Melanie Pilkington

The Coordination Chemistry of 3,3′-Diamino-2,2′-bipyridine and Its Dication: Exploring the Role of the Amino Groups by X-ray Crystallography

The synthesis and structural chemistry of a series of new divalent transition metal complexes of the bis-bidentate ligand 3,3-diamino-2,2-bipyridine (L1) and its dication L1H2 are described. Ligand L1 reacts with salts of divalent transition metals to afford the (1:1) metal-ligand complexes (2a-2d) as well as the tris complexes (3a-3f). All complexes were fully characterised by spectroscopic methods and the following compounds [Cu(L1)Cl2]2 (2a), [Cu(L1)(OAc)2] (2b), [Zn(L1)3][OTf]2 (3a), and [Zn(L1)3][ZnCl4] (3e and 3f) were structurally characterised. Results from single crystal X-ray diffraction measurements indicate that formation of an intramolecular hydrogen bond between the two amino groups allows the ligand to coordinate divalent metal ions through their diimine binding sites. Furthermore, the structure of compound 2a reveals that it crystallises as a dimer in which each copper ion is bound to two pyridine nitrogen atoms and two chloride ions in a distorted square planar arrangement, with a long axial contact from a neighbouring amino group completing the approximately square-pyramidal geometry at CuII. Complexation of this ligand in acidic conditions afforded the compound [Cu(L1H2)Cl4] (4), as well as the two salts [L1H2][CuCl4] (5a) and [L1H2][ZnCl4] (5b). All three compounds have been structurally characterised and results indicate that the dication (L1H2) displays a different coordination preference for the chelation of metal ions. In all three cases, both of the heterocyclic N atoms of the ligand are protonated, thus preventing chelation to the metal ion, although for compound 4 crystallographic studies reveal that the two amino functionalities coordinate the copper(II) ion. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Spin−Spin Interactions in Porphyrin-Based Monoverdazyl Radical Hybrid Spin Systems

The spin-spin interactions in a complex consisting of a metalloporphyrin with a verdazyl radical attached at one of the beta positions of the porphyrin ring are investigated. The X-ray crystal structure of the copper porphyrin complex shows that the plane of the verdazyl moiety is oriented such that it is nearly perpendicular to the plane of the porphyrin ring so that weak magnetic interactions between the metal and radical are expected. Consistent with this expectation, magnetic susceptibility and continuous-wave electron paramagnetic resonance (EPR) measurements of the copper (d(9)) and vanadyl (d(1)) versions of the porphyrin show that the metal and radical are weakly antiferromagnetically coupled. Thus, the ground state is a singlet, but the triplet state is thermally accessible above approximately 5 K. Spin-polarized transient EPR measurements of the free-base analogue show that its lowest excited state is a quartet, indicating that the verdazyl radical couples ferromagnetically to the triplet excited state of the porphyrin. Low-temperature transient EPR measurements on the vanadyl porphyrin reveal that the lowest excited quintet state is populated. This implies that the antiferromagnetic coupling between the metal and radical observed in the ground state is switched to a ferromagnetic arrangement in the excited state by the presence of the unpaired electrons in the pi and pi* orbitals of the porphyrin.

New carbon-carbon bond forming reactions of cyclic sulfate esters and cyclic sulfamidates

Abstract Carbon–carbon bonding forming reactions of two cyclic sulfate esters and a cyclic sulfamidate are reported, the former with lithium dianions to give substituted tetrahydrofuran derivatives with displacement of sulfate, and the latter undergoes ring-opening monosubstitution reactions with stabilised organolithiums and an organocuprate species.

Placing a crown on DyIII – a dual property LnIII crown ether complex displaying optical properties and SMM behaviour

Two mononuclear DyIII crown ether complexes [Dy(15C5)(H2O)4](ClO4)3·(15C5)·H2O (1) and [Dy(12C4)(H2O)5](ClO4)3·H2O (2) have been prepared and characterized. X-ray diffraction studies show that both compounds crystallize as half sandwich type structures with muffin and pseudo-capped square antiprismatic geometries respectively. Despite the comparable local environments of the DyIII ions they display remarkably different dynamic magnetic properties with only 1 displaying SMM properties in zero field. The solid state emission spectra for both 1 and 2 display sharp bands associated with f–f transitions. From the fine structure of the 4F9/2→6H15/2 band, the Stark splitting of the 6H15/2 ground state permitted the energy difference between the ground and first excited state to be determined. For 1 this value (ΔE = 58.0 ± 3.0 cm−1) is in excellent agreement with ab initio calculations and the experimentally observed SMM behaviour. For 2, the photoluminescence data and theoretical calculations support a less well isolated ground state (ΔE = 30 ± 3.0 cm−1) in which a rapid relaxation process affords no SMM behaviour in zero-field.

