Asymmetric Twisting of C-Centered Octahedral Gold(I) Clusters by Chiral N-Heterocyclic Carbene Ligation.

Asymmetric induction of metal clusters by ligation of chiral ligands is intriguing in terms of the mechanism of chirality transfer and the stability of the resulting chiral structure. Here we report the asymmetric induction of C-centered hexagold(I) CAuI6 clusters into an asymmetrically twisted structure through monodentate, chiral benzimidazolylidene-based N-heterocyclic carbene (NHC) ligands. X-ray diffraction analysis revealed that the NHC-ligated CAuI6 cluster was diastereoselectively twisted with directionally selective, bond length expansion, and contraction of the Au···Au contacts and that the original cluster with high symmetry was transformed into an optically pure, asymmetric CAuI6 cluster with C1 symmetry. Moreover, the circular dichroism spectroscopy and the time-dependent density functional theory calculation confirmed that the asymmetrically twisted CAuI6 structure was maintained even in solution. Such asymmetric induction of configurationally stable metal clusters would greatly expand the molecular design possibilities of asymmetric catalysts and chiroptical materials by utilizing library chiral NHC ligands.

Multi-Stimuli-Responsive Interconversion between Bowl- and Capsule-Shaped Self-Assembled Zinc(II) Complexes.

Self-assembled metal-organic architectures have great potential to undergo major structural changes into different architectures. Such molecular transformation is widely applicable to responsive systems like drug delivery and allosteric catalysis. A great number of metal-organic architectures responsive to a specific stimulus have been reported so far. However, interconversion between a pair of distinct metal-organic structures in response to multiple stimuli is rarely reported despite its high versatility. Herein we report multi-stimuli-responsive interconversion between a bowl-shaped and a capsule-shaped self-assembled ZnII complexes, [ZnII4L3X6] and [ZnII4L4], respectively, which were found to form in equilibrium from porphyrin-based ligand L and ZnII ions with different stoichiometry. Specifically, this interconversion was induced by four distinct external stimuli: exogenous ligands, Brønsted base/acid, solvents, and guest molecules. The mechanisms of the interconversion system are discussed in detail focusing on the species included in the equilibria. Thus, these findings would provide a helpful clue to design principles for multi-stimuli-responsive systems with functional versatility.

Synthesis of Hetero-multinuclear Metal Complexes by Site-Selective Redox Switching and Transmetalation on a Homo-multinuclear Complex.

Hetero-multinuclear metal complexes are a promising class of compounds applicable to photoluminescence, magnetism, and catalysis. In this work, we have developed a synthetic method for hetero-tetranuclear metal complexes by combining advantages of site-selective redox switching and transmetalation. First, a homo-tetranuclear CoII4 complex was converted to a mixed-valence CoIIICoII3 complex by site-selective oxidation, which was then transmetalated from CoII to NiII to form a heterometallic CoIIINiII3 complex. Finally, a CoIINiII3 complex was synthesized by metal-selective reduction on the CoIII site. The basic structural frameworks of the main products in the whole process starting from the CoII4 complex are isostructural. Notably, the CoIINiII3 complex was not accessible by direct mixing of ligand, CoII, and NiII. This method would provide an alternative strategy for highly selective synthesis of hetero-multinuclear metal complexes.

A Circularly Arranged Sextuple Triptycene Gear Molecule.

Herein we report the synthesis of a circularly arranged sextuple triptycene gear molecule, hexakis(10-dodecyloxy-9-triptycyl)ethynylbenzene, via the trimerization of the corresponding triyne with a cobalt catalyst. The six triptycene gears are closely engaged with each other as confirmed by single crystal X-ray structure analysis, and their motion in solution was established by NMR spectroscopy. Notably, when one bulky RuCp* complex was attached to one triptycene gear, the whole movement of the six gears was highly restricted via their mechanical engagement. Development of such a multigear molecule would provide a structural basis for molecular motion transmission systems with a switching function.

