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Time-resolved dynamic light scattering unveiled the multiscale self-assembly mechanism of metal–organic polyhedra and allowed for inducing asymmetry in a porous gel., Spatial heterogeneity and gradients within porous materials are key for controlling their mechanical properties and mass/energy transport, both in biological and synthetic materials. However, it is still challenging to induce such complexity in well-defined microporous materials such as crystalline metal–organic frameworks (MOFs). Here we show a method to generate a continuous gradient of porosity over multiple length scales by taking advantage of the amorphous nature of supramolecular polymers based on metal–organic polyhedra (MOPs). First, we use time-resolved dynamic light scattering (TRDLS) to elucidate the mechanism of hierarchical self-assembly of MOPs into colloidal gels and to understand the relationship between the MOP concentrations and the architecture of the resulting colloidal networks. These features directly impact the viscoelastic response of the gels and their mechanical strength. We then show that gradients of stiffness and porosity can be created within the gel by applying centrifugal force at the point of colloidal aggregation. These results with the creation of asymmetric and graded pore configuration in soft materials could lead to the emergence of advanced properties that are coupled to asymmetric molecule/ion transport as seen in biological systems.

Symmetry control using DABCO generates a large easy-plane magnetic anisotropy with zero rhombic anisotropy., The magnetic properties of 3d monometallic complexes can be tuned through geometric control, owing to their synthetic accessibility and relative structural simplicity. Monodentate ligands offer great potential for fine-tuning the coordination environment to engineer both the axial and rhombic zero-field splitting (ZFS) parameters. In [CoCl3(DABCO)(HDABCO)] (1), the trigonal bipyramidal Co(ii) centre has two bulky axial ligands and three equatorial chloride ligands. An in-depth experimental and theoretical study of 1 reveals a large easy-plane magnetic anisotropy (+ve D) with a negligible rhombic zero-field splitting (E) due to the strict axial symmetry imposed by the C3 symmetric ligand and trigonal space group. The large easy-plane magnetic anisotropy (D = +44.5 cm–1) is directly deduced using high-field EPR and frequency-domain magnetic resonance (FDMR) studies. Ab initio calculations reveal a large positive contribution to the D term arising from ground state/excited state mixing of the 4E′′ states at ∼4085 cm–1 and a minor contribution from the spin–flip transition as well. The nature of the slow relaxation in 1 is elucidated through analysis of the rates of relaxation of magnetisation, taking into account Raman and direct spin–lattice relaxation processes and Quantum Tunnelling of the Magnetisation (QTM). The terms relating to the direct process and QTM were found based on the fit of the field-dependence of τ at 2 K. Subsequently, these were used as fixed parameters in the fit of the temperature-dependence of τ to obtain the Raman terms. This experimental–theoretical investigation provides further insight into the power of FDMR and ab initio methods for the thorough investigation of magnetic anisotropy. Thus, these results contribute to design criteria for high magnetic anisotropy systems.

Assemblies of lantern-type metal–organic polyhedra are found to show gate-opening gas adsorption depending on how they are treated with solvents., Gate-opening gas sorption is known for metal–organic frameworks, and is associated with structural flexibility and advantageous properties for sensing and gas uptake. Here, we show that gate-opening is also possible for metal–organic polyhedra (MOPs), and depends on the molecular organisation in the lattice. Thanks to the solubility of MOPs, several interchangeable solvatomorphs of a lantern-type MOP were synthesised via treatment with different solvents. One phase obtained through use of methanol induced a gate-opening effect in the lattice in response to carbon dioxide uptake. The sorption process was thoroughly investigated with in situ powder X-ray diffraction and simultaneous adsorption experiments. Meanwhile, solution processing of this flexible phase using THF led to a permanently porous phase without a gate-opening effect. Furthermore, we find that we can change the metallic composition of the MOP, and yet retain flexibility. By showing that gate-opening can be switched on and off depending on the solvent of crystallisation, these findings have implications for the solution-based processing of MOPs.

