A robust heterodimeric bis-Rh(III)-porphyrin macrocycle for the self-assembly of a kinetically stable [2]-rotaxane.

We report the self-assembly of a robust di-nuclear tetralactam macrocyle based on two symmetric components: a Rh(III) bis-porphyrin and a bis-pyridyl ligand. We probe the binding properties of the tetralactam macrocycle with adipamide derivatives using 1H NMR spectroscopy. On the one hand, we show that the binding of the adipamide having linear alkyl chains that can thread through the intact macrocycle's cavity produces a weakly bound 1 : 1 complex stabilized by four intermolecular hydrogen bonds and featuring a preferred binding geometry of [2]pseudorotaxane topology. On the other hand, we detect the formation of two different complexes in the binding of an analogous adipamide possessing bulky stoppers (dumb-bell axle). The initial addition of the dumb-bell guest induces the formation of a 1 : 1 complex featuring fast exchange kinetics on the 1H chemical shift timescale and exo-cyclic (non-threaded) binding geometry. Notably, in the presence of a large excess of the dumb-bell guest and at suitable concentrations of the macrocycle (>5 mM) we observe the emergence of a second species displaying slow exchange kinetics. This observation allows the undisputed assignment of a [2]rotaxane topology to the second complex. The significant increase in kinetic stability featured by the di-nuclear Rh(III) [2]rotaxane complex contrasts with its reduction in thermodynamic stability (more than one order of magnitude) compared to the previously described di-nuclear Zn(II) counterpart.

Negative Homotropic Cooperativity in Guest Binding of a Trisporphyrin Double Cleft.

Invited for the cover of this issue is the group of Takeharu Haino at Hiroshima University. The image depicts the host-guest complex of a trisporphyrin double cleft with an electron-deficient aromatic molecule, which shows negative cooperativity in guest binding. Read the full text of the article at 10.1002/chem.202300107.

Negative Homotropic Cooperativity in Guest Binding of a Trisporphyrin Double Cleft.

The negative homotropic allostery of a triple-layered trisporphyrin cleft with two guest binding sites is confirmed. The X-ray crystal structures of the 1 : 2 host-guest complexes showed that the trisporphyrin accommodated two guest molecules within the cleft through π-π stacking and donor-acceptor interactions. In solution, 1 H NMR and Job plots showed 1 : 2 host-guest complexes. Isothermal titration calorimetry (ITC) and UV/vis absorption spectroscopy were employed to evaluate the binding constants and cooperativities. The guest binding of the trisporphyrin showed negative cooperativity and non-cooperativity depending on the structures of the guest molecules. The correlations between the interaction parameters (α) and Hill constants were determined. ITC experiments showed that the host-guest complexation of trisporphyrin with electron-deficient guests incurred an enthalpy penalty in the successive guest binding process. DFT calculations revealed that binding of the first guest reduced the electron density of the central porphyrin plane, which led to an energetic penalty that weakened the successive binding process.

Host-Guest Complexation of Bisporphyrin Cleft and Electron-Deficient Aromatic Guests.

The host-guest complexation of a bisporphyrin cleft with various electron-deficient guest molecules was studied in solution and in the solid-state. X-ray crystal structures of a bisporphyrin cleft with naphthalene dianhydride and 2,4,7-trinitrofluorenone reveal that these guest molecules were located within the bisporphyrin cleft and formed ideal π-π stacking interactions in a host-guest ratio of 1:1. Isothermal titration calorimetry determined the binding constants and thermodynamic parameters for the 1:1 host-guest complexations in 1,2-dichloroethane and toluene. Two types of enthalpy-entropy compensation effects were found: (1) The tightly stacked host-guest structures restrict guest movement within the cleft, which results in significant desolvation with large intrinsic entropies. (2) The loosely bound guests maintain their molecular freedom within the bisporphyrin cleft, which leads to less desolvation with small intrinsic entropies. Chiral guest encapsulation directed the clockwise and anticlockwise twisted conformations of the bisporphyrin units, which induced bisignate CDs.

A dual redox-responsive supramolecular polymer driven by molecular recognition between bisporphyrin and trinitrofluorenone.

A dual redox-responsive supramolecular polymer driven by molecular recognition between bisporphyrin (bisPor) and trinitrofluorenone (TNF) has been developed. The supramolecular polymer was degraded into monomers in response to both oxidation and reduction stimuli.

Helicity of a polyacetylene directed by molecular recognition of biscalixarene and fullerene.

Chiral biscalixarenes and a C60-appended polyphenylacetylene have been synthesized. The chiral biscalixarene encapsulated the C60 unit of the phenylpolyacetylene, which induced the preferred helicity of the polyacetylene.