Double Helicate Research Articles

Dinuclear Double‐Stranded Metallosupramolecular Ruthenium Complexes: Potential Anticancer Drugs

Just add a twist: Metallosupramolecular architecture design is used to create anti-cancer agents with high activities in human breast cancer cell lines. Three isomers, which comprise a nonhelical metallocyclophane, an unsaturated double helicate, and a new type of double helicate, bridge the fields of metallosupramolecular architecture and anticancer drug design (picture: double-helical trans/cis isomer; red Ru, green Cl).

Messages in Molecules: Ligand/Cation Coding and Self‐Recognition in a Constitutionally Dynamic System of Heterometallic Double Helicates

Double helicates are known to exhibit self-recognition characteristics determined by the coordination geometry of the metal involved as well as by the topicity of the ligands. Combining tridentate (terpyridine, T) or bidentate (bipyridine, B) subunits in a tritopic strand affords a set of ligands able to assemble by pairs to form double helicates, homo- or heterostranded, homo- or heterotopic, depending on the coordination properties of the metals involved. The four ligand strands, BBB, TTT, BBT, and TBT form constitutionally dynamic sets of double helicates with the metal ions Cu(I), Cu(II), and Zn(II); these helicates correspond to the correct coding of the BB, BT, and TT pairs for tetra-, penta-, and hexacoordinate Cu(I), Cu(II), and Zn(II) cations, respectively.

A Dynamic Tricopper Double Helicate

The reaction between 8-aminoquinoline, 1,10-phenantholine-2,9-dicarbaldehyde, and copper(I) tetrafluoroborate gave a quantitative yield of a tricopper double helicate. The presence of dynamic covalent imine (C=N) bonds allowed this assembly to participate in two reactions not previously known in helicate chemistry: 1) It could be prepared through subcomponent substitution from a dicopper double helicate that contained aniline residues. An electron-poor aniline was quantitatively displaced; a more electron-rich aniline competed effectively with the aminoquinoline, setting up an equilibrium between dicopper and tricopper helicates that could be displaced towards the tricopper through the addition of further copper(I). 2) Both dicopper and tricopper helicates could be prepared simultaneously from a mixture of phenanthroline dialdehyde, aniline, and aminoquinoline, which contained all possible imine condensation products in equilibrium. Following the addition of copper(I), thermodynamic equilibration on both covalent and coordinative levels eliminated all partially-formed and mixed imine ligands from the mixture, leaving the helicates as exclusive products.

Synthesis and Optical Resolution of a Double Helicate Consisting of ortho‐Linked Hexaphenol Strands Bridged by Spiroborates

Synthesis and Optical Resolution of a Double Helicate Consisting of <i>ortho</i>‐Linked Hexaphenol Strands Bridged by Spiroborates

Synthesis and Optical Resolution of a Double Helicate Consisting of ortho‐Linked Hexaphenol Strands Bridged by Spiroborates

Rechtsherum wie linksherum: Das erste Spiroborat-Helicat erwies sich sowohl im Festkörper als auch in Lösung als stabil. 1H-NMR-Spektroskopie, ESI-Massenspektrometrie und eine Röntgenstrukturanalyse charakterisierten die Doppelstrang-Struktur (siehe Bild). Die racemischen Helicate wurden durch Bildung diastereomerer Ammoniumsalze getrennt.

Mixed ligand helicates and mesocates

A 1 : 1 mixture of the homoleptic double helicates [MII(L1)2] and [MII(L2)2] [M = Cu, Zn; (L1)2− and (L2)2− are bis-bidentate ligands containing two pyrazolyl-phenolate termini but with different spacers separating them] affords the mixed ligand complexes [M(L1)(L2)] in high yield, indicative of a favourable inter-ligand interaction in the mixed-ligand complexes. Whereas [Cu(L1)(L2)] is a double helicate, [Zn(L1)(L2)] is a mesocate with a ‘face to face’ arrangement of the two ligands.

