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International audience; Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy, however, the excited state mechanisms by which photochemical ligand release operate remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)2(N^N)]2+ (bpy = 2,2'-bipyridyl; N^N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)) in which we probe the contribution to the promotion of photochemical N^N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N^N ligand. Complexes 2 to 6 all release the ligand N^N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)2(NCMe)2]2+ with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (3MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of 3MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a 3MCcis state but promotes the population of 3MCtrans states which rapidly deactivate 3MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of 3MCcis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Diimine metal complexes have significant relevance in the development of photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) applications. In particular, complexes of the TAP ligand (1,4,5,8-tetraazaphenanthrene) are known to lead to photoinduced oxidation of DNA, while TAP- and triazole-based complexes are also known to undergo photochemical ligand release processes relevant to PACT. The photophysical and photochemical properties of heteroleptic complexes [Ru(TAP)n(btz)3-n]2+ (btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, n = 1 (1), 2 (2)) have been explored. Upon irradiation in acetonitrile, 1 displays analogous photochemistry to that previously observed for [Ru(bpy)(btz)2]2+ (bpy = 2,2'-bipyridyl) and generates trans-[Ru(TAP)(btz)(NCMe)2]2+ (5), which has been crystallographically characterized, with the observation of the ligand-loss intermediate trans-[Ru(TAP)(κ2-btz)(κ1-btz)(NCMe)]2+ (4). Complex 2 displays more complicated photochemical behavior with not only preferential photorelease of btz to form cis-[Ru(TAP)2(NCMe)2]2+ (6) but also competitive photorelease of TAP to form 5. Free TAP is then taken up by 6 to form [Ru(TAP)3]2+ (3) with the proportion of 5 and 3 observed to progressively increase during prolonged photolysis. Data suggest a complex set of reversible photochemical ligand scrambling processes in which 2 and 3 are interconverted. Computational DFT calculations have enabled optimization of geometries of the pro-trans 3MCcis states with repelled btz or TAP ligands crucial for the formation of 5 from 1 and 2, respectively, lending weight to recent evidence that such 3MCcis states play an important mechanistic role in the rich photoreactivity of Ru(II) diimine complexes.

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)

One topical area of supramolecular chemistry is the binding of anionic species but despite the importance of anions in diverse cellular processes and for cancer development, anion receptors or ‘binders’ have received little attention as potential anti-cancer therapeutics. Here we report self-assembling trimetallic cryptands (e.g. [L2(Metal)3]6+ where Metal = Cu2+, Zn2+ or Mn2+) which can encapsulate a range of anions and which show metal-dependent differences in chemical and biological reactivities. In cell studies, both [L2Cu3]6+ and [L2Zn3]6+ complexes are highly toxic to a range of human cancer cell lines and they show significant metal-dependent selective activity towards cancer cells compared to healthy, non-cancerous cells (by up to 2000-fold). The addition of different anions to the complexes (e.g. PO43ˉ, SO42ˉ or PhOPO32ˉ) further alters activity and selectivity allowing the activity to be modulated via a self-assembly process. The activity is attributed to the ability to either bind or hydrolyse phosphate esters and mechanistic studies show differential and selective inhibition of multiple kinases by both [L2Cu3]6+ and [L2Zn3]6+ complexes but via different mechanisms., Artificial self-assembling systems such as anion receptors or ‘binders’ are largely unexplored for therapeutic applications. Here, the authors report self-assembling trimetallic cryptands containing copper, zinc or manganese that encapsulate a range of anions, are highly toxic to human cancer cell lines and show metal-dependent selectivity towards cancer vs. healthy cells linked to the selective inhibition of multiple kinases.

