Triple Helicate Research Articles

Supramolecular Modulation of Spin Crossover in an Fe(II) Dinuclear Triple Helicate.

A spin-crossover (SCO) active dinuclear Fe(II) triple helicate of the form [Fe2L3]4+ was combined with additional supramolecular components in order to manipulate the interhelical separation and steric congestion and to study the magneto-structural effects on the ensuing composite materials. A more separated array of SCO units produced more extensive spin-transitions, while a tightly arranged lattice environment stabilized the low-spin state. This study highlights the important interplay between crystal packing, intermolecualr interactions, and the magentic behavior of SCO materials.

Fine‐Tuning the Spring‐Like Motion of an Anion‐Based Triple Helicate by Tetraalkylammonium Guests

AbstractSupramolecular springs are a class of molecular devices that may provide implications to the macroscopic spring behavior from the molecular level. Helical structures are suitable molecular springs because the specific twisting of the helical strands can cause spring‐like (extension‐contraction) movement along the axis upon external stimuli. Herein we report an anion‐based triple‐helicate spring, which can undergo reversible contraction–extension motion through introduction and removal of tetraalkylammonium cations, including TMA+ and analogous irregular tetrahedral cations bearing different alkyl chains, while the relative orientation of the two phosphate ions changes to facilitate guest inclusion. Notably, the degree of contraction (shortening of the helical pitch) can be fine‐tuned by varying the shape and size of guest cation. However, with the larger cations (TEA+, TPA+ and TBA+), the meso‐helicate configuration is obtained, which interconverts with the helicate by addition/removal of TMA+ ion.

Fine-Tuning the Spring-Like Motion of an Anion-Based Triple Helicate by Tetraalkylammonium Guests.

Supramolecular springs are a class of molecular devices that may provide implications to the macroscopic spring behavior from the molecular level. Helical structures are suitable molecular springs because the specific twisting of the helical strands can cause spring-like (extension-contraction) movement along the axis upon external stimuli. Herein we report an anion-based triple-helicate spring, which can undergo reversible contraction-extension motion through introduction and removal of tetraalkylammonium cations, including TMA+ and analogous irregular tetrahedral cations bearing different alkyl chains, while the relative orientation of the two phosphate ions changes to facilitate guest inclusion. Notably, the degree of contraction (shortening of the helical pitch) can be fine-tuned by varying the shape and size of guest cation. However, with the larger cations (TEA+ , TPA+ and TBA+ ), the meso-helicate configuration is obtained, which interconverts with the helicate by addition/removal of TMA+ ion.

The kinetics and mechanism of interconversion within a system of [Fe2L3]4+ helicates and [Fe4L6]8+ cages.

Nature builds simple molecules into highly complex assemblies, which are involved in all fundamental processes of life. Some of the most intriguing biological assemblies are those that can be precisely reconfigured to achieve different functions using the same building blocks. Understanding the reconfiguration of synthetic self-assembled systems will allow us to better understand the complexity of proteins and design useful artificial chemical systems. Here we have prepared a relatively simple system in which two distinct self-assembled structures, a [Fe2L3]4+ helicate and a [Fe4L6]8+ cage that are formed from the same precursors, coexist at equilibrium. We have measured the rates of interconversion of these two species and propose a mechanism for the transformation.

Dinuclear helicate or mononuclear pincer lanthanide complexes from one ligand: stereo-controlled assembly and catalysis

By varying the metal/ligand ratio, either dinuclear triple helicate or mononuclear pincer can be stereo-selectively obtained from the same chiral ligand. Their catalytic properties have been demonstrated with the asymmetric FC alkylation reactions.

Selective Anion Binding Drives the Formation of AgI8L6 and AgI12L6 Six-Stranded Helicates.

