Controlled Synthesis of Nanoscopic Metal Cages

Magnetic Relaxations of Chromium Nitride Porphyrinato Complexes Driven by the Anisotropic g-Factor

Molecule-based magnetic materials are useful candidates as the spin qubit due to their long coherence time and high designability. The anisotropy of the g-values of the metal complexes can be utilized to access the individual spin of the metal complexes, making it possible to achieve the scalable molecular spin qubit. For this goal, it is important to evaluate the effect of g-value anisotropy on the magnetic relaxation behaviour. This study reports the slow magnetic relaxation behaviour of chromium nitride (CrN2+ ) porphyrinato complex (1), which is structurally and magnetically similar with the vanadyl (VO2+ ) porphyrinato complex (2) which is known as the excellent spin qubit. Detailed analyses for vibrational and dynamical magnetism of 1 and 2 revealed that g-value anisotropy accelerates magnetic relaxations greater than the internal magnetic field from nuclear spin does. These results provide a design criterion for construction of multiple spin qubit based on g-tensor engineering.

Gold(i) bridged dimeric and trimeric heterometallic {Cr7Ni}-based qubit systems and their characterization

Gold(i) bridged dimeric and trimeric structures of a ground state spin S = 1/2 heterometallic {Cr₇Ni} wheel have been prepared and studied by continuous wave (CW) and pulsed wave EPR spectrometry. The {Cr₇Ni} relaxation time constants (T₁ and T_m) show rates matching well with previous observations. Four pulse Double Electron Resonance (DEER) studies suggest presence of more than one conformations. Small Angle X-ray Scattering (SAXS) in conjunction with Molecular Dynamic (MD) Simulations were performed to look at the possible conformations in solution. In line with DEER results, simulation data further indicated more flexible molecular geometry in solution than the one in solid state.

The Influence of Halide Substituents on the Structural and Magnetic Properties of Fe6Dy3 Rings

We report the synthesis and magnetic properties of three new nine-membered Fe(III)-Dy(III) cyclic coordination clusters (CCCs), with a core motif of [Fe₆Dy₃(μ-OMe)₉(vanox)₆(X-benz)₆] where the benzoate ligands are substituted in the para-position with X = F (1), Cl (2), Br (3). Single crystal X-ray diffraction structure analyses show that for the smaller fluorine or chlorine substituents the resulting structures exhibit an isostructural Fe₆Dy₃ core, whilst the 4-bromobenzoate ligand leads to structural distortions which affect the dynamic magnetic behavior. The magnetic susceptibility and magnetization of 1-3 were investigated and show similar behavior in the dc (direct current) magnetic data. Additional ac (alternating current) magnetic measurements show that all compounds exhibit frequency-dependent and temperature-dependent signals in the in-phase and out-of-phase component of the susceptibility and can therefore be described as field-induced SMMs. The fluoro-substituted benzoate cluster 1 shows a magnetic behavior closely similar to that of the corresponding unsubstituted Fe₆Dy₃ cluster, with U_eff = 21.3 K within the Orbach process. By increasing the size of the substituent toward 4-chlorobenzoate within 2, an increase of the energy barrier to U_eff = 36.1 K was observed. While the energy barrier becomes higher from 1 to 2, highlighting that the introduction of different substituents on the benzoate ligand in the para-position has an impact on the magnetic properties, cluster 3 shows a significantly different SMM behavior where U_eff is reduced in the Orbach regime to only 4.9 K.

Close Encounters of the Weak Kind: Investigations of Electron-Electron Interactions between Dissimilar Spins in Hybrid Rotaxanes

