Stabilization of Lantern-Type Metal-Organic Cages (MOCs) by Protective Control of Ligand Exchange Rates.

Self-assembling systems in nature display remarkable complexity with assemblies of different sub-units to generate functional species. Synthetic analogues of such systems are a challenge, often requiring the ability to bias distributions that are under thermodynamic assembly control. Using lantern-type MOCs (metal-organic cages) as a prototypical self-assembling system, herein we explore the role that steric bulk plays in controlling the exchange rate of ligands in paddlewheel-based assemblies, and thus the stability of cages, in competitive self-assembling scenarios. The effective lifetime of the lantern-type MOCs varies over an order of magnitude depending on the steric bulk proximal to the metal nodes with lifetimes of the cages ranging from tens of minutes to several hours. The bulk of the coordinating solvents likewise reduces the rate of ligand exchange, and thus yields longer-lived species. Understanding this subtle effect has implications for controlling the stability of complex assemblies in competitive environments with implications for guest release and application.

A Study on Auto-Catalysis and Product Inhibition: A Nucleophilic Aromatic Substitution Reaction Catalysed within the Cavity of an Octanuclear Coordination Cage.

The ability of an octanuclear cubic coordination cage to catalyse a nucleophilic aromatic substitution reaction on a cavity-bound guest was studied with 2,4-dinitrofluorobenzene (DNFB) as the guest/substrate. It was found that DNFB undergoes a catalysed reaction with hydroxide ions within the cavity of the cubic cage (in aqueous buffer solution, pH 8.6). The rate enhancement of kcat/kuncat was determined to be 22, with cavity binding of the guest being required for catalysis to occur. The product, 2,4-dinitrophenolate (DNP), remained bound within the cavity due to electrostatic stabilisation and exerts two apparently contradictory effects: it initially auto-catalyses the reaction when present at low concentrations, but at higher concentrations inhibits catalysis when a pair of DNP guests block the cavity. When encapsulated, the UV/Vis absorption spectrum of DNP is red-shifted when compared to the spectrum of free DNP in aqueous solution. Further investigations using other aromatic guests determined that a similar red-shift on cavity binding also occurred for 4-nitrophenolate (4NP) at pH 8.6. The red-shift was used to determine the stoichiometry of guest binding of DNP and 4NP within the cage cavity, which was confirmed by structural analysis with X-ray crystallography; and was also used to perform catalytic kinetic studies in the solution-state.

Amine-Based MOF for Precious Metal Remediation.

Due to the continuous growth rate of the electronic industry, hi-tech companies depend on mining and extracting precious metals to meet the public demand. The high turnover of modern devices generates an alarming amount of electronic waste (e-waste), which contains more precious metals than mined ores and therefore needs efficient recovery procedures. A highly stable homopiperazine-derived Cd-MOF, poly-[Cd(H2L)]·9H2O, with a protonated amine ligand core, exists as a twofold interpenetrated 3D framework with 1D channels into which the N+-H bond is directed. The geometry of these channels appears to be suitable to host square planar metalate complexes. Under acidic conditions, [MCl4]x- anions containing Au, Cu, Ni, and Pt, representing common components of e-waste under extraction conditions, were tested for capture and recovery. Cd-MOF exhibits remarkable selectivity and uptake performance toward Au with an adsorbent capacity of 25 mg g-1ads and shows a marked selectivity for Au over Cu in competitive experiments. The adsorption mechanism of Au appears to be predominantly physical adsorption at the surface of the material.

Pushing the Boundary of Covalency in Lanthanoid-Tellurium Bonds: Insights from the Synthesis, Molecular and Electronic Structures of Low-Coordinate, Monomeric Europium(II) and Ytterbium(II) Tellurolates.

