Formation Of Helicates Research Articles

Crucial Role of the Heteroatom and Bite Angle Influencing Spin States and Dinuclear Fe(II) Helicate Formation in 1,3,4-Thia/Oxadiazole Ligand

Crucial Role of the Heteroatom and Bite Angle Influencing Spin States and Dinuclear Fe(II) Helicate Formation in 1,3,4-Thia/Oxadiazole Ligand

Theoretical investigation of the lack of chiral self-sorting behaviour of a molecular cinquefoil knot

ABSTRACT Despite the profound implications of chiral self-sorting in various fields such as biology, materials, catalysis, and supramolecular chemistry, the elucidation of intrinsic factors that affect the efficiency of this process remains a formidable challenge. Here, we present a theoretical analysis based on a recent example of a molecular cinquefoil knot. By introducing various amino acids into the chiral ligands, we probe the potential factors contributing to the absence of chiral self-sorting. Energy profile shows that the thermodynamic stability of the diastereoisomeric helicates is considerably influenced by the ratio and arrangement of chiral ligands. From our calculations, the interaction strength linked to chiral ligands exerts a greater influence on determining the chiral self-sorting outcome compared to structural or steric effects. A better understanding of the mechanism of chiral self-sorting formation of circular helicates could aid in the design of topologically chiral molecules with diverse functionalities.

Steric and Geometrical Frustration Generate Two Higher-Order CuI12L8 Assemblies from a Triaminotriptycene Subcomponent.

The use of copper(I) in metal-organic assemblies leads readily to the formation of simple grids and helicates, whereas higher-order structures require complex ligand designs. Here, we report the clean and selective syntheses of two complex and structurally distinct CuI12L8 frameworks, 1 and 2, which assemble from the same simple triaminotriptycene subcomponent and a formylpyridine around the CuI templates. Both represent new structure types. In T-symmetric 1, the copper(I) centers describe a pair of octahedra with a common center but whose vertices are offset from each other, whereas in D3-symmetric 2, the metal ions form a distorted hexagonal prism. The syntheses of these architectures illustrate how more intricate CuI-based complexes can be prepared via subcomponent self-assembly than has been possible to date through consideration of the interplay between the subcomponent geometry and solvent and electronic effects.

Stereoselective Assembly of Hydrogen-Bonded Anionic Cages Dictated by Organophosphate-Based Chiral Nodes.

Inspired by the signal transduction function of organophosphates in biological systems, bioactive organophosphates were utilized for the first time as chiral nodes to dictate the stereoselective assembly of hydrogen-bonded anionic cages. Phosphonomycin (antibiotics), tenofovir (antivirals), adenosine monophosphate (natural product, AMP) and clindamycin phosphate (antibiotics) were assembled with an achiral bis-monourea ligand, thereby leading to the stereoselective formation of quadruple or triple helicates. The extent of the stereoselectivity could be enhanced by either lowering the temperature or adding stronger-binding cations as templates. With the chiral anionic cages as the host, some enantioselectivity was achieved when binding chiral quaternary ammonium cations.

Stereoselective Assembly of Hydrogen‐Bonded Anionic Cages Dictated by Organophosphate‐Based Chiral Nodes

AbstractInspired by the signal transduction function of organophosphates in biological systems, bioactive organophosphates were utilized for the first time as chiral nodes to dictate the stereoselective assembly of hydrogen‐bonded anionic cages. Phosphonomycin (antibiotics), tenofovir (antivirals), adenosine monophosphate (natural product, AMP) and clindamycin phosphate (antibiotics) were assembled with an achiral bis‐monourea ligand, thereby leading to the stereoselective formation of quadruple or triple helicates. The extent of the stereoselectivity could be enhanced by either lowering the temperature or adding stronger‐binding cations as templates. With the chiral anionic cages as the host, some enantioselectivity was achieved when binding chiral quaternary ammonium cations.

Formation of Metallosupramolecular Helicates and Mesocates from Poly-N-Heterocyclic Carbene Ligands.