The synergistic combination of high pressure techniques with ab initio methods creates a powerful tool to understand giant magnetic anisotropy., Understanding and controlling magnetic anisotropy at the level of a single metal ion is vital if the miniaturisation of data storage is to continue to evolve into transformative technologies. Magnetic anisotropy is essential for a molecule-based magnetic memory as it pins the magnetic moment of a metal ion along the easy axis. Devices will require deposition of magnetic molecules on surfaces, where changes in molecular structure can significantly alter magnetic properties. Furthermore, if we are to use coordination complexes with high magnetic anisotropy as building blocks for larger systems we need to know how magnetic anisotropy is affected by structural distortions. Here we study a trigonal bipyramidal nickel(ii) complex where a giant magnetic anisotropy of several hundred wavenumbers can be engineered. By using high pressure, we show how the magnetic anisotropy is strongly influenced by small structural distortions. Using a combination of high pressure X-ray diffraction, ab initio methods and high pressure magnetic measurements, we find that hydrostatic pressure lowers both the trigonal symmetry and axial anisotropy, while increasing the rhombic anisotropy. The ligand–metal–ligand angles in the equatorial plane are found to play a crucial role in tuning the energy separation between the dx2–y2 and dxy orbitals, which is the determining factor that controls the magnitude of the axial anisotropy. These results demonstrate that the combination of high pressure techniques with ab initio studies is a powerful tool that gives a unique insight into the design of systems that show giant magnetic anisotropy.

Two flexible, branched, and sterically constrained di- and tripodal side arms around a phenol backbone were utilized in ligands H3L1 and H5L2 to isolate {Mn6} and {Mn3} coordination aggregates. 2,6-Bis{(1-hydroxy-2-methylpropan-2-ylimino)methyl}-4-methylphenol (H3L1) gave trinuclear complex [Mn3(μ-H2L1)2(μ1,3-O2CCH3)4(CH3OH)2](ClO4)2·4CH3OH (1), whereas 2,6-bis[{1-hydroxy-2-(hydroxymethyl)butan-2-ylimino}methyl]-4-methylphenol (H5L2) provided hexanuclear complex [Mn6(μ4-H2L2)2(μ-HL3)2(μ3-OH)2(μ1,3-O2CC2H5)4](ClO4)2·2H2O (2). Binding of acetates and coordination of {H2L1}− provided a linear MnIIIMnIIMnIII arrangement in 1. A MnIII6 fused diadamantane-type assembly was obtained in 2 from propionate bridges, coordination of {H2L2}3–, and in situ generated {HL3}2–. The magnetic characterization of 1 and 2 revealed the properties dominated by intramolecular anti-ferromagnetic exchange interactions, and this was confirmed using density functional theory calculations. Complex 1 exhibited field-induced slow magnetic relaxation at 2 K due to the axial anisotropy of MnIII centers. Both the complexes show effective solvent-dependent catechol oxidation toward 3,5-di-tert-butylcatechol in air. The catechol oxidation abilities are comparable from two complexes of different nuclearity and structure.

Materials that demonstrate long-range magnetic order are synonymous with information storage and the electronics industry, with the phenomenon commonly associated with metals, metal alloys or metal oxides and sulfides. A lesser known family of magnetically ordered complexes are the monometallic compounds of highly anisotropic d-block transition metals; the ‘transformation' from isolated zero-dimensional molecule to ordered, spin-canted, three-dimensional lattice being the result of through-space interactions arising from the combination of large magnetic anisotropy and spin-delocalization from metal to ligand which induces important intermolecular contacts. Here we report the effect of pressure on two such mononuclear rhenium(IV) compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, with Tc controlled by the strength of the intermolecular interactions. As these are determined by intermolecular distance, ‘squeezing' the molecules closer together generates remarkable enhancements in ordering temperatures, with a linear dependence of Tc with pressure., Materials that demonstrate long-range magnetic order are synonymous with information storage. Here, the authors report the effect of pressure on two mononuclear rhenium compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, where T c is proportional to pressure.

