Homo- and Heteroligand Poly-NHC Metal Assemblies: Synthesis by Narcissistic and Social Self-Sorting

Post‐Synthetic Modification of a Porous Hydrocarbon Cage to Give a Discrete Co 24 Organometallic Complex**

A new alkyne‐based hydrocarbon cage was synthesized in high overall yield using alkyne‐alkyne coupling in the cage forming step. The cage is porous and displays a moderately high BET surface area (546 m² g⁻¹). The cage loses crystallinity on activation and thus is porous in its amorphous form, while very similar cages have been either non‐porous, or retained crystallinity on activation. Reaction of the cage with Co₂(CO)₈ results in exhaustive metalation of its 12 alkyne groups to give the Co₂₄(CO)₇₂ adduct of the cage in good yield.

Development of Transition-Metal-Free Lewis Acid-Initiated Double Arylation of Aldehyde: A Facile Approach Towards the Total Synthesis of Anti-Breast-Cancer Agent

This work describes a mild and robust double hydroarylation strategy for the synthesis of symmetrical /unsymmetrical diaryl- and triarylmethanes in excellent yields using Lambert salt (0.2-1.0 mol%). Despite the anticipated challenges associated with controlling selective product formation, unsymmetrical diaryl- and triarylmethanes products are obtained unprecedentedly. A highly efficient gram scale reaction has also been reported (TON for symmetrical product=475 and for unsymmetrical product=390). The synthetic utility of the methodology is demonstrated by the preparation of several unexplored diaryl- and triarylmethane-based biologically relevant molecules, such as arundine, vibrindole A, turbomycin B, and certain anti-inflammatory agents. A total synthesis of an anti-breast-cancer agent is also demonstrated. Control experiments, Hammett analysis, HRMS and GC-MS studies reveal the reaction intermediates and reaction mechanism.

Self-Assembly of Octanuclear PtII/PdII Coordination Barrels and Uncommon Structural Isomerization of a Photochromic Guest in Molecular Space

Two tetragonal molecular barrels TB1 and TB2 were successfully synthesized by coordination-driven self-assembly of a tetrapyridyl donor (L) of the thiazolo[5,4-d]thiazole backbone with cis-blocked 90° Pd(II) and Pt(II) acceptors, respectively. The single-crystal structure analysis of TB1 revealed the formation of a two-face opened tetragonal Pd8 molecular barrel architecture. In contrast, the isostructural Pt(II) barrel (TB2) is water-soluble. The large confined hydrophobic molecular cavity including wide open windows and good water solubility of the barrel TB2 made it a potential molecular container for the encapsulation of guests with different sizes and properties. This has been exploited to encapsulate and stabilize the open form of a photochromic molecule (G2) in water, while the same photochromic molecule exists exclusively in a cyclic zwitterionic form in aqueous medium in the absence of the barrel TB2. This cyclic form is very stable in water and does not go back to its parent open form under common external stimuli. Surprisingly, reverse switching of the cyclic form to a colored hydrophobic open form was also possible instantly in water upon addition of the solid barrel TB2 into an aqueous solution of G2. Such a fast reverse isomerization of an irreversible process in aqueous medium by utilizing host-guest interaction of the barrel TB2 and the guest G2 is interesting. The barrel TB2 was also capable of encapsulating the water-insoluble radical initiator G1 in aqueous medium.

A Highly Ordered Quantum Dot Supramolecular Assembly Exhibiting Photoinduced Emission Enhancement

Multicomponent supramolecular assembly systems enable the generation of materials with outstanding properties, not obtained from single-component systems, via a synergetic effect. Herein, we demonstrate a novel supramolecular coassembly system rendering highly ordered quantum dot (QD) arrangement structures formed via the self-assembly of azobenzene derivatives, where the photocontrollable photoluminescence (PL) properties of the QDs are realized based on photoisomerization. Upon mixing the assembled azobenzene derivatives and QDs in apolar media, a time-evolution coaggregation into hierarchical nanosheets with a highly ordered QD arrangement structure occurs. Upon photoirradiation, the nanosheets transform into ill-defined aggregates without arranged QDs together with enhancing the PL intensity. In days, the photoirradiated coaggregates undergo recovery of the PL properties corresponding to the arranged QDs through thermal isomerization.