The tris(1,2,3-triazol-4-yl)methane framework offers a highly versatile architecture for ligand design, yet the coordination chemistry of this class of ligand remains largely unexplored. We report here the synthesis and characterisation of the homoleptic complexes [M(ttzm)

The ligands L1 and L2 form trinuclear self‐assembled complexes with Cu2+ (i.e. [(L1)2Cu3]6+ or [(L2)2Cu3]6+) both of which act as a host to a variety of anions. Inclusion of long aliphatic chains on these ligands allows the assemblies to extract anions from aqueous media into organic solvents. Phosphate can be removed from water efficiently and highly selectively, even in the presence of other anions., The ligands L1 and L2 form trinuclear self‐assemblies with Cu2+ (i.e. [L 2Cu3]6+) both of which act as hosts to a variety of anions. Inclusion of long aliphatic chains on these ligands allows the self‐assemblies to extract anions from aqueous media into organic solvents. Phosphate anions can be removed from water efficiently and highly selectively in the presence of other anions.

A Re(I) complex of 3,3′-diamino-2,2′-bipyridine reacts irreversibly with aldehydes and unhindered ketones in water to form bis-aminal cyclised derivatives with the solid-state structure of the reaction with benzaldehyde reported. This reaction produces a Re(I) complex which is significantly more emissive than the starting compound.

A convenient, highly regioselective synthesis of five-membered cyclic vinyl(aryl)iodonium salts directly from β-iodostyrenes is presented. An X-ray crystal structure confirms the identity of these heterocycles. These λ3-iodanes can be converted rapidly into functionalized arylacetylenes by treatment with mild base or undergo SNV reactions with nonbasic nucleophiles.

International audience; Re(i) complexes bearing thermally reversible photochromic naphthopyran axial ligands undergo highly efficient, reversible phosphorescence quenching actuated by either visible or UV irradiation. The photoinduced quenching of the triplet metal-to-ligand charge-transfer (3MLCT) emission is interpreted based on changes in the relative energies of the excited states.

A small series of novel isoflavone/benzo-δ-sultam hybrids was synthesised and evaluated as potential anti-inflammatory and neuroprotective drugs in LPS-activated BV2 microglia. The benzo-δ-sultam core was constructed in a two-step reaction by coupling 2-halobenzenesulfonamide derivatives with terminal alkynes, followed by a 6-endo-dig cyclisation. The synthesised compounds, including precursors and hybrids, were tested for their ability to inhibit NO and TNF-α production in LPS-stimulated BV2 microglial cells, and the results are promising. The most potent hybrid reduces the NO production to 41%, and the TNF-α to 34% at 20 µM final concentration in the well.

Diversely substituted 1,2-oxathiine 2,2-dioxides, including 3,5,6-triaryl-, 3,6-diaryl-, 3,5-diaryl-, 5,6-diaryl- and selected fused heterocyclic analogues, have been efficiently obtained by the application of a mild Cope elimination of a 4-amino moiety from the requisite 4-amino-3,4-dihydro-1,2-oxathiine 2,2-dioxides, which themselves were readily obtained by the addition of sulfenes to enaminoketones.

The series of complexes [Os(bpy)3- n(pytz) n][PF6]2 (bpy = 2,2'-bipyridyl, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole, 1 n = 0, 2 n = 1, 3 n = 2, 4 n = 3) were prepared and characterized and are rare examples of luminescent 1,2,3-triazole-based osmium(II) complexes. For 3 we present an attractive and particularly mild preparative route via an osmium(II) η6-arene precursor circumventing the harsh conditions that are usually required. Because of the high spin-orbit coupling constant associated with the Os(II) center the absorption spectra of the complexes all display absorption bands of appreciable intensity in the range of 500-700 nm corresponding to spin-forbidden ground-state-to-3MLCT transitions (MLCT = metal-to-ligand charge transfer), which occur at significantly lower energies than the corresponding spin-allowed 1MLCT transitions. The homoleptic complex 4 is a bright emitter (λmaxem = 614 nm) with a relatively high quantum yield of emission of ∼40% in deoxygenated acetonitrile solutions at room temperature. Water-soluble chloride salts of 1-4 were also prepared, all of which remain emissive in aerated aqueous solutions at room temperature. The complexes were investigated for their potential as phosphorescent cellular imaging agents, whereby efficient excitation into the 3MLCT absorption bands at the red side of the visible range circumvents autofluorescence from biological specimens, which do not absorb in this region of the spectrum. Confocal microscopy reveals 4 to be readily taken up by cancer cell lines (HeLa and EJ) with apparent lysosomal and endosomal localization, while toxicity assays reveal that the compounds have low dark and light toxicity. These complexes therefore provide an excellent platform for the development of efficient luminescent cellular imaging agents with advantageous photophysical properties that enable excitation and emission in the biologically transparent region of the optical spectrum.