Formation of a solubilized cobalt block oligomer from a M2L-type double helicate

The multi-dentate ligand, 2,3,5,6-tetrakis(2,2'-bipyridyl)pyrazine (L) and divalent cobalt self-assemble to a block co-polymer-like oligomer in solution, which contains at least the L(7)Co(8) fragment. The extent of oligomerization is sensitive to the water content in acetonitrile solution. In the solid state, the simple monomer [LCo(2)(CH(3)CN)(4)][ClO(4)](4) is isolated. The X-ray structure of the crystallized material (containing four CH(3)CN solvate molecules) reveals a double-helical complex with two heptadentate Co(II) sites, and a helical pitch of approximately 28.1 A. Coupled Co(I/II) redox processes are observed between the two metal centres.

Spontaneous resolution of silver double helicates consisting of achiral ligands with several aromatic rings

Symmetric breaking of silver double helicates through spontaneous crystallization was achieved via incorporation of C-H...pi and pi-pi stacking interactions of achiral ligands consisting of several aromatic rings.

Complexes of Ag(i), Hg(i) and Hg(ii) with multidentate pyrazolyl-pyridine ligands: from mononuclear complexes to coordination polymers via helicates, a mesocate, a cage and a catenate

The coordination chemistry of a series of di- and tri-nucleating ligands with Ag(I), Hg(I) and Hg(II) has been investigated. Most of the ligands contain two or three N,N'-bidentate chelating pyrazolyl-pyridine units pendant from a central aromatic spacer; one contains three binding sites (2 + 3 + 2-dentate) in a linear sequence. A series of thirteen complexes has been structurally characterised displaying a wide range of structural types. Bis-bidentate bridging ligands react with Ag(I) to give complexes in which Ag(I) is four-coordinate from two bidentate donors, but the complexes can take the form of one-dimensional coordination polymers, or dinuclear complexes (mesocate or helicate). A tris-bidentate triangular ligand forms a complicated two-dimensional coordination network with Ag(I) in which Ag...Ag contacts, as well as metal-ligand coordination bonds, play a significant role. Three dinuclear Hg(I) complexes were isolated which contain an {Hg2}2+ metal-metal bonded core bound to a single bis-bidentate ligand which can span both metal ions. Also characterised were a series of Hg(II) complexes comprising a simple mononuclear four-coordinate Hg(II) complex, a tetrahedral Hg(II)4 cage which incorporates a counter-ion in its central cavity, a trinuclear double helicate, and a trinuclear catenated structure in which two long ligands have spontaneously formed interlocked metallomacrocyclic rings thanks to cyclometallation of two of the Hg(II) centres.

This facile interconversion of dinuclear double helicates and side-by-side species: a reprogrammable ligand with potential sensor applications

The ligand L1 forms a dinuclear double helicate with Cu+ but upon addition of Ba2+ to the system a side-by-side species is formed both in solution and in the solid state; in the presence of Na+ both the helicate and the side-by-side species are formed in roughly equal amounts in solution.

Syntheses and Structures of Dinuclear Double-Stranded Helicates of Divalent Manganese, Iron, Cobalt, and Zinc

The syntheses and solid-state and solution structures of a series of unusually volatile, charge neutral, [4 + 4] double-stranded helical complexes of divalent manganese, iron, cobalt, and zinc are described. Deprotonation of the N4-donor iminopyrrole ligand H2L by KH cleanly generates the salt K2(THF)2L, which displays both sigma and pi interactions between K and iminopyrrolyl fragments in the X-ray crystal structure. Transamination, salt elimination, and protonolysis reactions were found to be versatile and, in general, high-yielding routes to the dinuclear double helicates [M2(L)2] (M = Mn, Fe, Co, and Zn). These compounds are isomorphous in the solid state by X-ray crystallography and adopt dinuclear cleft motifs as a result of pi stacking between opposing iminopyrrolyl fragments. This motif was also observed in the solution structures of [Fe2(L)2] and [Zn2(L)2] below 230 and 200 K, respectively (DeltaG++ = approximately 46 and 39.0 kJ mol(-1), respectively).

