Gold(I) Metallo-Tweezers for the Recognition of Functionalized Polycyclic Aromatic Hydrocarbons by Combined π-π Stacking and H-Bonding

A catalytic system based on π-π stacking interactions between a pyrene substituted gold NHC catalyst and amphiphilic polymers for alkyne hydration reactions

Pyrene-substituted amphiphilic ATRP polymers (P1) were used to stabilize the pyrene-substituted gold(I) NHC complex (Au-1) within the hydrophobic compartment of micellar structures in a methanol/water mixture through non-covalent π-π stacking interactions. The stacking interactions between pyrene groups of the polymer and the catalyst were investigated by means of fluorescence spectroscopy by comparing excimer and monomer emission signals of the pyrene moiety. The supported catalyst (Au-1@P1) formed spherical micellar structures in a water/methanol mixture with an average size of 55.6 (±13.1) nm as confirmed from TEM analysis. The performance of the catalytic system was tested on alkyne hydration reactions of alkynes. When compared to the unsupported analog, the supported catalyst showed an improved performance as a result of stabilization through π-stacking interactions within the hydrophobic compartment in the micellar structure.

"Lock and Key" and "Induced-Fit" Host-Guest Models in Two Digold(I)-Based Metallotweezers

Two different metallotweezers, each with two pyrene-imidazolylidene-gold(I) arms, were used as hosts for a series of planar aromatic guests. The metallotweezer with a dibenzoacridinebis(alkynyl) spacer (1) orients the two pyrene-imidazolylidene-gold(I) arms in a parallel disposition, with an interpanel distance of about 7 Å. The second metallotweezer (2) contains a carbazolylbis(alkynyl) spacer that directs the two pyrene panels in a diverging orientation. Determination of the association constants via ¹H NMR titrations demonstrates that the binding strength shown by 1 is significantly larger than that found by 2, with binding affinities as large as 10⁴ M^-1 (in CDCl₃), for the encapsulation of N,N'-dimethylnaphthalenetetracarboxydiimide with 1. The differences in the binding affinities are due to binding models associated with formation of the related host-guest complexes. While 1 operates via a "lock and key" model, in which the host does not suffer distortions upon formation of the inclusion complex, 2 operates via a guest-induced fit model. The large association constants shown by 1 with two planar guests were used for promotion of the template-directed synthesis of 1, which in the absence of an external template is produced in an equimolecular mixture with its self-aggregated congener, clippane [1₂]. This observation strongly suggests that the mechanically interlocked clippane is formed through a self-template-directed mechanism, while bonds are broken/formed during the synthetic protocol.

Molecular Tetris by sequence-specific stacking of hydrogen bonding molecular clips

A face-to-face stacking of aromatic rings is an effective non-covalent strategy to build functional architectures, as elegantly exemplified with protein folding and polynucleotide assembly. However, weak, non-directional, and context-sensitive van der Waals forces pose a significant challenge if one wishes to construct well-organized π-stacks outside the confines of the biological matrix. To meet this design challenge, we have devised a rigid polycyclic template to create a non-collapsible void between two parallel oriented π-faces. In solution, these shape-persistent aromatic clips self-dimerize to form quadruple π-stacks, the thermodynamic stability of which is enhanced by self-complementary N-H···N hydrogen bonds, and finely regulated by the regioisomerism of the π-canopy unit. With assistance from sufficient electrostatic polarization of the π-surface and bifurcated hydrogen bonds, a small polyheterocyclic guest can effectively compete against the self-dimerization of the host to afford a triple π-stack inclusion complex. A combination of solution spectroscopic, X-ray crystallographic, and computational studies aided a detailed understanding of this cooperative vs competitive process to afford layered aromatics with extraordinary structural regularity and fidelity.

The New Di-Gold Metallotweezer Based on an Alkynylpyridine System

We developed a simple method to prepare one gold-based metallotweezer with two planar Au-pyrene-NHC arms bound by a 2,6-bis(3-ethynyl-5-tert-butylphenyl)pyridine unit. This metallotweezer is able to bind a series of polycyclic aromatic hydrocarbons through the π-stacking interactions between the polyaromatic guests and the pyrene moieties of the NHC ligands. The metallotweezer was also used as a host for the encapsulation of planar metal complexes, such as the Au(III) complex [Au(C^N^C)(C≡CC₆H₄-OCH₃-p)], for which there is a large binding constant of 946 M⁻¹.