The self-assembled trimetallic species [L2 Cu3 ]6+ contains a cavity that acts as a host to many different anions. By using X-ray crystallography, ESI-MS, and UV/Vis spectroscopy we show that these anions are encapsulated both in the solid state and aqueous systems. Upon encapsulation, the anions Br- , I- , CO32- , SiF62- , IO63- , VO43- , WO42- , CrO42- , SO42- , AsO43- , and PO43- are all precipitated from aqueous solution and can be removed by filtration. Furthermore, the cavity can be tuned to be selective to either phosphate or sulfate anions by variation of the pH. Phosphate anions can be removed from water, even in the presence of other common anions, reducing the concentration from 1000 to

Fundamental insights into the mechanism of triplet-excited-state interligand energy transfer dynamics and the origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C

1,2-Oxathiine 2,2-dioxides have been obtained from their respective 3,4-dihydro-4-dimethylamino precursors, for the first time, by a mild Cope elimination of the 4-dimethylamino function. The application of the 1,2-oxathiine 2,2-dioxide scaffold in materials chemistry is exemplified by the efficient P-type photochromism of the 5,6-bis(2,5-dimethyl-3-thienyl) substituted oxathiine 2,2-dioxides.

The ligand L2,2 contains two bidentate domains separated by a 3,3-diamino-2,2'-biphenyl spacer unit and with Cu(ClO4)2 a mononuclear species is formed (e.g. [Cu(L2,2)]2+). Upon coordination with Cu(triflate)2 the ligand undergoes reaction with an acetonitrile solvent, producing a different ligand with unsymmetrical bidentate and tridentate domains (L2,3). This new ligand results in the formation of a tetranuclear head-to-tail circular helicate [Cu4(L2,3)4]8+ showing that in the presence of the triflate anion the ligand denticity is changed.

A new application of Pd-catalysed allylation is reported that enables the synthesis of a range of branched sp3-functionalised sulfonamides, a compound class for which few reported methods exist. By reacting benzyl sulfonamides with allylic acetates in the presence of Pd0 catalysts and base at room temperature, direct allylation was efficiently performed, yielding products that are analogues of structural motifs seen in biologically active small molecules. The reaction was performed under mild conditions and could be applied to nanomolar sigma-receptor binders, thus enabling a late-stage functionalisation and efficient expansion of drug-like chemical space.

Dithizone, since its discovery in 1878, has become essential in colorimetric assays for numerous transition metal ions. However, despite it being a vital reagent its coordination chemistry is not fully understood. Here we give insight into the binary complexes of dithizone, which contain two different metal ions, and the secondary complexes with Cu(II) which can either involve reduction of the metal ion or double deprotonation of dithizone giving the self-assembled [(H-DPTC)8Cu8] and [(DPTC)4Cu4] species respectively.

