An iron(ii) spin-crossover metallacycle from a back-to-back bis-[dipyrazolylpyridine]

The syntheses of 4-mercapto-2,6-di(pyrazol-1-yl)pyridine (bppSH) and bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide (bppSSbpp) are reported. In contrast to previously published “back-to-back” bis-[2,6-di(pyrazol-1-yl)pyridine] derivatives, which form coordination polymers with transition ions that are usually insoluble, bppSSbpp yields soluble oligomeric complexes with iron(II) and zinc(II). Mass spectrometry and DOSY data show that [{Fe(μ-bppSSbpp)}n]2n+ and [{Zn(μ-bppSSbpp)}n]2n+ form tetranuclear metallacycles in nitromethane solution (n = 4), although 1H NMR and conductivity measurements imply the iron compound may undergo more fragmentation than its zinc congener. Both [{Fe(bppSH)2]2+ and [{Fe(μ-bppSSbpp)}n]2n+ exhibit thermal spin-crossover in CD3NO2 solution, with midpoint temperatures near 245 K. The similarity of these equilibria implies there is little cooperativity between the iron centres in the metallacyclic structures

Crystal engineering and bifunctionality in spin-crossover materials

This thesis focuses on crystal engineering metal salts of 2,2’:6’,2’’-terpyridine and 2,6-di(pyrazol-1’-yl)pyridine to tune and better understand the cooperative spin crossover in the solid state and incorporation of a second functionality towards bifunctional spin crossover materials. Chapter 1 can be considered an introduction to the phenomenon of spin crossover in first row transition metal complexes and outlines its discovery, recent developments and progression towards real-world application. Chaper 2 is a discussion of the methodology of synthetic efforts undertaken in the acquisition of the wealth of tris-chelating organic compounds whose coordination chemistry is the subject of later chapters. Chapter 3 details the spectroscopic, electrochemical and magnetic properties of cobalt(II) and iron(II) complexes of tris-azinyl analogues of 2,2’:6’,2’’-terpyridine in efforts towards tuning their electronic spin-equilibria. Chapter 4 presents a series of novel ruthenium(II) based materials exhibiting enhanced room temperature emission and their incorporation into an inert spin crossover host material. Retention of parent functionalities is screened for over an operable temperature regime. Chapter 5 concerns iron(II) complex salts of 2,6-di(pyrazol-1’-yl)pyridine derivatives, whose ligand backbones are modified in an attempt to tune the solid state spin crossover behaviour both electronically and sterically. Chapter 6 is an account of all synthetic procedures carried out during this work, their standard characterisation and details the instrumentation performed on all materials which are the subject of discussion in this thesis

Scrolling in Supramolecular Gels: A Designer’s Guide

Gelation by small molecules is a topic of enormous importance in catalysis, nanomaterials, drug delivery, and pharmaceutical crystallization. The mechanism by which gelators self-organize into a fibrous gel network is poorly understood. Herein, we describe the crystal structures and gelation properties of a library of bis­(urea) compounds and show, via molecular dynamics simulations, how gelator aggregation progresses from a continuous pattern of supramolecular motifs to a homogeneous fiber network. Our model suggests that lamellae with asymmetric surfaces scroll into uniform unbranched fibrils, while sheets with symmetric surfaces undergo stacking to form crystals. The self-assembly of asymmetric lamellae is associated with specific molecular features, such as the presence of narrow and flexible end groups with high packing densities, and likely represents a general mechanism for the formation of small-molecule gels

Iron(II) complexes of 4-sulfanyl-, 4-sulfinyl- and 4-sulfonyl-2,6-dipyrazolylpyridine ligands. A subtle interplay between spin-crossover and crystallographic phase changes.

Oxidation of 4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (LSMe) with hydrogen peroxide or mCPBA yields 4-(methylsulfinyl)-2,6-di(pyrazol-1-yl)pyridine (LSOMe) and 4-(methylsulfonyl)-2,6-di(pyrazol-1-yl)-pyridine (LSO2Me), respectively. Solid [Fe(LSMe)2][ClO4]2 (1[ClO4]2) is high-spin at room temperature, and exhibits an abrupt spin-transition at T1/2 = 256 K. A shoulder on the cooling side of the χMT vs. T curve is associated with a hysteretic crystallographic phase change, occurring around T↓ = 245 K and T↑ = 258 K. The phase change involves a 180° rotation of around half the methylsulfanyl substituents in the crystal. This contrasts with the previously reported BF4 − salt of the same compound, which is isostructural to 1[ClO4]2 at room temperature but transforms to a different crystal phase in its low-spin state. Solid [Fe(LSOMe)2][BF4]2 (2[BF4]2) and [Fe(LSO2Me)2][BF4]2 (3[BF4]2) both exhibit gradual spin-crossover equilibria centred significantly above room temperature. Solution measurements show that the oxidised sulphur centers in 2[BF4]2 and 3[BF4]2 stabilise the low spin states of those complexes

Community Intercomparison Suite (CIS) v1.4.0: a tool for intercomparing models and observations

The Community Intercomparison Suite (CIS) is an easy-to-use command-line tool which has been developed to allow the straightforward intercomparison of remote sensing, in situ and model data. While there are a number of tools available for working with climate model data, the large diversity of sources (and formats) of remote sensing and in situ measurements necessitated a novel software solution. Developed by a professional software company, CIS supports a large number of gridded and ungridded data sources "out-of-the-box", including climate model output in NetCDF or the UK Met Office pp file format, CloudSat, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization), MODIS (MODerate resolution Imaging Spectroradiometer), Cloud and Aerosol CCI (Climate Change Initiative) level 2 satellite data and a number of in situ aircraft and ground station data sets. The open-source architecture also supports user-defined plugins to allow many other sources to be easily added. Many of the key operations required when comparing heterogenous data sets are provided by CIS, including subsetting, aggregating, collocating and plotting the data. Output data are written to CF-compliant NetCDF files to ensure interoperability with other tools and systems. The latest documentation, including a user manual and installation instructions, can be found on our website (http://cistools.net). Here, we describe the need which this tool fulfils, followed by descriptions of its main functionality (as at version 1.4.0) and plugin architecture which make it unique in the field

Humans rather than climate the primary cause of Pleistocene megafaunal extinction in Australia.

