Iñigo J. Vitorica-Yrezabal

Spin-Crossover Modification through Selective CO2 Sorption

We present a spin-crossover Fe(II) coordination polymer with no permanent channels that selectively sorbs CO2 over N2. The one-dimensional chains display internal voids of ∼9 Å diameter, each being capable to accept one molecule of CO2 at 1 bar and 273 K. X-ray diffraction provides direct structural evidence of the location of the gas molecules and reveals the formation of O═C═O(δ(-))···π interactions. This physisorption modifies the spin transition, producing a 9 K increase in T1/2.

Allosteric initiation and regulation of catalysis with a molecular knot

Catalysis gets all tied up in knots Over the past decade, chemists have used metal ion templating to prepare a wide variety of knotted molecular strands. Marcos et al. now show that one such pentafoil knot can be applied to catalysis. When held taut by zinc ions, the knot can capture a chloride or bromide ion from a halocarbon, thereby unleashing the reactivity of the residual cation for applications such as Lewis acid catalysis. Removing the zinc ions lowers the knots affinity for the halides, offering a reversible modulation mechanism for the catalysis. Science, this issue p. 1555 Molecular knots, held taut by zinc ions, can capture bromide by cleavage of carbon-bromine bonds and thereby promote catalysis. Molecular knots occur in DNA, proteins, and other macromolecules. However, the benefits that can potentially arise from tying molecules in knots are, for the most part, unclear. Here, we report on a synthetic molecular pentafoil knot that allosterically initiates or regulates catalyzed chemical reactions by controlling the in situ generation of a carbocation formed through the knot-promoted cleavage of a carbon-halogen bond. The knot architecture is crucial to this function because it restricts the conformations that the molecular chain can adopt and prevents the formation of catalytically inactive species upon metal ion binding. Unknotted analogs are not catalytically active. Our results suggest that knotting molecules may be a useful strategy for reducing the degrees of freedom of flexible chains, enabling them to adopt what are otherwise thermodynamically inaccessible functional conformations.

A modular design of molecular qubits to implement universal quantum gates

The physical implementation of quantum information processing relies on individual modules—qubits—and operations that modify such modules either individually or in groups—quantum gates. Two examples of gates that entangle pairs of qubits are the controlled NOT-gate (CNOT) gate, which flips the state of one qubit depending on the state of another, and the gate that brings a two-qubit product state into a superposition involving partially swapping the qubit states. Here we show that through supramolecular chemistry a single simple module, molecular {Cr7Ni} rings, which act as the qubits, can be assembled into structures suitable for either the CNOT or gate by choice of linker, and we characterize these structures by electron spin resonance spectroscopy. We introduce two schemes for implementing such gates with these supramolecular assemblies and perform detailed simulations, based on the measured parameters including decoherence, to demonstrate how the gates would operate.

Braiding a molecular knot with eight crossings

Three strands ironed closely together It is not uncommon when braiding hair or bread to intertwine three different strands. At the molecular level, however, synthetic knots have thus far been restricted to architectures accessible from two-strand braids. Danon et al. used iron ion coordination to guide three organic ligand strands to form a knot geometry with eight separate crossings. Science, this issue p. 159 Iron coordination helps to template a molecular knot formed from three distinct strands. Knots may ultimately prove just as versatile and useful at the nanoscale as at the macroscale. However, the lack of synthetic routes to all but the simplest molecular knots currently prevents systematic investigation of the influence of knotting at the molecular level. We found that it is possible to assemble four building blocks into three braided ligand strands. Octahedral iron(II) ions control the relative positions of the three strands at each crossing point in a circular triple helicate, while structural constraints on the ligands determine the braiding connections. This approach enables two-step assembly of a molecular 819 knot featuring eight nonalternating crossings in a 192-atom closed loop ~20 nanometers in length. The resolved metal-free 819 knot enantiomers have pronounced features in their circular dichroism spectra resulting solely from topological chirality.

Crystallographic studies of gas sorption in metal–organic frameworks

Adsorption and separation of gases is one of the primary applications of the class of materials known as metal–organic frameworks (MOFs). The role of crystallography in characterizing adsorbed gas molecules and changes in framework structure upon gas sorption is reviewed.

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.

Synthesis and polymorphism of (4-ClpyH)2[CuCl4]: solid–gas and solid–solid reactions

Reaction of blue crystalline solid trans-[CuCl2(4-Clpy)2] 1 (4-Clpy = 4-chloropyridine) with anhydrous HCl gas yields yellow crystalline salt (4-ClpyH)2[CuCl4] 2 with quantitative conversion. The reaction has been followed in situ by synchrotron powder X-ray diffraction using a specially designed gas-handling rig and demonstrates that the initial product is a C-centred monoclinic form of (4-ClpyH)2[CuCl4] (phase II), which then converts in the solid state to a primitive monoclinic form (phase I). The conversion has been quantified at each stage by mixed-phase Rietveld refinement of the powder diffraction data. The two polymorphic forms exhibit different patterns of N–H⋯Cl(Cu) hydrogen bonding and C–Cl⋯Cl(Cu) halogen bonding. Evidence from solution-phase crystallisations of 2 suggests that phase II is a kinetic product and phase I is the thermodynamic product. The salt 2 can also be prepared mechanochemically, by grinding for 2 min green-blue solid CuCl2·2H2O with white solid [4-ClpyH]Cl in a 1 : 2 stoichiometric ratio. The product was identified by powder diffraction as phase I of compound 2 in 97.5% yield, with the remaining material being compound 1.

Making hybrid [n]?-?rotaxanes as supramolecular arrays of molecular electron spin qubits

Quantum information processing (QIP) would require that the individual units involved—qubits—communicate to other qubits while retaining their identity. In many ways this resembles the way supramolecular chemistry brings together individual molecules into interlocked structures, where the assembly has one identity but where the individual components are still recognizable. Here a fully modular supramolecular strategy has been to link hybrid organic–inorganic [2]- and [3]-rotaxanes into still larger [4]-, [5]- and [7]-rotaxanes. The ring components are heterometallic octanuclear [Cr7NiF8(O2CtBu)16]– coordination cages and the thread components template the formation of the ring about the organic axle, and are further functionalized to act as a ligand, which leads to large supramolecular arrays of these heterometallic rings. As the rings have been proposed as qubits for QIP, the strategy provides a possible route towards scalable molecular electron spin devices for QIP. Double electron–electron resonance experiments demonstrate inter-qubit interactions suitable for mediating two-qubit quantum logic gates.

Controlled Synthesis of Nanoscopic Metal Cages

Here we show an elegant and general route to the assembly of a giant {M12C24} cage from 12 palladium ions (M) and 24 heterometallic octanuclear coordination cages (C = {Cr7Ni-Py2}). The molecule is 8 nm in size, and the methods for its synthesis and characterization provide a basis for future developments at this scale.

Lanthanide Template Synthesis of Trefoil Knots of Single Handedness

We report on the assembly of 2,6-pyridinedicarboxamide ligands (1) with point chirality about lanthanide metal ion (Ln(3+)) templates, in which the helical chirality of the resulting entwined 3:1 ligand:metal complexes is covalently captured by ring-closing olefin metathesis to form topologically chiral molecular trefoil knots of single handedness. The ligands do not self-sort (racemic ligands form a near-statistical mixture of homoleptic and heteroleptic lanthanide complexes), but the use of only (R,R)-1 leads solely to a trefoil knot of Λ-handedness, whereas (S,S)-1 forms the Δ-trefoil knot with complete stereoselectivity. The knots and their isomeric unknot macrocycles were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography and the expression of the chirality that results from the topology of the knots studied by circular dichroism.