We report a family of hybrid [2]rotaxanes based on inorganic [Cr7NiF8(O2CtBu)16]- ("{Cr7Ni}") rings templated about organic threads that are terminated at one end with pyridyl groups. These rotaxanes can be coordinated to [Cu(hfac)2] (where Hhfac = 1,1,1,5,5,5-hexafluoroacetylacetone), to give 1:1 or 1:2 Cu:{Cr7Ni} adducts: {[Cu(hfac)2](py-CH2NH2CH2CH2Ph)[Cr7NiF8(O2CtBu)16]}, {[Cu(hfac)2][py-CH2NH2CH2CH3][Cr7NiF8(O2CtBu)16]}, {[Cu(hfac)2]([py-CH2CH2NH2CH2C6H4SCH3][Cr7NiF8(O2CtBu)16])2}, {[Cu(hfac)2]([py-C6H4-CH2NH2(CH2)4Ph][Cr7NiF8(O2CtBu)16])2}, and {[Cu(hfac)2]([3-py-CH2CH2NH2(CH2)3SCH3][Cr7NiF8(O2CtBu)16])2}, the structures of which have been determined by X-ray diffraction. The {Cr7Ni} rings and CuII ions both have electronic spin S = 1/2, but with very different g-values. Continuous-wave EPR spectroscopy reveals the exchange interactions between these dissimilar spins, and hence the communication between the different molecular components that comprise these supramolecular systems. The interactions are weak such that we observe AX or AX2 type spectra. The connectivity between the {Cr7Ni} ring and thread terminus is varied such that the magnitude of the exchange interaction J can be tuned. The coupling is shown to be dominated by through-bond rather than through-space mechanisms.

A [13]rotaxane assembled via a palladium molecular capsule

Molecules that are the size of small proteins are difficult to make. The most frequently examined route is via self-assembly, and one particular approach involves molecular nanocapsules, where ligands are designed that will enforce the formation of specific polyhedra of metals within the core of the structure. Here we show that this approach can be combined with mechanically interlocking molecules to produce nanocapsules that are decorated on their exterior. This could be a general route to very large molecules, and is exemplified here by the synthesis and structural characterization of a [13]rotaxane, containing 150 metal centres. Small angle X-ray scattering combined with atomistic molecular dynamics simulations demonstrate the compound is intact in solution.

Palladium(II)-Based Self-Assembled Heteroleptic Coordination Architectures: A Growing Family

Metal-driven self-assembly is one of the most effective approaches to lucidly design a large range of discrete 2D and 3D coordination architectures/complexes. Palladium(II)-based self-assembled coordination architectures are usually prepared by using suitable metal components, in either a partially protected form (PdL') or typical form (Pd; charges are not shown), and designed ligand components. The self-assembled molecules prepared by using a metal component and only one type of bi- or polydentate ligand (L) can be classified in the homoleptic series of complexes. On the other hand, the less explored heteroleptic series of complexes are obtained by using a metal component and at least two different types of non-chelating bi- or polydentate ligands (such as L^a and L^b ). Methods that allow the controlled generation of single, discrete heteroleptic complexes are less understood. A survey of palladium(II)-based self-assembled coordination cages that are heteroleptic has been made. This review article illustrates a systematic collection of such architectures and credible justification of their formation, along with reported functional aspects of the complexes. The collected heteroleptic assemblies are classified here into three sections: 1) [(PdL')_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is equal; 2) [(PdL')_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is different; and 3) [Pd_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is equal. Representative examples of some important homoleptic architectures are also provided, wherever possible, to set a background for a better understanding of the related heteroleptic versions. The purpose of this review is to pave the way for the construction of several unique heteroleptic coordination assemblies that might exhibit emergent supramolecular functions.

Formation of an interlocked double-chain from an organic-inorganic [2]rotaxane

Here we show that a structure containing a polymeric interlocking daisy chain is obtained from the reaction of an inorganic-organic [2]rotaxane [HB{CrIII7NiII(μ-F)8(O2CtBu)16}], where B is an organic thread terminated with a bi-pyridyl unit, with an oxo-centered metal carboxylate triangle [FeIII2CoII(μ3-O)(O2CtBu)6(HO2CtBu)3].

Chemical Transformations in Confined Space of Coordination Architectures

The scholastic significance of supramolecular chemistry continues to grow with the recent development of catalytic transformations in confined space of supramolecular architectures. It has come a long way from a natural cavity containing molecules to modern smart materials capable of manipulating reaction pathways. The rise of self-assembled coordination complexes provided a diverse array of host structures. Starting from purely organic compounds to metalloligand surrogates, supramolecular host cavities were tuned according to the requirement of the reactions. The understanding of their participation in a reaction led to better usage of those assemblies for specific reaction sequences. Commencing from cyclodextrin, a wide range of organic molecules was used for cage-catalyzed organic transformations. However, difficulties in synthesis and a tedious purification procedure led chemists to choose a different pathway of metal-ligand coordination-driven self-assembly. The latter stood out as a potential replacement of the organic cages, overcoming the previous drawbacks. In the glut of different transition-metal assemblies used for catalytic transformations, many of them showed chemo- and stereoselective products. However, the small cavity size in some of them led to premature failure of the reaction. In that context, "molecular barrels" showed good efficacy for the catalytic reaction sequence. The large cavity size and bigger orifice for intake of the substrate and easy release of the product made them a better choice for catalysis. Additionally these are mostly used in aqueous media, which reinforces the idea of green and environmentally nonhazardous chemistry. In this Viewpoint, we discuss the use of metal-ligand coordination-driven self-assembled molecular containers used for catalysis with special emphasis on molecular barrels. This paper built on existing literature provides a thorough development of the fertile ground of the coordination architecture for catalysis and its future direction of propagation.