In this work, a series of poly-NHC-based tetranuclear silver helicates and mesocates were synthesized from the silver-mediated self-assembly of the ligands involving multiple tridentate CNC-type pincer units and NHC coordination sites. The silver helicate was found to be transferred to a gold mesocate upon metal exchange reaction. The metallosupramolecular helicates and mesocates have been fully characterized by single-crystal X-ray crystallography, mass spectrometry, and multinuclear nuclear magnetic resonance spectroscopies. This study provides an example of the selective preparation of poly-NHC-based helicates or mesocates depending on the size of metal ions and the steric effect of ligands.

Positive Cooperativity Induced by Interstrand Interactions in Silver(I) Complexes with α,α'-Diimine Ligands.

The allosteric positive cooperativity accompanying the formation of compact [CuI(α,α′‐diimine)2]+ building blocks contributed to the historically efficient synthesis of metal‐containing catenates and knotted assemblies. However, its limited magnitude can easily be overcome by the negative chelate cooperativity that controls the overall formation of related polymetallic multistranded helicates and grids. Despite the more abundant use of analogous dioxygen‐resistant [AgI(α,α′‐diimine)2]+ units in modern entangled metallo‐supramolecular assemblies, a related thermodynamic justification was absent. Solid‐state structural characterizations show the successive formation of [AgI(α,α′‐diimine)(CH3CN)][X] and [AgI(α,α′‐diimine)2][X] upon the stepwise reactions of α,α′‐diimine=2,2′‐bipyridine (bpy) or 1,10‐phenanthroline (phen) derivatives with AgX (X=BF4 −, ClO4 −, PF6 −). In room‐temperature, 5–10 mM acetonitrile solutions, these cationic complexes exist as mixtures in fast exchange on the NMR timescale. Spectrophotometric titrations using the unsubstituted bpy and phen ligands point to the statistical (=non‐cooperative) binding of two successive bidentate ligands around AgI, a mechanism probably driven by the formation of hydrophobic belts, that overcomes the unfavorable decrease in the positive charge borne by the metallic cation. Surprisingly, the addition of methyl groups adjacent to the nitrogen donors (6,6′ positions in dmbpy; 2,9 positions in dmphen) induces positive cooperativity for the formation of [Ag(dmbpy)2]+ and [Ag(dmphen)2]+, a trend assigned to additional stabilizing interligand interactions. Adding rigid and polarizable phenyl side arms in [Ag(Brdmbpy)2]+ further reinforces the positively cooperative process, while limiting the overall decrease in metal–ligand affinity.

The role of ancillary ligand on regulating photoluminescence properties of Eu(III) helicates

Three binuclear Eu3+ helicates, [Eu2(OBTA)3(H2O)3(CH3COCH3)]·1.5CH3COCH3·H2O (1), [Eu2(OBTA)3(Bpy)2]·2CH3COCH3·CH2Cl2(2), [Eu2(OBTA)3(Phen)2]·2CH3COCH3(3) based a new bis-β-diketone, 4,4′-bis(4,4,4-trifluoro-1,3-butanedione) diphenyl ether (OBTA) was prepared in order to insight into the effects of complexes structure on Ln3+ ion emission properties. X-ray single crystal diffraction analyses confirmed the formation of binuclear triple-stranded helicates. The introduction of different ancillaries (Solvent molecular H2O and CH3COCH3; 2,2′-bipyridine, Bpy and 1,10-phenanthroline, Phen) to helicates regulate the coordination environments around Eu3+ ions, and lead to the obvious variations of photophysical properties. Combination the structural analyses and photophysical experiment results, the significance of spatial tension of helicate is emphasized.

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.

Synergistic Self-Assembly of Oxoanions and d-Block Metal Ions with Heteroditopic Receptors into Triple-Stranded Helicates.

Cooperativity effects among the interconnected anion and cation binding sites can profoundly alter the performance of heteroditopic receptors in selective ion pair recognition, processes that are oftentimes pertinent to biological systems and chemical separations. This work reports the effect of the linker that connects both binding sites on self-assembly of heteroditopic receptors in the presence of divalent first-row transition metal salts in solution and solid phase. Introduction of backbone flexibility in the receptor results in the formation of triple-stranded ion-pair helicates with an extraordinary selectivity towards CuSO4 through an anion-induced fit.

