Luminescent transition-metal-containing cyclophanes (“molecular squares”): covalent self-assembly, host-guest studies and preliminary nanoporous materials applications

Coordinating nature of M6L12 double-stranded macrocycles: co-ligand competition of perchlorate, water, and acetonitrile depending on metal(II) ions

Self-assembly of M(ClO4)2 (M(II) = Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)) with dicyclopentyldi(pyridine-3-yl)silane (L) as a donor in a mixture of acetonitrile and toluene produces crystals consisting of M6L12 double-stranded macrocycles. The geometry around the M(II) cations is a typical octahedral arrangement, but the metallamacrocycles' outer axial coordination environment is sensitive to the M(II) cations. The conformation of the unique metallamacrocycles is informatively dependent on the nature of the coordination around the M(II) cations via subtle co-ligand competition among perchlorate anions, water, and acetonitrile. Both the coordinated acetonitriles and the solvate molecules of the crystals are removed at 170 °C, thereby transforming the crystals into new crystals that return to their original form in the mixture of toluene and acetonitrile. Catalytic oxidation of 3,5-di-tert-butylcatechol using [Cu6(ClO4)8(CH3CN)4L12]4ClO4·5C7H8 is much faster than those using the transformed product, [Cu(ClO4)2L2], and a simple mixture of Cu(ClO4)2 + L.

3D Cyclophane for the Selective Conversion of Epoxide to Cyclic Carbonate

A novel three-dimensional (3D) cyclophane molecule 1 was synthesized and fully characterized. Cyclophane 1, which can form a N heterocyclic carbene, was tested for conversion of certain epoxides (3-6) [scheme 2] to cyclic carbonates in the presence of CO2. Propylene oxide (3) was found to have more reactivity with cyclophane 1 compared to the other epoxides. The theoretical calculations based on N,N'-disubstituted imidazol(in)ium-2-carboxylates derived from N,N' disubstituted imidazole as the source of N-heterocyclic carbene show lower activation energy in the case of the reactivity of epoxides 5 and 6 as compared to 3 and 4. However, cyclophane 1, which possesses a 3D geometry, can form the open intermediate with CO2 and propylene oxide more feasibly than the other three epoxides, which have larger sizes as compared to propylene oxide. Hence, the reaction of propylene oxide, CO2, and cyclophane 1 can follow the mechanistic path 1, whereas the epoxides 4-6 can follow a different mechanistic path 2. Cyclophane 1 is the first example of a cyclophane to act as an organocatalyst for the conversion of CO2 and epoxide to cyclic carbonate via the N heterocyclic carbene pathway.

Self-Assembly of Rhenium(I) Double-Stranded Helicate and Mesocate from Flexible Ditopic Benzimidazolyl/Naphthanoimidazolyl N-Donor and Rigid Bis-Chelating Hydroxyphenylbenzimidazolyl N∩OH-Donor Ligands: Synthesis, Characterization, and Photophysical and B-DNA Docking Studies

The self-assembly of three rheniumtricarbonyl core-based supramolecular coordination complexes (SCCs), fac-[Re(CO)3(μ-L)(μ-L')Re(CO)3] (1-3) was carried out using Re2(CO)10, rigid bis-chelating ligand (HO∩N-Ph-N∩OH (L1) (where HO∩N = 2-hydroxyphenylbenzimidazolyl), and flexible ditopic N-donor ligands (L2 = bis(3-((1H-benzoimidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane, L3 = bis(3-((1H-naphtho[2,3-d]imidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane, L4 = bis(4-(naphtho[2,3-d]imidazol-1-yl-methyl)phenyl)methane) via a one-pot solvothermal approach. In the solid state, the dinuclear SCCs adopt heteroleptic double-stranded helicate and meso-helicate architectures. The supramolecular structures of the complexes are retained in the solution based on the 1H NMR and electrospray ionization (ESI)-mass analysis. The spectral and photophysical properties of the complexes were studied both experimentally and using time-dependent density functional theory (TDDFT) calculations. All of the supramolecules exhibited emission in both solution and solid states. Theoretical studies were conducted to determine the chemical reactivity parameters, molecular electrostatic potential surface plots, natural population, and Hirshfeld analysis for complexes 1-3. Additionally, molecular docking studies were carried out for complexes 1-3 with B-DNA.

