Cyclic ether and anhydride ring opening copolymerisation delivering new ABB sequences in poly(ester- alt -ethers) †

Poly(ester-alt-ethers) are interesting as they combine the ester linkage rigidity and potential for hydrolysis with ether linkage flexibility. This work describes a generally applicable route to their synthesis applying commercial monomers and yielding poly(ester-alt-ethers) with variable compositions and structures. The ring-opening copolymerisation of anhydrides (A), epoxides (B) and cyclic ethers (C), using a Zr(iv) catalyst, produces either ABB or ABC type poly(ester-alt-ethers). The catalysis is effective using a range of commercial anhydrides (A), including those featuring aromatic, unsaturated or tricyclic monomers, and with different alkylene oxides (epoxides, B), including those featuring aliphatic, alkene or ether substituents. The range of effective cyclic ethers (C) includes tetrahydrofuran, 2,5-dihydrofuran (DHF) or 1,4-bicyclic ether (OBH). In these investigations, the catalyst:anhydride loadings are generally held constant and deliver copolymers with degrees of copolymerisation of 25, with molar mass values from 4 to 11 kg mol−1 and mostly with narrow dispersity molar mass distributions. All the new copolymers are amorphous, they show the onset of thermal decomposition between 270 and 344 °C and variable glass transition temperatures (−50 to 48 °C), depending on their compositions. Several of the new poly(ester-alt-ethers) feature alkene substituents which are reacted with mercaptoethanol, by thiol–ene processes, to install hydroxyl substituents along the copolymer chain. This strategy affords poly(ether-alt-esters) featuring 30, 70 and 100% hydroxyl substituents (defined as % of monomer repeat units featuring a hydroxyl group) which moderate physical properties such as hydrophilicity, as quantified by water contact angles. Overall, the new sequence selective copolymerisation catalysis is shown to be generally applicable to a range of anhydrides, epoxides and cyclic ethers to produce new families of poly(ester-alt-ethers). In future these copolymers should be explored for applications in liquid formulations, electrolytes, surfactants, plasticizers and as components in adhesives, coatings, elastomers and foams

Self-assembly of a rare high spin FeII/PdII tetradecanuclear cubic cage constructed via the metalloligand approach

Polynuclear heterobimetallic coordination cages in which different metal cations are con-nected within a ligand scaffold are known to adopt a variety of polyhedral architectures, many of which display interesting functions. Within the extensive array of coordination cages incorporating Fe(II) centres reported so far, the majority contain low-spin (LS) Fe(II), with high-spin (HS) Fe(II) being less common. Herein, we present the synthesis and characterisation of a new tetradecanu-clear heterobimetallic [Fe8 Pd6 L8 ](BF4 ]28 (1) cubic cage utilising the metalloligand approach. Use of the tripodal tris-imidazolimine derivative (2) permitted the formation of the tripodal HS Fe(II) metalloligand [FeL](BF4)2·CH3 OH (3) that was subsequently used to form the coordination cage 1. Magnetic and structural analyses gave insight into the manner in which the HS environment of the metalloligand was transferred into the cage architecture along with the structural changes that accompanied its occupancy of the eight corners of the discrete cubic structure

Investigation of the structure-function relationships in a range of designed metallosupramolecular systems

