Show Abstract ▼

Abstract: This report investigates homoleptic iron(II) complexes of thiazolinyl analogues of chiral PyBox tridentate ligands: 2,6-bis(4-phenyl-4,5-dihydrothiazol-2-yl)pyridine (L1Ph), 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine (L1iPr), and 2,6-bis(4-tert-butyl-4,5-dihydrothiazol-2-yl)pyridine (L1t-Bu). Crystallographic data imply the larger and more flexible thiazolinyl rings reduce steric clashes between the R substituents in homochiral [Fe((R)-L1R)2]2+ or [Fe((S)-L1R)2]2+ (R = Ph, iPr, or t-Bu), compared to their PyBox (L2R) analogues. Conversely, the larger heterocyclic S atoms are in close contact with the R substituents in heterochiral [Fe((R)-L1Ph)((S)-L1Ph)]2+, giving it a more sterically hindered ligand environment than that in [Fe((R)-L2Ph)((S)-L2Ph)]2+ (L2Ph = 2,6-bis(4-phenyl-4,5-dihydrooxazol-2-yl)pyridine). Preformed [Fe((R)-L1Ph)((S)-L1Ph)]2+ and [Fe((R)-L1iPr)((S)-L1iPr)]2+ do not racemize by ligand redistribution in CD3CN solution, but homochiral [Fe(L1iPr)2]2+ and [Fe(L1t-Bu)2]2+ both undergo partial ligand displacement in that solvent. Homochiral [Fe(L1Ph)2]2+ and [Fe(L1iPr)2]2+ exhibit spin-crossover equilibria in CD3CN, centered at 344 ± 6 K and 277 ± 1 K respectively, while their heterochiral congeners are essentially low-spin within the liquid range of the solvent. These data imply that the diastereomers of [Fe(L1Ph)2]2+ and [Fe(L1iPr)2]2+ show a greater difference in their spin-state behaviors than was previous found for [Fe(L2Ph)2]2+. Gas-phase DFT calculations (B86PW91/def2-SVP) of the [Fe(L1R)2]2+ and [Fe(L2R)2]2+ complexes reproduce most of the observed trends, but they overstabilize the high-spin state of SCO-active [Fe(L1iPr)2]2+ by ca. 1.5 kcal mol–1. This might reflect the influence of intramolecular dispersion interactions on the spin states of these compounds. Attempts to model this with the dispersion-corrected functionals B97-D2 or PBE-D3 were less successful than our original protocol, confirming that the spin states of sterically hindered molecules are a challenging computational problem.

Open Access

Show Abstract ▼

Abstract: Chemical purifications are critical processes across many industries, requiring 10–15% of humanity’s global energy budget. Coordination cages are able to catch and release guest molecules based upon their size and shape, providing a new technological basis for achieving chemical separation. Here, we show that aqueous solutions of FeII4L6 and CoII4L4 cages can be used as liquid membranes. Selective transport of complex hydrocarbons across these membranes enabled the separation of target compounds from mixtures under ambient conditions. The kinetics of cage-mediated cargo transport are governed by guest binding affinity. Using sequential transport across two consecutive membranes, target compounds were isolated from a mixture in a size-selective fashion. The selectivities of both cages thus enabled a two-stage separation process to isolate a single compound from a mixture of physicochemically similar molecules.

Open Access

Show Abstract ▼

Abstract: In this work we use high-resolution synchrotron X-ray diffraction for electron density mapping, in conjunction with ab initio modelling, to study short O—H⋯O and O+—H⋯O− hydrogen bonds whose behaviour is known to be tuneable by temperature. The short hydrogen bonds have donor–acceptor distances in the region of 2.45 Å and are formed in substituted urea and organic acid molecular complexes of N,N′-dimethylurea oxalic acid 2[thin space (1/6-em)]:[thin space (1/6-em)]1 (1), N,N-dimethylurea 2,4-dinitrobenzoate 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (2) and N,N-dimethylurea 3,5-dinitrobenzoic acid 2[thin space (1/6-em)]:[thin space (1/6-em)]2 (3). From the combined analyses, these complexes are found to fall within the salt-cocrystal continuum and exhibit short hydrogen bonds that can be characterised as both strong and electrostatic (1, 3) or very strong with a significant covalent contribution (2). An additional charge assisted component is found to be important in distinguishing the relatively uncommon O—H⋯O pseudo-covalent interaction from a typical strong hydrogen bond. The electron density is found to be sensitive to the extent of static proton transfer, presenting it as a useful parameter in the study of the salt–cocrystal continuum. From complementary calculated hydrogen atom potentials, we attribute changes in proton position to the molecular environment. Calculated potentials also show zero barrier to proton migration, forming an ‘energy slide’ between the donor and acceptor atoms. The better fundamental understanding of the short hydrogen bond in the ‘zone of fluctuation’ presented in a salt-cocrystal continuum, enabled by studies like this, provide greater insight into their related properties and can have implications in the regulation of pharmaceutical materials.

Show Abstract ▼

Abstract: A combination of charge density studies and solid state nuclear magnetic resonance (NMR) 1 J NC coupling measurements supported by periodic density functional theory (DFT) calculations is used to characterise the transition from an n - π * interaction to bond formation between a nucleophilic nitrogen atom and an electrophilic sp 2 carbon atom in a series of crystalline peri-substituted naphthalenes. As the N···C distance reduces there is a sharp decrease in the Laplacian derived from increasing charge density between the two groups at ca. N···C:1.8 Å, with the periodic DFT calculations predicting, and heteronuclear spin-echo NMR measurements confirming, the 1 J NC couplings of ~2-5 Hz for long C-N bonds (1.60-1.65 Å), and 1 J NC couplings of <1 Hz for N···C > 2.1 Å.

Open Access

Show Abstract ▼

Abstract: The octanuclear Co(II) cubic coordination cage system H (or HW if it bears external water-solubilising substituents) has two types of binding site for guests. These are (i) the partially-enclosed central cavity where neutral hydrophobic organic species can bind, and (ii) the six 'portals' in the centres of each of the faces of the cubic cage where anions bind via formation of a network of CH•••X hydrogen bonds between the anion and CH units on the positively-charged cage surface, as demonstrated by a set of crystal structures. The near-orthogonality of these guest binding modes provides the basis for an unusual dual-probe fluorescence displacement assay in which either a cavity-bound fluorophore (4-methyl-7-amino-coumarin, MAC; λem = 440 nm), or a surface-bound anionic fluorophore (fluorescein, FLU; λem = 515 nm), is displaced and has its emission 'switched on' according to whether the analyte under investigation is cavity-binding, surface binding, or a combination of both. A completely orthogonal system is demonstrated based on a Hw/MAC/FLU combination: addition of the anionic analyte ascorbate displaced solely FLU from the cage surface, increasing the 515 nm (green) emission component, whereas addition of a neutral hydrophobic guest such as cyclooctanone displaced solely MAC from the cage central cavity, increasing the 440 nm (blue) emission component. Addition of chloride resuts in some release of both components, and an intermediate colour change, as chloride is a rare example of a guest that shows both surface-binding and cavity-binding behaviour. Thus we have a colourimetric response based on differing contributions from blue and green emission components in which the specific colour change signals the binding mode of the analyte. Addition of a fixed red emission component from the complex [Ru(bipy)3]2+ (Ru) provides a baseline colour shift of the overall colour of the luminescence closer to neutral, meaning that different types of guest binding result in different colour changes which are easily distinguishable by eye.