Rotational Control of a Dirhodium-Centered Supramolecular Four-Gear System by Ligand Exchange.

Self-assembled molecular machines have great potential to enable noncovalent regulation of a coupled motion of the building blocks. Herein we report the synthesis and the rotational control of a lantern-type dirhodium complex with circularly arranged four 2,3,6,7,14,15-hexamethyltriptycene carboxylates as gears and two axial ligands as the rate control elements. The rotating rates in solution were markedly affected by the coordination ability and the bulkiness of axial ligands. Notably, the rate changes were closely correlated with the changes in the electronic states of the dirhodium center. Such ligand exchange-based control of rotational motions with color changes would advance stimulus-responsive metallo-molecular multirotors.

Single-gold etching at the hypercarbon atom of C-centred hexagold(I) clusters protected by chiral N-heterocyclic carbenes.

Chemical etching of nano-sized metal clusters at the atomic level has a high potential for creating metal number-specific structures and functions that are difficult to achieve with bottom-up synthesis methods. In particular, precisely etching metal atoms one by one from nonmetallic element-centred metal clusters and elucidating the relationship between their well-defined structures, and chemical and physical properties will facilitate future materials design for metal clusters. Here we report the single-gold etching at a hypercarbon centre in gold(I) clusters. Specifically, C-centred hexagold(I) clusters protected by chiral N-heterocyclic carbenes are etched with bisphosphine to yield C-centred pentagold(I) (CAuI5) clusters. The CAuI5 clusters exhibit an unusually large bathochromic shift in luminescence, which is reproduced theoretically. The etching mechanism is experimentally and theoretically suggested to be a tandem dissociation-association-elimination pathway. Furthermore, the vacant site of the central carbon of the CAuI5 cluster can accommodate AuCl, allowing for post-functionalisation of the C-centred gold(I) clusters.

A C60-templated tetrameric porphyrin barrel complex via zinc-mediated self-assembly utilizing labile capping ligands.

Coordination-driven self-assembly utilizing labile capping ligands has been exploited as a novel strategy for metallo-cage containers. Herein, we report a tetrameric porphyrin barrel complex [C60⊂Zn814(H2O)4(OTs)12](OTs)4 (2) (OTs = p-CH3C6H4SO3) formed from a tetrakis(bipyridyl)porphyrin ligand 1, Zn(OTs)2, and a template guest, C60 fullerene. The tetrameric-barrel 2 contains two kinds of bis(bpy) Zn(II) centers coordinated by TsO(-) anions which serve as labile capping ligands in the formation of the finite structure of 2.

A self-assembled multiporphyrin cage complex through three different zinc(II) center formation under well-balanced aqueous conditions.

Construction of a self-assembled cage complex through three different Zn(II) centers is achieved using a Zn porphyrin ligand with four 2,2'-bipyridin-5-yl (bpy) groups. The multiporphyrin cage encapsulates guest molecules unsymmetrically by π-π interactions. Well-balanced aqueous conditions, which allow the formation of both tris(bpy) and hydrated bis(bpy) Zn(II) units, result in the unsymmetrical yet well-defined supramolecular structure.

Temperature-Dependent Spontaneous Resolution of a Tetrahedral Chiral-at-Nickel(II) Complex under Supramolecular Control.

Although stereochemical control of carbon centers is a well-established technique in modern synthetic chemistry, that of tetrahedral metal complexes with a stereogenic metal center remains difficult due to the dynamic nature of their coordination bonds. Here we report the synthesis of a tetrahedral d8 high-spin chiral-at-nickel(II) complex composed exclusively of achiral ligands and the supramolecular control of its temperature-dependent spontaneous resolution in crystals. Under certain conditions, complex molecules with the same absolute configuration of the nickel(II) center grow into conglomerate crystals with a helically arranged structure due to intermolecular hydrogen bonding. This process is highly spontaneous, does not require any chiral sources, and is closely related to the origin of homochirality in biological systems. The obtained enantiopure nickel(II) complex will be a new type of redox-active chiral source for asymmetric synthetic chemistry.