Molecular spin qudits for quantum algorithms

Presently, one of the most ambitious technological goals is the development of devices working under the laws of quantum mechanics. One prominent target is the quantum computer, which would allow the processing of information at quantum level for purposes not achievable with even the most powerful computer resources. The large-scale implementation of quantum information would be a game changer for current technology, because it would allow unprecedented parallelised computation and secure encryption based on the principles of quantum superposition and entanglement. Currently, there are several physical platforms racing to achieve the level of performance required for the quantum hardware to step into the realm of practical quantum information applications. Several materials have been proposed to fulfil this task, ranging from quantum dots, Bose-Einstein condensates, spin impurities, superconducting circuits, molecules, amongst others. Magnetic molecules are among the list of promising building blocks, due to (i) their intrinsic monodispersity, (ii) discrete energy levels (iii) the possibility of chemical quantum state engineering, and (iv) their multilevel characteristics that lead to Qudits, where the dimension of the Hilbert space is d > 2. Herein we review how a molecular nuclear spin qudit, (d = 4), known as TbPc₂, gathers all the necessary requirements to perform as a molecular hardware platform with a first generation of molecular devices enabling even quantum algorithm operations.

Programmable self-assembly of water-soluble organo-heterometallic cages [M12M'4L12] using 3-(3,5-dimethyl-1H-pyrazol-4-yl)pentane-2,4-dione (H2L)

A bifunctional ligand H₂L featuring primary (pyrazole) and secondary (acetylacetone) coordination sites was preferentially reacted with dimetallic [M₂(NO₃)₂](NO₃)₂ linkers at the pyrazolyl end of H₂L, giving rise to dimetallic corners. Subsequently, the corners serve as the secondary site with M' to form water-soluble organo-heterometallic [M₁₂M'₄L₁₂] cages in a stepwise mode.

An Extensive Family of Heterometallic Titanium(IV)-Metal(III) Rings with Structure Control through Templates

A family of heterometallic [Cat][Ti_x MO_(x+1 )(O₂ C^t Bu)_2x+2 ] rings is reported where Cat=a secondary or tertiary alkyl ammonium ion, x=7, 8 or 9, and M=Fe^III , Ga^III , Cr^III , In^III and Al^III . The structures are regular polygons with eight, nine or ten vertices with each edge bridged by an oxide and two pivalates. The size of the ring formed is controlled by the alkylammonium cation present. In each case a homometallic by-product is found [Cat][Ti_x O_(x+1 )(O₂ C^t Bu)_2x-1 ].

Use of Supramolecular Assemblies as Lithographic Resists

A new resist material for electron beam lithography has been created that is based on a supramolecular assembly. Initial studies revealed that with this supramolecular approach, high-resolution structures can be written that show unprecedented selectivity when exposed to etching conditions involving plasmas.

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(O2C(t)Bu)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.

Self-assembly of a redox active water soluble Pd6L8 ‘molecular dice’

Self-assembly of a water soluble redox active Pd(ii) “molecular dice” was achieved employing a tri-cationic donor. The Pd₆ dice is quite stable in water in a wide range of temperatures despite the expected weaker donor ability of the cationic pyridyl donor.

Nine members of a family of nine-membered cyclic coordination clusters; Fe6Ln3 wheels (Ln = Gd to Lu and Y)

We report a family of isostructural nonanuclear Fe^III–Ln^III cyclic coordination clusters, [FeIII6LnIII3(μ-OMe)₉(vanox)₆ (benzoate)₆], with a planar core structure built up from three [Fe₂Ln] units to give a giant Ln₃ triangle embedded into an Fe₆ ring.

