Dimensionality of luminescent coordination polymers of magnesium ions and 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylic acid modulated by structural inducing agents

Solvothermal reactions of aromatic 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylic acid (H₄EBTC) and Mg²⁺ salts in the presence of different supporting ligands afforded the coordination polymers [Mg(H₂EBTC)(DMF)₂(H₂O)₂] (1), [Mg₃(HEBTC)₂(H₂O)₄]·solvent (2) and [Mg₂(EBTC)(H₂O)₅]·solvent (3). The crystal structures of 1–3 were determined by the single crystal X-ray diffraction technique, where CP 1 showed a one-dimensional zigzag MgO₆ coordination octahedral chain structure; 2 exhibited a two-dimensional MgO₆ coordination octahedral framework with trinuclear [Mg₃(COO)₆] SBUs, and 3 featured a three-dimensional MgO₆ coordination octahedral framework with binuclear [Mg₂O(COO)₂] SBUs. The various structures in CPs 1–3 of Mg²⁺ ions with the H₄EBTC ligand were ascribed to the conformational flexibility and the coordination mode diversity of the H₄EBTC ligand. Interestingly, the zwitterionic supporting ligand 2-aminoterephthalic acid or 4-aminobenzenesulphonic acid played a vital role in the initial formation process of nuclear crystals but only as a structural induction agent, which modulated the dimensionality of these Mg²⁺-based CPs. Additionally, the three CPs emitted bright blue luminescence at ambient conditions, and the emission lifetimes and absolute quantum yields were also investigated.

Crystal structure diversity and dimensionality of three Mg²⁺-based CPs, which emit bright ligand-based luminescence at ambient conditions, are modulated by reaction temperature and structural induction agents.     

A series of Ln4III clusters: Dy4 single molecule magnet and Tb4 multi-responsive luminescent sensor for Fe3+, CrO42−/Cr2O72− and 4-nitroaniline

Five tetranuclear lanthanide clusters of compositions [Ln₄L₄(NO₃)₂(Piv)₂]·2CH₃OH (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5); H₂L = 2-(((2-hydroxy-3-methoxybenzyl)imino)methyl)-6-methoxyphenol; Piv = pivalic acid) were synthesized under solvothermal conditions. The structures of 1–5 were characterized by single-crystal X-ray crystallography. Complexes 1–5 possess a zig-zag topology with [Ln₄O₆] cores being formed by the fusion of oxygen atom-bridged two [Ln₂O₂] moieties. Direct-current magnetic susceptibility studied in the 2–300 K range revealed weak antiferromagnetic interactions in 1, 2, 4, 5 and ferromagnetic interactions in 3. Complex 3 exhibits single molecule magnet (SMM) behavior. The luminescence studies indicated that complex 2 can serve as highly sensitive and selective luminescent materials for Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and 4-nitroaniline (4-NA), demonstrating that complex 2 should be a potential candidate for multi-responsive luminescent sensor.

Five tetranuclear lanthanide clusters were synthesized. Dy₄ complex exhibits single molecule magnet (SMM) behavior and Tb₄ compound shows sensing properties towards Fe³⁺, CrO₄²⁻, Cr₂O₇²⁻ and 4-nitroaniline (4-NA).     

Lanthanide complexes combined with chiral salen ligands: application in the enantioselective epoxidation reaction of α,β-unsaturated ketones

Readily available lanthanide amides Ln[N(SiMe₃)₂]₃ (Ln = Nd (1), Sm (2), Eu (3), Yb (4), La (5)), combined with chiral salen ligands H₂L^a ((S,S)-N,N′-di-(3,5-disubstituted-salicylidene)-1,2-cyclohexanediamine) and H₂L^b ((S,S)-N,N′-di-(3,5-disubstituted-salicylidene)-1,2-diphenyl-1,2-ethanediamine) were employed in the enantioselective epoxidation of α,β-unsaturated ketones. It was found that the salen–La complex shows the highest efficiency and enantioselectivity. A relatively broad scope of α,β-unsaturated ketones was investigated, and excellent yields (up to 99%) and moderate to good enantioselectivities (37–87%) of the target molecules were achieved.

The enantioselective epoxidation of α,β-unsaturated ketones was catalysed by readily available lanthanide amides La[N(SiMe₃)₂]₃ combined with chiral salen ligands.     