Synthesis and molecular structural studies of racemic chiral-at-vanadium(V) complexes using an unsymmetric achiral phenolic bidentate ligand.

Mononuclear oxovanadium(V) complexes [V(O)XL (1: X = Ot-Bu, 2: X = Cl)] [H2L: 2,2'-methylene bis(4,6-di-tert-butylphenol)(4'-tert-butyl-6'-(1-adamantyl)phenol)] directed towards asymmetric catalysis have been synthesised as racemic compounds using an unsymmetric and achiral phenolic bidentate ligand (H2L), and NMR and UV-vis absorption spectroscopies, single-crystal X-ray diffraction, and IR spectroscopy revealed their racemic chiral-at-vanadium structures in solution and in the crystal. In addition, theoretical calculations revealed that the HOMO-LUMO energy gap is smaller for unsymmetric ligands, which promotes d-orbital splitting of the metal centre.

Photoluminescence control by atomically precise surface metallization of C-centered hexagold(i) clusters using N-heterocyclic carbenes.

The properties of metal clusters are highly dependent on their molecular surface structure. The aim of this study is to precisely metallize and rationally control the photoluminescence properties of a carbon(C)-centered hexagold(i) cluster (CAuI6) using N-heterocyclic carbene (NHC) ligands with one pyridyl, or one or two picolyl pendants and a specific number of silver(i) ions at the cluster surface. The results suggest that the photoluminescence of the clusters depends highly on both the rigidity and coverage of the surface structure. In other words, the loss of structural rigidity significantly reduces the quantum yield (QY). The QY in CH2Cl2 is 0.04 for [(C)(AuI-BIPc)6AgI3(CH3CN)3](BF4)5 (BIPc = N-isopropyl-N'-2-picolylbenzimidazolylidene), a significant decrease from 0.86 for [(C)(AuI-BIPy)6AgI2](BF4)4 (BIPy = N-isopropyl-N'-2-pyridylbenzimidazolylidene). This is due to the lower structural rigidity of the ligand BIPc because it contains a methylene linker. Increasing the number of capping AgI ions, i.e., the coverage of the surface structure, increases the phosphorescence efficiency. The QY for [(C)(AuI-BIPc2)6AgI4(CH3CN)2](BF4)6 (BIPc2 = N,N'-di(2-pyridyl)benzimidazolylidene) recovers to 0.40, 10-times that of the cluster with BIPc. Further theoretical calculations confirm the roles of AgI and NHC in the electronic structures. This study reveals the atomic-level surface structure-property relationships of heterometallic clusters.

N-Heterocyclic carbene-based C-centered Au(I)-Ag(I) clusters with intense phosphorescence and organelle-selective translocation in cells.

Photoluminescent gold clusters are functionally variable chemical modules by ligand design. Chemical modification of protective ligands and introduction of different metals into the gold clusters lead to discover unique chemical and physical properties based on their significantly perturbed electronic structures. Here we report the synthesis of carbon-centered Au(I)-Ag(I) clusters with high phosphorescence quantum yields using N-heterocyclic carbene ligands. Specifically, a heterometallic cluster [(C)(AuI-L)6AgI2]4+, where L denotes benzimidazolylidene-based carbene ligands featuring N-pyridyl substituents, shows a significantly high phosphorescence quantum yield (Φ = 0.88). Theoretical calculations suggest that the carbene ligands accelerate the radiative decay by affecting the spin-orbit coupling, and the benzimidazolylidene ligands further suppress the non-radiative pathway. Furthermore, these clusters with carbene ligands are taken up into cells, emit phosphorescence and translocate to a particular organelle. Such well-defined, highly phosphorescent C-centered Au(I)-Ag(I) clusters will enable ligand-specific, organelle-selective phosphorescence imaging and dynamic analysis of molecular distribution and translocation pathways in cells.

