Metal Template Approach Research Articles

Cinquefoil Knot Possessing Dynamic and Tunable Metal Coordination.

While the majority of knots are made from the metal-template approach, the use of entangled, constrained knotted loops to modulate the coordination of the metal ions remains inadequately elucidated. Here, we report on the coordination chemistry of a 140-atom-long cinquefoil knotted strand comprising five tridentate and five bidentate chelating vacancies. The knotted loop is prepared through the self-assembly of asymmetric "3 + 2" dentate ligands with copper(II) ions that favor five-coordination geometry. The formation of the copper(II) pentameric helicate is confirmed by X-ray crystallography, while the corresponding copper(II) knot is characterized by XPS and LR-/HR ESI-MS. Upon removal of the original template, the knotted ligand facilitates zinc(II) ions, which typically form four- or six-coordination geometries, resulting in the formation of an otherwise inaccessible zinc(II) metallic knot with coordinatively unsaturated metal centers. The coordination numbers and geometries of the zinc(II) cations are undoubtedly determined by X-ray crystallography. Despite the kinetically labile nature and high reversibility of the zinc(II) complex preventing the detection of 5-to-6 coordination equilibrium in solution, the effects on metal-ion coordination induced by knotting hold promise for fine-tuning the coordination of metal complexes.

Balancing ring and stopper group size to control the stability of doubly threaded [3]rotaxanes.

Synthesizing doubly threaded [3]rotaxanes requires the use of larger rings than more traditional singly threaded [2]rotaxanes. A key challenge in accessing stable doubly threaded [3]rotaxanes with large rings is finding the right combination of ring to stopper size. In this study, a series of doubly threaded [3]rotaxanes derived from five different sized macrocycles in the size range of 40-48 atoms and two different stopper groups, which contain 1 or 2 tris(p-t-butylbiphenyl)methyl moieties, were prepared and their kinetic stability examined. These interlocked compounds were synthesized using a metal-templated approach and fully characterized utilizing a combination of mass spectrometry, NMR spectroscopy, and size-exclusion chromatography techniques. The effect of ring size on the stability of the doubly threaded [3]rotaxane was investigated via kinetic stability tests monitored using 1H-NMR spectroscopy. By tightening the macrocycle systematically every 2 atoms from 48 to 40 atoms, a wide range of doubly threaded interlocked molecules could be accessed in which the rate of room temperature slippage of the macrocycle from the dumbbells could be tuned. Using the larger stopper group with a 48-atom ring results in no observable rotaxane, 46-44 atom macrocycles result in metastable rotaxane species with a slippage half-life of ∼5 weeks and ∼9 weeks, respectively, while macrocycles of 42 atoms or smaller yield a stable rotaxane. The smaller sized stopper is not able to fully stabilize any of the [3]rotaxane structures but metastable [3]rotaxanes are obtained with slippage half-lives of 25 ± 2 hours and 13 ± 1 days using macrocycles with 42 or 40 atoms, respectively. These results highlight the dramatic effect that relatively small ring size changes can have on the structure of doubly threaded [3]rotaxanes and lay the synthetic groundwork for a range of higher order doubly threaded interlocked architectures.

Synthesis and Structures of Iron(II) Metallacycles Based on a PNPNP Framework

A series of iron(II) metallacycles based on a saturated PNPNP framework has been prepared by a simple metal-templated approach, incorporating various central phosphorus donor groups. The reaction of [Cp*(CO)Fe{P(OMe)2(NPh)}2]Li (Cp* = η5-C5Me5) with RPCl2 (R = Ph, OMe, NEt2) afforded a pair of isomeric metallacycle complexes, [Cp*(CO)Fe{κ2-P1,P3-P1(OMe)2N(Ph)P2(R)N(Ph)P3(OMe)2}]PF6. For all cases of R, the configurational isomers were successfully separated, and their geometric arrangements were established through single-crystal X-ray analyses. UV light irradiation of the isomers bearing R = Ph provided the same product, [Cp*Fe{κ3-P1,P2,P3-P(OMe)2N(Ph)P(Ph)N(Ph)P(OMe)2}]PF6, via intramolecular coordination of the central phosphorus atom to the iron center. The isomers also reacted with ClAu(tht) (tht = tetrahydrothiophene) to provide the corresponding iron–gold bimetallic complexes via κ2-P1,P3-κ1-P2-coordination of the PNPNP ligand, revealing their utility as building blocks for multimetallic architectures.

DNA Origami Catenanes Templated by Gold Nanoparticles.

