Gavin A. Craig

Coupled Crystallographic Order-Disorder and Spin State in a Bistable Molecule: Multiple Transition Dynamics

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H(4)L, has led to an Fe(II) spin-crossover (SCO) complex, [Fe(H(4)L)(2)][ClO(4)](2)⋅H(2)O⋅2 (CH(3))(2)CO (1), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T(0.5) of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high-spin state may also be thermally trapped, exhibiting a very large T(TIESST) (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO(4)(-) and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe-N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order-disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising-like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.

Field-Induced Slow Relaxation in a Monometallic Manganese(III) Single-Molecule Magnet

High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted Mn(III) 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 Mn(III) centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field.

Local Coordination Geometry and Spin State in Novel FeII Complexes with 2,6-Bis(pyrazol-3-yl)pyridine-Type Ligands as Controlled by Packing Forces: Structural Correlations

A substituted 2,6-bis(pyrazol-3-yl)pyridine (3-bpp) ligand, H(4)L, created to facilitate intermolecular interactions in the solid, has been used to obtain four novel Fe(II) complexes: [Fe(H(4)L)(2)](ClO(4))(2)⋅2 CH(3)NO(2)⋅2 H(2)O, [Fe(H(4)L)(H(2)LBF(2))](BF(4))⋅5 C(3)H(6)O (H(2)LBF(2) is an in situ modified version of H(4)L), [Fe(H(4)L)(2)](ClO(4))(2)⋅2 C(3)H(7)OH and [Fe(H(4)L)(2)](ClO(4))(2)⋅4 C(2)H(5)OH. Changing of spin-inactive components (solvents, anions or distant ligand substituents) causes differences to the coordination geometry of the metal that are key to the magnetic properties. Magnetic measurements show that, contrary to the previously published complex [Fe(H(4)L)(2)](ClO(4))(2)⋅H(2)O⋅2 CH(3)COCH(3), the newly synthesised compounds remain in the high-spin (HS) state at all temperatures (5-300 K). A member of the known family of Fe(II)/3-bpp complexes, [Fe(3-bpp)(2)](ClO(4))(2)⋅1.75 CH(3)COCH(3)⋅1.5 Et(2)O, has also been prepared and characterised structurally. In the bulk, this compound exhibits a gradual and incomplete spin transition near 205 K. The single-crystal structure is consistent with it being HS at 250 K and partially low spin at 90 K. Structural analysis of all these compounds reveals that the exact configuration of intermolecular interactions affects dramatically the local geometry at the metal, which ultimately has a strong influence on the magnetic properties. Along this line, the geometry of Fe(II) in all published 3-bpp compounds of known structure has been examined, both by calculating various distortion indices (Σ, Θ, θ and Φ) and by continuous shape measures (CShMs). The results reveal correlations between some of these parameters and indicate that the distortions from octahedral geometry observed on HS systems are mainly due to strains arising from intermolecular interactions. As previously suggested with other related compounds, we observe here that strongly HS-distorted systems have a larger tendency to remain in that state.

Synthesis of a novel heptacoordinated Fe(III) dinuclear complex: experimental and theoretical study of the magnetic properties.

A new functionalized bis-pyrazol-pyridine ligand has been prepared by reaction with hydrazine of the corresponding bis-β-diketone precursor, also unprecedented. The aerobic reaction of this ligand with ferrous thiocyanate in the presence of ascorbic or oxalic acid affords the dinuclear complex of seven-coordinate Fe(III), [Fe₂(H₄L2)₂(ox)(NCS)₄] (1), as revealed by single crystal X-ray diffraction. This may represent an entry into a new family of [Fe₂] compounds with heptacoordinate metal centres. The capacity of this unusual chromophore to undergo magnetic super-exchange was investigated by means of bulk magnetization and DFT calculations. Both approaches confirmed the presence of antiferromagnetic interactions within the molecule. The theoretical investigation has served to describe the magnetic orbitals of Fe(III) in this unusual coordination geometry, as well as the exchange mechanism. A brief review of the scarce number of iron heptacoordinate complexes reported in the literature is also included and discussed.

