Supramolecular Helix-to-Helix Induction: A 3D Anionic Framework Containing Double-Helical Strands Templated by Cationic Triple-Stranded Cluster Helicates

Janus-Type Mixed-Valent Copper-Cyanido Honeycomb Layers

The synthesis of Janus-type layers, which possess front and back sides that consist of different structures, remains a major challenge in the field of two-dimensional materials. In this study, two Janus-type layered coordination polymers, namely, CuII(NEtH2)(NMe2H·H2O)CuI(CN)3 (1) and CuII(NMe2H)(NMe2H·H2O)CuI(CN)3 (2), were synthesized via a simple one-pot procedure using copper(II) nitrate and sodium cyanido in mixed solutions of dimethylamine and ethylamine. Uniquely, 1 and 2 were composed of cyanido-bridged neutral layers and exhibited a CuICuII mixed-valent state. Meanwhile, using a solution of pure dimethylamine for the synthesis yielded the monovalent three-dimensional framework (NMe2H2)[CuI2(CN)3] (3). Results indicated that the simultaneous use of two mixed amines gave rise to the controlled reduction of CuII ions during the reaction. In addition, each face of the layers was coordinated by different amines on the axial positions of the CuII sites, resulting in anisotropic Janus layers. Furthermore, the thermal expansion behavior of 2 was investigated, demonstrating that the neutral [CuICuII(CN)3] layer was relatively rigid compared with the analogous anionic [CuI2(CN)3]- layer.

Brønsted Acid/Base Site Isolated in a Pentanuclear Scaffold

The concept of Brønsted-Lowry acids and bases is long and widely recognized as the most reasonable theory to explain the behavior of H⁺ ions. Here, we report a Brønsted acid/base pair that does not follow this theory. Two heteronuclear metal complexes, in which Brønsted acid/base sites are sterically isolated, were synthesized and characterized. These sterically isolated sites exhibited anomalous behavior, wherein the H⁺ species encapsulated in the Brønsted acid site did not undergo a deprotonation reaction, and the corresponding protonation reaction at the Brønsted base site failed to proceed. As a result, two states that are in a relationship of a Brønsted acid/base pair stably exist over a wide pH range without any interconversion, generating a thermodynamically metastable state. Additionally, these two states exhibited distinct electron transfer abilities and reactivities. The system presented in this study is in sharp contrast with the traditional concept of Brønsted-Lowry acids and bases.

Pentanuclear Scaffold: A Molecular Platform for Small-Molecule Conversions

Small-molecule conversions involving multielectron transfer processes enable the conversion of earth-abundant materials into valuable chemicals and are regarded as a solution for environmental and energy shortage problems. In this context, the development of artificial catalysts that promote these reactions is an important research target. In nature, metalloenzymes that contain multinuclear metal complexes as active sites are known to efficiently catalyze reactions under mild conditions. Therefore, using multinuclear metal complexes as artificial catalysts can be an attractive strategy for small-molecule conversions involving multielectron transfer processes. However, multinuclear-metal-complex-based catalysts for these reactions have not been well established. In this Account, we describe our recent advances in the development of multinuclear metal complexes as catalysts for small-molecule conversion, mainly focusing on water oxidation. As small-molecule conversions involving multielectron transfer processes consists of two essential processes, (1) the transfer of multiple electrons and (2) the formation/cleavage of covalent bond(s), catalysts for these reactions should facilitate both steps. Therefore, we assumed that the assembly of redox-active metal ions and the cooperative effect of neighboring coordinatively unsaturated metal ions can promote these processes. On the basis of this assumption, we employed a pentanuclear metal complex as a molecular scaffold for the catalyst. The scaffold has a pentanuclear structure with quasi-D₃ symmetry and consists of a [M₃(μ₃-X)] core (X = O^2- or OH^-) wrapped by two [M(μ-bpp)₃] units (Hbpp = 3,5-bis(2-pyridyl)pyrazole). The metal ions in the triangular core are coordinatively unsaturated, whereas the metal ions at the apical positions are coordinatively saturated. In other words, the pentanuclear scaffold possesses multiple redox-active centers and coordinatively unsaturated sites. It should also be noted that the electron transfer ability of the complex changes dramatically depending on the identity of the constituent metal ions. The iron derivative of the pentanuclear scaffold was found to serve as an electrocatalyst for water oxidation (2H₂O → O₂ + 4e^- + 4H⁺) with a high reaction rate and excellent robustness. The substitution of metal ions in the pentanuclear scaffold to cobalt ions resulted in the development of a catalyst for CO₂ reduction. Furthermore, we investigated the effect of substituents on the ligands of the pentanuclear iron complex and succeeded in precisely manipulating the electron transfer possess. These results clearly demonstrate that the pentanuclear scaffold is an attractive platform for catalysts for small-molecule conversions. Additionally, the intrinsic features of the multinuclear catalytic system, which are totally different from those of conventional mononuclear-metal-complex-based catalysts, are disclosed. In reactions mediated by multinuclear complexes, the multinuclear core can initially accumulate the charge required for catalysis to reach the catalytically active state. Subsequently, the catalyst in the active state reacts with the substrate, initiating electron transfer to the substrate and rearrangement of covalent bonds in the substrate to afford the product. In such a mechanism, the desired number of electrons can be transferred to the substrates in an on-demand fashion, and the formation of undesired chemical species in the targeted catalysis may be prevented. This feature of multinuclear-metal-complex-based catalysts will achieve demanding small-molecule conversions with a high reaction rate, selectivity, and durability.

