Donor-acceptor interactions between cyclic trinuclear pyridinate gold(i)-complexes and electron-poor guests: nature and energetics of guest-binding and templating on graphite

A Broadened Class of Donor-Acceptor Stacked Macrometallacyclic Adducts of Different Coinage Metals

A yet-outstanding supramolecular chemistry challenge is isolation of novel varieties of stacked complexes with finely-tuned donor-acceptor bonding and optoelectronic properties, as herein reported for binary adducts comprising two different cyclic trinuclear complexes (CTC@CTC'). Most previous attempts focused only on 1-2 factors among metal/ligand/substituent combinations, resulting in heterobimetallic complexes. Instead, here we show that, when all 3 factors are carefully considered, a broadened variety of CTC@CTC' stacked pairs with intuitively-enhanced intertrimer coordinate-covalent bonding strength and ligand-ligand/metal-ligand dispersion are attained (dM-M' 2.868(2) Å; ΔE>50 kcal/mol, an order of magnitude higher than aurophilic/metallophilic interactions). Significantly, CTC@CTC' pairs remain intact/strongly-bound even in solution (Keq 4.67×105 L/mol via NMR/UV-vis titrations), and the gas phase (mass spectrometry revealing molecular peaks for the entire CTC@CTC' units in sublimed samples), rather than simple co-crystal formation. Photo-/electro-luminescence studies unravel metal-centered phosphorescence useful for novel all metal-organic light-emitting diodes (MOLEDs) optoelectronic device concepts. This work manifests systematic design of supramolecular bonding and multi-faceted spectral properties of pure metal-organic macrometallacyclic donor/acceptor (inorganic/inorganic) stacks with remarkably-rich optoelectronic properties akin to well-established organic/organic and organic/inorganic analogues.

Molecular imine cages with π-basic Au3(pyrazolate) faces

One tetrahedral and two trigonal prismatic cages with π-basic Au3(pyrazolate)3 faces were obtained by connection of pre-formed gold complexes via dynamic covalent imine chemistry. The parallel arrangement of the Au3(pyrazolate)3 complexes in the prismatic cages augments the interaction with π-acids, as demonstrated by the encapsulation of polyhalogenated aromatic compounds. The tetrahedral cage was found to act as a potent receptor for fullerenes. The structures of the three cages, as well as the structures of adducts with C60 and C70, could be established by X-ray crystallography.

Exploring π-π interactions and electron transport in complexes involving a hexacationic host and PAH guest: a promising avenue for molecular devices

Single isolated molecules and supramolecular host-guest systems, which consist of π-π stacking interactions, are emerging as promising building blocks for creating molecular electronic devices. In this article, we have investigated the noncovalent π-π interaction and intermolecular electron charge transport involved in a series of host-guest complexes formed between a cage-like host (H6+) and polycyclic aromatic hydrocarbon (PAH) guests (G1-G7) using different quantum chemical approaches. The host (H6+) consists of two triscationic π-electron-deficient trispyridiniumtriazine (TPZ3+) units that are bridged face-to-face by three ethylene-triazole-ethylene. Our theoretical calculations show that the perylene and naphthalene inclusion complexes G7⊂H and G1⊂H have the highest and lowest interaction energies, respectively. In addition, energy decomposition analysis (EDA) indicated that the dispersion interaction term, ΔEdisp, significantly contributes to the host-guest interaction and is correlated with the existence of π-π van der Waals interaction. Using the nonequilibrium Greens function (NEGF) method in combination with density functional theory (DFT), the current-voltage (I-V) curves of the complexes were estimated. The conductance values increased when the guests were embedded inside the host cavity. Notably, the complex G7⊂H has the maximum conductance value. Overall, this study provided the electron transport of the PAH inclusion host-guest complex through π-π interaction and provided a direction for the fabrication of future supramolecular molecular devices.

