ABSTRACT
We report on the use of the hydrogen-bond-accepting properties of neutral nitrone moieties to prepare benzylic amide macrocycle-containing [2]rotaxanes in yields as high as 70%. X-ray crystallography showed the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and the two nitrone groups of the thread. Dynamic (1)H NMR studies of the rates of macrocycle pirouetting in nonpolar solutions indicated that the amide-nitrone hydrogen bonds are particularly strong (approximately 1.3 and approximately 0.2 kcal mol(-1) stronger than similar amide-ester and amide-amide interactions, respectively). In addition to polarizing the N-O bond through hydrogen bonding, the rotaxane structure affects the chemistry of the nitrone groups in two significant ways: first, the intercomponent hydrogen bonding activates the nitrone groups to electrochemical reduction, a one-electron-reduction of the rotaxane being stabilized by a remarkable 400 mV (8.1 kcal mol(-1)) with respect to the same process in the thread; second, however, encapsulation protects the same functional groups from chemical reduction with an external reagent (and slows electron transfer to and from the electroactive groups in cyclic voltammetry experiments). Mechanical interlocking with a hydrogen-bonding molecular sheath thus provides a route to an encapsulated polarized functional group and radical anions of significant kinetic and thermodynamic stability.
ABSTRACT
The anthryl-substituted rhodium(III) and iridium(III) heteroleptic beta-ketoenolato derivatives of general formula [M(acac)(2)(anCOacac)] [acac = pentane-2,4-dionate; anCOacac = 3-(9-anthroyl)pentane-2,4-dionate], 3 (M = Rh) and 4 (M = Ir), and [M(acac)(2)(anCH(2)acac)] [anCH(2)acac = 3-(9-anthrylmethyl)pentane-2,4-dionate], 5 (M = Rh) and 6 (M = Ir), were prepared by reacting the corresponding tris(pentane-2,4-dionate)metal complexes, [M(acac)(3)], with 9-anthroyl chloride and 9-chloromethylanthracene, respectively, under Friedel-Crafts conditions. 3-6 were characterized by elemental analysis, ion spray mass spectrometry (IS-MS), (1)H NMR, and UV-vis spectroscopy. The structure of 3 was also elucidated by single-crystal X-ray analysis. When excited at 365 nm, 3-6 result to be poorly luminescent compounds; while the free diketone, i.e., 3-(9-anthrylmethyl)pentane-2,4-dione 1, whose structure was established also by single-crystal X-ray analysis, results to be a strongly light emitting molecule. The study of the electrochemical behavior of 3-6 as well as of the corresponding tris-acetylacetonates of rhodium(III) and iridium(III) allows a satisfactory interpretation of their electrode process mechanism, and gives information about the location of the redox sites along with the thermodynamic and kinetic characterization of the corresponding redox processes. All data are in agreement with the hypothesis that the quenching of the anthracene fluorescence, observed for compounds 3-6, can be due to an intramolecular electron transfer process between the anthryl moiety and the metal-beta-ketoenolato component. Moreover, a study was carried out of the redox behavior of the dyads 3-6 under chemical activation. The one-electron oxidation of compounds 3-6 by thallium(III) trifluoroacetate leads to the formation of the corresponding cation radicals, 3(+)-6(+), whose highly resolved X-band EPR spectra were fully interpreted by computer simulation as well as by semiempirical and DFT calculations of spin density distribution.