Here we describe the formation of an unexpected and unique family of hollow six-stranded helicates. The formation of these structures depends on the coordinative flexibility of silver and the 2-formyl-1,8-napthyridine subcomponent. Crystal structures show that these assemblies are held together by Ag4I, Ag4Br, or Ag6(SO4)2 clusters, where the templating anion plays an integral structure-defining role. Prior to the addition of the anionic template, no six-stranded helicate was observed to form, with the system instead consisting of a dynamic mixture of triple helicate and tetrahedron. Six-stranded helicate formation was highly sensitive to the structure of the ligand, with minor modifications inhibiting its formation. This work provides an unusual example of mutual stabilization between metal clusters and a self-assembled metal–organic cage. The selective preparation of this anisotropic host demonstrates new modes of guiding selective self-assembly using silver(I), whose many stable coordination geometries render design difficult.

Multiple Transformations among Anion-based A2nL3n Assemblies: Bicapped Trigonal Antiprism A8L12, Tetrahedron A4L6, and Triple Helicate A2L3 (A = Anion).

The construction of sophisticated, high-nuclearity polyhedral cages is an attractive yet challenging task in supramolecular chemistry. Herein we report the anion-coordination-driven assembly (ACDA) of a series of A2nL3n architectures ("A" denotes anion, L is ligand, n = 1, 2, 4) with a biphenylene-spaced bis-bis(urea) ligand including triple helicate A2L3 (H), tetrahedron A4L6 (T), and the octanuclear, bicapped trigonal antiprism (or parallelepiped) A8L12 (P). Among them, P is held by 96 hydrogen bonds, the largest number ever reported in a discrete polyhedron, and encapsulates multiple guests (three tetramethylammonium cations) in three compartments. Remarkably, multiple reversible transformations of these dynamic assemblies have been realized by alternation of the template guest, solvent, and concentration. Furthermore, a chiral ligand (L2S) with carbon stereocenters at both termini of the bis-bis(urea) backbone was designed and assembled with phosphate to form the enantio-pure triple helicate or tetrahedron. The chiral amplification effect in the tetrahedral complex is significantly larger than that in the triple helicate as a function of the point chirality.

Self-Assembly, Structural Transformation, and Guest-Binding Properties of Supramolecular Assemblies with Triangular Metal–Metal Bonded Units

The properties of supramolecular structures are highly dependent on their metal-centered building blocks and organic linkers, thus the search for novel systems will lead to new functions and applications for these unique assemblies. Here, two discrete triangular trimetallic sandwich building blocks were developed to construct supramolecular assemblies through coordination-driven self-assembly with organosulfur ligands. A series of tubelike (Tr2Pd3)4L6 assemblies (Tr = cycloheptatrienyl ring) were obtained from a discrete triangular tripalladium sandwich complex with bifunctional organosulfur ligands. By replacing the metal centers of the platinum analogue, the self-assembly process resulted in the clean formation of (Tr2Pt3)2L3 triple helicates instead of tubelike species. The trimetallic sandwich building blocks were also shown to form face-capped (Tr2M3)4L4 (M = Pd or Pt) tetrahedral cages when trifunctional organosulfur ligands were used. The supramolecular assemblies were comprehensively analyzed by X-ray crystallography. A metal-cluster-induced structural transformation between (Tr2Pd3)4L4 tubes and (Tr2Pt3)2L3 triple helicates was observed. Furthermore, the face-capped (Tr2Pd3)4L4 cage possesses a tetrahedral cavity allowing the encapsulation of a series of guests.

A Rare Example of a Complete, Incomplete, and Non-Occurring Spin Transition in a [Fe2L3]X4 Series Driven by a Combination of Solvent-and Halide-Anion-Mediated Steric Factors