Conjugated Nanohoops with Dibenzo[a,e]pentalenes as Non-alternant and Antiaromatic π-Systems

Conjugated nanohoops are excellent candidates to study structure-property relationships, as optoelectronic materials and as hosts for supramolecular chemistry. While carbon nanohoops containing aromatics are well studied, antiaromatic units had not been incorporated until recently by our group using dibenzo[a,e]pentalene (DBP). The non-alternant electronic character of the DBP units significantly influences the optoelectronic properties of such nanohoops. We herein summarize our synthetic strategies to DBP-containing nanohoops, their structural and electronic properties, chirality and host-guest chemistry. We demonstrate how incorporating antiaromatic units leads to unique properties and opens new synthetic avenues, making such nanohoops attractive as potential electronic materials.

Efficient Computation of Geometries for Gold Complexes

Computationally obtaining structural parameters along a reaction coordinate is commonly performed with Kohn‐Sham density functional theory which generally provides a good balance between speed and accuracy. However, CPU times still range from inconvenient to prohibitive, depending on the size of the system under study. Herein, the tight binding GFN2‐xTB method [C. Bannwarth, S. Ehlert, S. Grimme, J. Chem. Theory Comput. 2019, 15, 1652] is investigated as an alternative to produce reasonable geometries along a reaction path, that is, reactant, product and transition state structures for a series of transformations involving gold complexes. A small mean error (1 kcal/mol) was found, with respect to an efficient composite hybrid‐GGA exchange‐correlation functional (PBEh‐3c) paired with a double‐ζ basis set, which is 2–3 orders of magnitude slower. The outlined protocol may serve as a rapid tool to probe the viability of proposed mechanistic pathways in the field of gold catalysis.

Multinuclear Ag Clusters Sandwiched by Pt Complex Units: Fluxional Behavior and Chiral-at-Cluster Photoluminescence

Multinuclear Ag clusters sandwiched by Pt complex units were synthesized and characterized by single crystal X-ray diffraction and NMR studies. The sandwich-shaped multinuclear Ag complexes showed two different types of fluxional behavior in solution: rapid slippage of Pt complex units on the Ag₃ core and a reversible demetalation-metalation reaction by the treatment with Cl anion and Ag ion, respectively. The Ag₂ complex obtained by demetalation reaction from the Ag₃ complex displayed U to Z isomerization. These multinuclear Ag complexes showed strong photoluminescence whose properties depended on the existence of Pt→Ag dative bonds. The Ag₃ complex, identified to be "chiral-at-cluster", was optically resolved by the formation of a diastereomeric salt with a chiral anion. The enantiomers show circular dichroism (CD) and circularly polarized luminescence (CPL) properties which is unprecedented for compounds based on a chiral sandwich structure. Theoretical calculations allow to understand their structural features and photophysical properties.

ERα-agonist and ERβ-antagonist bifunctional next-generation bisphenols with no halogens: BPAP, BPB, and BPZ

As demonstrated for bisphenol AF (BPAF), the electrostatic halogen bond based on the London dispersion force of halogen atoms was found to be a major driving force of their bifunctional ERα-agonist and ERβ-antagonist activities. Because similar electronic effects are anticipated for hydrocarbon groups (alkyl or aryl groups), we hypothesized that bisphenol compounds consisting of such groups also work bifunctionally. In the present study, we examined bisphenol AP (BPAP), B (BPB), and Z (BPZ). After recognizing their considerably strong receptor binding affinities, we evaluated the abilities of BPAP, BPB, and BPZ to activate ERα and ERβ in a luciferase reporter gene assay. These bisphenols were fully active for ERα but completely inactive for ERβ. When we examined their inhibitory activities for 17β-estradiol in ERβ by two different qualitative and quantitative analytical methods, we found that those bisphenols worked as definite antagonists. Consequently, they were established as bifunctional ERα-agonists and ERβ-antagonists. The present structure-activity analyses revealed that the dispersion force works not only on the halogens but also on the hydrocarbon groups, and that it is a major driving force of bifunctional ERα-agonist and ERβ-antagonist activities.