Owing to the strict hard/soft dichotomy between the lanthanoids and tellurium atoms, and the strong affinity of lanthanoid ions for high coordination numbers, low-coordinate, monomeric lanthanoid tellurolate complexes have remained elusive as compared to the lanthanoid complexes with lighter group 16 elements (O, S, and Se). This makes the development of suitable ligand systems for low-coordinate, monomeric lanthanoid tellurolate complexes an appealing endeavor. In a first report, a series of low-coordinate, monomeric lanthanoid (Yb, Eu) tellurolate complexes were synthesized by utilizing hybrid organotellurolate ligands containing N-donor pendant arms. The reaction of bis[2-((dimethylamino)methyl)phenyl] ditelluride, 1 and 8,8'diquinolinyl ditelluride, 2 with Ln0 metals (Ln=Eu, Yb) resulted in the formation of monomeric complexes [LnII (TeR)2 (Solv)2 ] [R=C6 H4 -2-CH2 NMe2 ] [3: Ln=Eu, Solv=tetrahydrofuran; 4: Ln=Eu, Solv=acetonitrile; 5: Ln=Yb, Solv=tetrahydrofuran; 6: Ln=Yb, Solv=pyridine] and [EuII (TeNC9 H6 )2 (Solv)n ] (7: Solv=tetrahydrofuran, n=3; 8: Solv=1,2-dimethoxyethane, n=2), respectively. Complexes 3-4 and 7-8 represent the first sets of examples of monomeric europium tellurolate complexes. The molecular structures of complexes 3-8 are validated by single-crystal X-ray diffraction studies. The electronic structures of these complexes were investigated using Density Functional Theory (DFT) calculations, which revealed appreciable covalency between the tellurolate ligands and lanthanoids.

Electrochemical Hydrogenation of Furfural in Aqueous Acetic Acid Media with Enhanced 2-Methylfuran Selectivity Using CuPd Bimetallic Catalysts.

Herein, we report a series of CuPd catalysts for electrochemical hydrogenation (ECH) of furfural to 2-methylfuran (MF or FurCH3 where Fur=furyl) in aqueous 0.1 M acetic acid (pH 2.9). The highest faradaic efficiency (FE) for MF reached 75 % at -0.58 V vs. reversible hydrogen electrode with an average partial current density of 4.5 mA cm-2 . In situ surface-enhanced Raman spectroscopic and kinetic isotopic experiments suggested that electrogenerated adsorbed hydrogen (Hads ) was involved in the reaction and incorporation of Pd enhanced the surface coverage of Hads and optimized the adsorption pattern of furfural, leading to a higher FE for MF. Density functional theory calculations revealed that Pd incorporation reduced the energy barrier for the hydrogenation of FurCH2 * to FurCH3 *. Our study demonstrates that catalyst surface structure/composition plays a crucial role in determining the selectivity in ECH and provides a new strategy for designing advanced catalysts for ECH of bio-derived oxygenates.

A Multifunctional, Charge-Neutral, Chiral Octahedral M12 L12 Cage.

A chiral, octahedral M12 L12 cage, which is charge neutral and contains an internal void of about 2000 Å3 , is reported. The cage was synthesised as an enantiopure complex by virtue of amino-acid-based dicarboxylate ligands, which assemble around copper paddlewheels at the vertices of the octahedron. The cage persists in solution with retention of the fluorescence properties of the parent acid. The solid-state structure contains large pores both within and between the cages, and displays permanent porosity for the sorption of gases with retention of crystallinity. Initial tests show some enantioselectivity of the cage towards guests in solution.

Anisotropic Thermal and Guest-Induced Responses of an Ultramicroporous Framework with Rigid Linkers.

The interdependent effects of temperature and guest uptake on the structure of the ultramicroporous metal-organic framework [Cu3 (cdm)4 ] (cdm=C(CN)2 (CONH2 )- ) were explored in detail by using in situ neutron scattering and density functional theory calculations. The tetragonal lattice displays an anisotropic thermal response related to a hinged "lattice-fence" mechanism, unusual for this topology, which is facilitated by pivoting of the rigid cdm anion about the Cu nodes. Calculated pore-size metrics clearly illustrate the potential for temperature-mediated adsorption in ultramicroporous frameworks due to thermal fluctuations of the pore diameter near the value of the target guest kinetic diameter, though in [Cu3 (cdm)4 ] this is counteracted by a competing contraction of the pore with increasing temperature as a result of the anisotropic lattice response.

