Dinuclear Helicates Research Articles

Nickel, copper, and zinc dinuclear helicates: how do bulky groups influence their architecture?

The ligand design factors that may influence the isolation of metallosupramolecular helicates or mesocates still deserve to be investigated. In this sense, dinuclear nickel(II), copper(II) and zinc(II) compounds were obtained by electrochemical synthesis using a family of five Schiff base ligands, H2Ln (n = 1-5), derived from bisphenylmethane and functionalized with bulky tert-butyl groups in the periphery and ethyl groups in the spacer. Six of the new complexes were characterized by X-ray crystallography, thus demonstrating that the helicate structure is predominant in the solid state. 1H NMR studies were performed for the zinc complexes to analyze if the helical architecture of the metal complexes is retained in solution. These studies reveal that the presence of a tert-butyl group in the ortho position with respect to the OH group is an essential factor identified for the existence of a helicate conformation in solution.

Remarkable 980 nm circularly polarized luminescence from dinuclear Yb(iii) helicates with aD4symmetry

Chiral BINOL-modified dinuclear helicates Yb2(R/S-BTHP)4show the excellent NIR-CPL at 980 nm with theglumreaching up to 0.81 andBCPLbeing up to 821 M−1cm−1.

Copper-Templated Formation of Dihelical Oligothiophene–Phenanthroline Assemblies

We report the synthesis and comprehensive characterization of a series of (oligo)thiophene-bridged (bis)phenanthroline ligands. The complexation behavior of the ditopic ligands with Cu(I) was explored by high-resolution ESI mass spectrometry, UV-vis spectroscopy, and electrochemistry. Whereas ligands, in which the phenanthrolines are bridged by smaller (oligo)thiophene units, provided mainly mononuclear complexes, quaterthiophene-based ditopic ligands show a strong tendency to undergo metal-templated self-assembly into double-stranded, dinuclear helicates.

Frontispiece: Unravelling of a [High Spin—Low Spin] ↔ [Low Spin—High Spin] Equilibrium in Spin‐Crossover Iron(II) Dinuclear Helicates Using Paramagnetic NMR Spectroscopy

Spin-Crossover. A [high spin–low spin]/[low spin–high spin] equilibrium in spin-crossover iron(II) dinuclear helicates is unravelled through paramagnetic NMR spectroscopy by Guillem Aromí, Valentin V. Novikov et al. in their Communication (e202110310).

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.

Correlations between ligand field Δ o, spin crossover T 1/2 and redox potential E pa in a family of five dinuclear helicates.

A family of five new bis-bidentate azole–triazole Rat ligands (1,3-bis(5-(azole)-4-isobutyl-4H-1,2,4-triazol-3-yl)benzene), varying in choice of azole (2-imidazole, 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), and the corresponding family of spin-crossover (SCO) and redox active triply bridged dinuclear helicates, [FeII2L3]4+, has been prepared and characterised. X-ray crystal structures show all five Fe(ii) helicates are low spin at 100 K. Importantly, DOSY NMR confirms the intactness of these SCO-active dinuclear helicates in D3-MeCN solution, regardless of HS fraction: γHS(298 K) = 0–0.81. Variable temperature 1H NMR Evans and UV-vis studies reveal that the helicates are SCO-active in MeCN solution. Indeed, the choice of azole in the Rat ligand used in [Fe2L3]4+ tunes: (a) solution SCO T1/2 from 247 to 471 K, and (b) reversible redox potential, Em(FeII/III), from 0.25 to 0.67 V for four helicates, whilst one has an irreversible redox process, Epa = 0.78 V, vs. 0.01 M AgNO3/Ag. For the four reversible redox systems, a strong correlation (R2 = 0.99) is observed between T1/2 and Epa. Finally, the analogous Ni(ii) helicates have been prepared to obtain Δo, establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) ∼ 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) ∼ 4-oxazole (11 516) < 4-thiazole (11 804 cm−1), (b) that Δo ([NiII2L3]4+) strongly correlates (R2 = 0.87) with T1/2 ([FeII2L3]4+), and (c) interestingly that Δo strongly correlates (R2 = 0.98) with Epa for the four helicates with reversible redox, so the stronger the ligand field strength, the harder it is to oxidise the Fe(ii) to Fe(iii).

