N‑Heterocyclic Carbenes: A Door Open to Supramolecular Organometallic Chemistry

ConspectusThe field of metallosupramolecular chemistry
is clearly dominated
by the use of O-, N-, and P-donor Werner-type polydentate ligands.
These molecular architectures are of high interest because of their
wide range of applications, which include molecular encapsulation,
stabilization of reactive species, supramolecular catalysis, and drug
delivery, among others. Only recently, organometallic ligands have
allowed the preparation of a variety of supramolecular coordination
complexes, and the term supramolecular organometallic complexes (SOCs)
is gaining space within the field of metallosupramolecular chemistry.
While the early examples of SOCs referred to supramolecular architectures
mostly containing bisalkenyl, diphenyl, or bisalkynyl linkers, the
development of SOCs during the past decade has been boosted by the
parallel development of multidentate N-heterocyclic carbene (NHC)
ligands. The first examples of NHC-based SOCs referred to supramolecular
assemblies based on polydentate NHC ligands bound to group 11 metals.
However, during the last 10 years, several planar poly-NHC ligands
containing extended π-conjugated systems have facilitated the
formation of a large variety of architectures in which the supramolecular
assemblies can contain metals other than Cu, Ag, and Au. Such ligands
are Janus di-NHCs and trigonal-planar tris-NHCsmost of them
prepared by our research groupwhich have allowed the preparation
of a vast range of NHC-based metallosupramolecular compounds with
interesting host–guest chemistry properties. Although the number
of SOCs has increased in the past few years, their use for host–guest
chemistry purposes is still in its earliest infancy. In this Account,
we describe the achievements that we have made during the last 4 years
toward broadening the applications of planar extended π-conjugated
NHC ligands for the preparation of organometallic-based supramolecular
structures, including their use as hosts for some selected organic
and inorganic guests, together with the catalytic properties displayed
by some selected host–guest inclusion complexes. Our contribution
describes the design of several Ni-, Pd-, and Au-based metallorectangles
and metalloprisms, which we used for the encapsulation of several
organic substrates, such as polycyclic aromatic hydrocarbons (PAHs)
and fullerenes. The large binding affinities found are ascribed to
the incorporation of two cofacial panels with large π-conjugated
systems, which provide the optimum conditions for guest recognition
by π–π-stacking interactions. We also describe
a series of digold­(I) metallotweezers for the recognition of organic
and inorganic substrates. These metallotweezers were used for the
recognition of “naked” metal cations and polycyclic
aromatic hydrocarbons. The recognition properties of these metallotweezers
are highly dependent on the nature of the rigid connector and of the
ancillary ligands that constitute the arms of the tweezer. A peculiar
balance between the self-aggregation properties of the tweezer and
its ability to encapsulate organic guests is observed.