[M8L4]8+-Type Squares Self-Assembled by Dipalladium Corners and Bridging Aromatic Dipyrazole Ligands for Iodine Capture

Square-like metallamacrocyclic palladium(II) complexes [M₈L₄]⁸⁺ (1-7) were synthesized by reacting aromatic dipyrazole ligands (H₂L¹-H₂L³ with pyromellitic arylimide-, 1,4,5,8-naphthalenetetracarboxylic arylimide-, and anthracene-based aromatic groups, respectively) with dipalladium corners ([(bpy)₂Pd₂(NO₃)₂](NO₃)₂, [(dmbpy)₂Pd₂(NO₃)₂](NO₃)₂, or [(phen)₂Pd₂(NO₃)₂](NO₃)₂, where bpy = 2,2'-bipyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine, and phen = 1,10-phenanthroline) in aqueous solutions via metal-directed self-assembly. Metallamacrocycles 1-7 were fully characterized by ¹H and ¹³C nuclear magnetic resonance spectroscopy and electrospray ionization mass spectrometry, and the square structure of 7·8NO₃^- was further confirmed via single crystal X-ray diffraction. These square-like metallamacrocycles exhibit effective performance for iodine adsorption.

Thermally Activated Delayed Photoluminescence: Deterministic Control of Excited-State Decay

Thermally activated photophysical processes are ubiquitous in numerous organic and metal-organic molecules, leading to chromophores with excited-state properties that can be considered an equilibrium mixture of the available low-lying states. Relative populations of the equilibrated states are governed by temperature. Such molecules have been devised as high quantum yield emitters in modern organic light-emitting diode technology and for deterministic excited-state lifetime control to enhance chemical reactivity in solar energy conversion and photocatalytic schemes. The recent discovery of thermally activated photophysics at CdSe nanocrystal-molecule interfaces enables a new paradigm wherein molecule-quantum dot constructs are used to systematically generate material with predetermined photophysical response and excited-state properties. Semiconductor nanomaterials feature size-tunable energy level engineering, which considerably expands the purview of thermally activated photophysics beyond what is possible using only molecules. This Perspective is intended to provide a nonexhaustive overview of the advances that led to the integration of semiconductor quantum dots in thermally activated delayed photoluminescence (TADPL) schemes and to identify important challenges moving into the future. The initial establishment of excited-state lifetime extension utilizing triplet-triplet excited-state equilibria is detailed. Next, advances involving the rational design of molecules composed of both metal-containing and organic-based chromophores that produce the desired TADPL are described. Finally, the recent introduction of semiconductor nanomaterials into hybrid TADPL constructs is discussed, paving the way toward the realization of fine-tuned deterministic control of excited-state decay. It is envisioned that libraries of synthetically facile composites will be broadly deployed as photosensitizers and light emitters for numerous synthetic and optoelectronic applications in the near future.

Insights into the Gas Adsorption Mechanisms in Metal-Organic Frameworks from Classical Molecular Simulations

Classical molecular simulations can provide significant insights into the gas adsorption mechanisms and binding sites in various metal-organic frameworks (MOFs). These simulations involve assessing the interactions between the MOF and an adsorbate molecule by calculating the potential energy of the MOF-adsorbate system using a functional form that generally includes nonbonded interaction terms, such as the repulsion/dispersion and permanent electrostatic energies. Grand canonical Monte Carlo (GCMC) is the most widely used classical method that is carried out to simulate gas adsorption and separation in MOFs and identify the favorable adsorbate binding sites. In this review, we provide an overview of the GCMC methods that are normally utilized to perform these simulations. We also describe how a typical force field is developed for the MOF, which is required to compute the classical potential energy of the system. Furthermore, we highlight some of the common analysis techniques that have been used to determine the locations of the preferential binding sites in these materials. We also review some of the early classical molecular simulation studies that have contributed to our working understanding of the gas adsorption mechanisms in MOFs. Finally, we show that the implementation of classical polarization for simulations in MOFs can be necessary for the accurate modeling of an adsorbate in these materials, particularly those that contain open-metal sites. In general, molecular simulations can provide a great complement to experimental studies by helping to rationalize the favorable MOF-adsorbate interactions and the mechanism of gas adsorption.

Main-group metal cyclophane complexes with high coordination numbers

Density functional theory calculations using the PBE0-D3BJ hybrid functional have been employed to investigate the complexation of main-group metal-cations with [2.2.2]paracyclophane and deltaphane. Geometry optimization under symmetry constraints was performed to observe the mode of coordination that a metal-cation adopts when it resides inside the cyclophane cavity. Thermodynamic properties were investigated to note the trends of stability along a group of metals. To further investigate the bonding properties, Morokuma–Ziegler energy decomposition analysis, natural bond orbital analysis and Bader's analysis were employed. It was observed that most of the main-group metal complexes with cyclophanes prefer an η⁶η⁶η⁶ coordination mode where the metal-cation sits in the centre of the cyclophane cavity. There is an increased thermodynamic stability in [2.2.2]paracyclophane complexes compared to their deltaphane analogues while the reverse is true regarding the strength of coordination based on interaction energy.

