Jonathon E. Beves

A synthetic molecular pentafoil knot

Knots are being discovered with increasing frequency in both biological and synthetic macromolecules and have been fundamental topological targets for chemical synthesis for the past two decades. Here, we report on the synthesis of the most complex non-DNA molecular knot prepared to date: the self-assembly of five bis-aldehyde and five bis-amine building blocks about five metal cations and one chloride anion to form a 160-atom-loop molecular pentafoil knot (five crossing points). The structure and topology of the knot is established by NMR spectroscopy, mass spectrometry and X-ray crystallography, revealing a symmetrical closed-loop double helicate with the chloride anion held at the centre of the pentafoil knot by ten CH···Cl(-) hydrogen bonds. The one-pot self-assembly reaction features an exceptional number of different design elements-some well precedented and others less well known within the context of directing the formation of (supra)molecular species. We anticipate that the strategies and tactics used here can be applied to the rational synthesis of other higher-order interlocked molecular architectures.

Pentameric circular iron(II) double helicates and a molecular pentafoil knot.

We report on the synthesis of 11 pentameric cyclic helicates formed by imine condensation of alkyl monoamines with a common bis(formylpyridine)bipyridyl-derived building block and iron(II) and chloride ions. The cyclic double-stranded helicates were characterized by NMR spectroscopy, mass spectrometry, and in the case of a 2,4-dimethoxybenzylamine-derived pentameric cyclic helicate, X-ray crystallography. The factors influencing the assembly process (reactant stoichiometry, concentration, solvent, nature and amount of anion) were studied in detail: the role of chloride in the assembly process appears not to be limited to that of a simple template, and larger circular helicates observed with related tris(bipyridine) ligands with different iron salts are not produced with the imine ligands. Using certain chiral amines, pentameric cyclic helices of single handedness could be isolated and the stereochemistry of the helix determined by circular dichroism. By employing a particular diamine, a closed-loop molecular pentafoil knot was prepared. The pentafoil knot was characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography, confirming the topology and providing insights into the reasons for its formation.

The first example of a coordination polymer from the expanded 4,4 `-bipyridine ligand [Ru(pytpy)(2)](2+) (pytpy=4 `-(4-pyridyl)2,2 `: 6 `,2 ``-terpyridine)

The complex cation ligand [Ru(pytpy)2]2+ (pytpy = 4′-(4-pyridyl)-2,2′∶6′,2″-terpyridine) is an expanded 4,4′-bipyridine; we describe the first example of a coordination polymer in which [Ru(pytpy)2]2+ plays the role of a bridging bidentate ligand.

