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

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

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

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

Determination of Cu2+ in drinking water using a hydroxyjulolidine-dihydroperimidine colorimetric sensor

A new and highly efficient colorimetric Cu2+ chemosensor HL, synthesized by condensation between 8-hydroxyjulolidine-9-carboxaldehyde and 1,8-diaminonaphthalene, has been rationally designed and thoroughly studied. In a buffered aqueous methanol mixture, interactions between HL and Cu2+ produce an intense visible band at 421 nm, considerably red-shifted (~ 100 nm) from the peak maxima of HL (320 nm). Absorbance spectrophotometry experiments pointed to an exceptional 2.3 nM limit of detection (LoD) calculated from the ratiometric response upon Cu2+ binding. Furthermore, without the aid of instrumentation an impressive 0.5 µM LoD is possible by naked-eye observations, far below the 31.5 µM (2 mg/L) guidelines for drinking water established by the World Health Organization. Spectrophotometric pH titrations allowed the determination of the equilibrium constants and speciation plots for the formation of the various chemical species of HL in the absence and presence of Cu2+, with only mononuclear complexes being found. Additional studies highlighted the selectivity of HL to Cu2+ when in the presence of other metal ions, and a 1:1 (M:L) binding stoichiometry has also been confirmed with results from Cu2+ titrations, Job’s plot and ESI-HRMS in good agreement. The Cu2+ sensing mechanism was also found to be reversible by cycling with H2Na2EDTA

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

High-temperature spin crossover in FeIII N4O2 complexes incorporating an [R-sal2323] backbone

Of the multitude of [FeIII(R-sal2323)]X complexes reported in the literature, only four have demonstrated spin crossover (SCO). Herein, we report four additional examples of thermal spin crossover in [FeIII(R-sal2323)]X complexes (where R = Br, NEt2, and X = ClO4-, BF4-). Magnetic susceptibility measurements reveal gradual, high-temperature spin transitions in all four compounds with onsets near room temperature. To investigate the emergence of SCO behaviors being observed in these compounds, a range of intramolecular and intermolecular structural parameters were examined. The effect that ligand substituents may have on the electronic environment, as well as the effect of counterions and various intermolecular interactions on the crystal packing, were investigated and compared to the literature of [FeIII(R-sal2323)]X compounds for which magnetic measurements are reported. This comparison found that neither intramolecular subtleties nor intermolecular interactions have a large impact on whether or not these compounds are SCO active. Instead, it is shown and proposed that many compounds in the [FeIII(R-sal2323)]X family may demonstrate SCO activity if measured to higher temperatures (above 300 K). This would provide a wide range of FeIII compounds that are SCO active near or above room temperature to be explored in future work

Syntheses and structure investigations of 3d transition metal complexes with a flexible N4O2-donor hexadentate Schiff-base ligand

The syntheses and structure investigations of four new 3d transition metal complexes (1–4) with a flexible N4O2-donor hexadentate Schiff-base ligand are described; three complexes (1, 2, and 4) of FeIII, CoIII, and CuII metal ions have been investigated by UV-vis, FT-IR, high-resolution mass spectrometry (HR-MS), and scanning electron microscopy–electron dispersive spectroscopy, as well as single crystal X-ray diffraction. The X-ray structure of NiII complex 3 is also reported. The molecular structures of the complexes (1–3) demonstrate distorted octahedral coordination geometry, each exhibiting 1 : 1 (M : L) ratios and the CuII complex 4 shows a trinuclear structure with a CuII : L ratio of 3 : 2 in the solid state, which has been proven by X-ray diffraction. On the other hand, a mononuclear species of the CuII complex is formed in solution, which has been identified by electrospray ionization HR-MS

Synthesis and characterization of two Cu(I) metalloligands based on tetradentate tripodal ligands

Two new tetradentate tripodal ligands (L1 and L2) have been synthesized via Schiff base condensation of tris(2-aminoethyl)amine (tren) with 4-(4-pyridinyl)benzaldehyde or 4-(3-pyridinyl)benzaldehyde in ethanol. Four Cu(I) complexes [CuL1]PF6, [CuL1]I, [CuL2]PF6 and [CuL2]I (1-4) have been prepared and characterised by NMR, HR-MS, SEM-EDS, FT-IR, Raman and UV-Vis measurements. X-ray structures for 1 and 4 are presented. In both structures, the four-coordinate copper(I) centres are bound within the cavity defined by the tren backbone. In such Cu(I) complexes, steric considerations dictate that the three uncoordinated pyridine nitrogen donors will have their coordination vectors oriented in a mutually divergent manner suitable for coordination to three different metal centres and thus are preorganized to act as new tripodal metalloligands