Preparation and structural characterization of a dicopper prehelicate

In the reaction of 4′-(4-ethynylphenyl)-2,2′:6′,2″-terpyridine ( 3) with trans-[PtI 2(PEt 3) 2] and CuI, ligand 3 scavenges Cu(I) and forms [Cu 2(PPh 3) 2(μ-I)(μ- 3)]I, structural characterization of which shows that ligand 3 adopts a helical twist that parallels that found in related double helicates.

Tapes of Cyclic Water Tetramers in the Double‐Helical Complex[Cd2(bpa)2Cl4]·6 H2O

AbstractThe new complex [Cd2(bpa)2Cl4]·6 H2O [bpa = N,N'‐bis(picolinamide)azine] has been synthesized in which each bpa ligand is twisted into a spiral‐like conformation and the two CdII ions are ligated by two bpa ligands to form a double‐stranded binuclear helicate. Unprecedented water chains featuring cyclic water tetramers composed of three independent water molecules are trapped by [Cd2(bpa)2Cl4] double helicates by hydrogen bonds. The binuclear chiral helicates are interlinked into homochiral layers by hydrogen bonds with the water tapes. The layers are alternately stacked in a heterochiral fashion to yield the final racemic structure. The complex shows an endotherm at 65.7 °C in the DSC due to loss of water molecules, giving an enthalpy value of 47 kJ mol–1 per water molecule. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2005)

Bis-bidentate bridging ligands containing two N, O-chelating pyrazolyl-phenolate units; double helical complexes with Co(II), Cu(II) and Zn(II)

Two ligands have been prepared in which N,O-bidentate chelating pyrazolyl-phenolate units, based on 3-(2-hydroxyphenyl)pyrazole, are connected via methylene linkages to aromatic (1,4-phenylene or 3,3′-biphenylene) spacers. In each case the two N,O-donor units are too far apart to chelate to a single metal ion. Complexes of both ligands with Co(II), Cu(II) and Zn(II) were prepared and structurally characterised; in all cases the complexes are dinuclear double helicates M 2L 2, with each four-coordinate metal ion bound by a chelating unit from each of the two ligands in the complex. For Co(II) and Zn(II) the two M(NO) planes at each metal are close to perpendicular, indicative of a geometry which may be described as approximately distorted tetrahedral; for the Cu(II) complexes the angle between the two Cu(NO) planes is less, indicative of a distortion towards a more planar coordination geometry.

Hairpin helicates: a missing link between double-helicates and trefoil knots

We have prepared a novel ligand in which two qpy metal-binding domains are linked by a polyoxyethylene spacer. The new ligand reacts with copper(ii) salts to form dinuclear double helicates with a hairpin structure. The hairpin complex and a model complex have been structurally characterised. The hairpin structure is formed regioselectively as the head-to-tail (HT) stereoisomer. Detailed electrochemical and spectroelectrochemical studies of the complexes are reported.

Allosteric and electrostatic reprogramming of a ditopic ligand.

A potentially tetradentate pyridyl–thiazole ligand chain and an additional “external” crown ether binding site connect the two halves of a new ditopic ligand. The pyridyl-thiazole binding sites can be changed or “reprogrammed” by the addition of Ba2+: In the presence of Hg2+ and Na+ the ligand L1 forms a dinuclear double helicate ([Hg2(L1)2Na2]6+), whereas in the presence of Hg2+ and Ba2+ a mononuclear species is formed ([Hg(L1)Ba]4+).

Metal-assisted assembly of pyridine-containing arylene ethynylene strands to enantiopure double helicates.

Pyridine-containing arylene ethynylene strands were connected to the 2- and 2'-positions of (R)- and (S)-1,1'-binaphthyl templates. The arylene ethynylene moieties underwent intramolecular coordination with Ag(I) or Cu(I) ion to afford enantiopure double helicates. The double-helical structure was elucidated on the basis of circular dichroic (CD) spectra. The importance of intramolecular complexation of the double strands for the helicate formation was confirmed by comparison with a ligand bearing a single strand. Connection of the strands through an ether linkage enabled a sorting out of the Cotton effect induced by double-helical arylene ethynylene moieties. The CD exciton chirality method unambiguously proved that the termini of the strands approach each other upon complexation and that the sense of the induced helicity is the same as predicted by molecular modeling.