A trend between the degree of steric congestion of the Fe(II) coordination environment and the extent of spin transition (percentage completeness) has been observed in a series of halide salts of a dinuclear triple helicate architecture with the general form [Fe2L3]X4 (where X = Cl- for 1, Br- for 2, and (I-)3/I3- for 3, and L is (1E,1'E)-N,N'-(oxybis(4,1-phenylene))bis(1-(1H-imidazol-4-yl)methanimine). Crystal packing densities of adjacent helicates were found to decrease with increasing anion size. Greater steric congestion by neighboring helicates favored the [HS-HS] state of the dinuclear triple helicate architecture. As a result, the highly crowded Cl- salt (1) did not undergo spin-crossover (SCO), the more congested Br- salt (2) underwent an incomplete solvent-dependent transition, and the least crowded (I-)3/I3- analogue (3) exhibited a full SCO from the [HS-HS] ↔ [LS-LS] state. Furthermore, an interesting two-step transition was observed in the Br- salt, exhibiting a 28 K thermal hysteresis in the higher temperature step, the largest thermal hysteresis reported to date for a Fe(II) dinuclear triple helicate system. Variable-temperature single-crystal X-ray diffraction (SCXRD) analysis of 2 demonstrated that this two-step profile was found to be the result of crystallographic parameters evolving in a two-step manner with temperature, rather than a crystallographic phase change.

Diversity of Coordination Modes in a Flexible Ditopic Ligand Containing 2-Pyridyl, Carbonyl and Hydrazone Functionalities: Mononuclear and Dinuclear Cobalt(III) Complexes, and Tetranuclear Copper(II) and Nickel(II) Clusters

Syntheses, crystal structures and characterization are reported for four new complexes [Cu4Br2(L)4]Br2 (1), [Ni4(NO3)2(L)4(H2O)](NO3)2 (2), [Co2(L)3](ClO4)3 (3) and [Co(L)2](ClO4) (4), where L− is the monoanion of the ditopic ligand N′-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide (LH) built on a picolinoyl hydrazone core fragment, and possessing a bidentate and a tridentate coordination pocket. The tetranuclear cation of 1·0.8H2O·MeOH is a strictly planar, rectangular [2 × 2] grid. Two 2.21011 L− ligands bridge adjacent CuII atoms on the short sides of the rectangle through their alkoxide oxygen atoms, and two 2.11111 ligands bridge adjacent CuII atoms on the long sides of the rectangle through their diazine groups; two metal ions are 5-coordinate and two are 6-coordinate. The tetranuclear cation of 2·0.2H2O·3EtOH is a square [2 × 2] grid. The two 6-coordinate NiII atoms of each side of the square are bridged by the alkoxide O atom of one 2.21011 L− ligand. The dinuclear cation of 3·0.8H2O·1.3MeOH contains two low-spin octahedral CoIII ions bridged by three 2.01111 L− ligands forming a pseudo triple helicate. In the mononuclear cation [Co(L)2]+ of complex 4, the low-spin octahedral CoIII center is coordinated by two tridentate chelating, meridional 1.10011 ligands. The crystal structures of the complexes are stabilized by a variety of π–π stacking and/or H-bonding interactions. Compounds 2, 3 and 4 are the first structurally characterized nickel and cobalt complexes of any form (neutral or anionic) of LH. The 2.01111 and 1.10011 coordination modes of L−, observed in the structures of complexes 3 and 4, have been crystallographically established for the first time in coordination complexes containing this anionic ligand. Variable-temperature, solid-state dc magnetic susceptibility and variable-field magnetization studies at 1.8 K were carried out on complexes 1 and 2. Antiferromagnetic metal ion···metal ion exchange interactions are present in both complexes. The study reveals that the cation of 1 can be considered as a practically isolated pair of strongly antiferromagnetically coupled (through the diazine group of L−) dinulear units. The susceptibility data for 2 were fit to a single-J model for an S = 1 cyclic tetramer. The values of the J parameters have been rationalized in terms of known magnetostructural correlations. Spectral data (infrared (IR), ultraviolet/visible (UV/VIS), 1H nuclear magnetic resonance (NMR) for the diamagnetic complexes) are also discussed in the light of the structural features of 1–4 and the coordination modes of the organic and inorganic ligands that are present in the complexes. The combined work demonstrates the ligating flexibility of L−, and its usefulness in the synthesis of complexes with interesting structures and properties.

A Long-Lived Halogen-Bonding Anion Triple Helicate Accommodates Rapid Guest Exchange.