B12X11(H2)−: exploring the limits of isotopologue selectivity of hydrogen adsorption

We study the isotopologue-selective binding of dihydrogen at the undercoordinated boron site of B₁₂X₁₁⁻ (X = H, F, Cl, Br, I, CN) using ab initio quantum chemistry. With a Gibbs free energy of H₂ attachment reaching up to 80 kJ mol⁻¹ (ΔG at 300 K for X = CN), these sites are even more attractive than most undercoordinated metal centers studied so far. We thus believe that they can serve as an edge case close to the upper limit of isotopologue-selective H₂ adsorption sites. Differences of the zero-point energy of attachment average 5.0 kJ mol⁻¹ between D₂ and H₂ and 2.7 kJ mol⁻¹ between HD and H₂, resulting in hypothetical isotopologue selectivities as high as 2.0 and 1.5, respectively, even at 300 K. Interestingly, even though attachment energies vary substantially according to the chemical nature of X, isotopologue selectivities remain very similar. We find that the H–H activation is so strong that it likely results in the instantaneous heterolytic dissociation of H₂ in all cases (except, possibly, for X = H), highlighting the extremely electrophilic nature of B₁₂X₁₁⁻ despite its negative charge. Unfortunately, this high reactivity also makes B₁₂X₁₁⁻ unsuitable for practical application in the field of dihydrogen isotopologue separation. Thus, this example stresses the two-edged nature of strong H₂ affinity, yielding a higher isotopologue selectivity on the one hand but risking dissociation on the other, and helps define a window of adsorption energies into which a material for selective adsorption near room temperature should ideally fall.

The extreme H₂ affinity of B₁₂X₁₁⁻ gives a glimpse of how higher selectivities in adsorptive isotopologue separation may be achieved.     

TiO2–Au composite nanofibers for photocatalytic hydrogen evolution

TiO₂-based materials for photocatalytic hydrogen (H₂) evolution have attracted much interest as a renewable approach for clean energy applications. TiO₂–Au composite nanofibers (NFs) with an average fiber diameter of ∼160 nm have been fabricated by electrospinning combined with calcination treatment. In situ reduced gold nanoparticles (NPs) with uniform size (∼10 nm) are found to disperse homogenously in the TiO₂ NF matrix. The TiO₂–Au composite NFs catalyst can significantly enhance the photocatalytic H₂ generation with an extremely high rate of 12 440 μmol g⁻¹ h⁻¹, corresponding to an adequate apparent quantum yield of 5.11% at 400 nm, which is 25 times and 10 times those of P25 (584 μmol g⁻¹ h⁻¹) and pure TiO₂ NFs (1254 μmol g⁻¹ h⁻¹), respectively. Furthermore, detailed studies indicate that the H₂ evolution efficiency of the TiO₂–Au composite NF catalyst is highly dependent on the gold content. This work provides a strategy to develop highly efficient catalysts for H₂ evolution.

The H₂ production rate of TiO₂–Au nanofibers is dramatically improved to 12 440 μmol g⁻¹ h⁻¹, 10 times that of pure TiO₂.     

Hot exciton transition for organic light-emitting diodes: tailoring excited-state properties and electroluminescence performances of donor–spacer–acceptor molecules

The photophysical, electrochemical and electroluminescent properties of newly synthesized blue emitters with donor–π–acceptor geometry, namely, 4′-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (NSPI-TPA), 4′-(1-(2-methylnaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-(2-[1,1′-biphenyl]vinyl)-4-amine (MNSPI-TPA), 4-(2-(4′-(diphenylamino)-(2-[1,1′-biphenyl]vinyl)-4-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)-1-naphthalene-1-carbonitrile (SPNCN-TPA) and 4-(2-(4-(9H-carbazol-9-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)naphthalene-1-carbonitrile (SPNCN-Cz) were analyzed. The conjugation length in the emitters is not conducive to pure emission and hence, a molecular twisting strategy was adopted in NSPI-TPA, MNSPI-TPA, SPNCN-TPA and SPNCN-Cz to enhance pure emission. The emissive state (HLCT) of twisted D–π–A molecules containing both LE and CT (HLCT) states was tuned for high PL (η_PL) (LE) and high exciton utilization (η_s) (CT) efficiencies by replacing triphenylamine (strong donor) with carbazole (weak donor). Among strong donor compounds, namely, NSPI-TPA, MNSPI-TPA and SPNCN-TPA, the SPNCN-TPA-based device exhibited blue emission (451 nm) with CIE coordinates (0.15, 0.08), maximum current efficiency (η_c) of 2.32 cd A⁻¹, power efficiency (η_p) of 2.01 lm W⁻¹ and external quantum efficiency (η_ex) of 3.02%. The device with SPNCN-Cz emitter exhibited higher electroluminescence efficiencies than the SPNCN-TPA-based device, with pure blue emission (443 nm, CIE: 0.15,0.07), η_ex of 3.15%, η_c of 2.56 cd A⁻¹ and η_p of 2.45 lm W⁻¹.