Functionalised organosulfur donor molecules: Synthesis of racemic hydroxymethyl-, alkoxymethyl- and dialkoxymethyl-bis(ethylenedithio)tetrathiafulvalenes

Abstract Short synthetic routes to racemic derivatives of bis(ethylenedithio)tetrathiafulvalene carrying one hydroxymethyl (HMET), alkoxymethyl or dialkoxymethyl side chain are reported along with cyclic voltammetry measurements and conversion of HMET to (2:1) radical cation salts.

Synthesis and characterization of a TTF-π-verdazyl radical—a new building block for conducting and/or magnetic systems

The synthetic strategy for the preparation of a TTF donor, π-appended with a spin bearing verdazyl radical has been developed. UV-Vis, electrochemical and EPR studies reveal that this is an attractive magnetic building block for subsequent oxidation that could afford new conducting and/or magnetic systems.

An Alkyne Group with A Pair of Hydrogen-Bonds - the Crystal-Structure of 2,2'-Ethynylenedibenzeneboronic Acid at 122 K

For each boronic acid group in 1 one hydrogen atom makes a hydrogen bond to an alkyne carbon atom as well as an intermolecular O–H ⋯ O bond; structural effects indicate the boronic acid group to be σ-electron releasing and π-electron withdrawing.

A novel fully conjugated phenanthroline-appended phthalocyanine: synthesis and characterisation

The synthetic route to a new fully conjugated phenanthroline appended phthalocyanine is described. This compound has been fully characterised by elemental analysis, UV-VIS, IR, MS and 1H NMR spectroscopy.

Large Energy Barrier and Magnetization Hysteresis at 5 K for a Symmetric {Dy2} Complex with Spherical Tricapped Trigonal Prismatic DyIII Ions

The introduction of the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid (sacbH2) in 4f-metal chemistry has afforded a new dinuclear complex, [Dy2(NO3)4(sacbH)2(H2O)2(MeCN)2] (1), with the metal ions adopting a rare spherical tricapped trigonal prismatic coordination geometry. The deprotonated phenoxido O atoms of the organic chelate occupy the axial triangular faces of the prism and were found to be very close to the main anisotropy axes of the two DyIII ions. As a result, the {DyIII2} compound exhibits frequency- and temperature-dependent out-of-phase ac signals below ∼25 K in the absence of a static dc field, yielding an energy barrier of 109.3(1) K for the reversal of magnetization. Fast and efficient quantum tunneling of magnetization, attributed to the strong tails of signals below ∼15 K, was suppressed through the application of a small dc field, yielding entirely visible χM″ signals below 27 K. Single-crystal magnetic hysteresis studies confirmed the single-molecule magnet (SMM) behavior of 1; the hysteresis loops appear at temperatures below ∼5 K, which is one of the highest blocking temperatures in the field of 4f-SMMs to date. This joint magneto-structural and ab initio study demonstrates the ability of more common coordination numbers (i.e., 9), but with rare coordination geometries (i.e., spherical tricapped trigonal prismatic), to promote axiality that enhances the molecular anisotropy and subsequently the magnetization dynamics of the system.

Transition Metal Single-Molecule Magnets: A {Mn31} Nanosized Cluster with a Large Energy Barrier of ∼60 K and Magnetic Hysteresis at ∼5 K

The first {Mn31} cluster (1) has been prepared from carboxylate ions and the chelating/bridging ligand α-methyl-2-pyridine-methanol. Compound 1 possesses a unique nanosized structural topology with one of the largest energy barriers reported to-date for high-nuclearity 3d-metal clusters. Single-crystal magnetic hysteresis studies reveal the presence of hysteresis loops below 5 K, one of the highest temperatures below which molecular hysteresis has been observed for 3d-based SMMs.