Discrete metallosupramolecular complexes, including metallomacrocycles, helicates and cages, have emerged as the product of vast incremental improvements in coordination chemistry. These intricate chemical systems have potential applications in quantum data storage, catalysis, separation and trapping, sensing devices as well as drug delivery. The ability to design and synthesise spin-crossover systems, with controlled structures and properties, continues to evade the chemical and physical community. Currently, there is a lot of guess-and-check, lead-optimisation and post addendum methodology in the design of compounds with useful physical properties. This project aims to design and synthesise a range of supramolecular systems and investigate how their magnetic properties can be manipulated to explore the relationship between design, structure and function, or as it applies specifically to spin-crossover (SCO), the magneto-structural correlations. A deeper understanding of the factors affecting the design of more complex molecular systems with predictable structures and functions will enable chemists to reliably produce materials with intentional properties for use in future molecular devices. Further deep studies relating structure to function in a wide variety of chemical systems, are necessary to provide a more extensive collection of magneto-structural correlations, from which the design of future SCO systems can be better informed, and additionally, the development of new theories and models can be based. Firstly, a series of three Ln(III)-based complexes were synthesised via the Schiff-base condensation of N,N-diethylsalicylaldehyde (SAL) and tris(2-aminoethyl)amine (TREN) and characterised in order to assess the efficiency of the 4-position N-diethylamino electron-donating substituent as a modulator of the fluorescence and magnetic susceptibility of this commonly employed TRENSAL N4O3-donor cavity ligand. The Eu(III) compound exhibited efficient sensitisation of the metal centre and metal centred fluorescence, while the Dy(III) complex demonstrated signs of single molecule magnet behaviour in DC susceptibility measurements. Next, a mononuclear hexadentate complex, of the form [FeL](BF4)2, based on the 4-thioimidazole donor moiety, was designed to exhibit high-temperature SCO and was investigated using variable temperature X-ray photoelectron spectroscopy (VT-XPS). The aim of this study was to identify a method by which the high-spin (HS) fraction in the surface layers of a SCO material could be quantified using XPS, in order to allow the characterisation of future thin-film SCO materials and devices. Magnetic susceptibility measurements were used to calibrate XPS spectral fractions, and a HS-fraction curve was obtained that closely resembled that of the χmT Vs T results. Finally, three series of five Fe(II) dinuclear triple helicate compounds, of the form [Fe2L3], were synthesised, characterised and explored via magnetic susceptibility and single-crystal X-ray diffraction experiments. The general helicate architecture of each series differed by the steric nature of the central connecting atom of the ditopic bisbidentate imidazoleimine ligand donor (C, S or O). For each of the three helicate architectures, five counter ions were investigated; BF4-, ClO4-, I-/I3-, Br- and Cl-. In this way, fifteen analogous helicate materials, each with subtle changes in crystallographic structure, were analysed to determine the impact of various structural parameters on the magnetic susceptibility of these compounds—that is to identify magneto-structural correlations present in this chemical system—and compare these to previously reported results, with a particular focus on dinuclear helicate compounds. A range of relationships were found between selected magnetic and structural parameters. The most extensive of which were the observed dependence of the T1/2 on the strength of the anion-to-imidazole hydrogen-bonding at the external 4-position-imidazole H-N, and the relationship between the completion of SCO and the degree of intermolecular steric crowding of the Fe(II) coordination environment. As such, in this dinuclear triple helicate chemical system, the T1/2 can be systematically tuned by substitution of the hydrogen bond acceptor, and the extent of SCO—that is the HS Fe(II) fraction remaining at low temperatures—can be systematically tuned by manipulation of the packing of adjacent helicates throughout the crystal lattice. Encouraged by these results, co-crystallisation of a helicate compound with selected organic compounds (1,4-diiodo-2,3,5,6-tettrafluorobenzene (DITFB) and 1,3,5-benzenetricarboxylic acid respectively (BTC)) was performed in order to further investigate structure-function relationships in these dinuclear triple helicate compounds. This investigation demonstrated the various influences of steric congestion of the SCO centres on the completeness of the spin-transition in these compounds

A complementary characterisation technique for spin crossover materials : the application of X-ray photoelectron spectroscopy for future device applications