SPNCN-Cz device exhibits η_ex (3.15%), η_c (2.56 cd A⁻¹), η_p (2.45 lm W⁻¹) with CIE (0.15, 0.07).     

Determination of the Lewis acidity of amide–AlCl3 based ionic liquid analogues by combined in situ IR titration and NMR methods

A combinatorial method to determine both acidic strength and acidic amount of each Lewis acid site in amide–AlCl₃ based ionic liquid (IL) analogues was developed by the combination of in situ IR titration and NMR analysis. ³¹P NMR was used to distinguish effectively the acidic strength of each Lewis acid site in the amide–AlCl₃ based IL analogues. Nitrobenzene was used as a molecular probe to measure the total Lewis acidic amount of the amide–AlCl₃ based IL analogues by in situ IR titration. The acidic amount of each Lewis acid site in the amide–AlCl₃ based IL analogues was calculated with the assistance of ²⁷Al NMR analysis.

The Lewis acidic strength and amount of amide–AlCl₃ IL analogues are determined by the combination of in situ IR titration and NMR analysis.

Acidic amide–AlCl₃ based ionic liquid (IL) analogues have attracted significant attention as good alternatives to traditional imidazolium and pyridinium based halometallate ILs due to their broad acidity-adjusting range, high catalytic activity, low toxicity and cost, and easy preparation.^1–3 Amide–AlCl₃ based IL analogues exhibit a mixture of neutral molecular Al species, and cationic and anionic Al species in equilibrium, which contribute to the incomplete asymmetric splitting of Al₂Cl₆ under the induction of amide.^4,5 Therefore, multiple Lewis acidic species with catalytic activity exist in these ILs analogues.⁶ The Lewis acidity of the amide–AlCl₃ based IL analogues, including acidic strength and amount, is correlated with their catalytic activity and selectivity.^7–9 Hence, it is necessary to establish a suitable method to determine the acidic strength and amount of each Lewis acid in these IL analogues, which can guide corresponding acid-catalyzed reactions.For traditional ILs, the spectral measurement methods of the acidity are mainly UV-vis, NMR and IR spectroscopies. The UV-vis spectroscopy method determines semi-quantitatively the acidic strength of total Brønsted acid in ILs according to the Hammett function,^10–12 but it could not be applied in the analysis of Lewis acid in ILs, such as [Al₂Cl₇]⁻ in chloroaluminate ILs. The Lewis acidic strength can be quantified by the Gutmann acceptor number, which is directly proportional to the ³¹P NMR chemical shift of triethylphosphineoxide (TEPO) dissolved in ILs.^13,14 The ³¹P NMR method can distinguish effectively the acidic strength of each Lewis acid in ILs with multiple Lewis acids, but it could not measure the acidic amount of each Lewis acid.^15–17 The traditional KBr tabletting IR uses nitrogen-containing compounds as molecular probes, such as pyridine and ethanenitrile. The change in the IR frequencies of the molecular probes is correlated to the acidic strength of the acid species in ILs. The tabletting IR method can distinguish evidently the Brønsted and Lewis acid according to the wavenumber of the characteristic peaks.^18,19 For example, two peak at 1450 cm⁻¹ and 1540 cm⁻¹ were the indication of pyridine coordinated to Lewis and Brønsted acid, respectively.²⁰ But this method neither distinguishes easily the acidic strength of each Lewis acid in ILs with multiple Lewis acids because of the overlap of characteristic peaks, nor can it measure the acidic amount of each Lewis acid. In addition, infrared studies of ammonia adsorption and microcalorimetry were also used by Dupont Company to investigate the acidity of zeolite.