Chiral Dibenzopentalene-Based Conjugated Nanohoops through Stereoselective Synthesis

Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π-systems. They can possess unexpected optoelectronic properties and their radially oriented π-system makes them attractive for host-guest chemistry. Bending the π-subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel "corner unit" in the synthesis of the hoops. The [6]DBP[4]Ph-hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO-LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.

Stepwise Adsorption of Alkoxy-Pyrene Derivatives onto a Lamellar, Non-Porous Naphthalenediimide-Template on HOPG

The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC₃₃ -NDI-uC₃₃ , a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy-pyrene (PyrOR) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC₃₃ -NDI-uC₃₃ template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC₃₃ -NDI-uC₃₃ monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.

Au3-to-Ag3 coordinate-covalent bonding and other supramolecular interactions with covalent bonding strength

An efficient strategy for designing charge-transfer complexes using coinage metal cyclic trinuclear complexes (CTCs) is described herein. Due to opposite quadrupolar electrostatic contributions from metal ions and ligand substituents, [Au(μ-Pz-(i-C₃H₇)₂)]₃·[Ag(μ-Tz-(n-C₃F₇)₂)]₃ (Pz = pyrazolate, Tz = triazolate) has been obtained and its structure verified by single crystal X-ray diffraction – representing the 1^st crystallographically-verified stacked adduct of monovalent coinage metal CTCs. Abundant supramolecular interactions with aggregate covalent bonding strength arise from a combination of M–M′ (Au → Ag), metal–π, π–π interactions and hydrogen bonding in this charge-transfer complex, according to density functional theory analyses, yielding a computed binding energy of 66 kcal mol⁻¹ between the two trimer moieties – a large value for intermolecular interactions between adjacent d¹⁰ centres (nearly doubling the value for a recently-claimed Au(i) → Cu(i) polar-covalent bond: Proc. Natl. Acad. Sci. U.S.A., 2017, 114, E5042) – which becomes 87 kcal mol⁻¹ with benzene stacking. Surprisingly, DFT analysis suggests that: (a) some other literature precedents should have attained a stacked product akin to the one herein, with similar or even higher binding energy; and (b) a high overall intertrimer bonding energy by inferior electrostatic assistance, underscoring genuine orbital overlap between M and M′ frontier molecular orbitals in such polar-covalent M–M′ bonds in this family of molecules. The Au → Ag bonding is reminiscent of classical Werner-type coordinate-covalent bonds such as H₃N: → Ag in [Ag(NH₃)₂]⁺, as demonstrated herein quantitatively. Solid-state and molecular modeling illustrate electron flow from the π-basic gold trimer to the π-acidic silver trimer with augmented contributions from ligand-to-ligand’ (LL′CT) and metal-to-ligand (MLCT) charge transfer.

A stacked Ag₃–Au₃ bonded (66 kcal mol⁻¹) complex obtained crystallographically exhibits charge-transfer characteristics arising from multiple cooperative supramolecular interactions.

Coinage-Metal-Based Cyclic Trinuclear Complexes with Metal-Metal Interactions: Theories to Experiments and Structures to Functions

Among the d¹⁰ coinage metal complexes, cyclic trinuclear complexes (CTCs) or trinuclear metallocycles with intratrimer metal-metal interactions are fascinating and important metal-organic or organometallic π-acids/bases. Each CTC of characteristic planar or near-planar trimetal nine-membered rings consists of Au(I)/Ag(I)/Cu(I) cations that linearly coordinate with N and/or C atoms in ditopic anionic bridging ligands. Since the first discovery of Au(I) CTC in the 1970s, research of CTCs has involved several fundamental areas, including noncovalent and metallophilic interaction, excimer/exciplex, acid-base chemistry, metalloaromaticity, supramolecular assemblies, and host/guest chemistry. These allow CTCs to be embraced in a wide range of innovative potential applications that include chemical sensing, semiconducting, gas and liquid adsorption/separation, catalysis, full-color display, and solid-state lighting. This review aims to provide a historic and comprehensive summary on CTCs and their extension to higher nuclearity complexes and coordination polymers from the perspectives of synthesis, structure, theoretical insight, and potential applications.

