Gary S. Nichol

Increasing the dimensionality of cryogenic molecular coolers: Gd-based polymers and metal–organic frameworks

The magnetothermal properties of a coordination polymer and a metal-organic framework (MOF) based on Gd(3+) ions are reported. An equally large cryogenic magnetocaloric effect (MCE) is found, irrespective of the dimensionality. This combined with their robustness makes them appealing for widespread magnetic refrigeration applications.

New Chemistry from an Old Reagent: Mono- and Dinuclear Macrocyclic Uranium(III) Complexes from (U(BH4)3(THF)2)**

A new robust and high-yielding synthesis of the valuable U(III) synthon [U(BH4)3(THF)2] is reported. Reactivity in ligand exchange reactions is found to contrast significantly to that of uranium triiodide. This is exemplified by the synthesis and characterization of azamacrocyclic U(III) complexes, including mononuclear [U(BH4)(L)] and dinuclear [Li(THF)4][{U(BH4)}2(μ-BH4)(L(Me))] and [Na(THF)4][{U(BH4)}2(μ-BH4)(L(A))(THF)2]. The structures of all complexes have been determined by single-crystal X-ray diffraction and display two new U(III)2(BH4)3 motifs.

Maximizing Coordination Capsule–Guest Polar Interactions in Apolar Solvents Reveals Significant Binding

Guest encapsulation underpins the functional properties of self-assembled capsules yet identifying systems capable of strongly binding small organic molecules in solution remains a challenge. Most coordination capsules rely on the hydrophobic effect to ensure effective solution-phase association. In contrast, we show that using non-interacting anions in apolar solvents can maximize favorable interactions between a cationic Pd2 L4 host and charge-neutral guests resulting in a dramatic increase in binding strength. With quinone-type guests, association constants in excess of 108  m-1 were observed, comparable to the highest previously recorded constant for a metallosupramolecular capsule. Modulation of optoelectronic properties of the guests was also observed, with encapsulation either changing or switching-on luminescence not present in the bulk phase.

CO2 as a reaction ingredient for the construction of metal cages: a carbonate-panelled [Gd6Cu3] tridiminished icosahedron

A CO3(2-)-panelled [Gd(III)6Cu(II)3] cage conforming to a tridiminished icosahedron is synthesised by bubbling CO2 through a solution of Gd(III) and Cu(II) ions.

Directed secondary interactions in transition metal complexes of tripodal pyrrole imine and amide ligands.

Tripodal pyrrole imine and amide ligands provide platforms for combined primary and secondary coordination sphere interactions in square planar Pd and Cu, and octahedral Ti complexes.

[CrIII8MII6]12+ Coordination Cubes (MII=Cu, Co)

[CrIII8MII6]12+ (MII=Cu, Co) coordination cubes were constructed from a simple [CrIIIL3] metalloligand and a “naked” MII salt. The flexibility in the design proffers the potential to tune the physical properties, as all the constituent parts of the cage can be changed without structural alteration. Computational techniques (known in theoretical nuclear physics as statistical spectroscopy) in tandem with EPR spectroscopy are used to interpret the magnetic behavior.

From antiferromagnetic to ferromagnetic exchange in a family of oxime-based MnIII dimers: a magneto-structural study

The reaction of Mn(ClO4)2·6H2O, a derivatised phenolic oxime (R-saoH2) and the ligand tris(2-pyridylmethyl)amine (tpa) in a basic alcoholic solution leads to the formation of a family of cluster compounds of general formula [Mn(III)2O(R-sao)(tpa)2](ClO4)2 (1, R = H; 2, R = Me; 3, R = Et; 4, R = Ph). The structure is that of a simple, albeit asymmetric, dimer of two Mn(III) ions bridged through one μ-O(2-) ion and the -N-O- moiety of the phenolic oxime. Magnetometry reveals that the exchange interaction between the two Mn(III) ions in complexes 1, 3 and 4 is antiferromagnetic, but that for complex 2 is ferromagnetic. A theoretically developed magneto-structural correlation reveals that the dominant structural parameter influencing the sign and magnitude of the pairwise interaction is the dihedral Mn-O-N-Mn (torsion) angle. A linear correlation is found, with the magnitude of J varying significantly as the dihedral angle is altered. As the torsion angle increases the AF exchange decreases, matching the experimentally determined data. DFT calculations reveal that the dyz|π*|dyz interaction decreases as the dihedral angle increases leading to ferromagnetic coupling at larger angles.

Planar Ni(II), Cu(II) and Co(II) tetraaza[14]annulenes: structural, electronic and magnetic properties and application to field effect transistors

A series of planar Ni(II), Cu(II) and Co(II) tetraaza[14]annulenes has been prepared and studied optically, electrochemically and magnetically. Thin films of each of these complexes have been prepared by vacuum deposition to evaluate the field-effect transistor (FET) performance as well as the morphology and crystallinity of the film formed. The electrochemistry and UV/Vis absorption studies indicate the materials are redox active and highly coloured, with molar extinction coefficients as large as 80 000 M−1 cm−1 in the visible region. The paramagnetic complexes display weak antiferromagnetic interactions, fit to the Bonner–Fisher chain model. Each of the materials formed polycrystalline films when vacuum deposited and showed field-effect transistor behaviour, with charge carrier mobilities in the range of 10−5 to 10−9 cm2 V−1 s−1. SEM imaging of the substrates indicates that the central metal ion, and its sublimation temperature, has a crucial role in defining the morphology of the resulting film.

A new polymorph of metacetamol

Metacetamol is a structural isomer of the widely used drug paracetamol and is being considered as a promising alternative to the latter because of its lower toxicity. Due to the importance of the well-known polymorphism of paracetamol, an investigation of the polymorphism of metacetamol was successfully undertaken. A new polymorph of metacetamol has been discovered and extensively characterised using a variety of analytical techniques (IR- and Raman spectroscopy, UV-visible optical spectroscopy, X-ray powder and single-crystal diffraction, TGA and DSC). A procedure for the reliable and reproducible preparation of the new polymorph is described. Its properties and crystal structure are compared with those of the previously known polymorph, as well as with those of paracetamol.

Switching the orientation of Jahn-Teller axes in oxime-based Mn III dimers and its effect upon magnetic exchange: a combined experimental and theoretical study†

A family of Mn(III) dimers of general formula [Mn(R-sao)2(dpa)2](ClO4)2 (1-5) has been synthesised using derivatised phenolic oximes (R-saoH2, where R = H, Me, Et, Ph) in combination with di-(2-picolyl)-amine (dpa). Their structures reveal a double-oxime bridged [Mn(III)(NO)]2 magnetic core in which the Jahn-Teller axes lie perpendicular to the bridging plane, in contrast to two previously reported family members (6, 7). The switch in the orientation of the Jahn-Teller axes is enforced through the use of the chelating ligand which is present in 1-5 and absent in 6-7. Dc magnetic susceptibility measurements reveal that the exchange interactions between the Mn(III) metal centres in 1-5 are antiferromagnetic in contrast to that observed for 6 and 7 which are ferromagnetic. DFT calculations performed on complexes 1-6 reproduce both the sign and strength of the J values found experimentally. Molecular orbital analysis unlocks a common mechanism of magnetic coupling based upon the orientation of the Jahn-Teller axis, with the magneto-structural correlation also dependent upon the Mn-N-O-Mn angles--with ferromagnetic interactions at smaller dihedral angles.