Tetrameric Cyclic Double Helicates as a Scaffold for a Molecular Solomon Link

A Solomon link, colloquially termed a “Solomon knot” (a link in Alexander–Briggs notation[1]), is a topology of two interwoven rings that cross each other four times in the simplest representation (Figure 1).[2] Such doubly-entwined [2]catenanes are still rare,[3–5] with only two small-molecule examples with wholly organic backbones reported[4,5] to date. The Solomon link is the most complex topology to have been produced[4] using Sauvage’s pioneering route[6] of generating higher order interlocked structures through the connection of the termini of linear double-stranded metal helicates. In principle,[2b,d] cyclic double helicates[7] can provide the crossings required for a range of topologies, while simultaneously positioning connecting sites in close proximity to aid the macrocyclization reactions that can be problematic when employing long linear helicates[8] (Figure 1). A small-molecule pentafoil knot (five crossings) was recently prepared using a pentameric circular helicate scaffold.[9] Here we report on the use of a tetrameric circular helicate as the basis for a Solomon link, illustrating the general utility of this approach for the assembly of complex molecular topologies. Figure 1 Ring-closing cyclic metal double helicates for the formation of topologically complex molecules. A pentameric circular double helicate is the scaffold (five crossings) required for a pentafoil knot,[9] and a tetrameric circular double helicate (four crossings) ... The ligand used in our earlier synthesis of a pentafoil knot[9] was based on a tris(bipyridine) motif employed[7a,b,d] by Lehn to assemble penta- and hexameric cyclic helicates, but with both outer bipyridine units replaced by 2-formylpyridine groups that could condense with amines to form imines and generate tris(bidentate) ligand strands. As well as providing a convenient way of connecting metal binding components, imine bond formation is reversible, imparting an ‘error checking’ mechanism during the assembly process.[10] Incorporating an additional oxygen atom in the ethylene spacer between each bipyridine group of Lehn’s tris(bipyridine) ligand led to cyclic tetrameric helicates.[7b] Accordingly, in an attempt to generate the four crossings required for a Solomon link, we introduced a similar structural change to the ligand used in the pentafoil knot synthesis in the form of 1 (for the synthesis of 1 see the Supporting Information) and investigated its coordination chemistry with primary amines and FeII salts (Scheme 1). Scheme 1 Synthesis of cyclic and linear iron(II) helicates. Reaction conditions: a) FeX2, RCH2NH2, DMSO, 60 °C, 24 h; b) excess KPF6 (aq). DMSO=dimethyl sulfoxide. The reaction of 1 with n-hexylamine and FeCl2 (DMSO, 60 °C, 24 h, Scheme 1)[8] produced an intensely colored purple solution typical of low-spin iron(II) tris(diimine) complexes. After 24 hours, the product was isolated in 47 % yield as the hexafluorophosphate salt 2 by precipitation with aqueous KPF6. Electrospray ionization mass spectrometry (ESI-MS; see the Supporting Information, Figure S1) revealed that 2 was a metal–ligand tetramer with the formula [Fe4L4](PF6)8][11] (L=bis(imine) ligand resulting from the condensation of 1 with two molecules of n-hexylamine). 1H NMR spectroscopy (Figure 2 a) indicated that 2 was highly symmetrical, with the splitting of the diastereotopic CH2-O-CH2 protons consistent with the chiral (racemic) helicate topology shown in Scheme 1. The yield of 2 was increased to 71 % (yield of isolated product) when employing 4.4 equivalents of the iron(II) salt (see the Supporting Information, Figure S9). Figure 2 1H NMR spectra (CD3CN, 500 MHz) for a) cyclic tetramer 2, b) linear triple helicate 3 (green, signals marked * correspond to trace amounts of 2), c) a 1:1 mixture of cyclic tetramer 4 (black) and linear triple helicate ... The formation of the tetrameric cyclic helicate was not limited to the use of FeCl2 as the iron(II) salt (Scheme 1), both Fe(BF4)2 and Fe(ClO4)2 also produced 2, although in significantly lower yields (see the Supporting Information, Figure S13) and contaminated with other polymeric and oligomeric by-products. When FeBr2 was employed as the iron source, a different main product was obtained (Scheme 1), which was identified as the linear trinuclear triple helicate ([Fe3L3]6+) 3 by 1H NMR spectroscopy (Figure 2 b) and ESI-MS (see the Supporting Information, Figure S14). A linear triple helicate with a lifetime of a few minutes was previously observed as an intermediate during the formation of pentameric cyclic helicates using Lehn’s tris(bipyridine) ligand.[7d] While 3 is a much longer-lived species, it is not clear whether this is because the linear triple helicate is particularly stable as the bromide salt, or whether the assembly/disassembly/rearrangement of the various linear and circular helicates and oligomers is markedly slower using FeBr2, perhaps as a result of their limited solubility. Substituting n-hexylamine for 4-methylbenzylamine in the reaction of 1 with FeCl2 gave a mixture of two species (Figure 2 c), identified by ESI-MS (Supporting Information, Figures S3 and S5) as the cyclic tetramer 4 and the linear triple helicate ([Fe3L3]6+) 5 (Scheme 1). Using our standard reaction protocol with an initial concentration of 1 of 2.2 mm, the ratio of 4/5 was approximately 1:1, however the distribution of cyclic-double-helicate/linear-triple-helicate was significantly altered by small variations in concentration: using an initial concentration of 8.8 mm of 1, more than 95 % of the reaction product was the higher order (four ligands, four metal ions) circular helicate 4 after 24 hours, whereas starting with a concentration of 0.55 mm of 1, the reaction produced more than 85 % of the lower nuclearity (three ligands, three metal ions) linear helicate 5 over the same time period (Supporting Information, Figure S15).[12] In contrast, the yield of the analogous n-hexylamine-derived cyclic tetramer 2 was essentially invariant over this concentration range and no linear triple helicate was observed, illustrating the influence that subtle changes in the ligands can have over the outcomes of the self-assembly reactions. In order to link the end groups of the open cyclic helicate to generate a Solomon link, we employed 2,2′-(ethylenedioxy)bis(ethylamine), a diamine that is stereoelectronically predisposed to adopt low-energy turns.[9] The reaction of 1 with the diamine and FeCl2 in DMSO for 24 hours, with subsequent anion exchange with aqueous KPF6, generated the Solomon link 6 in 75 % yield of isolated product (Scheme 2).[13] Scheme 2 Synthesis of molecular Solomon link 6. Reaction conditions: a) FeCl2, 2,2′-(ethylenedioxy)bis(ethylamine), DMSO, 60 °C, 24 h; b) excess KPF6 (aq), 75 % (over two steps). The 1H NMR spectrum (CD3CN, 500 MHz, Figure 2 d) of 6 is very similar to that of the tetrameric cyclic helicate 2 derived from n-hexylamine (Figure 2 a), including the splitting pattern for the diastereotopic CH2-O-CH2 protons. ESI-MS (Supporting Information, Figure S7) confirmed that 6 had a structural formula consistent with a Solomon link. Single crystals of 6 suitable for X-ray crystallography were grown by slow diffusion of diethyl ether into a nitromethane solution of 6, and the structure was confirmed by X-ray crystallography (Figure 3). The solid-state structure shows the two organic macrocycles interlocked by the four crossings that define the topology of a Solomon link. The iron atoms are close-to-coplanar and lie on the vertices of a square with Fe–Fe distances of just over 1 nm. Despite the high yield, as for the related pentafoil knot,[9] the octahedral coordination geometry of the iron(II) centers is amongst the most distorted [Fe(N-ligand)6] structures in the Cambridge Structural Database[14] (see the Supporting Information for details). The -OCH2CH2O- units in the linking group adopt close-to-gauche conformations (59–73°). Two PF6− counter ions are positioned directly above and below the center of the helicate (Figure 3 a) and form bifurcated CH⋅⋅⋅F interactions with the eight Ha protons, which are particularly electron-poor because of the ligand coordination to the iron(II) dications (Supporting Information, Figures S16 and S17). Figure 3 X-Ray crystal structure of Solomon link 6. a) Viewed in the plane of FeII ions (all but two PF6− anions omitted); b) viewed from above the center of the macrocycle cavities (all PF6− anions omitted). The C atoms of one ... The one-pot synthesis of molecular Solomon link 6 assembles four iron(II) cations, four bis(aldehyde) and four bis(amine) building blocks to generate two interwoven 68-membered-ring macrocycles with four crossings in 75 % isolated yield. The assembly process for the tetrameric cyclic double helicate forms the basis for the Solomon link synthesis and is sensitive to structural changes in the amine, the concentration and the anion used (even though the reaction product is not the result of an anion-template mechanism). The synthesis of Solomon link 6 and the earlier pentafoil knot[9] show that cyclic helicates of different sizes can act as highly efficient and effective scaffolds for intricate molecular topologies.

