2+, Cu2+, and Zn2+ were determined and Fe-M2+ distances obtained: Fe-Co2+ 395.9, Fe-Ni2+ 385.4, Fe-Cu2+ 377.7, and Fe-Zn2+ 369.0 pm. The redox potential of FcCyclam is influenced in a characteristic manner by the complexation of M2+. A linear correlation of 1/r approximately/= deltaE [r = distance Fe-M2+ from crystal data, deltaE=-E1/2([M(FcCyclam)]2+) - E1/2(FcCyclam)] was found; this is indicative of a mainly Coulomb type interaction between the two metal centers. The nature of the Fe...M2+ interaction was also investigated by determining deltaE in several solvents (mixtures) of different dielectric constants epsilon. The expected relation of deltaE approximately/= 1/epsilon was only found at very high values of epsilon. At epsilon < 40 increased ion-pairing appears to reduce the effective positive charge at M2+ leading to progessively smaller values of deltaE with lowered epsilon. The dependence of deltaE and epsilon can be calculated semiquantitatively by combining the Fuoss ion-pairing theory with the Coulomb model.
ABSTRACT
The palladium-catalyzed Sonogashira reaction can be used to build optically active, oligomeric 1,2,3-substituted ferrocenes up to the tetramer, as well as polymers, by sequential coupling of optically active (ee > 98 %), planar chiral iodoferroceneacetylenes and ferroceneacetylenes. (SFC)-1-Iodoferrocene-2-carbaldehyde (1) was reduced to the alcohol and methylated to give the corresponding methyl ether, which was Sonogashira-coupled with HC(triple bond)CSiEt3, resulting in (RFc)-1-(C(triple bond)CSiEt3)-2-methoxymethylferrocene (4) (79%, three steps). Orthometalation with tBuLi followed by quenching with 1,2-diodoethane gave (RFc)-1-(C(triple bond)CSiEt3)-2-methoxymethyl-3-iodoferrocene (5). Deprotection of the acetylene with nBu4NF resulted in (RFc)-1-ethynyl-2-methoxymethyl-3-iodoferrocene (6), which was Sonogashira-coupled with itself to produce an optically active polymer. Deprotection of 4 with nBu4NF and Sonogashira coupling of the product with 5 resulted in the dinuclear ferrocene 9. Deprotection of 9 and coupling with 5, followed by deprotection of the resulting acetylene 11, gave the trinuclear ferrocene 12. Another such sequence involving 11 and 5 produced a tetranuclear ferrocene 13. To study the electronic communication in such oligomers in more detail, two symmetrical, closely interrelated, trinuclear ferrocenes 18 and 19 were synthesized. The redox potentials of all the ferrocenes and the ferroceneacetylene polymer were determined by cyclic and square-wave voltammetry. All the metallocenes were investigated by UV/Vis spectroscopy. A linear relationship was found between lambdamax and l/n (n=number of ferrocene units in the oligomer). The polymer displayed two redox waves in the cyclic voltammogram, at 0.65 and 0.795 V. The corresponding mixed-valence oligoferrocene cations were synthesized from four ferroceneacetylenes, and their metal-metal charge transfer bands were examined by UV/Vis-NIR. The resonance exchange integrals Had, calculated on the basis of spectral information from the metal - metal charge transfer (MMCT) bands, were between 290 and 552 cm-1.