²¹In this communication, we first establish a combinatorial method to determine the acidity of amide–AlCl₃ based IL analogues with multiple Lewis acids by combining in situ IR titration with NMR analysis. This method not only distinguished effectively the acidic strength of each Lewis acid in amide–AlCl₃ based IL analogues, but also measured the acidic amount of each Lewis acid.Firstly, ³¹P NMR was used to identify the acidic strength of each Lewis acid in amide–AlCl₃ based IL analogues, as shown in Fig. 1. A single peak at 83.48 ppm was observed in the ³¹P NMR spectra of molecular probe (TEPO) dissolved in neat Et₃NHCl–AlCl₃ IL (molar ratio of Et₃NHCl to AlCl₃ was 0.65), which was assigned to the coordination of TEPO to Lewis acid. This result indicated that neat Et₃NHCl–AlCl₃ IL only contained single Lewis acid, namely [Al₂Cl₇]⁻. However, two peaks at 83.48 and 84.92 ppm were observed in neat NMA–AlCl₃ IL analogue (molar ratio of N-methylacetamide to AlCl₃ was 0.65, marked as 0.65NMA–1.0AlCl₃) with the addition of TEPO. This phenomenon indicated that another Lewis acid in addition to [Al₂Cl₇]⁻ existed in 0.65NMA–1.0AlCl₃. The peak at 84.92 ppm was assigned to the cationic Al species because the molecule Al species was neutral.⁵ Meanwhile, the acidic strength of cationic Al species located in low field was stronger than that of [Al₂Cl₇]⁻.Open in a separate windowFig. 1 ³¹P NMR spectra of three amide–AlCl₃ based IL analogues and Et₃NHCl–AlCl₃ IL with 1 mol% TEPO (ligand/AlCl₃ molar ratio was 0.65).Subsequently, in situ IR titration method was used to measure the acidic mount of two Lewis acids in neat 0.65NMA–1.0AlCl₃. The principle of this method is based on the online monitoring of the variation in the characteristic peaks formed by the coordination of indicator (nitrobenzene) with 0.65NMA–1.0AlCl₃.^22,23 A quantitative measurement of the acidic amount of 0.65NMA–1.0AlCl₃ was made based on the typical procedure. 0.65NMA–1.0AlCl₃ (10 g) was placed into a 25 mL two-necked flask equipped with a stirrer. The silicon probe of the in situ IR apparatus was inserted into the 0.65NMA–1.0AlCl₃, and then the data on the IR spectra were collected. Next, nitrobenzene (0.25 g) was added dropwise to the flask and IR spectra were collected continuously until the absorbance of the characteristic peaks remained constant, meanwhile, the peaks of nitrobenzene itself were observed. The aforementioned steps were repeated until the absorbance of the characteristic peaks did not change with the addition of nitrobenzene. This point was marked as the terminal point of titration, the total mass of nitrobenzene added into 0.65NMA–1.0AlCl₃ was collected.As a premise of the in situ IR titration method, the characteristic peak formed by the coordination of nitrobenzene with 0.65NMA–1.0AlCl₃ and the peak of nitrobenzene itself needed to be marked. Fig. 2 shows the IR spectra of neat nitrobenzene, neat 0.65NMA–1.0AlCl₃, and the mixture of 0.65NMA–1.0AlCl₃ with nitrobenzene. Two peaks at 1520 and 1346 cm⁻¹ were observed in neat nitrobenzene, which were assigned to the υ_as(O–N–O) and υ_s(O–N–O) stretching vibration of –NO₂ group, respectively.^24,25 A new peak at 1260 cm⁻¹ was observed in the mixture of 0.65NMA–1.0AlCl₃ with nitrobenzene, which should be assigned to the coordination of nitrobenzene with Lewis acids. Meanwhile, the υ_as(O–N–O) stretching vibration at 1520 cm⁻¹ shifted to higher wavenumber 1537 cm⁻¹. The υ_s(O–N–O) stretching vibration at 1346 cm⁻¹ appeared only in the case that excess nitrobenzene were added into 0.65NMA–1.0AlCl₃. Therefore, the peaks at 1260 and 1346 cm⁻¹ were chosen as the characteristic peaks to observe in the following in situ IR titration method.Open in a separate windowFig. 2IR spectra of (a) pure nitrobenzene; (b) neat 0.65NMA–1.0AlCl₃ based IL analogue; (c) nitrobenzene + 0.65NMA–1.0AlCl₃ based IL analogue (1 : 10 by mass ratio); (d) nitrobenzene + 0.65NMA–1.0AlCl₃ based IL analogue (1 : 4 by mass ratio). Fig. 3 shows the variation of the characteristic peaks at 1260 cm⁻¹ and 1346 cm⁻¹ from the coordination of nitrobenzene with 0.65NMA–1.0AlCl₃ and υ_s(O–N–O) stretching vibration of nitrobenzene, respectively. A surface plot was generated during the continuous addition of nitrobenzene into 0.65NMA–1.0AlCl₃ (Fig. 4). The absorbance of the peak at 1260 cm⁻¹ increased with the increasing addition of nitrobenzene, while the absorbance of the peak at 1346 cm⁻¹ remained almost constant before the terminal point, which attributed that the Lewis acidic amount of 0.65NMA–1.0AlCl₃ was continuously consumed by nitrobenzene. When the Lewis acidic amount of 0.65NMA–1.0AlCl₃ was used up, the absorbance of the peak at 1346 cm⁻¹ had a significantly increase with the addition of nitrobenzene. The total mass of nitrobenzene from start to terminal point was recorded, and “the molar consumption of nitrobenzene per 1000 g IL analogue” was defined as “activity index” to evaluate the acidic amount of 0.65NMA–1.0AlCl₃.