Heterocyclic architectures offer powerful creative possibilities to a range of chemistry end-users. This is particularly true of heterocycles containing a high proportion of sp3 -carbon atoms, which confer precise spatial definition upon chemical probes, drug substances, chiral monomers and the like. Nonetheless, simple catalytic routes to new heterocyclic cores are infrequently reported, and methods making use of biomass-accessible starting materials are also rare. Here, we demonstrate a new method allowing rapid entry to spirocyclic bis-heterocycles, in which inexpensive iron(III) catalysts mediate a highly stereoselective C-C bond-forming cyclization cascade reaction using (2-halo)aryl ethers and amines constructed using feedstock chemicals readily available from plant sources. Fe(acac) 3 mediates the deiodinative cyclization of (2-halo)aryloxy furfuranyl ethers, followed by capture of the intermediate metal species by Grignard reagents, to deliver spirocycles containing two asymmetric centres. The reactions offer potential entry to key structural motifs present in bioactive natural products.

Whilst [Os(N^N)3 ](2+) complexes are supposed to be photochemically inert to ligand loss, the complex [Os(btz)3 ](2+) (btz=1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl) undergoes unprecedented photolytic reactivity to liberate free btz (Φ363 ≈1.2 %). Further, both cis and trans isomers of the photodechelated ligand-loss solvento intermediate [Os(κ(2) -btz)2 (κ(1) -btz)(NCMe)](2+) are unambiguously observed and characterized by NMR spectroscopy and mass spectrometry.

An attempted synthesis of the indolizidine natural product castanospermine resulted in the successful addition of cyclopropenone to a sugar-derived poly-hydroxylated cyclic imine to give an indolizidinone product, but with the installation of an extra hydroxy group at the castanospermine 8a-bridgehead position. This was also observed in our previous approach to the australine and hyacinthacine pyrrolizidine natural products. The same oxidative phenomenon occurred during the synthesis of pyrrolo[1,2-a]isoquinolines from the reaction of aldimine dihydroisoquinolines with cyclopropenones, whereas ketimine based dihydroisoquinolines gave pyrrolo[1,2-a]isoquinolines without bridgehead oxidation. These results may have some significance for the origins of the bridgehead hydroxy natural products jenamidine B1/B2, clazamycin A/B and legonmycin A/B. The precursor cyclic aldimine for the synthesis of the indolizino[8,7-b]indoles gave dimeric indolizino[8,7-b]indoles, whereas the corresponding cyclic ketimines behaved as expected and gave the indolizino[8,7-b]indole core after reaction with cyclopropenones.

The tripodal ligand L forms a trinuclear assembly [L2Cu3]6+ in which large and unsymmetrical anions are encapsulated, with the X-ray structures of [L2Cu3(SeO4)]4+ and [L2Cu3(S2O3)]4+ reported. In the presence of the trigonal pyramidal SeO32− anion, both a trinuclear [L2Cu3(SeO3)]4+ and an octametallic assembly [L4Cu8(SeO3)4]8+ are formed and are analysed by X-ray crystallography. The formation of the octanuclear species is a consequence of maximising both the –NH⋯SeO32− interactions and the Cu2+ coordination bonds.

The synthesis and characterisation of a photoresponsive 3,3-diphenyl-3H-pyrano[3,2-f]quinoline ligand which contains both quinoline and thiazole N-donor moieties is described. This ligand acts as a bidentate N-donor ligand and the solid-state structure of a Ag(I) complex is reported. Whereas the free ligand exhibits typical photochromic behaviour, coordination with Ag(I) results in complete inhibition of the photochromic response. However, excitation wavelength dependent emission spectra demonstrated an increase in fluorescence response of the new Ag(I) complex.

Ligands L1 and L2 both form dinuclear assemblies with Cu(ii) and these react with dihydrogen phosphate to give [Cu2L2(H2PO4)]3+. However, in the presence of tetrafluoroborate anions the phosphate undergoes reaction with the anion forming [Cu3(L1)3(O3POBF3)]3+ and [Cu2(L2)2(O2P(OBF3)2)]+.