Clippane: A Mechanically Interlocked Molecule (MIM) Based on Molecular Tweezers

In this study we report the preparation of a new mechanically interlocked molecule formed by the self-aggregation of two metallotweezers composed by two pyrene-imidazolylidene gold(I) arms and a pyridine-centered pentacyclic bis-alkynyl linker. The mechanically interlocked nature of this molecule arises from the presence of the bulky tert-butyl groups attached to the sides of the pyrene moieties of the arms of the tweezer, which act as stoppers avoiding the dissociation of the self-aggregated metallotweezer dimer once it is formed. By combining experimental techniques, we were able to confirm the mechanically interlocked nature of this molecule in solution, in the gas phase and in the solid state. The behavior of the tert-butyl substituted tweezer differs greatly form that shown by the tweezer lacking of these bulky groups, whose dimeric structure is in equilibrium with the monomeric structure, therefore not showing any mechanical coercion that avoids the disassembly of the self-aggregated structure.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

Shape-Adaptability and Redox-Switching Properties of a Di-Gold Metallotweezer

The use of a carbazolyl‐connected di‐gold(I) metallotweezer for the encapsulation of several electron‐poor organic substrates, and a planar Au(III) complex containing a CNC pincer ligand, is described. The binding affinity of the receptor depends on the electron‐deficient character of the planar guest, with larger association constants found for the more electron‐poor guests. The X‐ray diffraction molecular structures of two host:guest adducts show that the host approaches its arms in order to facilitate the optimum interaction with the surface of the planar guests, in a clear example of an guest‐induced fit conformational arrangement. The electrochemical studies of the encapsulation of N,N’‐dimethyl‐naphthalenetetracarboxy diimide (NTCDI) show that the redox active guest is released from the receptor upon one electron reduction, thus constituting an example of redox‐switchable binding.

Bright and Robust Phosphorescence Achieved by Non-Covalent Clipping

Phosphorescent materials with bright emission in versatile media are important for their practical applications, which require to lower the susceptibility of triplet excitons to surroundings. Herein a non-covalent clipping strategy has been developed to attain this objective, by designing a tweezer receptor to encapsulate Pt^II -based triplet emitters through two-fold π-stacking interactions. The Pt^II emitters display robust phosphorescence by virtue of synergistic rigidifying and shielding effects, which are hardly influenced by emitter concentration, oxygen content, and solvent polarity changes. The phosphorescent colors are elaborately modulated by varying ligand substitutes on Pt^II emitters. Circularly polarized phosphorescence is further amplified for chiral Pt^II emitters, by taking advantage of dual phosphorescence and chirality enhancement upon non-covalent tweezer complexation. Overall, the clipping approach paves the way for the development of high-performance phosphorescent materials with bright emission, environmental robustness, and facile color tunability.

Acridine N-Heterocyclic Carbene Gold(I) Compounds: Tuning from Yellow to Blue Luminescence

The synthesis and the luminescence features of three gold(I)-N-heterocyclic carbene (NHC) complexes are presented to study how the n-alkyl group can influence the luminescence properties in the crystalline state. The mononuclear gold(I)-NHC complexes, [(L¹ )Au(Cl)] (1), [(L² )Au(Cl)] (2), and [(L³ )Au(Cl)] (3) were isolated from the reactions between [(tht)AuCl] and corresponding NHC ligand precursors, [N-(9-acridinyl)-N'-(n-butyl)-imidazolium chloride, (L¹ .HCl)], [N-(9-acridinyl)-N'-(n-pentyl)-imidazolium chloride, (L² .HCl)] and [N-(9-acridinyl)-N'-(n-hexyl)-imidazolium chloride, (L³ .HCl)]. Their single-crystal X-ray analysis reveals the influence of the n-alkyl groups on solid-state packing. A comparison of the luminescence features of 1-3 with n-alkyl substituents is explored. The molecules 1-3 depicted blue emission in the solution state, while the yellow emission (for 1), greenish-yellow emission (for 2), and blue emission (for 3) in the crystalline phase. This paradigm emission shift arises from n-butyl to n-pentyl and n-hexyl in the crystalline state due to the carbon-carbon rotation of the n-alkyl group, which tends to promote unusual solid packing. Hence n-alkyl group adds a novel emission property in the crystalline state. Density Functional Theory and Time-Dependent Density Functional Theory calculations were carried out for monomeric complex, N-(9-acridinyl)-N'-(n-heptyl)imidazole-2-ylidene gold(I) chloride and dimeric complex, N-(9-acridinyl)-N'-(n-heptyl)imidazole-2-ylidene gold(I) chloride to understand the structural and electronic properties.

Supramolecular Chirogenesis Induced by Platinum(II) Tweezers with Excellent Environmental Tolerance

Supramolecular chirogenesis has emerged as an effective strategy to access symmetry breaking in artificial systems. However, the chirogenic signals suffer from high susceptibility toward environmental variations. An effective strategy has been developed to address this issue by constructing platinum(II)-based tweezer/guest complexes stabilized by two-fold donor-acceptor and Pt^II -Pt^II metal-metal interactions. Upon guest encapsulation, the two pincers on the achiral Pt^II tweezer undergo a stereospecific twist to minimize steric repulsion, thus locking tweezer/guest complexes into the preferred chiral conformations. The induced chiroptical effects display outstanding solvent and temperature tolerance, ascribed to the balance between electrostatic and desolvation effects for the involved non-covalent interactions. Moreover, hierarchical and multi-component supramolecular assembly of tweezer/guest complexes provide a convenient way to modulate chirogeneic signals for their intensities.