Binding of Mono- and Dianions within Silver Thiazolylurea Tweezers and Capsules.

A silver thiazolylurea complex, [Ag(TUTh)2]+, has been used as a host species for geometrically differently shaped mono- and dianions: trigonal planar (NO3-), tetrahedral (SO42-), and octahedral (SiF62-). In the presence of nitrate a 1:1 near-planar tweezer host-guest species is formed, with poor binding in solution despite excellent geometric complementarity being found between the host and the anion in the solid state. In the presence of either SO42- or SiF62- a 2:1 host-guest species is formed, whereby the guest is held in a capsulelike arrangement stabilized by an array of eight NH hydrogen-bond donors, as confirmed by X-ray crystallographic studies. Solution studies in DMSO-d6 support the host-guest stoichiometry seen in the solid state. The binding constant between SO42- and [Ag(TUTh)2]+ was calculated to be K21 = 2511 M-2 and was shown to be the dominant species in solution, in excellent agreement with the solid-state studies. However, upon the addition of SiF62- ions different speciation is observed: H2·G (capsule), H·G (tweezer), and H·G2 during the course of the study.

Coordination Chemistry and Structural Dynamics of a Long and Flexible Piperazine-Derived Ligand.

A long and highly flexible internally functionalized dipyridyl ligand α,α'-p-xylylenebis(1-(4-pyridylmethylene)-piper-4-azine), L, has been employed in the synthesis of a series of coordination polymer materials with Co(II), Cd(II), and Ag(I) ions. In poly-[Cd(L)(TPA)] 1 and poly-[Co(L)(IPA)], 2, (TPA = terephthalate, IPA = isophthalate) the ligand adopts a similar linear conformation to that seen in the structure of the unbound molecule and provides a long (2.6 nm) metal-metal bridging distance. Due to the mismatch of edge lengths with that provided by the carboxylate coligands, geometric distortions from the regular dia and (4,4) network geometries for 1 and 2, respectively, are observed. In poly-[Ag2(CF3SO3)2(L)], 3, the ligand coordinates through both pyridine groups and two of the four piperazine nitrogen donors, forming a high-connectivity 2-dimensional network. The compound poly-[Ag2(L)](BF4)2·2MeCN, 4, a porous 3-dimensional cds network, undergoes a fascinating and rapid single-crystal-to-single-crystal rearrangement on exchange of the acetonitrile guests for water in ambient air, forming a nonporous hydrated network poly-[Ag2(L)](BF4)2·2H2O, 5, in which the well-ordered guest water molecules mediate the rearrangement of the tetrafluoroborate anions and the framework itself through hydrogen bonding. The dynamics of the system are examined in greater detail through the preparation of a kinetic product, the dioxane-solvated species poly-[Ag2(L)](BF4)2·2C4H8O2, 6, which undergoes a slow conversion to 5 over the course of approximately 16 h, a transition which can be monitored in real time. The reverse transformation can also be observed on immersing the hydrate 5 in dioxane. The structural features and physical properties of each of the materials can be rationalized based on the flexible and multifunctional nature of the ligand molecule, as well as the coordination behavior of the chosen metal ions.

Porous Polyrotaxane Coordination Networks Containing Two Distinct Conformers of a Discontinuously Flexible Ligand.

A new divergent homopiperazine-derived ligand N,N'-bis(4-carboxyphenyl)-1,4-diazacycloheptane H2L has been prepared, containing a semirigid saturated heterocyclic core which is capable of providing multiple distinct bridging geometries. Reaction of H2L with zinc acetate in DMSO gives a two-dimensional parallel interpenetrated polyrotaxane structure 1 in which the loops and rods are formed by the bent cis-(eq,ax) twist boat and trans-(ax,ax) twist chair conformers, respectively. By matching the distances between the solvated metal sites in the structure of 1, a related material 2 can be prepared incorporating the pillaring ligand trans-1,2-bis(4-pyridyl)ethylene bpe. Compound 2 displays a similar polyrotaxane interpenetration mode, permitted by the presence of both cis and trans ligand conformers, but displays a three-dimensional 2.69 topology related to the dia diamondoid network. The guest exchange and gas adsorption properties of both materials were investigated; while compound 1 displays poor stability to guest exchange and negligible gas uptake, the higher connectivity microporous compound 2 shows facile guest exchange and a surprisingly high CO2 capacity of 12 wt % at 1 bar and 273 K, and a zero-loading enthalpy of adsorption of -32 kJ mol-1. High-pressure adsorption isotherms also show moderate physisorption of H2 and CH4 within the material.