Anion-templated helical structures are emerging as a dynamic and tractable class of supramolecules that exhibit anion-switchable self-assembly. We present the first kinetic studies of an anion helicate by utilizing halogen-bonding m-arylene-ethynylene oligomers. These ligands formed high-fidelity triple helicates in solution with surprisingly long lifetimes on the order of seconds even at elevated temperatures. We propose an associative ligand-exchange mechanism that proceeded slowly on the same timescale. In contrast, intrachannel anion exchange occurred rapidly within milliseconds or faster as determined by stopped-flow visible spectroscopy. Additionally, the helicate accommodated bromide in solution and the solid state, while the thermodynamic stability of the triplex favored larger halide ions (bromide≈iodide≫chloride). Taken together, we elucidate a new class of kinetically stable helicates. These anion-switchable triplexes maintain their architectures while accommodating fast intrachannel guest exchange.

A Long‐Lived Halogen‐Bonding Anion Triple Helicate Accommodates Rapid Guest Exchange

Anion-templated helical structures are emerging as a dynamic and tractable class of supramolecules that exhibit anion-switchable self-assembly. We present the first kinetic studies of an anion helicate by utilizing halogen-bonding m-arylene-ethynylene oligomers. These ligands formed high-fidelity triple helicates in solution with surprisingly long lifetimes on the order of seconds even at elevated temperatures. We propose an associative ligand-exchange mechanism that proceeded slowly on the same timescale. In contrast, intrachannel anion exchange occurred rapidly within milliseconds or faster as determined by stopped-flow visible spectroscopy. Additionally, the helicate accommodated bromide in solution and the solid state, while the thermodynamic stability of the triplex favored larger halide ions (bromide≈iodide≫chloride). Taken together, we elucidate a new class of kinetically stable helicates. These anion-switchable triplexes maintain their architectures while accommodating fast intrachannel guest exchange.

Solvent Effects on the Spin-Transition in a Series of Fe(II) Dinuclear Triple Helicate Compounds

This work explores the effect of lattice solvent on the observed solid-state spin-transition of a previously reported dinuclear Fe(II) triple helicate series 1–3 of the general form [FeII2L3](BF4)4(CH3CN)n, where L is the Schiff base condensation product of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively, and 1 is the complex when L = L1, 2 when L = L2 and 3 when L = L3 (Craze, A.R.; Sciortino, N.F.; Bhadbhade, M.M.; Kepert, C.J.; Marjo, C.E.; Li, F. Investigation of the Spin Crossover Properties of Three Dinuclear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type. Inorganics. 2017, 5 (4), 62). Desolvation of 1 and 2 during measurement resulted not only in a decrease in T1/2 and completeness of spin-crossover (SCO) but also a change in the number of steps in the spin-profile. Compounds 1 and 2 were observed to change from a two-step 70% complete transition when fully solvated, to a single-step half complete transition upon desolvation. The average T1/2 value of the two-steps in the solvated materials was equivalent to the single T1/2 of the desolvated sample. Upon solvent loss, the magnetic profile of 3 experienced a transformation from a gradual SCO or weak antiferromagnetic interaction to a single half-complete spin-transition. Variable temperature single-crystal structures are presented and the effects of solvent molecules are also explored crystallographically and via a Hirshfeld surface analysis. The spin-transition profiles of 1–3 may provide further insight into previous discrepancies in dinuclear triple helicate SCO research reported by the laboratories of Hannon and Gütlich on analogous systems (Tuna, F.; Lees, M. R.; Clarkson, G. J.; Hannon, M. J. Readily Prepared Metallo-Supramolecular Triple Helicates Designed to Exhibit Spin-Crossover Behaviour. Chem. Eur. J. 2004, 10, 5737–5750 and Garcia, Y.; Grunert, C. M.; Reiman, S.; van Campenhoudt, O.; Gütlich, P. The Two-Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy. Eur. J. Inorg. Chem. 2006, 3333–3339).