Novel multidentate pyridyl/thiazolyl ligands containing terpyridine units: formation of dinuclear and trinuclear double helicate complexes.

Three new multidentate N-donor ligands L 1–L 3, which contain a combination of pyridyl and thiazolyl donor units, have been prepared starting from 2,2′:6′,2″-terpyridine. The extended linear sequence of ortho-linked N-donor heterocycles (five for L 1; six for L 2; and seven for L 3) is reminiscent of the well-known oligopyridines, although they have different coordination properties due to the presence of five-membered thiazolyl rings which result in natural breaks of the ligand backbone into distinct bidentate or terdentate domains. Thus, the pentadentate ligand L 1 partitions into terdentate and bidentate domains to give a dinuclear double helicate [M 2(L 1) 2] 4+ with Cu(II). The hexadentate ligand L 2 partitions into two terdentate domains to give dinuclear double helicates [M 2(L 2) 2] 4+ with six-coordinate first-row transition metals dications (M=Ni and Co). The heptadentate ligand L 3 partitions into two terminal bidentate and a central terdentate domains to give a trinuclear double helicate [Cd 3(L 3) 2] 6+ with Cd(II). The formation of these complexes highlights the fact that the thiazole group is instrumental in instructing the partitioning of the ligand, which in turn controls the formation of the helicate complexes.

A novel left-handed double helicate constructed from L-tartrate bridged molybdenum(vi) and gadolinium(iii) atoms.

The one-dimensional double helicate, [NH4][Mo2O4Gd(H2O)6(L-C4H2O6)2] x 4H2O (1), which was synthesized by the reaction of GdCl3, L-tartaric acid and ammonium molybdate in acidified water solution, is built up by two heft-handed single-helical chains, linked up further by eight-coordinated GdIII pieces in an enantiopure left-handed double helical configuration, of which each helix is formed by L-tartrate bridged six-coordinated MoVI atoms.

Cadmium-containing pyridyl–thiazole complexes: crystal structures and solution behaviour of mononuclear, dinuclear double helicate and dinuclear triple helicate complexes

Reaction of Cd(ClO 4) 2 with the potentially tetra- (L 1), penta- (L 2) and hexadentate (L 3) pyridine–thiazole-containing ligands gives [Cd 2(L 1) 3(H 2O)][ClO 4] 4 (a dinuclear triple helicate), mononuclear [Cd(L 2)(ClO 4) 2], and [Cd 2(L 3) 2(ClO 4)(CH 3CN)][ClO 4] 3 (a dinuclear double helicate), respectively. In [Cd 2(L 1) 3(H 2O)][ClO 4] 4 two of the ligands L 1 partition into two bidentate pyridyl–thiazole domains whereas the remaining ligand partitions into a bidentate (pyridyl–thiazole) and monodentate (coordinating pyridyl unit with a pendant thiazole) unit; one Cd(II) centre is coordinated by three bidentate ligand fragments, whereas the other is coordinated by two bidentate and one monodentate ligand fragments as well as a water molecule. This low-symmetry arrangement is retained in solution. In [Cd(L 2)(ClO 4) 2], L 2 acts as a planar pentadentate equatorial ligand with perchlorate anions coordinated at the axial sites; the ligand has a shallow helical twist to minimise steric interactions between the terminal pyridyl H 6 protons, which are directed towards each other. In [Cd 2(L 3) 2(ClO 4)(CH 3CN)][ClO 4] 3, the potentially hexadentate ligand L 3 is partitioned into terdentate (pyridyl–thiazole–pyridyl) and bidentate (pyridyl–thiazole) coordination domains with a non-coordinated terminal pyridyl unit; each Cd(II) centre is coordinated by one terdentate and one bidentate ligand fragment, with the sixth site being occupied by MeCN at one Cd(II) site and a perchlorate anion at the other. Again, the low symmetry coordination mode of the ligands is retained in solution although the two metal centres become equivalent.