Synthesis and structure of new lanthanoid carbonate "lanthaballs".

New insights into the synthesis of high-nuclearity polycarbonatolanthanoid complexes have been obtained from a detailed investigation of the preparative methods that initially yielded the so-called "lanthaballs" [Ln(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)] Cl(3)(CO(3))·25H(2)O [α-1Ln; Ln = La, Ce, Pr; phen = 1,10-phenanthroline; ccnm = carbamoylcyanonitrosomethanide]. From this investigation, we have isolated a new pseudopolymorph of the cerium analogue of the lanthaball, [Ce(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)]·C(l3)·CO(3) (ß-1Ce). This new pseudopolymorph arose from a preparation in which fixation of atmospheric carbon dioxide generated the carbonate, and the ccnm ligand was formed in situ by the nucleophilic addition of water to dicyanonitrosomethanide. From a reaction of cerium(III) nitrate, instead of the previously used chloride salt, with (Et4N)(ccnm), phen, and NaHCO(3) in aqueous methanol, the new complex Na[Ce(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)](NO(3))(6)·20H(2)O (2Ce) crystallized. A variant of this reaction in which sodium carbonate was initially added to Ce(NO(3))(3), followed by phen and (Et(4)N)(ccnm), also gave 2Ce. However, an analogous preparation with (Me4N)(ccnm) gave a mixture of crystals of 2Ce and the coordination polymer [CeNa(ccnm)4(phen)3]·MeOH (3), which were manually separated. The use of cerium(III) acetate in place of cerium nitrate in the initial preparation did not give a high-nuclearity complex but a new coordination polymer, [Ce(ccnm)(OAc)(2)(phen)] (4). The first lanthaball to incorporate neodymium, namely, [Nd(13)(ccnm)(4)(CO(3))14(NO(3))(4)(H(2)O)(7)(phen)(15)](NO(3))(3)·10H(2)O (5Nd), was isolated from a preparation similar to that of the second method used for 2Ce, and its magnetic properties showed an antiferromagnetic interaction. The identity of all products was established by X-ray crystallography.

Liquid-phase enantioselective chromatographic resolution using interpenetrated, homochiral framework materials.

Effective separation of mixtures of enantiomers is of continuing interest in analytical and preparative chromatography, with new materials frequently designed and tested. We report two new enantiomerically pure 2D→3D interpenetrated materials used as stationary liquid chromatographic (LC) phases that are shown to resolve selected racemic mixtures with enantiomeric and chemical selectivity. Dicarboxylate ligands derived from amino acids on naphthalene and perylene cores form 2D frameworks that interpenetrate to give 3D structures. Selectivity is initially tested by uptake from solution; subsequent LC methods show that the materials exhibit resolution of racemic analytes in 'micro-columns' and that the two closely related materials show markedly different selectivity for different analytes with much greater activity than the ligands alone. Comparison with a close-packed analogue suggests that the separation activity is largely due to surface effects.

Structure, magnetic behavior, and anisotropy of homoleptic trinuclear lanthanoid 8-quinolinolate complexes.