Catecholate‐Based Helicates

Catechol is a bidentate ligand for metal coordination which is well suited for the formation of helicates and related metallosupramolecular complexes. The development of the self‐assembly and stereochemistry of helicates based on catechol ligands in the last 25 years is discussed. Unsaturated, heteronuclear and hierarchical helicates as well as molecular switches based on catecholate helicates are covered.

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.

Remarkable self-sorting selectivity in covalently linked homochiral and heterochiral pairs driven by Pd2L4 helicate formation.

Imidazole-based ditopic ligands bearing two chiral alkyl groups (LRR, LSS, and LRS) were synthesized. The ligands formed Pd2L4 helicates with palladium ions (Pd2+). Self-sorting occurred between LRR and LRS to form (Pd2+)2(LRR)4 and (Pd2+)2(LRS)4 homoligand assemblies, whereas mixing of LRR and LSS with Pd2+ gave a near statistical mixture.

Self-Sorting Assembly of Molecular Trefoil Knots of Single Handedness

We report on the stereoselective synthesis of trefoil knots of single topological handedness in up to 90% yield (over two steps) through the formation of trimeric circular helicates from ligand strands containing either imine or, unexpectedly, amide chelating units and metal ion templates of the appropriate coordination character (zinc(II) for imines; cobalt(III) for amides). The coordination stereochemistry of the octahedral metal complexes is determined by asymmetric carbon centers in the strands, ultimately translating into trefoil knots that are a single enantiomer, both physically and in terms of their fundamental topology. Both the imine-zinc and amide-cobalt systems display self-sorting behavior, with racemic ligands forming knots that individually contain only building blocks of the same chirality. The knots and the corresponding trimeric circular helicate intermediates (Zn(II)3 complex for the imine ligands; Co(III)3 complex for the amide ligands) were characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and X-ray crystallography. The latter confirms the trefoil knots as 84-membered macrocycles, with each of the metal ions sited at a crossing point for three regions of the strand. The stereochemistry of the octahedral coordination centers imparts alternating crossings of the same handedness within each circular helicate. The expression of chirality of the knotted molecules was probed by circular dichroism: The topological handedness of the demetalated knots was found to have a greater effect on the CD response than the Euclidean chirality of an individual chiral center.

Metal-Ion Tuning in Triple-Stranded Helicate-Based Metallosupramolecules.

Effective incorporation of multiple types of ligands in a given coordination structure provides structural complexity and functional diversity to the resultant coordination-driven assembly. One of the most widely used synthetic approaches is the utilization of the molecular symmetry principle to combine multiple ligands and specific metallic centers in a preferred manner. The variation of metal ions can be helpful to understand the importance of symmetry for the generation of structurally hierarchical supramolecular platforms. We describe the synthesis and characterization of isostructural supramolecular helicates, [M8 (PDA)6 (AIP)3 (DMF)6-x (H2 O)x ] (M=Ni (1), Co (2), and Mn (3); PDA=2,6-pyridinedicarboxylate; AIP=5-aminoisophthalate; x=0 for 1, and x=4 for 2 and 3). The effect of metal variation on the formation of supramolecular helicates and their solid-state crystal packing are discussed. Despite the disparity in the ionic radii and distinct coordination-geometry preferences of Co2+ , Ni2+ , and Mn2+ , all metal centers engaged in the assembly with the heteroleptic ligands in the same manner to form isostructural supramolecular helicates.

A Design Strategy for Single-Stranded Helicates using Pyridine-Hydrazone Ligands and PbII.