The ligands L1 and L2 both form separable dinuclear double-stranded helicate and mesocate complexes with RuII . In contrast to clinically approved platinates, the helicate isomer of [Ru2 (L1 )2 ]4+ was preferentially cytotoxic to isogenic cells (HCT116 p53-/- ), which lack the critical tumour suppressor gene. The mesocate isomer shows the reverse selectivity, with the achiral isomer being preferentially cytotoxic towards HCT116 p53+/+ . Other structurally similar RuII -containing dinuclear complexes showed very little cytotoxic activity. This study demonstrates that alterations in ligand or isomer can have profound effects on cytotoxicity towards cancer cells of different p53 status and suggests that selectivity can be "tuned" to either genotype. In the search for compounds that can target difficult-to-treat tumours that lack the p53 tumour suppressor gene, [Ru2 (L1 )2 ]4+ is a promising compound for further development.

This work shows how multiple non-covalent interactions are employed to control metallosupramolecular architectures and we demonstrate that a ligand, which contains two bidentate domains separated by a ArOH spacer, forms a mesocate when complexed with Ag(I). However, changing this to an ArOCH2CH2Ph spacer unit results in a 1-dimensional helical polymer upon reaction with the same cation. Reaction of Ag(I) with the ArOMe derivative gives a hexanuclear circular helicate which forms inter-assembly Ag⋯Ag interactions resulting in a 3-dimensional honeycomb-like polymer of hexanuclear circular helicates.

3-Benzoylnaphtho[1,2-b]furan-5-ol forms a photochromic benzochromene upon reaction with a 1,1-diarylprop-2-yn-1-ol affording a red coloured photomerocyanine, with a half-life of 2.3 min, upon UV-irradiation. Retention of the initial 1,4-oxygenation pattern of the naphthalene moiety through replacement of the furan unit with a methoxy group led to a benzochromene, which developed a similar red colour upon UV-irradiation but was more persistent with a half-life of over 42 min. Treatment of the 3-benzoylnaphtho[1,2-b]furan-5-ol with a 1,1,3-triarylprop-2-yn-1-ol similarly afforded a benzochromene, which did not display any photochromism at ambient temperature as a consequence of steric interactions in the photomerocyanines. The synthesis of N-(5-hydroxynaphtho[1,2-d]thiazol-2-yl)acetamide, as a precursor to a photochromic hetero-fused benzochromene, by the mild selective deprotection of the O-acetyl group of 2-acetamidonaphtho[1,2-d]thiazol-5-yl acetate was complicated by a facile, competitive, oxidative dimerisation to afford a novel [2,2′-binaphthothiazole]-1,1′-diol.

The amine-containing ligand L, composed of two bidentate pyridyl-thiazole moieties linked by a 1,3-diaminophenylene unit, reacts with copper(II) ions to form a dinuclear double helicate [Cu2 L2 ](4+) . Reaction of [Cu2 L2 ](4+) with dihydrogen phosphate (0.5 equivalents) gives the unsaturated dinuclear double helicate [Cu2 L2 (OPO3 H2 )](3+) . [Cu2 L2 (OPO3 H2 )](3+) further reacts with another 0.5 equivalents of dihydrogen phosphate to give a trinuclear circular helicate which then self-assembles into a hexameric cluster [{Cu3 L3 (OPO3 H2 )3 }](26+) .