Environmental histories that span the last full glacial cycle and are representative of regional change in Australia are scarce, hampering assessment of environmental change preceding and concurrent with human dispersal on the continent ca. 47,000 years ago. Here we present a continuous 150,000-year record offshore south-western Australia and identify the timing of two critical late Pleistocene events: wide-scale ecosystem change and regional megafaunal population collapse. We establish that substantial changes in vegetation and fire regime occurred ∼70,000 years ago under a climate much drier than today. We record high levels of the dung fungus Sporormiella, a proxy for herbivore biomass, from 150,000 to 45,000 years ago, then a marked decline indicating megafaunal population collapse, from 45,000 to 43,100 years ago, placing the extinctions within 4,000 years of human dispersal across Australia. These findings rule out climate change, and implicate humans, as the primary extinction cause

The role of symmetry breaking in the structural trapping of light-induced excited spin states

Light-Induced Excited Spin State Trapping (LIESST) data are reported for seven isostructural solvate salts from the iron(II)/2,6-di(pyrazol-1-yl)pyridine family. A complicated relationship between their spin-crossover T1/2 and T(LIESST) values may reflect low-temperature thermal and light-induced symmetry breaking, which is shown by one of the compounds but not by two others

Spin-Crossover and the LIESST Effect in [FexCo1–x(bpp)2][BF4]2 (1.00 ⩽ x ⩽ 0.77). Comparison with Bifunctional Solid Solutions of Iron and Cobalt Spin-Crossover Centers

Co-crystallization of [Fe(bpp)2][BF4]2 and [Co(bpp)2][BF4]2 (bpp = 2,6-di{pyrazol-1-yl}pyridine) from nitromethane-diethyl ether yields homogeneous polycrystalline materials analysing as [FexCo1–x(bpp)2][BF4]2 (1.00 ⩽ x ⩽ 0.77). Thermal spin-crossover in these materials only involves the iron centers, and increasing the cobalt dopant concentration leads to a reduction in T½ and a loss of cooperativity. The materials exhibit the LIESST effect, with all three samples presenting the same T(LIESST) value. LIESST relaxation kinetics have a clear multistep character, which has not been detected before in samples derived from [Fe(bpp)2][BF4]2. Magnetic susceptibility and low-temperature crystallographic data are also presented for the pure precursor complex [Co(bpp)2][BF4]2

Bead-like structures and self-assembled monolayers from 2,6-dipyrazolylpyridines and their iron(II) complexes

Drop-casting acetone solutions of [Fe(bpp)2][BF4]2 (bpp = 2,6-di[pyrazol-1-yl]pyridine) onto a HOPG surface affords unusual chain-of-beads nanostructures. The beads in each chain are similar in size, with diameters in the range of 2–6 nm and heights of up to 10 Å, which is consistent with them containing between 10–50 molecules of the compound. The beads can be classified into two types, which exhibit different conduction regimes by current-imaging tunnelling spectroscopy (CITS) which appear to correlate with their positions in the chains, and may correspond to molecules containing high-spin and low-spin iron centres. Similarly drop-cast films of the complex on a gold surface contain the intact [Fe(bpp)2][BF4]2 compound by XPS. 4-Mercapto-2,6-di[pyrazol-1-yl]pyridine undergoes substantial decomposition when deposited on gold, forming elemental sulfur, but 4-(N-thiomorpholinyl)-2,6-di[pyrazol-1-yl]pyridine successfully forms SAMs on a gold surface by XPS and ellipsometry

High Yielding Flow Synthesis of a Macrocyclic Molecular Hinge

ABSTRACT: Many molecular machines are built from modular components with well-defined motile capabilities, such as axles and wheels. Hinges are particularly useful, as they provide the minimum flexibility needed for a simple and pronounced conformational change. Compounds with multiple stable conformers are common, but molecular hinges almost exclusively operate via dihedral rotations rather than truly hinge-like clamping mechanisms. An ideal molecular hinge would better reproduce the behavior of hinged devices, such as gates and tweezers, while remaining soluble, scalable and synthetically versatile. Herein, we describe two isomeric macrocycles with clamp-like open and closed geometries, which crystallize as separate polymorphs but interconvert freely in solution. An unusual one-pot addition cyclization reaction was used to produce the macrocycles on a multigram scale from inexpensive reagents, without supramolecular templating or high-dilution conditions. Using mechanistic information from NMR kinetic studies and at-line mass spectrometry, we developed a semi-continuous flow synthesis with maximum conversions of 85-93% and over 80% selectivity for a single isomer. The macrocycles feature voids that are sterically protected from guests, including reactive species such as fluoride ions, and could therefore serve as chemically inert hinges for adaptive supramolecular receptors and flexible porous materials.</jats:p