The synthesis of a new bioinspired dinucleating ligand scaffold based on a bridging pyrazolate with appended bis[2-(1-methylimidazolyl)methyl]aminomethyl chelate arms is reported. This ligand forms very stable copper complexes, and a series of different species is present in solution depending on the pH. Interconversions between these solution species are tracked and characterized spectroscopically, and X-ray crystallographic structures of three distinct complexes that correspond to the species present in solution from acidic to basic pH have been determined. Overall, this provides a comprehensive picture of the copper coordination chemistry of the new ligand system. Alterations in the protonation state are accompanied by changes in nuclearity and pyrazolate binding, which cause pronounced changes in color and magnetic properties. Antiferromagnetic coupling between the copper(II) ions is switched on or off depending on the pyrazole binding mode.

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Octahedral transition metal centers such as Fe(II), Co(II), and Co(III) have been used as templates in the construction of [3]pseudorotaxanes and [3]rotaxanes from various acyclic and macrocyclic fragments. The species obtained consist of a ring threaded by two string-like compounds. Such systems are relatively uncommon in the [3]rotaxane family, the most usual form being made up of a single axis threaded through two rings. The key structural feature of the present systems is the coordinating unit incorporated in the various organic fragments and used in conjunction with the metal to gather and thread the two filaments through the ring. This bidentate chelate is derived from 8,8'-diphenyl-3,3'-bi-isoquinoline, a very rare example of an endotopic but nonsterically hindering ligand. The stoppered [3]rotaxanes were obtained by using an open-chain fragment bearing azide groups as end functions, followed by click chemistry using a propargyl ether attached to a very bulky group. A particularly attractive X-ray structure was obtained for a cobalt(III)-complexed [3]pseudorotaxane consisting of a 41-membered ring and two crescent-shaped threaded components. The Fe(II) and Co(III) complexes were characterized by (1)H NMR and ES-MS. By taking advantage of the markedly different kinetic properties of the two oxidation states, Co(II) and Co(III), it was possible to proceed to fast coordination or decoordination reactions (for the divalent state) or, when needed, to "freeze" the complexes due to the kinetic inertness of the trivalent state and to study them by (1)H NMR. Finally, demetalation of the two stoppered compounds prepared was performed. This demetalation reaction was fast for the Co(II)-complexed [3]rotaxane, whereas decomplexation of the Fe(II) equivalent required harsh conditions which were not compatible with the stability of the metal-free rotaxane. Interestingly, the thermal stability of the free [3]rotaxane toward unthreading and formation of its constitutive elements was only limited. (1)H NMR measurements showed that the half-life of the rotaxane is about one week at room temperature in dichloromethane. A variable-temperature study revealed that the unthreading reaction leading to dissociation of the [3]rotaxane has a remarkably high entropy of activation, in agreement with the intuitive view that the unthreading process involves a highly ordered transition state.