Three complexes of the form [Ln(III)3(OQ)9] (Ln = Gd, Tb, Dy; OQ = 8-quinolinolate) have been synthesized and their magnetic properties studied. The trinuclear complexes adopt V-shaped geometries with three bridging 8-quinolinolate oxygen atoms between the central and peripheral eight-coordinate metal atoms. The magnetic properties of these three complexes differ greatly. Variable-temperature direct-current (dc) magnetic susceptibility measurements reveal that the gadolinium and terbium complexes display weak antiferromagnetic nearest-neighbor magnetic exchange interactions. This was quantified in the isotropic gadolinium case with an exchangecoupling parameter of J = -0.068(2) cm(-1). The dysprosium compound displays weak ferromagnetic exchange. Variable-frequency and -temperature alternating-current magnetic susceptibility measurements on the anisotropic cases reveal that the dysprosium complex displays single-molecule-magnet behavior, in zero dc field, with two distinct relaxation modes of differing time scales within the same molecule. Analysis of the data revealed anisotropy barriers of Ueff = 92 and 48 K for the two processes. The terbium complex, on the other hand, displays no such behavior in zero dc field, but upon application of a static dc field, slow magnetic relaxation can be observed. Ab initio and electrostatic calculations were used in an attempt to explain the origin of the experimentally observed slow relaxation of the magnetization for the dysprosium complex.

Coordination polymers of small cyano anions.

Small cyano anions, and their derivatives, have proved to be remarkably versatile building blocks for coordination polymers. They have led to magnetically ordered materials, structures with new entangled motifs or novel topologies, and a significant structural diversity, including polymorphism, induced by subtle changes in anion, cation, co-ligands, preparative conditions, or even desolvation.

The preservation of sarin and O,O'-diisopropyl fluorophosphate inside coordination cage hosts.

The host-guest chemistry of O,O'-diisopropyl fluorophosphate (DFP), a phosphonofluoridate G-series chemical warfare agent simulant, was investigated in the presence of a number of octanuclear cubic coordination cage hosts. The aim was to demonstrate cage-catalysed hydrolysis of DFP at near neutral pH: however, two octanuclear coordination cages, HPEG (containing water-solubilising PEG groups) and HW (containing water-solubilising hydroxymethyl groups), were actually found to increase the lifetime of DFP in aqueous buffer solution (pH 8.7). Crystallographic analysis of DFP with a structurally related host cage revealed that DFP binds to windows in the cage surface, not in the internal cavity. The phosphorus-fluorine bond is directed into the cavity rather than towards the external environment, with the cage/DFP association protecting DFP from hydrolysis. Initial studies with the chemical warfare agent (CWA) sarin (GB) with HPEG cage in a buffered solution also showed a drastically reduced rate of hydrolysis for sarin when bound in the host cage. The ability of these cages to inhibit hydrolysis of these P-F bond containing organophosphorus guests, by encapsulation, may have applications in forensic sample preservation and analysis.

Efficient synthetic route to heterobimetallic trinuclear complexes [Ln-Mn-Ln] and their single molecule magnetic properties.

A series of mononuclear lanthanoidate complexes isolated as [Bu4N][Ln(QCl4)] 1Ln (QCl = 5-chloro-8-quinolinolate; Ln = Eu, Gd, Tb, Dy, Ho, and Er) have been prepared, characterised, and used as facile precursors to obtain a series of new heterobimetallic complexes as crystalline materials. Reaction of 1Ln with manganese nitrate forms [Ln2Mn(QCl)8] (2Ln, where Ln = Tb, Dy, Er and Yb) which have been structurally characterised in the cases of 2Tb and 2Yb. The heteroleptic trinuclear complex [Dy3(QCl)8Cl(OH2)], 3, has also been obtained. Compounds 1Dy, 1Tb, and 1Er display slow relaxation of magnetisation below 10K, particularly for the prolate Er3+ ion. These results also suggest that the positive effects of the change from mononuclear to trinuclear lanthanoid complexes enhance their single molecule magnetic (SMM) behaviour, as evidenced by the well resolved frequency dependent AC out-of-phase susceptibility maxima seen in the 2Ln systems, that have been analysed quantitatively. The synthesis used here provides a promising strategy in obtaining heterobimetallic complexes with quinolinolate ligands and also constructing efficient heterobimetallic SMMs.

Coordination polymers of hexacyanotrimethylenecyclopropanediide and its monoanionic radical: synthesis, structure, and magnetic properties.

