Multifunctional Guest-Hosting Triple-Stranded Helicates: From Anion Recognition to Quantum Information Applications

ConspectusThe growing field of coordination supramolecular chemistry constitutes a fruitful avenue for accessing a variety of multifunctional materials with a range of applications. Their versatility is enhanced if they have the ability to encapsulate guest molecules, opening opportunities for host/guest synergies. One of the most paradigmatic categories of such assemblies is coordination supramolecular helicates, which exhibit a central cavity for the potential allocation of small species, provided that their symmetry and volumes are compatible. The presence of noncovalent interactions (NCIs) between host and guest strongly contributes to the thermodynamic stability of these edifices, sometimes giving rise to a template effect. All those features are exploited for the case of triple-stranded helicates, which are predictably obtained from reactions of metal ions that adopt an octahedral coordination geometry with ligands made of two chelating moieties sufficiently separated by a spacer. The properties of the cavity of the helicate can be tuned by adjusting the central spacer of the ligand, which in turn, may incorporate functionalities facilitating NCIs with potential guests, such as hydrogen bonds. In this manner, a collection of pyrazolylpyridine (or -quinoline) ligands (L) has given rise to a large family of (G@[M₂L₃])ⁿ⁺ species (where G represents various guests), in which the encapsulated entities are firmly held in place by [N-H···G] hydrogen bonds. These assemblies can thus be employed for the selective recognition of anions or small coordination complexes, capitalizing on the specific architecture of the ligand strands. Furthermore, they have opened a plethora of possibilities for the investigation of synergic multifunctionality. The host can be made to exhibit molecular switching behavior (for example, spin-crossover, SCO, if M = Fe^II) or single-ion magnet (SIM) behavior (if M = Co^II) while the guest has been exploited to tune these properties or to incorporate new ones. More recently, anionic coordination complexes such as these from the series [M(ox)₃]^3- ("ox" being the oxalate anion and M = Fe, Cr, Al, Ru) have been efficiently trapped inside the metallo-helices. This has unveiled unprecedented phenomena resulting from encapsulation, such as the first manifestation of SIM behavior for Cr^III or the enhancement of the quantum coherence of a molecular qubit when acting as the guest. This family has been expanded with the inclusion of the anilate analogues of oxalates, opening unlimited options for multiproperty explorations (such as photophysical, redox chemistry, radical generation, etc.). More recently, within this group of systems, the guest has been employed as a template to selectively assemble specific combinations of two different ligands in the form of G@[M₂L_xL'_(3-x)]^m+ heteroleptic helicates, thus leading to a further opportunity of function tunability and enhancement. In this Account, we survey this and other related types of host/guest assemblies and place them in the general context of triple-stranded supramolecular helicates while assessing their impact in fields like molecular magnetism, quantum technologies, or anion recognition.