Mechanically interlocked molecules have marked a breakthrough in the field of topological chemistry and boosted the vigorous development of molecular machinery. As an archetypal example of the interlocked molecules, catenanes comprise macrocycles that are threaded through one another like links in a chain. Inspired by the transition metal-templated approach of catenanes synthesis, the hierarchical assembly of DNA origami catenanes templated by gold nanoparticles is demonstrated in this work. DNA origami catenanes, which contain two, three or four interlocked rings are successfully created. In particular, the origami rings within the individual catenanes can be set free with respect to one another by releasing the interconnecting gold nanoparticles. This work will set the basis for rich progress toward DNA-based molecular architectures with unique structural programmability and well-defined topology.

Heterorotaxanes.

Heterorotaxanes, in which at least two types of macrocycles were introduced as the wheel components in rotaxanes, have attracted more and more attention during the past few decades owing to their unique structural features and intriguing properties. The coexistence of varied macrocycles endows the resultant heterorotaxanes not only versatile shuttling and switching behaviors but also great potential for the construction of functional rotaxane systems for applications. In this feature article, a survey of the successful synthesis of heterorotaxanes will be provided based on the various strategies towards the synthesis of heterorotaxanes, i.e. orthogonal binding approach, self-sorting approach, cooperative capture approach, active metal template approach, etc.

Construction of mononuclear macrocyclic and dinuclear acyclic Schiff base complexes via cadmium(II)–ion template: Synthesis, characterisation, and crystal structures

Abstract Using a metal template approach a series of mono and dinuclear cadmium(II) were synthesized by Schiff base condensation of a pendant armed dialdehyde, i.e. 2-[3-(2-formylphenoxy)-2-hydroxypropoxy] benzaldehyde (PL) with selection of different diamines, diethylenetriamine, dipropylenetriamine, 1,2-diaminopropane and 1,2-diaminoethane. Depending on the nature of the diaminoalkane spacers and metal ion, 1:1 (metal:ligand) macrocyclic (L1OH and L2OH) or 2:1 (metal:ligand) acylic (L3O− and L4O−) complexes of cadmium(II) were obtained. With the longer spacers diethylenetriamine and dipropylenetriamine ring closure around the cadmium ion was observed and [1+1] macrocyclic Schiff base ligands (L1OH and L2OH) were formed. With the shorter spacers 1,2-diaminopropane and 1,2-diaminoethane ring closure was not observed. Interestingly, the dialdehyde PL reacted with two molecules of diamine, forming intermediate product acyclic Schiff base [1+2] ligands (L3O− and L4O−) that are coordinated to two cadmium ions to form dinuclear complexes. Crystal structures indicated that the complex [CdL1(NO3)]. NO3 contains a hexadentate macrocyclic ligand and a bidentate NO3− anion with an eight-coordinated cadmium in distorted dodecahedron geometry. The molecular structures of the complexes [Cd2(L3O)(NO3)3(H2O)] and [Cd2(L4O)(NO3)3], on the other hand, exhibit of two seven-coordinated cadmium centers, which are bridged by the deprotonated hydroxyl group. The coordination geometries of the cadmium ions can be described as a capped trigonal prism and/or capped octahedron.

Active metal template synthesis of a neutral indolocarbazole-containing [2]rotaxane host system for selective oxoanion recognition.

An active metal template strategy was used to synthesise a neutral indolocarbazole containing [2]rotaxane anion host system. 1H NMR anion binding investigations reveal that the [2]rotaxane recognises a range of monoanions in acetone-d5 : D2O 95 : 5 with an unusual interlocked host selectivity for acetate and dihydrogenphosphate oxoanions over halides. The rotaxane displays an overall selectivity for the sulfate dianion, favouring a 2 : 1 host : guest binding stoichiometry at low sulfate concentration and a 1 : 1 stoichiometry in the presence of excess sulfate. Fluorescence titration demonstrates that the [2]rotaxane is also capable of sensing guest anions via significant changes in its emission spectrum.

Metal-Templated Design: Enantioselective Hydrogen-Bond-Driven Catalysis Requiring Only Parts-per-Million Catalyst Loading.

Based on a metal-templated approach using a rigid and globular structural scaffold in the form of a bis-cyclometalated octahedral iridium complex, an exceptionally active hydrogen-bond-mediated asymmetric catalyst was developed and its mode of action investigated by crystallography, NMR, computation, kinetic experiments, comparison with a rhodium congener, and reactions in the presence of competing H-bond donors and acceptors. Relying exclusively on weak forces, the enantioselective conjugate reduction of nitroalkenes can be executed at catalyst loadings as low as 0.004 mol% (40 ppm), representing turnover numbers of up to 20 250. A rate acceleration by the catalyst of 2.5 × 10(5) was determined. The origin of the catalysis is traced to an effective stabilization of developing charges in the transition state by carefully orchestrated hydrogen-bonding and van der Waals interactions between catalyst and substrates. This study demonstrates that the proficiency of asymmetric catalysis merely driven by hydrogen-bonding and van der Waals interactions can rival traditional activation through direct transition metal coordination of the substrate.