Organoplatinum(II)-Based Self-Complementary Molecular Tweezers with Guest-Induced Fluorochromic Behaviors

Cyclometalated organoplatinum(II) complexes have aroused tremendous interests due to their square-planar geometry and intriguing photophysics. To access multiplatinum systems with more than three cyclometalated organoplatinum(II) units, the traditional covalent synthetic approach suffers from tedious multistep reactions with low overall yield. In comparison, supramolecular assembly can be regarded as an effective strategy toward multiplatinum(II) architectures. Despite the progresses achieved, it is still challenging to fabricate well-ordered supramolecular assemblies with precise numbers of organoplatinum(II) units. Herein, self-complementary dimerized molecular tweezers with four cyclometalated platinum(II) units have been successfully constructed by taking advantage of dual roles of the incorporated 2,2':6',2''-terpyridine unit (serving as the rigid spacer and encapsulated guest). Furthermore, addition of electron-rich carbazoles leads to conversion of the self-complementary structure to molecular tweezer/guest complexes. Such a structural transformation gives rise to the concomitant luminescent color change. The unique guest-induced fluorochromic phenomena, which are seldom reported in the previous host-guest systems, would be promising as tunable luminescent and ratiometric sensing materials.

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

The 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.

Mild and Efficient Synthesis of Diverse Organo-AuI -L Complexes in Green Solvents

An exceptionally mild and efficient method was developed for the preparation of (hetero)aryl-Au^I -L complexes using ethanol or water as the reaction medium at room temperature and Ar-B(triol)K boronates as the transmetalation partner. The reaction does not need an exogeneous base or other additives, and quantitative yields can be achieved through a simple filtration as the only required purification method, which obviates considerable waste associated with alternative workup methods. A broad reaction scope was demonstrated with respect to both the L and (hetero)aryl ligands on product Au complexes. Despite the polar reaction medium, large polycyclic aromatic hydrocarbon units can be incorporated on the Au complexes in very good to excellent yields. The approach was demonstrated for the chemoselective manipulation of orthogonally protected aryl boronates to afford a new class of N-heterocyclic carbene-Au-aryl complexes. A mechanistic rationale was proposed.

Dimensional Matching versus Induced-Fit Distortions: Binding Affinities of Planar and Curved Polyaromatic Hydrocarbons with a Tetragold Metallorectangle

A tetragold(I) rectangle-like metallocage containing two pyrene-bis-imidazolylidene ligands and two carbazolyl-bis-alkynyl linkers is used for the encapsulation of a series of polycyclic aromatic hydrocarbons (PAHs), including corannulene. The binding affinities obtained for the encapsulation of the planar PAHs guests in CD₂ Cl₂ are found to exponentially increase with the number of π-electrons of the guest (1.3 > logK >6.6). For the bowl-shaped molecule of corannulene, the association constant is much lower than the expected one according to its number of electrons. The molecular structure of the host-guest complex formed with corannulene shows that the molecule of the guest is compressed, while the host is expanded, thus showing an interesting case of artificial mutual induced-fit arrangement.

Discrete Supramolecular Stacks Based on Multinuclear Tweezer-Type Rhodium Complexes

By taking advantage of self-complementary π-π stacking and CH-π interactions, a series of discrete quadruple stacks were constructed through the self-aggregation of U-shaped dirhodium metallotweezer complexes featuring various planar polyaromatic ligands. By altering the conjugate stacking strength and bridging ligands, assemblies with a range of topologies were obtained, including a binuclear D-shaped macrocycle, tetranuclear open-ended cagelike frameworks, and duplex metallotweezer stacking structures. Furthermore, a rare stacking interaction resulting in selective C-H activation was observed during the self-assembly process of these elaborate architectures.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Process-tracing study on the post-assembly modification of poly-NHC-based metallosupramolecular cylinders with tunable aggregation-induced emission

A process-tracing and aggregation-induced emission (AIE) study of a covalent post-assembly modification (PAM) process of the Au^I–C_NHC cylinders was presented.

4-Halo-1,2,3-triazolylidenes: stable carbenes featuring halogen bonding

Synthesis and coordination of 4-halo-1,2,3-triazolylidenes have been developed. These novel ligands featured the character with σ-donation at carbon and a σ-hole at the halogen. Halogen bonding was observed by single crystal X-ray diffraction in their coinage metal complexes. The electronic properties of 4-iodo-1,2,3-triazolylidene were studied by both Ir-CO frequencies of the Tolman electronic parameter (TEP) and Huynh's electronic parameter (HEP) method, which suggested similar electronic properties to those of imidazolylidenes. During HEP tests, an interesting tunability was observed when different electron donors were employed.

Modulating Pt⋯Pt metal–metal interactions through conformationally switchable molecular tweezer/guest complexation

Pt(ii)⋯Pt(ii) metal–metal interactions can be modulated for molecular tweezer/guest complexation systems in response to pH variation.