We report the unprecedented observation and unequivocal crystallographic characterization of the meta-stable ligand loss intermediate solvento complex trans-[Ru(bpy)(κ2-btz)(κ1-btz)(NCMe)]2+ (1 a) that contains a monodentate chelate ligand. This and analogous complexes can be observed during the photolysis reactions of a family of complexes of the form [Ru(equation image)(btz)2]2+ (1 a–d: btz=1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl; equation image=a) 2,2′-bipyridyl (bpy), b) 4,4′-dimethyl-2,2′-bipyridyl (dmbpy), c) 4,4′-dimethoxy-2,2′-bipyridyl (dmeobpy), d) 1,10-phenanthroline (phen)). In acetonitrile solutions, 1 a–d eventually convert to the bis-solvento complexes trans-[Ru(equation image)(btz)(NCMe)2]2+ (3 a–d) along with one equivalent of free btz, in a process in which the remaining coordinated bidentate ligands undergo a new rearrangement such that they become coplanar. X-ray crystal structure of 3 a and 3 d confirmed the co-planar arrangement of the equation image and btz ligands and the trans coordination of two solvent molecules. These conversions proceed via the observed intermediate complexes 2 a–d, which are formed quantitatively from 1 a–d in a matter of minutes and to which they slowly revert back on being left to stand in the dark over several days. The remarkably long lifetime of the intermediate complexes (>12 h at 40 °C) allowed the isolation of 2 a in the solid state, and the complex to be crystallographically characterized. Similarly to the structures adopted by complexes 3 a and d, the bpy and κ2-btz ligands in 2 a coordinate in a square-planar fashion with the second monodentate btz ligand coordinated trans to an acetonitrile ligand.

The syntheses of a series of new ketohydrazone dyes and pigment lakes with a unique structural motif derived from 4-morpholino-2-naphthol are described. The morpholine substituent is responsible for the hypsochromically shifted absorption maxima with significantly enhanced molar extinction coefficients of the new dyes; this conclusion is supported by time-dependent DFT (TD-DFT) simulations. The location of the morpholine substituent at a sterically demanding site in the pigment lake obstructs further coordination of the bridging sulfonate groups in the solid state and prevents the formation of a 1-dimensional polymeric system, which is common for other pigment lakes.

A new application of Pd-catalysed allylation is reported that enables the synthesis of a range of branched sp

We report the synthesis, characterization, and photochemical reactivity of the triazole-containing complex [Ru(pytz)(btz)2](2+) (1, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole, btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl). The UV-vis absorption spectrum of 1 exhibits pytz- and btz-centered (1)MLCT bands at 365 and 300 nm, respectively. Upon photoexcitation, acetonitrile solutions of 1 undergo conversion to the ligand-loss intermediate, trans-[Ru(pytz)(κ(2)-btz)(κ(1)-btz)(NCMe)](2+) (2, Φ363 = 0.013) and ultimately to the ligand-loss product trans-[Ru(pytz)(btz)(NCMe)2](2+) (3), both of which are observed and characterized by (1)H NMR spectroscopy. Time-dependent density functional theory calculations reveal that the S1 state of the complex has primarily HOMO → LUMO pytz-based (1)MLCT character. Data show that the (3)MLCT and (3)MC states are in close energetic proximity (≤0.11 eV to 2 d.p.) and that the T1 state from a single-point triplet state calculation at the S0 geometry suggests (3)MC character. Optimization of the T1 state of the complex starting from the ground state geometry leads to elongation of the two Ru-N(btz) bonds cis to the pytz ligand to 2.539 and 2.544 Å leading to a pseudo-4-coordinate (3)MC state rather than the (3)MLCT state. The work therefore provides additional insights into the photophysical and photochemical properties of ruthenium triazole-containing complexes and their excited state dynamics.

Sixteen new isothiazoloisoxazole 1,1-dioxides, one new isothiazolotriazole and one new isothiazolopyrazole have been synthesised by using 1,3-dipolar cycloadditions to isothiazole 1,1-dioxides. One sub-set of these isothiazoloisoxazoles showed low μM activity against a human breast carcinoma cell line, whilst a second sub-set plus the isothiazolotriazole demonstrated an interesting restricted rotation of sterically hindered bridgehead substituents. A thiazete 1,1-dioxide produced from one of the isothiazole 1,1-dioxides underwent conversion into an unknown 1,2,3-oxathiazolin-2-oxide upon treatment with Lewis acids, but was inert towards 1,3-dipoles and cyclopropenones. Six supporting crystal structures are included.