Helix-mediated over 1 nm-range chirality recognition by ligand-to-ligand interactions of dinuclear helicates.

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (dLn–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively) used as mediators. Both 2Th and 3Th form one-dimensional (1D) helical structures upon terminal binding of two chiral guest co-ligands (LR or LS). Long-range chiral self-recognition is achieved in self-assembly of 2Th with LR and LS to preferentially form homochiral assemblies, 2Th-LR·LR and 2Th-LS·LS, whereas there is no direct molecular interaction between the two guest ligands at the terminal edges. X-ray crystal structure analysis and density functional theory studies reveal that long-range chiral recognition is achieved by terminal ligand-to-ligand interactions between the bis-diketonate ligands and chiral guest co-ligands. Conversely, in self-assembly of 3Th with a longer helix length, statistical binding of LR and LS occurs, forming heterochiral (3Th-LR·LS) and homochiral (3Th-LR·LR and 3Th-LS·LS) assemblies in an almost 1 : 1 ratio. When phenyl side arms of the chiral guest co-ligands are replaced by isopropyl groups (L′R and L′S), chiral self-recognition is also achieved in the self-assembly process of 3Th with the longer helix length to generate homochiral (3Th-L′R·L′R and 3Th-L′S·L′S) assemblies as the favored products. Thus, subtle modification of the chiral guests is capable of achieving over 1.4 nm-range chirality recognition.

Helicates with Ether‐Substituted Catechol Esters as Ligands

Mono‐ or biscatechol esters with ether‐type substituents or spacers form either triple lithium bridged dimeric helicates or triple stranded helicates with the ability to bind three lithium cations in their interior. Hierarchical helicates with ether or thioether substituents show in solution a monomer‐dimer equilibrium which is independent of the heteroatom in the ester substituent. However, dimerization constants are significantly lower than for corresponding alkyl derivatives. Dinuclear helicates with oligoether spacers are well obtained in the presence of lithium cations. Upon removal of the cations the helicates expand and successive addition of LiCl results in compression again.

Supramolecular interactions in a family of dinuclear helicates in the solid-state

ABSTRACT Metallo-helicates are a commonly encountered assembly within supramolecular chemistry. Interest in these architectures stems from their inherent helical chirality which positions them for a diverse range of applications such as catalysis and non-linear optics (NLO). The current study uses Co(II) dinuclear double helicates as versatile supramolecular synthons. The ditopic ligand, L, features two tridentate quinolinyl-hydrazone binding sites imparting it with hydrogen bond donors and π-faces for secondary supramolecular interactions. Incorporation of L into [Co2(L)2]4+ helical assemblies results in a helical cationic supramolecular synthon with moieties predisposed to forming π-π stacking and hydrogen bond interactions. The single-crystal X-ray structures of [Co2(L)2]X4 (X = ClO4 −, BF4 − and CF3SO3 −) revealed a variety of anion dependent hydrogen bond networks arising through the interaction of the hydrazide hydrogen with the anion. These interactions in turn strongly influence the nature of the π-π stacking interactions of the quinoline moieties which can be analysed via the Hirshfield surface.

Pyrrophens: Pyrrole-Based Hexadentate Ligands Tailor-Made for Uranyl (UO22+) Coordination and Molecular Recognition.

Derivatives of a novel pyrrole-containing Schiff base ligand system (called "pyrrophen") are presented which feature substituted phenylene linkers (R1 = R2 = H (H2L1); R1 = R2 = CH3 (H2L2)) and a binding pocket modeled after macrocyclic species. These ligands bind neutral CH3OH in the solid state through pyrrolic hydrogen-bonding. The interaction of the uranyl cation (UO22+) and H2L1-2 yields planar hexagonal bipyramdial uranyl complexes, while the Cu2+ and Zn2+ complexes were found to self-assemble as dinuclear helicate complexes (M2L2) with H2L1 under identical conditions. The favorable binding of UO22+ over Zn2+ provides insight into the molecular recognition of uranyl over other metal species. Structural features of these complexes are examined with special attention to features of the UO22+ coordination environment which distinguish them from other related salophen and porphyrinoid complexes.

