Self-assembly of tetrahedral and trigonal antiprismatic clusters [Fe4(L4)4] and [Fe6(L5)6] on the basis of trigonal tris-bidentate chelators

In a one-pot reaction, the tetranuclear iron chelate complex [Fe4(L4)4] 6 was generated from benzene-1,3,5-tricarboxylic acid trichloride (4), bis-tert-butyl malonate (5a), methyllithium, and iron(II) dichloride under aerobic conditions. Alternatively, hexanuclear iron chelate complex [Fe(L5)6] 7 was formed starting from bis-para-tolyl malonate (5b) by employing identical reaction conditions to those applied for the synthesis of 6. The clusters 6 and 7 are present as racemic mixtures of homoconfigurational (delta,delta,delta,delta)/(lambda,lambda,lambda,lambda)-fac or (delta,delta,delta,delta,delta,delta)/(lambda,lambda,lambda,lambda,lambda,lambda)-fac stereoisomers. The structures of 6 and 7 were unequivocally resolved by single-crystal X-ray analyses. The all-iron(III) character of 6 and 7 was determined by M?ssbauer spectroscopy.     

Exploiting host–guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages

Reaction of Ni(OTf)₂ with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [Ni^II₄L₆]⁸⁺. By selectively exchanging the bound triflate from [OTf⊂Ni^II₄L₆](OTf)₇ (1), we have been able to prepare a series of host–guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [M^IIX₄⊂Ni^II₄L₆](OTf)₆, where M^IIX₄²⁻ = MnCl₄²⁻ (2), CoCl₄²⁻ (5), CoBr₄²⁻ (6), NiCl₄²⁻ (7), and CuBr₄²⁻ (8) or [M^IIIX₄⊂Ni^II₄L₆](OTf)₇, where M^IIIX₄⁻ = FeCl₄⁻ (3) and FeBr₄⁻ (4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host–guest complexes 1–8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN₆} vertexes within a single Ni₄L₆ unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [Ni^II₄] complex, and between the host and the MX₄ⁿ⁻ guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX₄ⁿ⁻ guest molecules lead to stronger host–guest magnetic exchange interactions.

The tetrahedral [Ni^II₄L₆]⁸⁺ cage can reversibly bind paramagnetic MX₄^1/2− guests, inducing magnetic exchange interactions between host and guest.     

The L Spectrum of Fe and Fe3O4

The intensities of the Fe L lines/bands l = L₃M₁, η = L₂M₁, α_1,2 = L₃M_4,5, β₁ = L₂M₄, and β_3,4 = L₁M_2,3 were measured for pure Fe and Fe₃O₄ using a TAP crystal as the dispersing element. The energy of the exciting electrons, E₀, was varied in the range 5 ≤ E₀ ≤ 25 keV. For pure Fe the following results were obtained. The net peak height ratio Ll/Lα remains relatively constant with varying E₀ at approximately 14%. The E₀ dependence of Lη is similar to that of Ll, although Lη is less intense than Ll by a factor of 7. Lβ₁/Lα decreases from 20% for E₀ = 5 keV to about 5% for 25 keV. Lβ_3,4 behaves like Lβ₁ but is weaker by a factor of 15. For Fe₃O₄ a much weaker intensity of Lα was observed which can be partially explained by its stronger absorption. Again, the E₀ dependence of Ll and Lη is similar with Ll/Lα = 19% and Lη/Lα = 4%. Lβ₁ and Lβ_3,4 show a comparable E₀ dependence. Lβ₁/Lα decreases from 50% for E₀ = 5 keV to 34% for 25 keV. Lβ_3,4 is weaker than Lβ₁ by a factor of about 25. The observed E₀ dependence of the different lines was used to estimate a set of mass absorption coefficients. Our value for Lα in Fe agrees well with other data which were deduced from variable E₀ measurements but differs considerably from data given by Heinrich and Henke.     

Transfer of stereochemical information in a minimal self-replicating system

[structure: see text]. The rational design, synthesis, and characterization of a minimal self-replicating system based on a 1,3-dipolar cycloaddition between a nitrone and a maleimide is presented. The importance of molecular recognition in this system is demonstrated using a competitive inhibitor. Doping experiments demonstrate that only one of the two diastereoisomeric products of the cycloaddition reaction is capable of acting as an efficient template for its own formation, accelerating the reaction between the nitrone and maleimide and controlling the stereochemical outcome of the reaction.     

A three dimensional porous metal-organic framework [Fe4L6.(DMF)3.(H2O)10] constructed from neutral discrete Fe4L6 pyramids [H2L = 1,3-benzodihydroxamix acid

A 3-D porous zeolite-like metal-organic framework surviving guest removal is assembled from a well-defined tetrahedral Fe(4)L(6) cavity by the cooperativity of hydrogen bonds and [small pi]-[small pi] stacking.     