^26,27Open in a separate windowFig. 3Trend of the characteristic peaks at 1260 cm⁻¹ and 1346 cm⁻¹ for the addition of nitrobenzene into 0.65NMA–1.0AlCl₃ based IL analogue.Open in a separate windowFig. 4Surface plot in the 1390–1185 cm⁻¹ range for the 0.65NMA–1.0AlCl₃ based IL analogue with the addition of nitrobenzene.The Lewis acidic amount of several amide–AlCl₃ based IL analogues with different amide structures and amide/AlCl₃ molar ratios were measured by in situ IR titration method, as shown in Fig. 5. The amide structure affected the Lewis acidic amount of amide–AlCl₃ based IL analogues,²⁸ for example, the Lewis acidic amount of amide–AlCl₃ based IL analogues (NMA–AlCl₃ and DMA–AlCl₃) with bidentate coordination was higher than that of amide–AlCl₃ based IL analogues (AA–AlCl₃ and Ur–AlCl₃) with monodentate coordination under the same amide/AlCl₃ molar ratio.²⁹ This phenomenon was attributed to the fact that the bidentate coordination was more favorable to the asymmetric splitting of AlCl₃ than the monodentate coordination with the same amide/AlCl₃ molar ratio, resulting in the more active Lewis species (anionic Al species and cationic Al species). On the other hand, the amide/AlCl₃ molar ratio also affected the Lewis acidic amount of amide–AlCl₃ based IL analogues. The Lewis acidic amount of amide–AlCl₃ based IL analogues increased with the decreasing amide/AlCl₃ molar ratio. For amide–AlCl₃ based IL analogues, the balance between neutral molecular Al species and ionic Al species was readily broken with the change of amide/AlCl₃ molar ratio. The asymmetric splitting degree of Al₂Cl₆ increased and the molecular species transformed into ionic species as the amide/AlCl₃ molar ratio decreased, so the Lewis acidic amount of amide–AlCl₃ based IL analogue also increased.Open in a separate windowFig. 5Activity index of four amide–AlCl₃ based IL analogues at different amide/AlCl₃ molar ratios.The total Lewis acidic amount of amide–AlCl₃ based IL analogues could be measured by in situ IR titration method, but the acidic amount of anionic Al species and cationic Al species needed to be further determined. ²⁷Al NMR is a good tool to distinguish these Al species, and the peaks at 102.75, 89.30 and 77.05 ppm should be assigned to the anionic Al species ([Al₂Cl₇]⁻ and [AlCl₄]⁻), molecular Al species [AlCl₃L_n], and cationic Al species [AlCl₂L_n]⁺, respectively.²⁹ The integral area ratio of anionic Al species ([Al₂Cl₇]⁻, [AlCl₄]⁻) to cationic Al species ([AlCl₂L_n]⁺) was obtained by the normalization method of the peak areas, as shown in Fig. 6. The integral area represented the number of Al nucleus (note: [Al₂Cl₇]⁻ had two Al nuclei). Therefore, the integral area ratio of anionic Al species to cationic Al species represented the molar ratio of 2 × [Al₂Cl₇]⁻ + [AlCl₄]⁻ to [AlCl₂L_n]⁺. The mole of [Al₂Cl₇]⁻ + [AlCl₄]⁻ was equal to that of [AlCl₂L_n]⁺ according to the conservation law of charge, so the molar ratio of [Al₂Cl₇]⁻ to [AlCl₂L_n]⁺ could be calculated. Taking NMA–AlCl₃ based IL analogue with different NMA/AlCl₃ molar ratios as an example, the acidic amount of two Lewis acids ([Al₂Cl₇]⁻ and [AlCl₂L_n]⁺) in NMA–AlCl₃ based IL analogue was calculated from the results of both in situ IR titration and ²⁷Al NMR analysis, as listed in Open in a separate windowFig. 6 ²⁷Al NMR spectra of the NMA–AlCl₃ based IL analogue with different NMA/AlCl₃ molar ratios.Molar ratio of 2 × [Al₂Cl₇]⁻ + [AlCl₄]⁻, [Al₂Cl₇]⁻, and [AlCl₄]⁻ to [AlCl₂L_n]⁺; and acidic amount of two Lewis acids ([Al₂Cl₇]⁻ and [AlCl₂L_n]⁺) in NMA–AlCl₃ based IL analogue with different NMA/AlCl₃ molar ratios