Versatile coordination architectures of products generated by the in situ reaction of a doubly bis(2-pyridyl)pyrazolate bridged dinuclear copper(ii) complex with tetracyanoethylene

Reaction of doubly bpypz⁻ bridged Cu(ii) dinuclear complex and TCNE gave three versatile coordination assemblies with TCNE solvolysis derivatives, depending on reaction solvents.

An in Situ Embedded Square-Planar CuII/NiIIN4 Metalloligand in Coordination Polymers for Visible-Light Photocatalysis

Three coordination polymers (CPs) with square-planar Cu^II/Ni^IIN₄ subunits were formed in one step by subcomponent self-assembly, giving rise to an unprecedented linking variety of in situ embedded metalloligands and Cu^I clusters. All CPs exhibit unusual visible-light adsorption. Enhanced photocatalytic activity and high selectivity were observed in the oxidation of benzene under visible-light irradiation.

Tuning the Spin States of Two Apical Iron(II) Ions in a Pentanuclear Iron(II) Cluster Helicate through the Choice of Anions

Spin-crossover clusters with iron(II) high nuclearity are rare. By using 3,5-bis(pyridin-2-yl)-1,2,4-triazole (bptH) as the ligand with two bidentate chelating sites, we successfully obtained three pentanuclear iron(II) cluster helicate compounds [{FeII(μ-bpt)3}2FeII3(μ3-O)][FeII2(μ-Br)(μ-bpt)(NCS)4(H2O)]·2H2O·C2H5OH (1), [{FeII(μ-bpt)3}2FeII3(μ3-O)][FeII2(μ-bpt)2(NCS)4] (2), and [{FeII(μ-bpt)3}2FeII3(μ3-O)]I2·2C2H5OH (3). Research on single-crystal structure and magnetism has indicated that tuning the spin state of two iron(II) ions in axial direction is successfully realized by regulating the different counter anions: one apical [FeII(bpt)3]− unit exhibits spin-crossover behavior while the other [FeII(bpt)3]− unit remains in low spin state in 1, both apical ions are of high spin states in 2, and are of low spin states in 3.

Flexible and rigid dicarboxylic acids enable the assembly of achiral and chiral 3D Co(ii) metal–organic frameworks

The homochirality of the 3D Co(ii) MOFs may arise from the rotation of the two phenyl rings in the biphenyl-4,4′-dicarboxylic acid ligand.