Synthesis of [3]Rotaxanes by the Combination of Copper-Mediated Coupling Reaction and Metal-Template Approach.

[3]Rotaxanes with two axle components and one ring component were synthesized by the combination of a coupling reaction using a transition-metal catalyst and a metal-template approach. Thus, [2]rotaxanes were prepared by the oxidative dimerization of alkyne promoted by macrocyclic phenanthroline-CuI complexes. The [2]rotaxane was reacted with a Cu(I) salt and an acyclic ligand to generate a tetrahedral Cu(I) complex. Metal-free [3]rotaxane was isolated by the end-capping reaction of the acyclic ligand, followed by the removal of Cu(I) ion. The stability of the tetrahedral Cu(I) complexes depended on the size of both the ring component and the acyclic ligand, which was correlated with the yield of the corresponding [3]rotaxane.

Topologically complex molecules obtained by transition metal templation: it is the presentation that determines the synthesis strategy

Topological constructions made from closed curves range from simple links to intricate knots and started to capture the chemists' attention in the early sixties. These mathematical objects result from particular embeddings of a single or a set of closed curves in the three-dimensional space that show an infinite variety of presentations. Simple catenanes, higher order interlocked macrocycles, and molecular knots can be synthesized via the metal template approach, just as simple macrocycles. However, this requires that rigid presentations with appropriate geometrical characteristics be identified prior to molecular design, and those selected for the metal-templated synthesis of some of these fascinating molecules are reviewed here.

Cooperative self-assembly of a macrocyclic Schiff base complex

A metal template approach affords in high yield a tetra-Zn(salphen) macrocycle (3) which shows strong and cooperative self-assembly mediated by the formation of Zn(salphen) dimer units held together via μ(2)-phenoxo interactions. A cooperative binding mode for the tetranuclear Zn(4) macrocycle 3 is supported by comparison of UV-vis and fluorescence titration data recorded for 3 when compared with respective mononuclear and dinuclear Zn(salphen) model compounds. UV-vis dilution experiments carried out for Zn(4) macrocycle 3 and its Pd(4) analogue 4, as well as comparative TEM studies involving the same tetranuclear macrocycles further support the strong assembly behavior of 3. This self-assembly seems to be primarily dictated by its ability to form multiple, self-assembled dimeric [Zn(salphen)](2) units.

Architectural Diversity via Metal Template-Assisted Polymer Synthesis: A Macroligand Chelation Approach to Linear and Star-Shaped Polymeric Ruthenium Tris(bipyridine) Complexes

Polymeric metal complexes are constructed by combining living polymerization techniques with coordination chemistry. These metal-centered linear and star-shaped materials combine the film-forming properties of polymers with optical and other features of metal complexes. A metal template approach described herein offers a versatile alternative to the metalloinitiator method previously employed to generate Ru tris(bipyridine)-centered polystyrenes. Specifically, 4,4‘-bis(chloromethyl)-2,2‘-bipyridine and 4-chloromethyl-2,2‘-bipyridine were utilized as initiators for both the bulk and solution polymerization of styrene using atom transfer radical polymerization (ATRP). Narrow dispersity polystyrenes with bipyridine (bpy) binding sites at the end (bpyPS) or center (bpyPS2) of the chains result. These bpyPSn macroligands were chelated to Ru precursor complexes, RuL2Cl2 (L = bpy, phen) or Ru(DMSO)4Cl2, to form complexes with one or three bpyPSn macroligands, respectively. Linear polymers, [RuL2(bpyPSn)]2+, with Ru chromophores at the end or center of the chains, as well as Ru-centered star-shaped polymers, [Ru(bpyPSn)3]2+, with three and six arms were produced. In all cases, dehalogenation with AgPF6 was crucial for efficient macroligand chelation. The relative efficiency of these reactions was estimated by UV/vis spectroscopy. Molecular weight determination by GPC was coupled with in-line diode array UV/vis spectroscopy to confirm the presence of the Ru chromophores in the eluting polymer fractions. The convergent macroligand chelation approach to star-shaped polymeric metal complexes typically works best for polymers of low to moderate molecular weights (<∼65K), with higher molecular weights possible for systems with a single macroligand coordinated. Specific molecular weight thresholds encountered are determined by the number of macroligands, the position of the bpy on the polystyrene chain, and the total number of arms emanating from the metal core.