The nature of metal–metal bonding in Re-, Ru- and Os-corrole dimers

Studies of multiple bonding between transition metal complexes offer fundamental insight into the nature of bonding between metal ions and facilitate predictions of the physical properties and the reactivities of metal complexes containing metal–metal multiple bonds. Here we report a computational interrogation on the nature of the metal–metal bonding for neutral, oxidized, and reduced forms of dinuclear rhenium and osmium corrole complexes, [{Re[TpXPC]}₂]^0/1+/1− and [{Os[TpXPC]}₂]^0/1+/1−, using a complete active space self-consistent (CASSCF) methodology and density functional theory (DFT) calculations. For [{Re[TpXPC]}₂]⁰, [{Ru[TpXPC]}₂]⁰, and [{Os[TpXPC]}₂]⁰, CASSCF calculations shows that the effective bond order is 3.29, 2.63, and 2.73, respectively. On their oxidized forms, [{Re[TpXPC]}₂]¹⁺, [{Ru[TpXPC]}₂]¹⁺, and [{Os[TpXPC]}₂]¹⁺ molecules, the results indicate an electron removal from a ligand-based orbital, where [{Re[TpXPC]}₂]¹⁺ gives slightly different geometry from its neutral form due to populating the δ* orbital. In this regard, the CASSCF calculations give an effective bond order of 3.25 which is slightly lower than in the [{Re[TpXPC]}₂]⁰. On their reduced forms, the electron addition appears to be in the metal-based orbital for [{Re[TpXPC]}₂]¹⁻ and [{Ru[TpXPC]}₂]¹⁻ whereas in the ligand-based orbital for the Os-analogue which has no effect on the Os–Os bonding, an effective bond order of 3.18 and 2.17 is presented for the [{Re[TpXPC]}₂]¹⁻ and [{Ru[TpXPC]}₂]¹⁻, respectively, within the CASSCF simulations. These results will further encourage theoreticians and experimentalists to design metalloporphyrin dimers with distinct metal–metal bonding.

Our CASSCF calculations on the first oxidation and reduction processes of the synthesised [{Re[TpXPC]}₂]⁰, [{Ru[TpXPC]}₂]⁰, and [{Os[TpXPC]}₂]⁰ molecules revealed a pronounced effect on the nature of the metal–metal bonding.     

Aerobically stable and substitutionally labile α-diimine rhenium dicarbonyl complexes

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes are described. The molecules are prepared in high yield from the cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br₂]⁻ anion (2, where ^tBu₂bpy is 4,4′-di-tert-butyl-2,2′-bipyridine), which can be isolated from the one electron reduction of the corresponding 17-electron complex (1). Compound 2 is stable in the solid state, but in solution it is oxidized by molecular oxygen back to 1. Replacement of a single bromide of 2 by σ-donor monodentate ligands (Ls) yields stable neutral 18-electron cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO)₂(^tBu₂bpy)Br(L)] species can be further reacted with Ls to prepare stable cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes in good yield. Ligand substitution of Re(i) complexes proceeds via pentacoordinate intermediates capable of Berry pseudorotation. In addition to the cis–cis–trans-complexes, cis–cis–cis- (all cis) isomers are also formed. In particular, cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes establish an equilibrium with all cis isomers in solution. The solid state crystal structure of nearly all molecules presented could be elucidated. The molecules adopt a slightly distorted octahedral geometry. In comparison to similar fac-[Re(CO)₃]⁺complexes, Re(i) diacarbonyl species are characterized by a bend (ca. 7°) of the axial ligands towards the α-diimine unit. [Re(CO)₂(^tBu₂bpy)Br₂]⁻ and [Re(CO)₂(^tBu₂bpy)Br(L)] complexes may be considered as synthons for the preparation of a variety of new stable diamagnetic dicarbonyl rhenium cis-[Re(CO)₂]⁺ complexes, offering a convenient entry in the chemistry of the core.

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes offer a convenient entry in the chemistry of the cis-[Re(CO)₂]⁺ core.     