Spin crossover (SCO) materials have long been studied for their inherent electronic switchability, which has been well investigated for potential application in electronic and switching devices. As the technologies for the fabrication of thin films and monolayers continue to develop at an exceedingly rapid pace, an emerging challenge for the SCO community has become the characterisation of spin transitions in the surface layers of a material, as well as understanding the origins of discrepancies observed between SCO in thin films and that of the bulk material. For the manufacture of such devices to become a reality, it is crucial to understand how spin crossover is affected by interactions with the substrate material and within thin films. As such, detailed analysis of the surface layers without interference from the substrate material emerged as a critical area of characterisation for future developments in SCO devices. In this regard, X-ray Photoelectron Spectroscopy (XPS) has emerged as a complementary technique for the analysis of SCO in the surface layers of a material, becoming an essential part of a multi-technique protocol that is driving advances in the field. Here we describe the complementary application of XPS to a variety of SCO materials, review major developments and provide illustrative examples of innovations made through surface analysis with XPS

Solvent Effects on the Spin-Transition in a Series of Fe(II) Dinuclear Triple Helicate Compounds

This work explores the effect of lattice solvent on the observed solid-state spin-transition of a previously reported dinuclear Fe(II) triple helicate series 1–3 of the general form [FeII2L3](BF4)4(CH3CN)n, where L is the Schiff base condensation product of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively, and 1 is the complex when L = L1, 2 when L = L2 and 3 when L = L3 (Craze, A.R.; Sciortino, N.F.; Bhadbhade, M.M.; Kepert, C.J.; Marjo, C.E.; Li, F. Investigation of the Spin Crossover Properties of Three Dinuclear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type. Inorganics. 2017, 5 (4), 62). Desolvation of 1 and 2 during measurement resulted not only in a decrease in T1/2 and completeness of spin-crossover (SCO) but also a change in the number of steps in the spin-profile. Compounds 1 and 2 were observed to change from a two-step 70% complete transition when fully solvated, to a single-step half complete transition upon desolvation. The average T1/2 value of the two-steps in the solvated materials was equivalent to the single T1/2 of the desolvated sample. Upon solvent loss, the magnetic profile of 3 experienced a transformation from a gradual SCO or weak antiferromagnetic interaction to a single half-complete spin-transition. Variable temperature single-crystal structures are presented and the effects of solvent molecules are also explored crystallographically and via a Hirshfeld surface analysis. The spin-transition profiles of 1–3 may provide further insight into previous discrepancies in dinuclear triple helicate SCO research reported by the laboratories of Hannon and Gütlich on analogous systems (Tuna, F.; Lees, M. R.; Clarkson, G. J.; Hannon, M. J. Readily Prepared Metallo-Supramolecular Triple Helicates Designed to Exhibit Spin-Crossover Behaviour. Chem. Eur. J. 2004, 10, 5737–5750 and Garcia, Y.; Grunert, C. M.; Reiman, S.; van Campenhoudt, O.; Gütlich, P. The Two-Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy. Eur. J. Inorg. Chem. 2006, 3333–3339)

A rare example of a complete, incomplete, and non-occurring spin transition in a [Fe2L3]X4 series driven by a combination of solvent-and halide-anion-mediated steric factors

A trend between the degree of steric congestion of the Fe(II) coordination environment and the extent of spin transition (percentage completeness) has been observed in a series of halide salts of a dinuclear triple helicate architecture with the general form [Fe2L3]X4 (where X = Cl- for 1, Br- for 2, and (I-)3/I3 - for 3, and L is (1E,1′E)-N,N′-(oxybis(4,1-phenylene))bis(1-(1H-imidazol-4-yl)methanimine). Crystal packing densities of adjacent helicates were found to decrease with increasing anion size. Greater steric congestion by neighboring helicates favored the [HS-HS] state of the dinuclear triple helicate architecture. As a result, the highly crowded Cl- salt (1) did not undergo spin-crossover (SCO), the more congested Br- salt (2) underwent an incomplete solvent-dependent transition, and the least crowded (I-)3/I3 - analogue (3) exhibited a full SCO from the [HS-HS] ↔ [LS-LS] state. Furthermore, an interesting two-step transition was observed in the Br- salt, exhibiting a 28 K thermal hysteresis in the higher temperature step, the largest thermal hysteresis reported to date for a Fe(II) dinuclear triple helicate system. Variable-temperature single-crystal X-ray diffraction (SCXRD) analysis of 2 demonstrated that this two-step profile was found to be the result of crystallographic parameters evolving in a two-step manner with temperature, rather than a crystallographic phase change