The first examples of polymeric complexes that contain the polynitrile dianion hexacyanotrimethylenecyclopropanediide (HCTMCP(2-)) were isolated and their magnetic properties have been explored. Complexes of the form (n-TBA)(2)[M(HCTMCP)(2)(H(2)O)(2)] (1) (M = Mn(II), Fe(II), Co(II), Cd(II)) possess (4,4) sheet structures with large cavities that contain the tetra-n-butylammonium (n-TBA) countercations. Synthesis using sodium as the countercation yields a family of products with the general form [M(S)(4)M(S)(2)(HCTMCP)(2)] (S = EtOH, M = Fe(II) (2); S = MeOH, M = Co(II) or Zn(II) (3)). These complexes adopt a variety of two-dimensional (2D) structures. The complex [Mn(3)(HCTMCP)(2)(H(2)O)(12)](HCTMCP)·6(H(2)O) (4) contains cationic (6,3) sheets with the counteranion and solvent molecules encapsulated within the hexagonal windows. Complexes 1-4 display weak antiferromagnetic coupling in all cases. The first example of a complex that contains the CN-coordinated monoanionic radical HCTMCP (•-), [Cu(HCTMCP)(MeCN)(2)] (5) is described. This one-dimensional (1D) coordination polymer, containing tetrahedral Cu(I) centers, displays medium strength antiferromagnetic coupling that is mediated through π-interactions between the radical anions on adjacent chains.

Self-Assembly of a Redox Active, Metallosupramolecular [Pd3 L6 ]6+ Complex Using a Rotationally Flexible Ferrocene Ligand.

A new ferrocene-containing [Pd3 (L4EFc )6 ]6+ (X- )6 (C ⋅ BF4 and C ⋅ SbF6 where X=BF4 - or SbF6 - ) self-assembled double-walled triangle has been synthesized from the known, rotationally flexible, 1,1'-bis(4-pyridylethynyl)ferrocene ligand (L4EFc ), and characterized by 1 H, 13 C and diffusion ordered (DOSY) NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), X-ray crystallography and cyclic voltammetry (CV). The molecular structures confirmed that double-walled triangle cage systems (C ⋅ BF4 and C ⋅ SbF6 ) were generated. C ⋅ BF4 was shown to interact with the anionic guest, p-toluenesulfonate. CV experiments revealed that the triangles were redox active, however addition of the guest did not influence the redox potentials.

Steric control of sorting regimes in self-assembled cages.

Control of self-sorting regimes is achieved through adjustment of steric interactions in self-assembled coordination cages. The self-assembly regime of dynamic mixtures of heteroleptic cages is followed by HPLC to show that statistical or biased distributions can be selectively obtained, including isolation of a single heteroleptic species.

Chalcogen Bonds in Selenocysteine Seleninic Acid, a Functional GPx Constituent, and in Other Seleninic or Sulfinic Acid Derivatives.

The controlled oxidation reaction of L-selenocystine under neutral pH conditions affords selenocysteine seleninic acid (3-selenino-L-alanine) which is characterized also by means of single-crystal X-ray diffraction. This technique shows that selenium forms three chalcogen bonds (ChBs), one of them being outstandingly short. A survey of seleninic acid derivatives in the Cambridge Structural Database (CSD) confirms that the C-Se(=O)O- functionality tends to act as a ChB donor robust enough to systematically influence the interactional landscape in the solid. Quantum Theory of Atom in Molecules (QTAIM) analysis proves the attractive nature of the short contacts observed in crystals containing the seleninic functionality and calculation of surface molecular electrostatic potential (MEP) reveals that remarkably positive σ-holes can frequently be found opposite to the covalent bonds at selenium. Both CSD searches and QTAIM and MEP approaches show that also the sulfinic acid moiety can function as a ChB donor, albeit less frequently than the seleninic acid one. These findings may contribute to a better understanding, at the atomic level, of the mechanism of action of the enzymes that control oxidative stress and ROS deactivation and that contain selenocysteine seleninic acid and cysteine sulfinic acid in the active site.