Antioxidant and UV-radiation absorption activity of aaptamine derivatives – potential application for natural organic sunscreens

Antioxidant and UV absorption activities of three aaptamine derivatives including piperidine[3,2-b]demethyl(oxy)aaptamine (C1), 9-amino-2-ethoxy-8-methoxy-3H-benzo[de][1,6]naphthyridine-3-one (C2), and 2-(sec-butyl)-7,8-dimethoxybenzo[de]imidazo[4,5,1-ij][1,6]-naphthyridin-10(9H)-one (C3) were theoretically studied by density functional theory (DFT). Direct antioxidant activities of C1–C3 were firstly evaluated via their intrinsic thermochemical properties and the radical scavenging activity of the potential antioxidants with the HOO˙/HO˙ radicals via four mechanisms, including: hydrogen atom transfer (HAT), single electron transfer (SET), proton loss (PL) and radical adduct formation (RAF). Kinetic calculation reveals that HOO˙ scavenging in water occurs via HAT mechanism with C1 (k_app, 7.13 × 10⁶ M⁻¹ s⁻¹) while RAF is more dominant with C2 (k_app, 1.40 × 10⁵ M⁻¹ s⁻¹) and C3 (k_app, 2.90 × 10⁵ M⁻¹ s⁻¹). Antioxidant activity of aaptamine derivatives can be classified as C1 > C3 > C2. Indirect antioxidant properties based on Cu(i) and Cu(ii) ions chelating activity were also investigated in aqueous phase. All three studied compounds show spontaneous and favorable Cu(i) ion chelating activity with ΔG⁰ being −15.4, −13.7, and −15.7 kcal mol⁻¹, whereas ΔG⁰ for Cu(ii) chelation are −10.4, −10.8, and −2.2 kcal mol⁻¹ for C1, C2 and C3, respectively. In addition, all compounds show UVA and UVB absorption; in which the excitations are determined mostly as π–π* transition. Overall, the results suggest the potential applications of the aaptamines in pharmaceutics and cosmetics, i.e. as a sunscreen and antioxidant ingredient.

Antioxidant and UV absorption activities of three aaptamine derivatives were theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT).     

Preparation of biomimetic non-iridescent structural color based on polystyrene–polycaffeic acid core–shell nanospheres

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties. In this work, caffeic acid was introduced into the preparation of photonic band gap materials. By controlling the variables, a reasonable preparation method of polystyrene (PS) @polycaffeic (PCA)–Cu(ii) core–shell microspheres was achieved: 1 mmol L⁻¹ cupric chloride anhydrous (CuCl₂), 3 mmol L⁻¹ sodium perborate tetrahydrate (NaBO₃·4H₂O), 2 mmol L⁻¹ CA and 2 g L⁻¹ polystyrene (PS) were reacted at 50 °C for 10 min to prepare PS@PCA–Cu(ii) core–shell microspheres through rapid oxidative polymerization of CA coated PS of different particle diameters. The amorphous photonic crystal structure was self-assembled through thermal assisted-gravity sedimentation, resulting in structural color nanomaterials with soft and uniform color, no angle dependence, stable mechanical fastness and excellent UV resistance.

Caffeic acid (CA), as a natural plant-derived polyphenol, has been widely used in surface coating technology in recent years due to its excellent properties.     

Experimental and theoretical interpretation of the magnetic behavior of two Dy(iii) single-ion magnets constructed through β-diketonate ligands with different substituent groups (–Cl/–OCH3)

Two Dy(iii) single-ion magnets, formulated as [Dy(Phen)(Cl-tcpb)₃] (Cl-1) and [Dy(Phen)(CH₃O-tmpd)₃] (CH₃O-2) were obtained through β-diketonate ligands (Cl-tcpb = 1-(4-chlorophenyl)-4,4,4-trifluoro-1,3-butanedione and CH₃O-tmpd = 4,4,4-trifluoro-1-(4-methoxyphenyl)-1,3-butanedione) with different substituent groups (–Cl/–OCH₃) and auxiliary ligand, 1,10-phenanthroline (Phen). The Dy(iii) ions in Cl-1 and CH₃O-2 are eight-coordinate, with an approximately square antiprismatic (SAP, D_4d) and trigonal dodecahedron (D_2d) N₂O₆ coordination environment, respectively, in the first coordination sphere. Under zero direct-current (dc) field, magnetic investigations demonstrate that both Cl-1 and CH₃O-2 display dynamic magnetic relaxation of single-molecule magnet (SMM) behavior with different effective barriers (U_eff) of 105.4 cm⁻¹ (151.1 K) for Cl-1 and 132.5 cm⁻¹ (190.7 K) for CH₃O-2, respectively. As noted, compound CH₃O-2 possesses a higher effective barrier than Cl-1. From ab initio calculations, the energies of the first excited state (KD₁) are indeed close to the experimental U_eff as 126.7 cm⁻¹vs. 105.4 cm⁻¹ for Cl-1 and 152.8 cm⁻¹vs. 132.5 cm⁻¹ for CH₃O-2. The order of the calculated energies of KD₁ is same as that of the experimental U_eff. The superior SIM properties of CH₃O-2 could have originated from the larger axial electrostatic potential (ESP_(ax)) felt by the central Dy(iii) ion when compared with Cl-1. The larger ESP_(ax) of CH₃O-2 arises from synergic effects of the more negative charge and shorter Dy–O distances of the axial O atoms of the first sphere. These charges and distances could be influenced by functional groups outside the first sphere, e.g., –Cl and –OCH₃.

