Microwave-Mediated Synthesis of Bulky Lanthanide Porphyrin–Phthalocyanine Triple-Deckers: Electrochemical and Magnetic Properties

Five heteroleptic lanthanide porphyrin–bis-phthalocyanine triple-decker complexes with bulky peripheral groups were prepared via microwave-assisted synthesis and characterized in terms of their spectroscopic, electrochemical, and magnetic properties. These compounds, which were easily obtained under our preparative conditions, would normally not be accessible in large quantities using conventional synthetic methods, as a result of the low yield resulting from steric congestion of bulky groups on the periphery of the phthalocyanine and porphyrin ligands. The electrochemically investigated triple-decker derivatives undergo four reversible one-electron oxidations and three reversible one-electron reductions. The sites of oxidation and reduction were assigned on the basis of redox potentials and UV–vis spectral changes during electron-transfer processes monitored by thin-layer spectroelectrochemistry, in conjunction with assignments of electronic absorption bands of the neutral compounds. Magnetic susceptibility measurements on two derivatives containing Tb^III and Dy^III metal ions reveal the presence of ferromagnetic interactions, probably resulting from magnetic dipolar interactions. The Tb^III derivative shows SMM behavior under an applied field of 0.1 T, where the direct and Orbach process can be determined, resulting in an energy barrier of <i>U</i>_eff = 132.0 K. However, Cole–Cole plots reveal the presence of two relaxation processes, the second of which takes place at higher frequencies, with the data conforming to a 1/<i>t</i> ∝ <i>T</i>⁷ relation, thus suggesting that it can be assigned to a Raman process. Attempts were made to form two-dimensional (2D) self-assembled networks on a highly oriented pyrolytic graphite (HOPG) surface but were unsuccessful due to bulky peripheral groups on the two Pc macrocycles

Spin Exchange Monitoring of the Strong Positive Homotropic Allosteric Binding of a Tetraradical by a Synthetic Receptor in Water

The flexible tetranitroxide <b>4T</b> has been prepared and was shown to exhibit a nine line EPR spectrum in water, characteristic of significant through space spin exchange (<i>J</i>_<i>ij</i>) between four electron spins interacting with four nitrogen nuclei (<i>J</i>_<i>ij</i> ≫ <i>a</i>_N). Addition of CB[8] to <b>4T</b> decreases dramatically all the <i>J</i>_<i>ij</i> couplings, and the nine line spectrum is replaced by the characteristic three line spectrum of a mononitroxide. The supramolecular association between <b>4T</b> and CB[8] involves a highly cooperative asymmetric complexation by two CB[8] (<i>K</i>₁ = 4027 M^–1; <i>K</i>₂ = 202 800 M^–1; α = 201) leading to a rigid complex with remote nitroxide moieties. The remarkable enhancement for the affinity of the second CB[8] corresponds to an allosteric interaction energy of ≈13 kJ mol^–1, which is comparable to that of the binding of oxygen by hemoglobin. These results are confirmed by competition and reduction experiments, DFT and molecular dynamics calculations, mass spectrometry, and liquid state NMR of the corresponding reduced complex bearing hydroxylamine moieties. This study shows that suitably designed molecules can generate allosteric complexation with CB[8]. The molecule must (i) carry several recognizable groups for CB[8] and (ii) be folded so that the first binding event <i>reorganizes</i> the molecule (unfold) for a better subsequent recognition. The presence of accessible protonable amines and H-bond donors to fit with the second point are also further stabilizing groups of CB[8] complexation. In these conditions, the spin exchange coupling between four radicals has been efficiently and finely tuned and the resulting allosteric complexation induced a dramatic stabilization enhancement of the included paramagnetic moieties in highly reducing conditions through the formation of the supramolecular <b>4T</b>@CB­[8]₂ complex

Hosting Various Guests Including Fullerenes and Free Radicals in Versatile Organic Paramagnetic <b>bTbk</b> Open Frameworks

The dinitroxide bis­(TEMPO) bisketal (<b>bTbk</b>) was shown to crystallize into open frameworks whose structures were determined by single-crystal X-ray diffraction. We show that <b>bTbk</b> can be used as a supramolecular building block for the hosting of a plethora of guests inside the 1D channels of its paramagnetic framework, including other radicals such as TEMPO or 2-azaadamantane-<i>N</i>-oxyl. C₆₀ and C₇₀ were also found to be easily included in this open framework during its crystallization. This resulted in well-defined, nanostructured assemblies of composite radical crystals (<b>bTbk</b>/toluene/C₆₀ or C₇₀) or (<b>bTbk</b>/toluene/TEMPO) by a very simple dissolution/crystallization process with tunable guest content. Selective C₆₀ extraction was also demonstrated directly from fullerene soot