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

## TL;DR

This paper explores triple-stranded helicates that can host guest molecules, enabling applications in anion recognition and quantum technologies.

## Contribution

The paper introduces new host/guest helicate systems with tunable properties for multifunctional applications like molecular magnetism and quantum coherence.

## Key findings

- Triple-stranded helicates can selectively recognize anions and small complexes through tailored ligand architecture.
- Encapsulation of anionic guests like [M(ox)3]3– enhances quantum coherence and enables new magnetic behaviors.
- Heteroleptic helicates allow function tunability by combining different ligands using guest molecules as templates.

## Abstract

The 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 = FeII) or single-ion
magnet (SIM) behavior (if M = CoII) 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 CrIII 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
x
L′(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.