Urea-Functionalized Fe4L6 Cages for Supramolecular Gold Catalyst Encapsulation to Control Substrate Activation Modes

The excellent catalytic performances of enzymes in terms of activity and selectivity are an inspiration for synthetic chemists and this has resulted in the development of synthetic containers for supramolecular catalysis. In such containers the local environment and pre-organization of catalysts and substrates leads to control of the activity and selectivity of the catalyst. Herein we report a supramolecular strategy to encapsulate single catalysts in a urea-functionalized Fe₄L₆ cage, which can co-encapsulate a functionalized urea substrate through hydrogen bonding. Distinguished selectivity is obtained, imposed by the cage as site isolation only allows catalysis through π activation of the substrate and as a result the selectivity is independent of catalyst concentration. The encapsulated catalyst is more active than the free analogue, an effect that can be ascribed to transitionstate stabilization rather than substrate pre-organization, as revealed by the MM kinetic data. The simple strategy reported here is expected to be of general use in many reactions, for which the catalyst can be functionalized with a sulfonate group required for encapsulation.     

Configurational stability and transfer of stereochemical information in the reactions of enantioenriched organolithium reagents

The conversion of a prochiral methylene group into a stereogenic center by means of a lithiation/substitution sequence has emerged as a powerful synthetic transformation over the past 15 years. This reaction proceeds via a chiral organolithium intermediate, and the stereochemical fidelity of the overall reaction sequence is intimately dependent on the stereochemical behavior of the chiral organolithium as well as on the rate and stereochemical sense of the electrophilic substitution step. Chiral organolithium reagents were first reported by Letsinger, Curtin, and Applequist half a century ago. The lithiated intermediates in these early studies were not highly configurationally stable, and applications in stereoselective synthesis were not immediately forthcoming. The two decades that followed the 1980 report by Still and Sreekumar of a configurationally stable alpha-oxyorganolithium were marked by an increased interest in these reagents. As the synthetic applications of chiral organolithium reagents have grown, so have accompanying mechanistic studies of these intermediates which serve as the basis for this review.     

Computational study of the energetics of 3Fe(CO)4, 1Fe(CO)4 and 1Fe(CO)4(L), L = Xe, CH4, H2 and CO

Large basis CCSD(T) calculations are used to calculate the energetics of 3Fe(CO)4, 1Fe(CO)4 and 1Fe(CO)4(L), L = Xe, CH4, H2 and CO. . The relative energy of the excited singlet state of Fe(CO)4 with respect to the ground triplet state is not known experimentally, and various lower levels of theory predict very different results. Upon extrapolating to the infinite basis set limit, and including corrections for core-core and core-valence correlation, scalar relativity, and multi-reference character of the wavefunction, the best CCSD(T) estimate for the spin-state splitting in iron tetracarbonyl is 2 kcal mol(-1). Calculation of the dissociation energy of 1Fe(CO)4(L) into singlet fragments, taken together with known experimental behaviour of triplet Fe(CO)4, provides independent evidence for the fact that the spin-state splitting is smaller than 3 kcal mol(-1). These calculations highlight some of the challenges involved in benchmark calculations on transition metal containing systems.     

Self-assembly of tripyrazolate-linked macrotricyclic M(12)L(4) cages with dimetallic clips

[structure: see text]. The self-assembly in aqueous solution of tripyrazolate trianion linkers (H3L = 1 and 2) with coordinated dimetallic clips leads to the formation of Pd12L4 cages with interior cavities of 1800 Angstroms3 and 1500 Angstroms3, respectively, confirmed by 1H NMR, 13C NMR, cold spray ionization mass spectrometry (CSI-MS), and single-crystal X-ray analysis.     

Direct conversion of iron stearate into magnetic Fe and Fe3C nanocrystals encapsulated in polyhedral graphite cages

We report a direct salt-conversion approach for large-scale synthesis of carbon-encapsulated magnetic Fe and Fe3C nanoparticles.     

Hierarchically porous carbon cages synthesized through in situ migration of templates

Hierarchically porous networks have been fabricated through immigration of templates.     

Indolizinones as synthetic scaffolds: fundamental reactivity and the relay of stereochemical information

Indolizinones are under-explored N-heterocycles that react with exquisite chemo- and stereoselectivity. An exploration of the fundamental reactivity of these azabicycles demonstrates the potential to relay stereochemical information from the ring-fusion to newly formed stereocenters on the bicyclic core. The indolizinone diene undergoes selective hydrogenation and readily participates in Diels-Alder cycloadditions as well as ene reactions. The vinylogous amide embedded in the five-membered ring is resistant to reaction when the diene is in place. However, removal of the diene allows for diastereoselective hydrogenation of, and 1,4-additions to, the vinylogous amide. These fundamental reactions with indolizinones have provided a structurally diverse array of products that hold promise in the context of natural product synthesis.     