The self-sorting behavior of circular helicates and molecular knots and links.

We report on multicomponent self-sorting to form open circular helicates of different sizes from a primary monoamine, FeII ions, and dialdehyde ligand strands that differ in length and structure by only two oxygen atoms. The corresponding closed circular helicates that are formed from a diamine—a molecular Solomon link and a pentafoil knot—also self-sort, but up to two of the Solomon-link-forming ligand strands can be accommodated within the pentafoil knot structure and are either incorporated or omitted depending on the stage that the components are mixed.

Photochemical switching of luminescence and singlet oxygen generation by chemical signal communication

Photoluminescence in the far red spectral region and photosensitised generation of singlet oxygen, with associated near-IR emission, are reversibly controlled by near-UV or violet light in a communicating ensemble of molecular switches.

A one-dimensional copper(II) coordination polymer containing [Fe(pytpy)2]2+ (pytpy = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine) as an expanded 4,4′-bipyridine ligand : a hydrogen-bonded network penetrated by rod-like polymers

The one-dimensional coordination polymer [{(H2O)(NO3)2CuFe(1)2}2(NO3)4·2.15MeCN·5.85H2O]n assembles when the pendant pyridine donors in [Fe(1)2]2+ (1 = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine) are treated with Cu(NO3)2·3H2O, exemplifying [Fe(1)2]2+ as an expanded 4,4′-bipyridine ligand. In the solid state, the polymer chains are threaded through hydrogen-bonded macrocycles comprising coordinated and non-coordinated water molecules and nitrate ions, the macrocycles being motifs within interconnected sheets in the structure.

A Solomon Link through an Interwoven Molecular Grid

A molecular Solomon link was synthesized through the assembly of an interwoven molecular grid consisting of four bis(benzimidazolepyridyl)benzthiazolo[5,4-d]thiazole ligands and four zinc(II), iron(II), or cobalt(II) cations, followed by ring-closing olefin metathesis. NMR spectroscopy, mass spectrometry, and X-ray crystallography confirmed the doubly interlocked topology, and subsequent demetalation afforded the wholly organic Solomon link. The synthesis, in which each metal ion defines the crossing point of two ligand strands, suggests that interwoven molecular grids should be useful scaffolds for the rational construction of other topologically complex structures.

Strong and Selective Anion Binding within the Central Cavity of Molecular Knots and Links.

A molecular pentafoil knot and doubly and triply entwined [2]catenanes based on circular Fe(II) double helicate scaffolds bind halide anions in their central cavities through electrostatic and CH···X(-) hydrogen-bonding interactions. The binding is up to (3.6 ± 0.2) × 10(10) M(-1) in acetonitrile (for pentafoil knot [2·Cl](PF6)9), making these topologically complex host molecules some of the strongest synthetic noncovalent binders of halide anions measured to date, comparable in chloride ion affinity to silver salts.

The conjugate acid of bis{4′-(4-pyridyl)-2,2′:6′,2″-terpyridine}iron(II) as a self-complementary hydrogen-bonded building block

The protonation of [Fe(1)2]2+, where 1 = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine, generates a trication which forms a one-dimensional, hydrogen-bonded polymer. The single crystal structures of [Fe(1)(H1)][Fe(NCS)6]·2H2O, [Fe(1)(H1)][Fe(NCS)6]·MeCN and [Fe(1)(H1)][ClO4]3·EtOH are reported and illustrate that the packing of the {[Fe(1)(H1)]3+}n chains in the solid state can be modulated by changing the steric demands of the counter-ion. Subtle changes in packing on going from [Fe(1)(H1)][Fe(NCS)6]·2H2O to [Fe(1)(H1)][Fe(NCS)6]·MeCN result in a reorientation of the pendant pyridine rings with respect to the central pyridine ring of the tpy unit in [Fe(1)(H1)]3+.