Asymmetric construction of tetrahedral chiral zinc with high configurational stability and catalytic activity.

Chiral metal complexes show promise as asymmetric catalysts and optical materials. Chiral-at-metal complexes composed of achiral ligands have expanded the versatility and applicability of chiral metal complexes, especially for octahedral and half-sandwich complexes. However, Werner-type tetrahedral complexes with a stereogenic metal centre are rarely used as chiral-at-metal complexes because they are too labile to ensure the absolute configuration of the metal centre. Here we report the asymmetric construction of a tetrahedral chiral-at-zinc complex with high configurational stability, using an unsymmetric tridentate ligand. Coordination/substitution of a chiral auxiliary ligand on zinc followed by crystallisation yields an enantiopure chiral-only-at-zinc complex (> 99% ee). The enantiomer excess remains very high at 99% ee even after heating at 70 °C in benzene for one week. With this configurationally stable zinc complex of the tridentate ligand, the remaining one labile site on the zinc can be used for a highly selective asymmetric oxa-Diels-Alder reaction (98% yield, 87% ee) without substantial racemisation.

Enantioselective Henry (nitroaldol) reaction catalyzed by axially chiral guanidines.

The enantioselective activation of nitroalkanes was attempted on the basis of the complexation between chiral guanidinium and nitronate through two hydrogen bonds. The proposed enantioselective activation was applied to the diastereo- and enantioselective Henry (nitroaldol) reaction of nitroalkanes with aldehydes using axially chiral guanidine bases as the catalyst. Optically active nitroaldol products were obtained in acceptable yields with fairly good enantio- and diastereoselectivities at low temperature.

Metal-centred azaphosphatriptycene gear with a photo- and thermally driven mechanical switching function based on coordination isomerism.

Metal ions can serve as a centre of molecular motions due to their coordination geometry, reversible bonding nature and external stimuli responsiveness. Such essential features of metal ions have been utilized for metal-mediated molecular machines with the ability to motion switch via metallation/demetallation or coordination number variation at the metal centre; however, motion switching based on the change in coordination geometry remain largely unexplored. Herein, we report a PtII-centred molecular gear that demonstrates control of rotor engagement and disengagement based on photo- and thermally driven cis-trans isomerization at the PtII centre. This molecular rotary motion transmitter has been constructed from two coordinating azaphosphatriptycene rotators and one PtII ion as a stator. Isomerization between an engaged cis-form and a disengaged trans-form is reversibly driven by ultraviolet irradiation and heating. Such a photo- and thermally triggered motional interconversion between engaged/disengaged states on a metal ion would provide a selector switch for more complex interlocking systems.

Axially chiral guanidine as highly active and enantioselective catalyst for electrophilic amination of unsymmetrically substituted 1,3-dicarbonyl compounds.

A newly designed axially chiral guanidine is found to function as an effective platform for asymmetric induction at the alpha-carbon of unsymmetrically substituted 1,3-dicarbonyl compounds. Highly efficient and enantioselective electrophilic amination of various 1,3-dicarbonyl compounds with azodicarboxylate was successfully achieved using the present chiral guanidine catalyst, which provides efficient access to the construction of nitrogen-substituted quaternary stereocenters in an optically active form.

Axially chiral guanidine as enantioselective base catalyst for 1,4-addition reaction of 1,3-dicarbonyl compounds with conjugated nitroalkenes.

A new strategy for designing chiral guanidine molecules is presented, which features the introduction of an axially chiral binaphthyl backbone. The axially chiral guanidine catalysts thus developed facilitated the highly enantioselective 1,4-addition reaction of 1,3-dicarbonyl compounds with a broad range of conjugated nitroalkenes and showed extremely high catalytic activity.