A Chiral 3 D Net with 2 D Cairo Pentagonal Tiling Projection in Site-Modified CuCN/CuSCN Networks

A novel isohedral 3-periodic net (i.e. a net with one kind of tile), showing a Cairo pentagonal tiling projection on the Euclidean plane, has been identified in a series of site-modified CuCN/CuSCN networks, namely, [Cu₆ (SCN)₂ (CN)₆ ⋅Ni(tpy)₂ ]_n (1), [Cu₆ (SCN)(CN)₇ ⋅Ni(tpy)₂ ]_n (2) and [Cu₆ (CN)₈ ⋅Ni(tpy)₂ ]_n (3) (tpy=2,2':6'',2''-terpyridine). These infinite inclusion compounds were prepared via a solvothermal sulfur migration reaction with subtle modification to fine-tune the sulfur content. The chiroptical activity of single crystals, which arises from the intrinsically chiral net, was confirmed by X-ray diffraction and solid-state circular dichroism spectra.

Intramolecular Electron Transfers Thwart Bistability in a Pentanuclear Iron Complex

With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{Mn(II)(μ-bpp)3}2Mn(III)Mn(II)2(μ3-O)](3+) (2(3+)), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{Fe(II)(μ-bpp)3}2Fe(III)Fe(II)2(μ3-O)](3+) complex (1(3+)). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 1(4+), 1(5+), and 1(6+) with a 3Fe(II)2Fe(III), 2Fe(II)3Fe(III), and 1Fe(II)4Fe(III) composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the μ3-oxo core except for the 1(4+) to 1(5+) process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [Fe(III)Fe(II)2(μ3-O)](5+) oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.

Understanding and tuning the properties of redox-accumulating manganese helicates

The multiple redox transitions of pentanuclear Mn clusters and the tuning of their redox potentials by ligand design are investigated computationally.

A lanthanide salen "square prism" and a wrapped exo-lanthanide salen "double decker"

An erbium(iii) salen "square prism" and a supramolecular aggregate of exo-erbium(iii) salen "double-decker" cations wrapped by an anionic cuprous cyanide network were prepared from N,N'-ethylene bis[4-(diethylamino)salicylideneimine. Both erbium(iii) edifices show fine-structure near-infrared (NIR) luminescence under the excitation in the visible light area. Here we provide a novel and efficient method for stabilizing non-isolable lanthanide edifices in the solid state.

A chiral coordination polymer with double coaxially nested helical chains exhibiting spin-canting antiferromagnetism

A chiral coordination polymer is reported, which contains rarely two types of double coaxially nested helical chains, exhibiting spin-canting antiferromagnetism.

Spontaneous symmetry breaking of Co(ii) metal–organic frameworks from achiral precursors via asymmetrical crystallization

Enantioenriched 3D pseudo-enantiomorphs integrating porosity, chirality and magnetism together with different colour and morphology were obtained through spontaneous symmetry breaking.

Influence of ionic liquids on the syntheses and structures of Mn(ii) coordination polymers based on multidentate N-heterocyclic aromatic ligands and bridging carboxylate ligands

[RMI]Br probably serves as a template directing agent for the formation of a cluster helicate in 2, while it serves as a mineralizing agent in 3–6. Additionally, [RMI]Br could obviously improve the crystal yield of 7.

Facile preparation and dual catalytic activity of copper(i)–metallosalen coordination polymers

Three copper(i)–metallosalen coordination polymers (CPs), {[Ni^II(SalImCy)]₂(Cu^ICN)₉}_n (1), {[Cu^II(SalImCy)]₂(Cu^ICN)₉}_n (2) and {[Ni^II(SalImCy)](Cu^II)₂·DMF}_n (3) were prepared by direct combination of Ni^II/Cu^II(salen) motifs with [Cu^ICN]_n chains and Cu₂I₂ clusters via the metalloligand strategy.

Different interpenetrated coordination polymers based on flexible dicarboxylate ligands: topological diversity and magnetism

This work evidently indicates that the effect of the secondary ligands, molar ratios and geometric preferences of the metal centers are critical on the resulting entangled motifs.