Crystallization and single molecule magnetic behavior of quadruple-stranded helicates: tuning the anisotropic axes.

It has been successfully proven that the bis-β-diketone ligands with proper lengths and flexibility are essential for the construction of multiple-stranded helicates, where two Dy3+ centers in subtly different environments allow the tuning of the anisotropic axes. Based on our previous work, we have designed a flexible bis-β-diketone ligand BTT (BTT = 3,3''-bis(4,4,4-trifluoro-1,3-dioxobutyl)-m-terphenyl), which is successfully utilized to construct quadruple-stranded helicates with the formula of [C6H16N]2[Ln2(BTT)4]·2CH2Cl2·4CH3OH [Ln = Ce (1), Dy (2)]. Structural analysis indicates that they crystallize in the tetragonal space group P4/n, and each Ln3+ center is chelated by four diketonate moieties from four ligands, giving rise to a dinuclear quadruple-stranded helicate. Magnetic measurements show that 2 displays single molecular magnet behavior under an applied DC field of 2000 Oe. Further investigations indicate that the anisotropic axes of the Dy3+ centers can be tuned depending on the bis-β-diketonate ligands used to assemble the Dy helicates.

From Hierarchical Helicates to Functional Supramolecular Devices.

Catechol ligands with aldehyde, ketone or ester groups attached in 3-position form, in the presence of titanium(IV) triscatecholate, titanium(IV) complexes. If lithium cations are the counterions, they can bind in a successive step to the salicylate units of the complex and form a dimeric triple-lithium-bridged dinuclear helicate. In solution, the dimer is in equilibrium with the monomer and the thermodynamics of the dimerization can be easily evaluated. Thus, the hierarchically assembled titanium(IV) helicates represent a lithium-dependent molecular switch. The investigation of different derivatives of the complex allows for an estimation of the influence of side chain functionalities on the energetics of the dimerization. Thus, the hierarchically assembled helicates can be used as a kind of molecular balance to determine weak interaction energies (solvophobic effects and even dispersive effects). In addition, tethering of two ligands leads to "classical" helicates. Removal or addition of lithium cations allows for a reversible switching between a compressed and expanded state, which in the case of chiral ligands can be even performed stereospecifically.

Dinuclear helicate and tetranuclear cage assembly using appropriately designed ditopic triazole-azine ligands.

Four new, symmetrical, bis-bidentate ditopic Rdpt-type, Rat (R azine-triazole), ligands have been prepared, Lnpym-meta/para (n = 2 or 4), which contain bidentate n-pyrimidine/triazole binding pockets connected through an appropriate aromatic spacer, meta/para-phenyl, to enable assembly into dinuclear helicates or tetranuclear tetrahedral cages, respectively. The 3 : 2 self assembly reactions of each Lnpym-meta/para ligand with iron(ii) tetrafluoroborate gave the desired complexes, as shown by X-ray crystal structure determinations of the pair of helicates [FeII2(Lnpym-meta)3(BF4)4]·6CH3CN, with n = 2 (1·6CH3CN) or 4 (2·6CH3CN), and the pair of Td cages [FeII4(Lnpym-para)6(BF4)8]·xsolvent, with n = 2 (3·xsolvent) or 4 (4·xsolvent). Reversible FeII/III processes at Em = 0.95 ± 0.05 V vs. 0.01 M AgNO3/Ag in MeCN are a feature of 1-4, with little variation in the redox potential as a function of nuclearity, architecture or choice of n-pyrimidine isomer. In all four complexes the iron(ii) centres are low spin, despite having employed the weakest field diazines, the 2- and 4-pyrimidines, in these ditopic Rat ligands. Nevertheless this is an exciting proof of principle that Rdpt-type ligands (previously used to generate about 50 spin crossover-active complexes) can be extended into ditopic forms that are suitable for supramolecular assembly of helicates and cages.