The Use of a Bis(phenylpyrazolyl)pyridyl Ligand to Prepare [Mn4] and [Mn10] Cage Complexes

Many branches in molecular magnetism live on the production of novel transition-metal (TM) coordination clusters. This has been the source of high-spin molecules, most single-molecule magnets (SMMs), or the possible future qubits for quantum computing. Without doubt, the great advances made in these areas have benefited, for the most part, from the preparation of molecules through so-called “serendipitous self-assembly”. Even though the designed preparation of clusters with predetermined structures and properties is highly desirable, it continues to be a huge synthetic challenge. Therefore, there is no reason to think that the “nonpredictable” approach will cease to be instrumental in the near future. For this, synthetic coordination chemists will keep providing original methods of preparation of polynuclear TM complexes. Typically, these are produced from reactions involving one or more small ligands with donor atoms capable of bridging metals, normally exhibiting several potential coordination modes. By contrast, the synthesis of large multidentate ligands with numerous donor atoms is far less common in this context. We have recently prepared a new ligand with two aligned hydroxyphenylpyrazolyl units separated by a pyridine group (2,6-bis[5-(2-hydroxyphenyl)pyrazol-3-yl]pyridine, H4L, Scheme 1). [10] Hydroxyphenylpyrazolyl derivatives or related pyrazolinoles have proven to be excellent ligands for the assembly of TM clusters with novel structures. The build up of more than one such moiety on the same molecule promises to be an open door to a variety of unprecedented architectures. Ligand H4L is well suited, for example, to stimulate the aggregation of a sequence of five closely spaced metals within a cluster (Scheme 2B). We report herein very promis-

Multimetastability in a Spin-Crossover Compound Leading to Different High-Spin-to-Low-Spin Relaxation Dynamics

The relaxation kinetics of both the thermally trapped and photoinduced high-spin (HS) states of the spin-crossover compound [Fe(H4L)2](ClO4)2·H2O·2(CH3)2CO (1) were measured and found to differ significantly. Calorimetry measurements then demonstrated that relaxation of the thermally trapped phase was concurrent with two separate processes, not previously detected as such. Determination of the photogenerated HS structure revealed a new metastable HS state of the system, much closer structurally to the low-spin phase than the thermally trapped one. This difference is proposed as the root of the disparate kinetic behavior, which is proposed to require two processes in the case of the structurally more complex thermally trapped state. Therefore, light irradiation is shown as a mechanism to decouple effectively the structural and magnetic phase transitions that occur in 1 during the course of its spin crossover.

Linear or cyclic clusters of Cu(II) with a hierarchical relationship

The polydentate ligand 2,6-bis(5-(2-hydroxyphenyl)-pyrazol-3-yl)-pyridine, H4L, exhibits a series of coordination pockets favoring the establishment of metal sequences with predetermined motifs, together with a degree of flexibility for the formation of clusters with various overall topologies. With Cu(II) under strong basic conditions it has a marked tendency to stabilize a cyclic [Cu16L8] cluster. The sequential formation of this compound via [Cu7L8](2-) intermediates, recognized in its structure, is suggested by crystallographic evidence, which shows the persistent formation of the complex salt (NBu4)2[Cu7L8] in the presence of the organic cation. Also, the crystallographic identification of the related cluster [Cu11L5(OH)2(py)12] from similar reaction conditions underscores the rich multiplicity of species attainable from this simple reaction system.

Dangling and Hydrolyzed Ligand Arms in [Mn3] and [Mn6] Coordination Assemblies: Synthesis, Characterization, and Functional Activity

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.

Molecular [Co(III)Co(II)] × 2 assemblies of a new bis-phenol/pyrazolyl ligand

Use of a novel multinucleating ligand in aerobic reactions with Co(II) affords two novel coordination assemblies exhibiting a rare linear [CoIII2CoII2] structure and novel structural features. The magnetic exchange within these unprecedented moieties is investigated.

Self-assembly of metal–organic polyhedra into supramolecular polymers with intrinsic microporosity

Designed porosity in coordination materials often relies on highly ordered crystalline networks, which provide stability upon solvent removal. However, the requirement for crystallinity often impedes control of higher degrees of morphological versatility, or materials processing. Herein, we describe a supramolecular approach to the synthesis of amorphous polymer materials with controlled microporosity. The strategy entails the use of robust metal–organic polyhedra (MOPs) as porous monomers in the supramolecular polymerization reaction. Detailed analysis of the reaction mechanism of the MOPs with imidazole-based linkers revealed the polymerization to consist of three separate stages: nucleation, elongation, and cross-linking. By controlling the self-assembly pathways, we successfully tuned the resulting macroscopic form of the polymers, from spherical colloidal particles to colloidal gels with hierarchical porosity. The resulting materials display distinct microporous properties arising from the internal cavity of the MOPs. This synthetic approach could lead to the fabrication of soft, flexible materials with permanent porosity.Porosity in metal–organic materials typically relies on highly ordered crystalline networks, which hinders material processing and morphological control. Here, the authors use metal–organic polyhedra as porous monomers in supramolecular polymerization to produce colloidal spheres and gels with intrinsic microporosity.