Synthesis and chiral recognition of nickel(II) macrocyclic complex with (R)-naphthylethyleneamine pendant groups and its self-assembled framework

A novel nickel(II) hexaaza macrocyclic complex, [Ni(L(R,R))](ClO(4))(2) (1), containing chiral pendant groups was synthesized by an efficient one-pot template condensation and characterized (L(R,R) ═1,8-di((R)-α-methylnaphthyl)-1,3,6,8,10,13-hexaazacyclotetradecane). The crystal structure of compound 1 was determined by single-crystal X-ray analysis. The complex was found to have a square-planar coordination environment for the nickel(II) ion. Open framework [Ni(L(R,R))](3) [C(6)H(3)(COO)(3)](2) (2) was constructed from the self-assembly of compound 1 with deprotonated 1,3,5-benzenetricarboxylic acid, BTC(3-). Chiral discrimination of rac-1,1'-bi-2-naphthol and rac-2,2,2-trifluoro-1-(9-anthryl)ethanol was performed to determine the chiral recognition ability of the chiral complex (1) and its self-assembled framework (2). Binaphthol showed a good chiral discrimination on the framework (2). The optimum experimental conditions for the chiral discrimination were examined by changing the weight ratio between the macrocyclic complex 1 or self-assembled framework 2 and racemates. The detailed synthetic procedures, spectroscopic data including single-crystal X-ray analysis, and the results of the chiral recognition for the compounds are described.

Endogenous arene hydroxylation promoted by copper(I) cluster helicates

A novel neutral triple-stranded hexanuclear copper(I) cluster helicate [Cu(I)(6)L(3)]·2CH(3)CN derived from a thiosemicarbazone ligand could be synthesized and crystallographically characterized. The MALDI mass spectrum of this complex suggests that the tetranuclear copper(I) cluster helicate [Cu(I)(4)L(2)] is also present in solution. These copper(I) cluster helicates are capable, in the presence of O(2), of hydroxylating the arene linker of their supporting ligand strands. The resulting dinuclear complex [Cu(II)(2)L'(OH)] is formed by two copper(II) centers, a new ligand arising from the hydroxylation reaction, and one hydroxide group. The magnetic investigation of this compound shows a strong antiferromagnetic coupling between the two Cu(II) centers. The kinetic studies for the hydroxylation process show values of ΔH(≠)=-70 kJ mol(-1), similar to those mediated by the tyrosinase enzymes.

Combining azide, carboxylate, and 2-pyridyloximate ligands in transition-metal chemistry: ferromagnetic Ni(II)5 clusters with a bowtie skeleton

The combined use of the anion of phenyl(2-pyridyl)ketone oxime (ppko(-)) and azides (N(3)(-)) in nickel(II) carboxylate chemistry has afforded two new Ni(II)(5) clusters, [Ni(5)(O(2)CR')(2)(N(3))(4)(ppko)(4)(MeOH)(4)] [R' = H (1), Me (2)]. The structurally unprecedented {Ni(5)(μ-N(3))(2)(μ(3)-N(3))(2)}(6+) cores of the two clusters are almost identical and contain the five Ni(II) atoms in a bowtie topology. Two N(3)(-) ions are end-on doubly bridging and the other two ions end-on triply bridging. The end-on μ(3)-N(3)(-) groups link the central Ni(II) atoms with the two peripheral metal ions on either side of the molecule, while the Ni···Ni bases of the triangles are each bridged by one end-on μ-N(3)(-) group. Variable-temperature, solid-state direct- (dc) and alternating-current (ac) magnetic susceptibility, and magnetization studies at 2.0 K were carried out on both complexes. The data indicate an overall ferromagnetic behavior and an S = 5 ground state for both compounds. The ac susceptibility studies on 1 reveal nonzero, frequency-dependent out-of-phase (χ(M)") signals at temperatures below ~3.5 K; complex 2 reveals no χ(M)" signals. However, single-crystal magnetization versus dc field scans at variable temperatures and variable sweep rates down to 0.04 K on 1 reveal no noticeable hysteresis loops, except very minor ones at 0.04 K assignable to weak intermolecular interactions propagated by nonclassical hydrogen bonds.