Synthesis and reactions of N-heterocycle functionalised variants of heterometallic {Cr7Ni} rings

Here we present a series of linked cage complexes of functionalised variants of the octametallic ring {Cr₇Ni} with the general formula [ⁿPr₂NH₂][Cr₇NiF₈(O₂C^tBu)₁₅(O₂CR)], where HO₂CR is a N-heterocycle containing carboxylic acid.

A high-nuclearity metal-cyanide cluster [Mo6Cu14] with photomagnetic properties

A high-nuclearity metal-cyanide cluster [Mo₆Cu₁₄] has been prepared and its photomagnetic properties investigated.

Supramolecular aggregates of single-molecule magnets: exchange-biased quantum tunneling of magnetization in a rectangular [Mn3]4 tetramer

Exchange-biased QTM within a magnetically-supramolecular tetramer of Mn₃ single-molecule magnets with spin S = 6 has been analyzed.

The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn₃ units are reported: dimeric [Mn₆O₂(O₂CMe)₈(CH₃OH)₂(pdpd)₂] (3) and [Mn₆O₂(O₂CMe)₈(py)₂(pdpd)₂](ClO₄)₂ (4), and tetrameric [Mn₁₂O₄(O₂CR)₁₂(pdpd)₆](ClO₄)₄ (R = Me (5), ^tBu (6)). They were all obtained employing 3-phenyl-1,5-di(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH₂), either in direct synthesis reactions involving oxidation of Mn^II salts or in metathesis reactions with the preformed complex [Mn₃O(O₂CMe)₆(py)₃](ClO₄) (1); complex 6 was then obtained by carboxylate substitution on complex 5. Complexes 3 and 4 contain two [MnIII2Mn^II(μ₃-O)]⁶⁺ and [MnIII3(μ₃-O)]⁷⁺ units, respectively, linked by two pdpd^2– groups. Complexes 5 and 6 contain four [MnIII3(μ₃-O)]⁷⁺ units linked by six pdpd^2– groups into a rectangular tetramer [MnIII3]₄. Solid-state dc magnetic susceptibility studies showed that the Mn₃ subunits in 3 and 4 have a ground-state spin of S = 3/2 and S = 2, respectively, while the Mn₃ subunits in 5 and 6 possess an S = 6 ground state. Complexes 5 and 6 exhibit frequency-dependent out-of-phase (χ′′_M) ac susceptibility signals indicating 5 and 6 to be tetramers of Mn₃ single-molecule magnets (SMMs). High-frequency EPR studies of a microcrystalline powder sample of 5·2CH₂Cl₂ provided precise spin Hamiltonian parameters of D = –0.33 cm^–1, |E| = 0.03 cm^–1, B04 = –8.0 × 10^–5 cm^–1, and g = 2.0. Magnetization vs. dc field sweeps on a single crystal of 5·xCH₂Cl₂ gave hysteresis loops below 1 K that exhibit exchange-biased quantum tunneling of magnetization (QTM) steps with a bias field of 0.19 T. Simulation of the loops determined that each Mn₃ unit is exchange-coupled to the two neighbors linked to it by the pdpd^2– linkers, with an antiferromagnetic inter-Mn₃ exchange interaction of J/k_B = –0.011 K (Ĥ = –2Jŝ_i·ŝ_j convention). The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum computing.

Heterometallic Rings: Their Physics and use as Supramolecular Building Blocks

An enormous family of heterometallic rings has been made. The first were Cr7 M rings where M = Ni(II), Zn(II), Mn(II), and rings have been made with as many as fourteen metal centers in the cyclic structure. They are bridged externally by carboxylates, and internally by fluorides or a penta-deprotonated polyol. The size of the rings is controlled through templates which have included a range of ammonium or imidazolium ions, alkali metals and coordination compounds. The rings can be functionalized to act as ligands, and incorporated into hybrid organic-inorganic rotaxanes and into molecules containing up to 200 metal centers. Physical studies reported include: magnetic measurements, inelastic neutron scattering (including single crystal measurements), electron paramagnetic resonance spectroscopy (including measurements of phase memory times), NMR spectroscopy (both solution and solid state), and polarized neutron diffraction. The rings are hence ideal for understanding magnetism in elegant exchange-coupled systems.