ABSTRACT
Although circular helicates can be assembled with a range of labile transition-metal centers, solely "chiral-at-metal" examples (i.e., systems without chiral ligands) and heterometallic (i.e., mixed metal systems, racemic or chiral) circular helicates both remain unexplored. Here, we report on the enantioselective synthesis of a heterometallic (Ir2Zn4) hexameric circular helicate and its elaboration into the corresponding triply interlocked Star of David [2]catenane. The relative inertness of Ir(III) enables enantiospecific synthesis of the hexameric circular helicate using chiral-at-metal building blocks. The resulting Star of David [2]catenane, which is a chiral 6-2-1 link, is formed as a single topological enantiomer. The X-ray crystal structure of the (Ir2Zn4)-catenane shows each of the two 95-atom-long macrocycles entwined around the six metal octahedral metal ions and each other, forming a triply interlocked circular double helix. Two PF6- anions reside above and below the central cavity. The Star of David [2]catenane, both with and without coordinated Zn(II) ions, retains the photophysical properties characteristic of cyclometalated Ir(III) complexes. The synthetic strategy opens up new research directions and opportunities for the assembly of other chiral knots, links, and heterometallic circular helicates.
ABSTRACT
The "off" state for aminocatalysis by a switchable [2]rotaxane is shown to correspond to an "on" state for anion-binding catalysis. Conversely, the aminocatalysis "on" state of the dual-function rotaxane is inactive in anion-binding catalysis. Switching between the different states is achieved through the stimuli-induced change of position of the macrocycle on the rotaxane thread. The anion-binding catalysis results from a pair of triazolium groups that act together to CH-hydrogen-bond to halide anions when the macrocycle is located on an alternative (ammonium) binding site, stabilizing the in situ generation of benzhydryl cation and oxonium ion intermediates from activated alkyl halides. The aminocatalysis and anion-binding catalysis sites of the dual-function rotaxane catalyst can be sequentially concealed or revealed, enabling catalysis of both steps of a tandem reaction process.
ABSTRACT
Unique donor-π-acceptor phthalocyanines have been synthesized through the asymmetric functionalization of an ABAB phthalocyanine, crosswise functionalized with two iodine atoms through Pd-catalyzed cross-coupling reactions with adequate electron-donor and electron-acceptor moieties. These push-pull molecules have been optically and electrochemically characterized, and their ability to perform as chromophores for dye-sensitized solar cells has been tested.
ABSTRACT
Molecular knots occur in DNA, proteins, and other macromolecules. However, the benefits that can potentially arise from tying molecules in knots are, for the most part, unclear. Here, we report on a synthetic molecular pentafoil knot that allosterically initiates or regulates catalyzed chemical reactions by controlling the in situ generation of a carbocation formed through the knot-promoted cleavage of a carbon-halogen bond. The knot architecture is crucial to this function because it restricts the conformations that the molecular chain can adopt and prevents the formation of catalytically inactive species upon metal ion binding. Unknotted analogs are not catalytically active. Our results suggest that knotting molecules may be a useful strategy for reducing the degrees of freedom of flexible chains, enabling them to adopt what are otherwise thermodynamically inaccessible functional conformations.
ABSTRACT
ABAB-type Zn(II) phthalocyanines, crosswise-functionalized with two and four iodine atoms, respectively, have been efficiently prepared using statistical condensation procedures. Key to the selective preparation of the opposite ABAB isomers versus the adjacent AABB ones is the use of bulky 3,6-(3',5'-bis(trifluoromethyl)phenyl)phthalonitrile with hampered self-condensation capabilities.