Artificial allosterism: A ditopic ligand, which contains both bipyridine and tetra-aza-crown binding sites, coordinates one Cu(II) through all four N-donors of the tetradentate aza-crown unit. Reaction at the bipyridine site with another Cu(II) allosterically changes the aza-crown from being a tetradentate to a tridentate N-donor unit, a change in coordination mode that causes the successive binding of two Cu(II) ions to proceed with severe negative cooperativity (see scheme).

The syntheses of two novel polydentate pyridyl-thiazolyl ligands which contain an N-oxide unit on the central pyridyl ring are reported. Reaction of either of these ligands with Cd(ClO4)2·6H2O results in the formation of a dinuclear double helicate. The solid state structures of these transition metal helicates demonstrate the ability of the N-oxide unit to partition the ligands into two separate binding domains.

This review describes the self-assembly of anion receptors from organic ligands and transition metal ions. These metal-assembled anion receptors can be synthesised from a number of different species; bidentate ligands with metals that prefer octahedral coordination geometries and monodentate ligands with metals that prefer square planar geometries are common. Anion binding transition metal helicates and systems where the coordination of metal ions results in the formation of an anion receptor by conformational locking are also reported. The effect of anion binding on the different properties of these complexes is discussed.

Reaction of the potentially tetradentate N-donor ligand 6,6′-bis(4-methylthiazol-2-yl)-2,2′-bipyridine (L1) with the transition metal dications CoII, NiII, CuII, CdII and HgII results in the formation of mononuclear [M(L1)]2+ complexes, in which a planar ligand coordinates to the metals via all four N-donors. In contrast, reaction of L1 with CuI and AgI monocations, affords dinuclear double stranded helicate species [M2(L1)2]2+ (where M = CuI or AgI), in which partitioning of the ligand into two bis-bidentate pyridyl-thiazole chelating units allows each ligand to bridge both metal centres. X-Ray crystallography, electrospray mass spectroscopy and NMR spectroscopy reveal that the complexes [Mn(L1)m]z+ (where n = 1, m = 1 and z = 2, when M = CoII, NiII, CuII, CdII and HgII; n = 2, m = 2 and z = 2, when M = CuI), retain their solid-state structures in solution. Conversely, whilst 1H NMR studies suggest that combination of equimolar amounts of Ag(X)(where ) and L1 (in either nitromethane or acetonitrile) results in the formation of a helicate in solution, in the solid-state, an anion-templating effect gives rise to either mononuclear or dinuclear helicate structures [Agn(L1)n][X]n (where n = 2 when X = OTf−; n = 1 when ).

Three new multidentate N-donor ligands L1–L3, 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 L1; six for L2; and seven for L3) 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 L1 partitions into terdentate and bidentate domains to give a dinuclear double helicate [M2(L1)2]4+ with Cu(II). The hexadentate ligand L2 partitions into two terdentate domains to give dinuclear double helicates [M2(L2)2]4+ with six-coordinate first-row transition metals dications (M=Ni and Co). The heptadentate ligand L3 partitions into two terminal bidentate and a central terdentate domains to give a trinuclear double helicate [Cd3(L3)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.

Reaction of Cd(ClO4)2 with the potentially tetra- (L1), penta- (L2) and hexadentate (L3) pyridine–thiazole-containing ligands gives [Cd2(L1)3(H2O)][ClO4]4 (a dinuclear triple helicate), mononuclear [Cd(L2)(ClO4)2], and [Cd2(L3)2(ClO4)(CH3CN)][ClO4]3 (a dinuclear double helicate), respectively. In [Cd2(L1)3(H2O)][ClO4]4 two of the ligands L1 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(L2)(ClO4)2], L2 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 H6 protons, which are directed towards each other. In [Cd2(L3)2(ClO4)(CH3CN)][ClO4]3, the potentially hexadentate ligand L3 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.