Investigation of the complexation of main-group metal-cations with [2.2.2]paracyclophane and deltaphane.

Excited-State Triplet Equilibria in a Series of Re(I)-Naphthalimide Bichromophores

We present the synthesis, structural characterization, electronic structure calculations, and the ultrafast and supra-nanosecond photophysical properties of a series of five bichromophores of the general structural formula [Re(5-R-phen)(CO)₃(dmap)](PF₆), where R is a naphthalimide (NI), phen = 1,10-phenanthroline, and dmap is 4-dimethylaminopyridine. The NI chromophores were systematically modified at their 4-positions with -H (NI), -Br (BrNI), phenoxy (PONI), thiobenzene (PSNI), and piperidine (PNI), rendering a series of metal-organic bichromophores (Re1-Re5, respectively) featuring variability in the singlet and triplet energies in the pendant NI subunit. Five closely related organic chromophores as well as [Re(phen)(CO)₃(dmap)](PF₆) (Re6) were investigated in parallel to appropriately model the photophysical properties exhibited in the bichromophores. The excited state processes of all molecules in this study were elucidated using a combination of transient absorption spectroscopy and time-resolved photoluminescence (PL) spectroscopy, revealing the kinetics of the energy transfer processes occurring between the appended chromophores. The spectroscopic analysis was further supported by electronic structure calculations which identified the origin of many of the experimentally observed electronic transitions.

fac-Re(CO)3-based neutral heteroleptic tetrahedrons

Four new flexible ditopic nitrogen donors possessing a xylene spacer and 2-phenylbenzimidazolyl or its derivatives as a coordinating unit and one rigid bis-chelating ligand consisting of two 2-hydroxyphenylbenzimidazolyl motifs and a central phenylene spacer were synthesized and further used with Re₂(CO)₁₀ for making a new family of neutral, heteroleptic tetrahedral-shaped supramolecular coordination complexes via a one-pot approach. The new ligands and the complexes were characterized using various analytical and spectroscopic methods. The molecular structures of the complexes were determined using single crystal X-ray diffraction analysis, which reveal that four rhenium cores are arranged in the vertices, and four ligands are at the edges of the tetrahedron.

Suite of Organoplatinum(II) Triangular Metallaprism: Aggregation-Induced Emission and Coordination Sequence Induced Emission Tuning

A series of triangular metallaprisms with a kinetically inert Pt-N bond have been synthesized from the stepwise assembly of a Pt-corner, linear linker 4,4'-bipy (4,4'-bipy = 4,4'-bipyridine) and triangular ligand [tpb or tpt, tpb = tris(4-pyridyl)benzene, tpt = tris(4-pyridyl)triazine]. The use of an unsymmetrical [Pt(HL)]-corner (H₂L = 2,6-diphenylpyridine) leads to novel isostructural products. Phenyl rotation at the metal-corners endows these complexes with good aggregation-induced emission (AIE) function, with varied activities across the isostructural complexes. The coordination sequence of electron-deficient ligand tpt also imparts significant influence on the complex emission. These organoplatinum triangular metallaprisms thus provide a good model to study the influence of building blocks and coordination sequence on the luminescence of supramolecules.

Bifunctional photo- and vapochromic behaviors of a novel porous zwitterionic metal–organic framework

A novel porous zwitterionic metal–organic framework shows photo- and vapochromic behaviors with high sensitivity for sensing NH₃, ethylamine, and n-propylamine.

Crystal structure of fac-tri-carbonyl-chlorido-bis-(4-hy-droxy-pyridine)-rhenium(I)-pyridin-4(1H)-one (1/1)

The asymmetric unit of the title compound, [ReCl(C5H5NO)2(CO)3]·C5H5NO, contains one mol-ecule of the complex fac-[ReCl(4-pyOH)2(CO)3] (where 4-pyOH represents 4-hy-droxy-pyridine) and one mol-ecule of pyridin-4(1H)-one (4-HpyO). In the mol-ecule of the complex, the Re atom is coordinated to two N atoms of the two 4-pyOH ligands, three carbonyl C atoms, in a facial configuration, and the Cl atom. The resulting geometry is slightly distorted octa-hedral. In the crystal structure, both fragments are associated by hydrogen bonds; two 4-HpyO mol-ecules bridge between two mol-ecules of the complex using the O=C group as acceptor for two different HO- groups of coordinated 4-pyOH from two neighbouring metal complexes. The resulting square arrangements are extented into infinite chains by hydrogen bonds involving the N-H groups of the 4-HpyO mol-ecule and the chloride ligands. The chains are further stabilized by π-stacking inter-actions.

Photofunctional multinuclear rhenium(i) diimine carbonyl complexes

In this review, we summarize the synthesis, photophysical properties and applications of a wide variety of multinuclear complexes consisting of Re(i)-diimine-carbonyl units.