Direct monitoring of spin transitions in a dinuclear triple-stranded helicate iron(ii) complex through X-ray photoelectron spectroscopy.

A dinuclear helical iron(ii) complex of a new ditopic thiazolylimine ligand (L) has been synthesised via supramolecular assembly. The resulting dinuclear helical cylinder [Fe2L3]·4BF4 was investigated by variable temperature X-ray crystallography, ESI high resolution mass spectrometry, CHN analysis, FT-IR and UV-Vis spectroscopy. The nature of the spin transition was investigated by magnetic susceptibility measurements, and confirmed by VT-SCXRD and X-ray photoelectron spectroscopy. [Fe2L3]·4BF4 displays a complete spin transition with a gradual-abrupt character at T1/2 = 348 K and represents a new example of a dinuclear iron(ii) complex exhibiting a spin transition at high temperature. Both VT-SCXRD and XPS measurements show excellent correlation with the magnetic susceptibility experiments, demonstrating the power of XPS not just to confirm, but also to clearly follow the spin-state transition in Fe(ii) SCO complexes.

Understanding the speciation of Ln(iii) complexes with octadentate tripodal ligands

Thorough speciation studies of Ln(iii) complexes with new dissymmetrical tripodal ligands were undertaken, which reveal the preferential formation of unsaturated dinuclear helicates.

Expanding the Size of Catecholesters – Modified Ligands for the Hierarchical Assembly of Dinuclear Titanium(IV) Helicates

AbstractFive 2,3‐dihydroxybenzoic acid derivatives 1 – 5 were used as starting materials to obtain the corresponding methyl and ethyl esters. Those were applied as ligands in the hierarchical self‐assembly of lithium‐bridged dinuclear titanium(IV) complexes 1a–4a, 1b–3b, and 5b. The equilibria between the mononuclear triscatecholate complexes (monomer) and the dinuclear helicates (dimer) were observed by 1H NMR spectroscopy in [D6]DMSO and [D4]MeOH at room temperature.

Post-Assembly Covalent Di- and Tetracapping of a Dinuclear [Fe2L3]4+ Triple Helicate and Two [Fe4L6]8+ Tetrahedra Using Sequential Reductive Aminations

The use of a highly efficient reductive amination procedure for the postsynthetic end-capping of metal-templated helicate and tetrahedral supramolecular structures bearing terminal aldehyde groups is reported. Metal template formation of a [Fe2L3](4+) dinuclear helicate and two [Fe4L6](8+) tetrahedra (where L is a linear ligand incorporating two bipyridine domains separated by one or two 1,4-(2,5-dimethoxyaryl) linkers and terminated by salicylaldehyde functions is described. Postassembly reaction of each of these "open" di- and tetranuclear species with excess ammonium acetate (as a source of ammonia) and sodium cyanoborohydride results in a remarkable reaction sequence whereby the three aldehyde groups terminating each end of the helicate, or each of the four vertices of the respective tetrahedra, react with ammonia then undergo successive reductive amination to yield corresponding fully tertiary-amine capped cryptate and tetrahedral covalent cages.

An NMR study on the 6,6′-(2-(diethylamino)ethylazanediyl)bis(methylene)bis(5-hydroxy-2-hydroxymethyl-4H-pyran-4-one) interaction with AlIII and ZnII ions