Coordination-Induced Emission from Tetraphenylethylene Units and Their Applications

Thanks to the potential of aggregation-induced emission (AIE) phenomena, improved stabilities, and the good selectivity and sensitivity of the chemical responses exhibited by the products, coordination-driven self-assembly with tetraphenylethylene (TPE) units has recently received much attention and has been widely investigated for application in chemical sensors, cell imaging agents, light-harvesting systems, and others. Several reviews have emerged on the topics of AIE chemistry and aggregation-induced emission luminogen (AIEgen)-based supramolecular assembles, however, there is still a distinct lack of full overviews of emission enhancement from the viewpoint of metal-coordination effects. Thus, this minireview offers recent advances that have been made in the design and application of TPE-based metallacycles, metallacages, metal-organic frameworks (MOFs) and coordination polymers (CPs).

Cavitand-Based Pd-Pyridyl Coordination Capsules: Guest-Induced Homo- or Heterocapsule Selection and Applications of Homocapsules to the Protection of a Photosensitive Guest and Chiral Capsule Formation

A 2 : 4 mixture of tetrakis[4-(4-pyridyl)phenyl]cavitand (1) or tetrakis[4-(4-pyridyl)phenylethynyl]cavitand (2) and Pd(dppp)(OTf)₂ self-assembles into a homocapsule {1₂  ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C1) or {2₂  ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C2), respectively, through Pd-Npy coordination bonds. A 1 : 1 : 4 mixture of 1, 2, and Pd(dppp)(OTf)₂ produced a mixture of homocapsules C1, C2, and a heterocapsule {1 ⋅ 2 ⋅ [Pd(dppp)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C3) in a 1 : 1 : 0.98 mole ratio. Selective formation (self-sorting) of homocapsules C1 and C2 or heterocapsule C3 was controlled by guest-induced encapsulation under thermodynamic control. Applications of Pd-Npy coordination capsules with the use of 1 were demonstrated. Capsule C1 serves as a guard nanocontainer for trans-4,4'-diacetoxyazobenzene to protect against the trans-to-cis photoisomerization by encapsulation. A chiral capsule {1₂  ⋅ [Pd((R)-BINAP)]₄ }⁸⁺  ⋅ (TfO^- )₈ (C5) was also constructed. Capsule C5 induces supramolecular chirality with respect to prochiral 2,2'-bis(alkoxycarbonyl)-4,4'-bis(1-propynyl)biphenyls by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.

Strategy for the Construction of Diverse Poly-NHC-Derived Assemblies and Their Photoinduced Transformations

A series of supramolecular assemblies of types [Ag₈ (L)₄ ](PF₆ )₈ and [Ag₄ (L)₂ ](PF₆ )₄ , obtained from the tetraphenylethylene (TPE) bridged tetrakis(1,2,4-triazolium) salts H₄ -L(PF₆ )₄ and Ag^I ions, is described. The assembly type obtained dependends on the N-wingtip substituents of H₄ -L(PF₆ )₄ . Changes in the lengths of the N4-wingtip substituents enables controlled formation of assemblies with either [Ag₄ (L)₂ ](PF₆ )₄ or [Ag₈ (L)₄ ](PF₆ )₈ stoichiometry. The molecular structures of selected [Ag₈ (L)₄ ](PF₆ )₈ and [Ag₄ (L)₂ ](PF₆ )₄ assemblies were determined by X-ray diffraction analyses. While H₄ -L(PF₆ )₄ does not exhibit fluorescence in solution, their tetra-NHC (NHC=N-heterocyclic carbene) assemblies do upon NHC-metal coordination. Upon irradiation, all assemblies undergo a light-induced, supramolecule-to-supramolecule structural transformation by an oxidative photocyclization involving phenyl groups of the TPE core, resulting in a significant change of the luminescence properties.

Synthesis, Characterization, and Properties of Tetraphenylethylene-Based Tetrakis-NHC Ligands and Their Metal Complexes

The development of highly emissive dinuclear Ag^I or Au^I complexes [M₂ L](PF₆ )₂ (L=2 a, 2 b; M=Ag, Au) derived from tetraphenylethylene (TPE)-based tetrabenzimidazolin-2-ylidene ligands is reported. The new complexes exhibit a remarkable fluorescence enhancement compared to their parent benzimidazolium salts. The quantum yield (Φ_F ) value for salt H₄ -2 a(PF₆ )₄ in dilute solution (c=10^-5  m) was found to be less than 1 %, whereas its metal complexes show Φ_F values up to 55 % at similar solution concentration. This observation can be attributed to the rigidification of the TPE skeleton upon metalation resulting in a restriction of the intramolecular rotation of the phenyl groups. Functionalization of the complexes [M₂ 2 b](PF₆ )₂ (M=Ag, Au) with terminal coumarin groups and subsequent photoirradiation yielded the complexes [M₂ 3 b](PF₆ )₂ (M=Ag, Au) bearing a new type of ligand with an unaffected TPE moiety.