Preparation of57Fe enriched K4[Fe(CN)6] and K3[Fe(CN)6]

A simple method to prepare⁵⁷Fe enriched K₄[Fe(CN)₆] and K₃[Fe(CN)₆] is described. The yields of the products are much better than those reported in the literature so far. The enrichment is essential for⁵⁷Fe Mössbauer investigation in a variety of Prussiate type complexes and other inorganic compounds which are conveniently prepared from K₄[Fe(CN)₆] and K₃[Fe(CN)₆]. K₄[Fe(CN)₆] was obtained by reacting freshly prepared Fe(OH)₃ with glacial acetic acid and treating with iron acetate in boiling aqueous solution of KCN. The novel feature of the procedure to obtain K₃[Fe(CN)₆] is that the oxidation of K₄[Fe(CN)₆] has been carried out in the solid state by passing chlorine gas over the powdered specimen. K₃[Fe(CN)₆] was crystallised from alkaline solution of this oxidised powder. The compounds were characterised by Mössbauer spectroscopy.     

Self-assembly of discrete M6L8 coordination cages based on a conformationally flexible tripodal phosphoric triamide ligand

Reactions of a tripodal ligand, N,N',N″-tris(3-pyridinyl)phosphoric triamide (TPPA), and a series of transition-metal ions result in the assembly of five discrete M(6)L(8) coordination cages [M(6)(TPPA)(8)(H(2)O)(12)](ClO(4))(12)·57H(2)O [M = Ni(2+) (1), Co(2+) (2), Zn(2+) (3), Cd(2+) (4)] and [Pd(6)(TPPA)(8)]Cl(12)·22H(2)O (5). X-ray structural analyses reveal that the cages have large internal cavities and flexible windows. The flexible ligand TPPA adopts the syn conformation in cages 1-4, but it transforms to the anti conformation in cage 5. Because of the conformational transformation, the sizes of the windows and the volume of the internal cavity of cage 5 are increased. (1)H NMR and electrospray mass spectrometric studies show that cage 5 maintains its structural integrity in solution. Additionally, compounds 3 and 4 exhibit strong blue fluorescent emissions, which are 1 order of magnitude higher than that of the free ligand.     

Spontaneous Resolution of an Electron‐Deficient Tetrahedral Fe4L4 cage

A highly electron‐deficient C₃‐symmetric tris(bipyridyl) ligand was prepared in four steps and used for the coordination of Fe(OTf)₂, thereby resulting in the homochiral assembly of a new family of robust tetrahedral M₄L₄ cages. This homochiral T‐symmetric cage containing a relatively large cavity of 330 ų is capable of encapsulating an anionic guest, as was determined by mass spectrometry, ¹⁹F NMR spectroscopy, and finally shown from its crystal structure. Moreover, crystallization of the cage from CH₃CN led to crystals containing both (ΔΔΔΔ and ΛΛΛΛ) enantiomers, while crystallization from CH₃OH resulted in crystals containing only the right‐handed (ΔΔΔΔ) cage. The difference in the crystal packing of the two crystal structures is discussed and a feasible explanation for the unique phenomenon among supramolecular cages—spontaneous resolution—is given.     

Programming the supramolecular helical polymerization of dendritic dipeptides via the stereochemical information of the dipeptide

Many natural biomacromolecules are homochiral and are built from constituents possessing identical handedness. The construction of synthetic molecules, macromolecules, and supramolecular structures with tailored stereochemical sequences can detail the relationship between chirality and function and provide insight into the process that leads to the selection of handedness and amplification of chirality. Dendritic dipeptides, previously reported from our laboratory, self-assemble into helical porous columns and serve as fundamental mimics of natural porous helix-forming proteins and supramolecular polymers. Herein, the synthesis of all stereochemical permutations of a self-assembling dendritic dipeptide including homochiral, heterochiral, and differentially racemized variants is reported. A combination of CD/UV-vis spectroscopy in solution and in film, X-ray diffraction, and differential scanning calorimetry studies in solid state established the role of the stereochemistry of the dipeptide on the thermodynamics and mechanism of self-assembly. It was found that the highest degree of stereochemical purity, enantiopure homochiral dendritic dipeptides, exhibits the most thermodynamically favorable self-assembly process in solution corresponding to the greatest degree of helical order and intracolumnar crystallization in solid state. Reducing the stereochemical purity of the dendritic dipeptide through heterochirality or by partially or fully racemizing the dendritic dipeptide destructively interferes with the self-assembly process. All dendritic dipeptides were shown to coassemble into single columns regardless of their stereochemistry. Because these columns exhibit no deracemization, the thermodynamic advantage of enantiopurity and homochirality suggests a mechanism for stereochemical selection and chiral amplification.     

Phase equilibria in the system FeVO4−Fe2WO6 in the solid state

A diagram of the phase equilibria established in the solid state in the system FeVO₄?Fe₂WO₆ was plotted on the basis of X-ray phase analysis and DTA. This system, one of the intersections of the three-component system Fe₂O₃?V₂O₅?WO₃, does not appear to be a real two-component system in the solid state.