Here we report about the complex formation among an amine-bearing bis-kojic acid, 6,6′-(2-(diethylamino)ethylazanediyl)bis(methylene)bis(5-hydroxy-2-hydroxymethyl-4H-pyran-4-one) and two metal ions, the trivalent hard and not essential metal ion AlIII and the borderline and essential divalent metal ion ZnII. We carried out a thorough NMR study in order to reach the indispensable structural information on the behavior of these complexes in solution. A combination of 1D, 2D total correlation spectroscopy, heteronuclear single quantum coherence spectroscopy, nuclear Overhauser enhancement spectroscopy and rotating-frame Overhauser effect spectroscopy experiments was used to assign the signals of both free and metal-bound ligand at different pH values. Our results highlighted the different coordination behaviors of the ligand towards the different metal ions, depending on their hard or borderline character. The trivalent metal ion, AlIII, mainly forms dinuclear helicate complexes of M2L3 stoichiometry, and the coordination only involves both hydroxypyrone (O,O)-donor atoms. NMR data are in agreement with the presence of a rigid and symmetric structure of L9–AlIII complexes up to physiological pH. On the contrary, with the divalent metal ion, NMR data showed the coexistence of several species in solution though ZnII forms complexes of ML stoichiometry at physiological pH, where the metal coordination involves the nitrogen atoms of both the linker and the side-chain amine groups together with the oxygen atoms of phenolate groups. The in solution study will be of interest for providing an insight on the ligand bioavailability and on its behavior in the chelation treatments.

A family of Fe(3+) based double-stranded helicates showing a magnetocaloric effect, and Rhodamine B dye and DNA binding activities.

Herein, the synthesis, structural characterization, magnetic properties and guest binding activities of four Fe(3+) based double-stranded helicates namely; [Fe2(L)2](ClO4)(Cl)·4(CH3OH)·2(H2O) (), [Fe2(L)2](BF4)2·2(H2O) (), [Fe2(L)2](NO3)2·3(CH3OH)·2(H2O) (), and [Fe2(L)2](Cl)2·2(CH3OH)·4(H2O) () are reported. Complexes have been synthesized using the hydrazide-based ligand H2L (H2L = N'1,N'4-bis(2-hydroxybenzylidene)succinohydrazide) and the corresponding Fe(2+) salts. Each of the independent cationic complexes [Fe2(L)2](2+) shows double-stranded helicates from the self-assembly of the ligand and metal ions in a 2 : 2 ratio, where the individual Fe(3+) centre is lying on a C2-axis and the ligand strands wrap around it. In , ligand L adopts "pseudo-C" conformations and forms a double-stranded dinuclear helicate with a small cage in between them. Moreover, in , each of the independent cationic complexes [Fe2L2](2+) is inherently chiral and possesses P for right-hand and M for left-hand helicity and as a consequence is a racemic solid. Detailed magnetic studies of all the complexes reveal that the Fe(3+) centres are magnetically isolated and isotropic in nature. Estimation of the Magnetocaloric Effect (MCE) from magnetization data unveils a moderate MCE at a temperature of 3 K with magnetic entropy changes (-ΔSm) of 22.9, 27.7, 24.1, 26.5 J kg(-1) K(-1) at a magnetic field of 7 T for complexes , respectively. Also, the variation of the -ΔSm values was justified by considering the parameter of magnetization per unit mass. Stability of all the complexes in solution phase was confirmed by ESI-mass spectrometric analysis and liquid phase FT-IR spectroscopy. Further, the interaction of the complexes with Rhodamine B dye was examined by UV-vis and fluorescence spectroscopic study. The observed blue-shift in the fluorescence study and hyperchromicity and hypochromicity with the appearance of two isosbestic points in the UV-vis study ascertain the interactions of the dye with the complexes. A DNA binding study by absorption spectral titration suggests the weak external intercalation of complex within the nucleotide of calf thymus DNA. Computational study supports the isotropic nature of the metal centres and the consequent high spin multiplicity, which assists the complexes to show significant magnetic entropy changes.

Kinetic model and mechanism of the acid dissociation of d-metal bis(dipyrrolylmethenates)

A kinetic model and a mechanism have been developed by quantum chemical simulation for the protolytic dissociation of dinuclear homoleptic bis(dipyrrolylmethenates) of d-metals (M) with the general formula [M2L2] and a double-helix structure. The reaction is described by a third-order equation that is first-order with respect to the complex and second-order with respect to the acid. The main contribution to the total dissociation rate is made by the rate-determining step, specifically, attack of the second acetic acid molecule on the nitrogen atom of one of the coordination sites of the helicate. The dinuclear helicates of 3,3′-bis(dipyrrolylmethenes) are more inert in an acidic medium than 2,3′- and 2,2′-bis(dipyrrolylmethenes).