A large dinuclear Fe(II) triple helicate demonstrating a two-step spin crossover

Reported herein, the synthesis as well as the structural and magnetic characterisation of the largest reported dinuclear Fe(II) triple helicate system to exhibit spin crossover—and also a rare example of a 273° helical twist using aromatic spacers—is presented, with exploration of the two-step spin-transition observed

Supramolecular Modulation of Spin Crossover in an Fe(II) Dinuclear Triple Helicate

A spin-crossover (SCO) active dinuclear Fe(II) triple helicate of the form [Fe2L3]4+ was combined with additional supramolecular components in order to manipulate the interhelical separation and steric congestion and to study the magneto-structural effects on the ensuing composite materials. A more separated array of SCO units produced more extensive spin-transitions, while a tightly arranged lattice environment stabilized the low-spin state. This study highlights the important interplay between crystal packing, intermolecualr interactions, and the magentic behavior of SCO materials.</p

Investigation of the spin crossover properties of three dinulear Fe(II) triple helicates by variation of the steric nature of the ligand type

The investigation of new spin-crossover (SCO) compounds plays an important role in understanding the key design factors involved, informing the synthesis of materials for future applications in electronic and sensing devices. In this report, three bis-bidentate ligands were synthesized by Schiff base condensation of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively. Their dinuclear Fe(II) triple helicates were obtained by complexation with Fe(BF4)2·6H2O in acetonitrile. The aim of this study was to examine the influence of the steric nature of the ligand central atom (–X–, where X = CH2, S or O) on the spin-crossover profile of the compound. The magnetic behaviours of these compounds were investigated and subsequently correlated to the structural information from single-crystal X-ray crystallographic experiments. All compounds [Fe2(L1)3](BF4)2 (1), [Fe2(L2)3](BF4)2 (2) and [Fe2(L3)3](BF4)2 (3), demonstrated approximately half-spin transitions, with T1/2↓ values of 155, 115 and 150 K respectively, corresponding to one high-spin (HS) and one low-spin (LS) Fe(II) centre in a [LS–HS] state at 50 K. This was also confirmed by crystallographic studies, for example, bond lengths and the octahedral distortion parameter (∑) at 100 K. The three-dimensional arrangement of the HS and LS Fe(II) centres throughout the crystal lattice was different for the three compounds, and differing extents of intermolecular interactions between BF4− counter ions and imidazole N–H were present. The three compounds displayed similar spin-transition profiles, with 2 (–S–) possessing the steepest nature. The shape of the spin transition can be altered in this manner, and this is likely due to the subtle effects that the steric nature of the central atom has on the crystal packing (and thus inter-helical Fe–Fe separation), intermolecular interactions and Fe–Fe intra-helical separations

Investigation of the high-temperature spin-transition of a mononuclear iron(ll) complex using X-ray photoelectron spectroscopy

This study presents a new mononuclear complex (1) of the form [FeL](BF4)2, incorporating the thiazolylimine donor moiety, which was found to exhibit a high-temperature spin-transition. The effect of scan rate was investigated, with magnetic susceptibility being measured at 4, 2, and 1 K min−1. The magnetic susceptibility results were confirmed by variable temperature X-ray photoelectron spectroscopy (XPS) (100, 270, 400, and 500 K) and single crystal X-ray diffraction (150 and 400 K) experiments. A rare example of a high-temperature (400 K) single crystal structure of 1 has been reported. The high-spin fraction was calculated indirectly from XPS data, presenting a method for analyzing the spin-state in the surface layers of spin-crossover materials