Reaction of the potentially tetradentate ligand L4 with Co(ClO4)2 gives the dinuclear triple helicate [Co2(L4)3](ClO4)4 with the ligand L4 partitioning into two N,N-bidentate units (phenanthroline and thiazolyl–pyridine); in contrast, reaction of L4 with Cu(ClO4)2 affords the mononuclear complex [Cu(L4)(OH2)(ClO4)](ClO4) in which L4 acts as a terdentate ligand via the phenanthroline and thiazolyl units, with the pyridyl group pendant but hydrogen-bonded to the water ligand.

Comparison of the crystal structures of the dinuclear double helicates [M2(L1)2][ClO4]4 (M = Ni, Zn; L1 is a potentially hexadentate ligand containing a py–th–py–py–th–py sequence, where ‘py’ denotes pyridyl and ‘th’ denotes thiazolyl) illustrates how L1 can show two different coordination modes: in [Zn2(L1)2][ClO4]4 the ligands L1 are bis-bidentate chelates (via the terminal py–th fragments, with the central bipyridyl unit not coordinated) such that the metal ions are four-coordinate, whereas in [Ni2(L1)2][ClO4]4 the ligand coordinates in a more usual bis-terdentate manner such that the metal ions are six-coordinate. Reaction of Ni(II), Cu(II) or Zn(II) salts with a 1 ∶ 1 mixture of the potentially hexadentate ligands L1 and L2 (where L2 contains a phen–th–th–phen sequence, ‘phen’ denoting a 1,10-phenanthroline unit) afforded in each case a mixture of helical complexes [M2(L1)2]4+, [M2(L1)(L2)]4+ and [M2(L2)2]4+ in different proportions according to the preferences of the different metal ions for different coordination numbers, and the actual denticity of the ligand. For example the mixed-ligand complex [M2(L1)(L2)]4+ was formed to the same extent (ca. 50%) for M = Ni and M = Cu, but hardly at all for M = Zn, indicating that self–self ligand recognition operates during assembly of L1 and L2 with Zn(II) such that the homoleptic complexes [Zn2(L1)2]4+ and [Zn2(L2)2]4+ are favoured more than simple statistical considerations would suggest.

Four new multidentate N-donor ligands L1–L4 have been prepared which contain a combination of pyridyl and thiazolyl donor units. The syntheses of these ligands are facile and high-yielding, being based on reaction of an α-bromoacetyl unit with a thioamide to form the thiazolyl ring. The extended linear sequence of ortho-linked N-donor heterocycles (four for L1, six for L2; five for L3; and six for L4) is reminiscent of the well-known linear oligopyridines, although these new ligands are much easier to make and have significantly different geometric coordination properties because the presence of the five-membered thiazolyl rings results in natural breaks of the ligand backbone into distinct bidentate or terdentate domains. Thus, the tetradentate ligand L1 partitions into two bidentate domains to give dinuclear triple helicates [M2(L1)3]4+ with six-coordinate first-row transition metal dications (M = Co, Cu, Zn). The hexadentate ligand L2 partitions into two terdentate domains to give dinuclear double helicates [M2(L2)2]4+ with six-coordinate metal ions (M = Cu, Zn). In the double helicate [Cu2(L3)2]4+ the pentadentate ligand L3 only uses its two terminal bidentate binding sites, resulting in four-coordinate Cu(II) centres and a non-coordinated pyridyl residue in the centre of each of the two ligand strands. These pendant pyridyl residues are directed towards each other to give a potentially two-coordinate cavity between the metal ions in the centre of the helicate. Similarly, in the double helicate [Cu2(L4)2]4+ the metal ions are only four-coordinate, with each ligand having its central bipyridyl unit un-coordinated. This results in a potentially four-coordinate cavity between the two metal ions in the centre of the helicate. These easy-to-prepare ligands offer a great deal of scope for the development of multinuclear helicates.

The extent of photoinduced energy-transfer between [Ru(bipy)3]2+ and [Os(bipy)3]2+ units attached to either end of a deca(oxoethylene) 33-atom chain can be used to show how the distribution of end-to-end distances in the ensemble of conformations changes in different solvents.