Guest effects on crystal structure and phosphorescence of a Cu6L3 prismatic cage

The crystal packing and photoluminescent properties of coordination cage can be fine-tuned via the encapsulation of aromatic guests with different substituents.

TriQuinoline

The bottom-up synthesis of structurally well-defined motifs of graphitic materials is crucial to understanding their physicochemical properties and to elicit new functions. Herein, we report the design and synthesis of TriQuinoline (TQ) as a molecular model for pyridinic-nitrogen defects in graphene sheets. TQ is a trimer of quinoline units concatenated at the 2- and 8-positions in a head-to-tail fashion, whose structure leads to unusual aromatisation behaviour at the final stage of the synthesis. The central atomic-sized void endows TQ with high proton affinity, which was confirmed empirically and computationally. TQ•H⁺ is a two-dimensional cationic molecule that displays both π-π and CH-π contact modes, culminating in the formation of the ternary complex ([12]cycloparaphenylene(CPP) ⊃ (TQ•H⁺/coronene)) that consists of TQ•H⁺, coronene (flat), and [12]cycloparaphenylene ([12]CPP) (ring). The water-miscibility of TQ•H⁺ allows it to serve as an efficient DNA intercalator for e.g. the inhibition of topoisomerase I activity.

The π-acidity/basicity of cyclic trinuclear units (CTUs): from a theoretical perspective to potential applications

Cyclic trinuclear units (CTUs) based on Au(i), Ag(i) and Cu(i) cations, featuring near planar nine-membered coordination rings, represent an important class of metal-organic π-acids/bases with highly adjustable π-acidity/basicity. Their superior π-acidity/basicity coupled with Lewis-acidic and metalmetal bonding sites offers excellent attraction for a wide range of acidic/basic species, and usually followed by noticeable changes of luminescence or charge transfer behaviors. A series of representative cases from the past two decades have been selected herein for such cyclic trinuclear units in both oligomeric and polymeric systems. Their fascinating and profound potential applications related to π-acidity/basicity are highlighted, including molecular absorption and separation, luminescence sensing and detection, organic light-emitting diodes (OLEDs), metal-organic field-effect transistors (MOFETs), molecular wires, and catalysis. The challenges in improving the performance for practical application will also be discussed.

Raising the Bar in Aromatic Donor-Acceptor Interactions with Cyclic Trinuclear Gold(I) Complexes as Strong π-Donors

Aromatic donor-acceptor interactions are of high importance in supramolecular chemistry, materials science and biology. Compared to other noncovalent interactions, such as hydrogen bonding, the binding is often weak. Here we show that strong donor-acceptor interactions between planar aromatics with binding free energies down to -10.1 kcal mol^-1 and association constants of up to 2.34 × 10⁷ L mol^-1 for 1:1 complexes can be realized using cyclic trinuclear complexes of gold(I) with pyridinate, imidazolate, or carbeniate ligands. Data were obtained through NMR and UV/vis absorption spectroscopic studies and supported by quantum chemical calculations for a variety of acceptors. By using a specifically designed bridged naphthalene diimide-based acceptor with only one binding site, we furthermore show that a 1:2 (donor:acceptor) binding model is best suited to quantify the donor and acceptor/complex equilibrium. Scanning electron microscopy on selected donor-acceptor pairs shows crystalline supramolecular assemblies. We anticipate this study to be relevant for the future design of supramolecular systems and chemical sensors and the determination of binding energies between planar donors and acceptors.