The reactions of py-hz ligands (L1-L5) with Pb(CF3 SO3 )2 ⋅H2 O resulted in some rare examples of discrete single-stranded helical PbII complexes. L1 and L2 formed non-helical mononuclear complexes [PbL1(CF3 SO3 )2 ]⋅CHCl3 and PbL2(CF3 SO3 )2 ][PbL2CF3 SO3 ]CF3 SO3 ⋅CH3 CN, which reflected the high coordination number and effective saturation of PbII by the ligands. The reaction of L3 with PbII resulted in a dinuclear meso-helicate [Pb2 L3(CF3 SO3 )2 Br]CF3 SO3 ⋅CH3 CN with a stereochemically-active lone pair on PbII . L4 directed single-stranded helicates with PbII , including [Pb2 L4(CF3 SO3 )3 ]CF3 SO3 ⋅CH3 CN and [Pb2 L4CF3 SO3 (CH3 OH)2 ](CF3 SO3 )3 ⋅2 CH3 OH⋅2 H2 O. The acryloyl-modified py-hz ligand L5 formed helical and non-helical complexes with PbII , including a trinuclear PbII complex [Pb3 L5(CF3 SO3 )5 ]CF3 SO3 ⋅3CH3 CN⋅Et2 O. The high denticity of the long-stranded py-hz ligands L4 and L5 was essential to the formation of single-stranded helicates with PbII .

Off-off-on chiroptical property switching of a pyrene luminophore by stepwise helicate formation.

Pyrene-based chiral ligands (LR and LS) consisting of two chiral imidazole sidearms at the 1,8-positions first form pyrene-bridged oligomers [(L(R or S))3(Zn2+)n], which are chiroptically inactive, with a low concentration of zinc ions (Zn2+). Upon addition of more Zn2+, chiroptically active coordination helicates ([(L(R or S))3(Zn2+)2]) are formed. The stepwise assembly process successfully facilitated an "off-off-on" chiroptical switching process of a pyrene luminophore controlled by the concentration of Zn2+. The resulting pyrene-bridged helicates exhibit high performance in terms of a dissymmetry factor of circular dichroism (|gCD| = 0.006) and circularly polarized luminescence (|gCPL| = 0.01).

Selective Metal-Assisted Assembly of Mesocates or Helicates with Tristhiosemicarbazone Ligands.

The effect of the ligand and/or metal-related factors on the formation of tristhiosemicarbazone metallosupramolecular complexes has been studied in this work. The crystal structures of zinc(II) and lead(II) tristhiosemicarbazone mesocates and a hydrolyzed cadmium(II) helicate let us better rationalize some factors involved in the selective formation of helicates or mesocates.

Steric and Electronic Factors Affecting the Conformation of Bimetallic CuI Complexes: Effect of the Aliphatic Spacer of Tetracoordinating Schiff-Base Ligands.

A family of six homoleptic [CuI (Ln )]2 (ClO4 )2 and six heteroleptic [CuI (Ln )(PPh3 )2 ]2 (ClO4 )2 bimetallic complexes, in which Ln are bis-Schiff base ligands with alkyl spacers of variable length (n=2-7 -CH2 -), were prepared to evaluate the role of the spacer on the formation of helicates or mesocates. In the homoleptic series, spectroscopic and theoretical studies indicate that preferences for a conformation are based on energetic parameters, mainly, the establishment of noncovalent interactions. The odd-even nature of the spacers preconditions the superposition of the aromatic rings to allow the juxtaposition necessary for noncovalent interactions, whereas the increase of the length reduces the strength of such interactions. Consequently, complexes with even-spacers of short length were identified as helicates in solution, [CuI (Ln )]22+ (n=2, 4). Complexes [CuI (Ln )]22+ (n=3-7) dissociate in solution to produce the monometallic complexes in equilibrium, [CuI (Ln )]+ . The stability of the bimetallic species is discussed in terms of their conformations. The set of heteroleptic complexes was prepared to evaluate the reach of the "odd-even rule" in the solid, which is based on the "zig-zag" arrangements of the spacers. Based on crystallographic information, "S-" and "C"-type conformations of Ln are related to even and odd spacers, respectively. This trend is considered in addition to other factors to explain preferences for either a mesocate or helicate conformation in the homoleptic series.

Symmetrical and dissymmetrical 2,2′-bis-dipyrrin ligands and Zn(ii) binuclear helicates

A series of amide functionalized symmetrical and dissymmetrical 2,2′-bis-dipyrrin ligands has been prepared and characterized and their assembly with Zn(ii) towards the formation of binuclear helicates has been investigated.