Four different carboxylato bridges have been efficiently utilized for growth of three tetranuclear nickel(II) complexes [Ni4(μ3-H2L)2(μ3-OH)2(μ1,3-CH3CO2)2](ClO4)2 (1), [Ni4(μ3-H2L)2(μ3-OH)2(μ1,3-C2H5CO2)2](ClO4)2·1/2H2O (2), and [Ni4(μ3-H2L)2(μ3-OH)2(μ1,3-O2C-C6H4-pNO2)2](ClO4)(p-NO2-C6H4-CO2)·DMF·5H2O (3) and one dinuclear nickel(II)-based chain complex {[Ni2(μ-H2L)(μ1,3-O2CCH2Ph)2(H2O)](ClO4)·1/2(CH3OH)}n (4). These were obtained via the reaction of Ni(ClO4)2·6H2O with H3L [2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol] and RCO2Na (R = CH3,C2H5, p-NO2C6H4, and PhCH2). This family of complexes is developed from {Ni2(μ-H2L)}3+ fragments following self-aggregation. The complexes were characterized by X-ray crystallography and magnetic measurements. The changes from acetate, propionate, and p-nitrobenzoate to phenylacetate groups resulted in two different types of coordination aggregation. These compounds are new examples of [Ni4] and [Ni2]n complexes where organization of the building motifs are guided by the type of the carboxylate groups responsible for in-situ generation and utilization of HO– bridges with alteration in the aggregation process within the same ligand environment. Studies on the magnetic behavior of the compounds reveal that the exchange coupling within 1–4 is predominantly antiferromagnetic in nature.

One of the determining factors in whether single-molecule magnets (SMMs) may be used as the smallest component of data storage, is the size of the barrier to reversal of the magnetisation, Ueff. This physical quantity depends on the magnitude of the magnetic anisotropy of a complex and the size of its spin ground state. In recent years, there has been a growing focus on maximising the anisotropy generated for a single 3d transition metal (TM) ion, by an appropriate ligand field, as a means of achieving higher barriers. Because the magnetic properties of these compounds arise from a single ion in a ligand field, they are often referred to as single-ion magnets (SIMs). Here, the synthetic chemist has a significant role to play, both in the design of ligands to enforce propitious splitting of the 3d orbitals and in the judicious choice of TM ion. Since the publication of the first 3d-based SIM, which was based on Fe(II), many other contributions have been made to this field, using different first row TM ions, and exploring varied coordination environments for the paramagnetic ions.

ACS AuthorChoice - This is an open access article published under a Creative Commons Attribution (CC-BY) License., High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted MnIII ion in Na5[Mn(l-tart)2]·12H2O (1; l-tart = l-tartrate) has a significant negative axial zero-field splitting and a small rhombic anisotropy (∼1% of D). Alternating-current magnetic susceptibility measurements demonstrate that 1, which contains isolated MnIII centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field., The U.K. Engineering and Physical Sciences Research Council are thanked for financial support (Grant EP/K033662/1). Work performed at the NHMFL was supported by the NSF (Grants DMR-1157490 and DMR-1309463) and the State of Florida.