A Rapid and Straightforward Supramolecular Self-Assembly Experiment To Prepare and Characterize a Triple Helicate Complex

This experiment prepares a large (Mr: 1821 g mol–1) triple helicate complex quickly in one pot from inexpensive starting materials. The protocol illustrates the power of self-assembly in a simple manner that is suitable for second-year general chemistry students using reversible imine bond and metal–ligand bond formation. Plastic molecular models and 1H NMR spectroscopy are used to elucidate the structure of the self-assembled product.

Inside Cover: Peripheral Templation‐Modulated Interconversion between an A4L6 Tetrahedral Anion Cage and A2L3 Triple Helicate with Guest Capture/Release (Angew. Chem. Int. Ed. 7/2018)

Inside Cover: Peripheral Templation‐Modulated Interconversion between an A₄L₆ Tetrahedral Anion Cage and A₂L₃ Triple Helicate with Guest Capture/Release (Angew. Chem. Int. Ed. 7/2018)

Innentitelbild: Peripheral Templation‐Modulated Interconversion between an A4L6 Tetrahedral Anion Cage and A2L3 Triple Helicate with Guest Capture/Release (Angew. Chem. 7/2018)

Innentitelbild: Peripheral Templation‐Modulated Interconversion between an A₄L₆ Tetrahedral Anion Cage and A₂L₃ Triple Helicate with Guest Capture/Release (Angew. Chem. 7/2018)

Peripheral Templation-Modulated Interconversion between an A4 L6 Tetrahedral Anion Cage and A2 L3 Triple Helicate with Guest Capture/Release.

An anion-coordination-based A4 L6 ("A" denotes anion and "L" is ligand) tetrahedral cage was constructed by a C2 -symmetric bis-bis(urea) ligand and phosphate anion, which showed reversible interconversion with the A2 L3 triple helicate as a response to the template, concentration, or solvent. Notably, an unusual "peripheral" templation was found to be critical to stabilize the tetrahedral structure. This peripheral effect was utilized to assemble an "empty" A4 L6 cage that allows the multi-stimuli-controlled capture/release of biologically important species such as choline and acetylcholine.

Peripheral Templation‐Modulated Interconversion between an A4L6 Tetrahedral Anion Cage and A2L3 Triple Helicate with Guest Capture/Release

AbstractAn anion‐coordination‐based A4L6 (“A” denotes anion and “L” is ligand) tetrahedral cage was constructed by a C2‐symmetric bis‐bis(urea) ligand and phosphate anion, which showed reversible interconversion with the A2L3 triple helicate as a response to the template, concentration, or solvent. Notably, an unusual “peripheral” templation was found to be critical to stabilize the tetrahedral structure. This peripheral effect was utilized to assemble an “empty” A4L6 cage that allows the multi‐stimuli‐controlled capture/release of biologically important species such as choline and acetylcholine.

Chirality sensing of choline derivatives by a triple anion helicate cage through induced circular dichroism.

Chirality sensing of choline derivatives is achieved by a self-assembled, racemic triple anion helicate cage which exhibits induced circular dichroism (ICD) upon encapsulation of a chiral guest. The host-guest interactions were illustrated by NMR, crystal structure, CD and DFT calculations. The absolute configurations and ee values were determined by ICD.

Varied spin crossover behaviour in a family of dinuclear Fe(ii) triple helicate complexes

Dinuclear triple-helicate complexes of the formula [Fe2L3](BF4)4·solv (solv = CH3CN, CHCl3, H2O) have been synthesised and structurally characterised. The bis-bidentate ligands, L, present either strong-field 2-pyridylimine (1) or weaker-field 2-imidazolylimine (2) and 4-imidazolylimine (3) coordination spheres about Fe(ii) centres in an octahedral geometry. Whereas 1 is pervasively diamagnetic, spin crossover (SCO) behaviour is observed in 2 and 3 and has been studied using variable-temperature structural, UV-visible spectroscopic, magnetic and photo-magnetic techniques. Variable-temperature (1.8-400 K) magnetic-susceptibility measurements reveal the T1/2 values of 2 and 3 to be strongly dependent upon the solvent and degree of solvation. Photomagnetic studies at 10 K under white-light irradiation revealed an inefficient photo-induced SCO in 2, but full switching in 3.