Rhenium-Based Molecular Trap as an Evanescent Wave Infrared Chemical Sensing Medium for the Selective Determination of Amines in Air

An evanescent wave infrared chemical sensor was developed to selectively detect volatile amines with heterocyclic or phenyl ring. To achieve this goal, a rhenium-based metallacycle with a "molecular-trap" structure was designed and synthesized as host molecules to selectively trap amines with heterocyclic or phenyl ring through Re-amine and π-π interactions. To explore the trapping properties of the material, a synthesized Re-based molecular trap was treated on an IR sensing element, and wide varieties of volatile organic compounds (VOCs) were examined to establish the selectivity for detection of amines. Based on the observed IR intensities, the Re-based molecular trap favors interaction with amines as evidenced by the variation of absorption bands of the Re molecular trap. With extra π-π interaction force, molecules, such as pyridine and benzylamine, could be detected. After optimization of the parameters for IR sensing, a rapid response in the detection of pyridine was observed, and the linear ranges were generally up to 10 mg/L with a detection limit around 5.7 μg/L. In the presence of other VOCs, the recoveries in detection of pyridine were all close to 100%.

Redox-Responsive Viologen-Mediated Self-Assembly of CB[7]-Modified Patchy Particles

Sulfonated surface patches of poly(styrene)-based colloidal particles (CPs) were functionalized with cucurbit[7]uril (CB[7]). The macrocycles served as recognition units for diphenyl viologen (DPV(2+)), a rigid bridging ligand. The addition of DPV(2+) to aqueous suspensions of the particles triggered the self-assembly of short linear and branched chainlike structures. The self-assembly mechanism is based on hydrophobic/ion-charge interactions that are established between DPV(2+) and surface-adsorbed CB[7]. DPV(2+) guides the self-assembly of the CPs by forming a ternary DPV(2+)⊂(CB[7])2 complex in which the two CB[7] macrocycles are attached to two different particles. Viologen-driven particle assembly was found to be both directional and reversible. Whereas sodium chloride triggers irreversible particle disassembly, the one-electron reduction of DPV(2+) with sodium dithionite causes disassembly that can be reversed via air oxidation. Thus, this bottom-up synthetic supramolecular approach allowed for the reversible formation and directional alignment of a 2D colloidal material.

Comparison of rhenium-porphyrin dyads for CO2 photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy

We report a study of the photocatalytic reduction of CO₂ to CO by zinc porphyrins covalently linked to [Re^I(2,2'-bipyridine)(CO)₃L]^+/0 moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C₆H₄NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C₆H₄NHC(O)C₆H₃(OMe)NHC(O), while Dyad 3 has a saturated bridge C₆H₄NHC(O)CH₂; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than [Dyad 1 pic]PF₆ or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re-Br units is very similar to that of the dyads with [Re-pic]⁺ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO₂ into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH₂Cl₂) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of ³MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.

Pore-controlled formation of 0D metal complexes in anionic 3D metal–organic frameworks

The host–guest chemistry between a series of anionic MOFs and their trapped counterions was investigated by single crystal XRD.

Self-assembly of oxamidato bridged ester functionalised dirhenium metallastirrups: synthesis, characterisation and cytotoxicity studies

Hetero-topic self-assembly of Re₂(CO)₁₀ with oxamide ligands and ester-functionalised flexible ditopic-tectons afforded dinuclear metallacycles resembling a stirrup. The metallastirrups showed promising cytotoxic activity against few cancer cell lines in vitro.

From chromium-chromium quintuple bonds to molecular squares and porous coordination polymers

Reaction of the quintuply bonded chromium(I) dimer [ApCrCrAp] (Ap = sterically demanding 2-aminopyridinate) with pyrazine yields a chromium(II) complex with a η(4):η(4) face-on coordinated pyrazine dianion. Reaction with 4,4'-bipyridine, on the other hand, completely cleaves the metal-metal bond, leading to a chromium(II)-based molecular square. XRD and magnetic measurements show ligand radical anions and a ferrimagnetic alignment of alternating metal and ligand magnetic moments. Controlled polymerization of the molecular square with pyrazine yields a porous coordination polymer featuring both reduced and nonreduced linkers.

Architectural spiroligomers designed for binuclear metal complex templating

The first structurally, spectroscopically, and electronically characterized metal-spiroligomer complexes are reported. The binuclear [M2L2](4+) ions (M = Mn, Zn) are macrocyclic "squares" and are characterized by X-ray diffraction, (1)H and (13)C NMR, electronic absorption, emission, and mass spectroscopies. The manganese complex contains two spin-independent Mn(II) ions and is additionally characterized using EPR and CD spectroscopies and CV.