Systems based on the organic ligand 3-bpp display a wide variety of magnetic behaviour, and have been studied in depth in the field of spin crossover (SCO). However, despite the apparent richness of SCO behaviour observed for 3-bpp systems, at the time of undertaking this thesis there were no published examples of the use of synthetic derivatives of 3-bpp in this field of research. This was remarked upon by Olguín and Brooker, who attributed this lack of 3-bpp analogues not to a lack of interest, but to “synthetic issues”. Therefore, the initial challenge set out was to develop a methodology that would allow the synthesis of functionalised 3-bpp ligands. Once obtained, these ligands would then be used for the generation of mononuclear Fe(II) compounds, with a view to study their macroscopic properties. Provided that this strategy was successful, and depending on the availability of structural data, an attempt would then be made to outline correlations between the observed magnetic properties and the relationship between the lattice entities, both spin-active and –inactive. The development of this work is detailed over the course of the ten Chapters. The first is an Introduction, primarily to the general field of SCO, the theory underlying its occurrence and its potential utility, but also describes the latest advances in the field before focussing on the family of 3-bpp systems. Chapter 2 contains the synthetic procedures and experimental details for the physical characterisation of the compounds obtained. Chapter 3 then describes the development of the novel polypyrazolyl ligands that have been designed for the subsequent investigation of their coordination chemistry. The magneto-structural properties of a new [Fe(3-bpp)2]2+ salt are described, and the compound shown to display a gradual spin crossover. Chapters 4 and 5 provide an in-depth study of a mononuclear compound, [Fe(H4L)2](ClO4)2•H2O•2(CH3)2CO (1), containing one of the polypyrazolyl ligands. A strongly cooperative spin transition is found to be intimately related to the structure and level of disorder within the lattice. This conclusion is supported by a description of the thermally trapped crystal structure. The photo-physical properties of 1 are then described, through the study of its meta-stable HS phase, which is distinct to the thermally trapped meta-stable HS phase. A Raman spectroscopy study of the system is then used to observe photo-switching within the hysteresis loop, and as a means of following the pressure-induced spin switch. Chapters 6 and 7 use compound 1 as the basis of an investigation into the effect of varying the anions and solvents within the lattice on the magnetic properties and crystal packing. A case where the spin crossover becomes more cooperative on ageing the compound is described. Chapter 8 details the compounds obtained using another 3-bpp derivative, which results in the formation of distorted structures that do not display SCO. In Chapter 9, the structural data provided by this thesis is collated with that found in the CSD for mononuclear Fe(II) systems with 3-bpp ligands. The data is then analysed from two points of view: a first, which involves previously defined parameters for the distortion of the cations, both in terms of their shape and in terms of the coordination sphere using Continuous Symmetry Measures. And a second, which uses Hirshfeld surface analysis to look at the bonding motifs and intermolecular interactions that are observed in the novel compounds obtained in this thesis. Chapter 10 offers conclusions based on the results obtained in this thesis., A pesar de la riqueza de los comportamientos de la transición de espín de los sistemas basados en el ligando 3-bpp, no se ha estudiado ningún derivado del ligando. Así pues, el reto inicial de esta tesis fue el de desarrollar ligandos derivados del 3-bpp. Estos se utilizarían en la síntesis de compuestos mononucleares de Fe(II), para estudiar sus propiedades macroscópicas. Este trabajo se detalla a lo largo de nueve capítulos. El capítulo 2 contiene los procedimientos experimentales para la síntesis y caracterización de los compuestos obtenidos. El capítulo 3 describe el desarrollo de los ligandos que fueron utilizados en la química de coordinación. Se detallan las propiedades magneto-estructurales de una nueva sal [Fe(3-bpp)2](ClO4)2 que presenta una transición de espín gradual. Los capítulos 4 y 5 estudian el compuesto [Fe(H4L)2](ClO4)2•H2O•2(CH3)2CO (1). Se observa que una transición de alta cooperatividad se asocia al desorden cristalográfico. Se procede a describir las propiedades foto-físicas del compuesto, mediante el estudio de su fase meta-estable de espín alto inducido por irradiación. Un estudio de espectroscopia de Raman se llevó a cabo, permitiendo así la observación de un proceso de “photo-switch” dentro del ciclo de histéresis. Los espectros obtenidos sirvieron para seguir la transición al aplicar una presión externa. Los capítulos 6 y 7 investigan el efecto de los aniones y disolventes sobre las propiedades magnéticas y el empaquetamiento estructural del compuesto 1. Se describe un caso dónde un proceso de envejecimiento del compuesto lleva a que se aumente su grado de cooperatividad. En el capítulo 8, se utiliza otro derivado del 3-bpp para preparar unos compuestos mononucleares de Fe(II). Debido a la deformación estructural presentada por estos compuestos, no se observa una transición de espín. En el capítulo 9, se recogen los datos estructurales obtenidos a lo largo de la tesis, y se combinan con los datos disponibles en el CSD para compuestos mononucleares de Fe(II). Se analizan los datos para medir la forma y distorsión de los cationes, y para estudiar los tipos de interacción intermolecular mediante los análisis de superficies de Hirshfeld. El capítulo 10 saca unas conclusiones basadas en los resultados descritos en los capítulos anteriores.