Further studies from the viewpoint of electrostatic potential demonstrate that the larger axial electrostatic potential (ESP) felt by the central Dy (iii) ion of CH₃O-2 is responsible for its better SIM property when compared with Cl-1.     

Control of Fe3+ coordination by excess Cl− in alcohol solutions

We spectroscopically investigated coordination state of Fe³⁺ in methanol (MeOH) and ethanol (EtOH) solutions against Cl⁻ concentration ([Cl⁻]). In both the system, we observed characteristic absorption bands due to the FeCl₄ complex at high-[Cl⁻] region. In the MeOH system, the proportion (r) of [FeCl₄]⁻ exhibits a stationary value of 0.2–0.3 in the intermediate region of 10 mM < [Cl⁻] < 50 mM, which is interpretted in terms of [FeCl_nL_6−n]³⁻ⁿ (n = 1 and 2). In the EtOH system, r steeply increases from 0.1 at [Cl⁻] = 1.5 mM to 0.7 at [Cl⁻] = 3.5 mM, indicating direct transformation from [FeL₆]³⁺ to [FeCl₄]⁻. We further found that the coordination change significantly decreases the redox potential of Fe²⁺/Fe³⁺.

Fe³⁺ coordination in alcohol solution can be controlled by the Cl⁻ concentration ([Cl⁻]). The coordination state changes from FeL₆ (L: solvent molecule) to FeCl₄ type via FeCl_nL_6−n with increases in [Cl⁻].     

Donor–acceptor duality of the transition-metal-like B2 core in core–shell-like metallo-borospherenes La3&[B2@B17]− and La3&[B2@B18]−

Transition-metal doping induces dramatic structural changes and leads to earlier planar → tubular → spherical → core–shell-like structural transitions in boron clusters. Inspired by the newly discovered spherical trihedral metallo-borospherene D_3h La₃&B₁₈⁻ (1) (Chen, et al., Nat. Commun., 2020, 11, 2766) and based on extensive first-principles theory calculations, we predict herein the first and smallest core–shell-like metallo-borospherenes C_2v La₃&[B₂@B₁₇]⁻ (2) and D_3h La₃&[B₂@B₁₈]⁻ (3) which contain a transition-metal-like B₂ core at the cage center with unique donor–acceptor duality in La₃&B_n⁻ spherical trihedral shells (n = 17, 18). Detailed energy decomposition and bonding analyses indicate that the B₂ core in these novel complexes serves as a π-donor in the equatorial direction mainly to coordinate three La atoms on the waist and a π/σ-acceptor in the axial direction mainly coordinated by two B₆ triangles on the top and bottom. These highly stable core–shell complexes appear to be spherically aromatic in nature in bonding patterns. The IR, Raman, and photoelectron spectra of 2 and 3 are computationally simulated to facilitate their spectroscopic characterizations.

The smallest core–shell-like metallo-borospherenes C_2v La3&[B₂@B₁₇]⁻ and D_3h La₃&[B₂@B₁₈]⁻ have been predicted at first-principles theory level which contain a transition-metal-like B₂ core with unique donor–acceptor duality.     

Theoretical hydrogen bonding calculations and proton conduction for Eu(iii)-based metal–organic framework

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation. The investigation of proton conduction shows that the conductivity of Eu(iii)-MOF obtained at 353 K and 98% RH is 3.5 × 10⁻³ S cm⁻¹, comparable to most of the Ln(iii)-MOF based proton conductors.

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation.

In recent years, with the aggravation of environmental pollution and growing depletion of petroleum, coal and other traditional fossil energy, the demand to exploit alternative cleaner energy is increasingly urgent. Compared to the dispersion of several developed new energy sources, such as solar energy, wind, geothermal heat, and so on,¹ the proton exchange membrane fuel cell (PEMFC) is recognized as a promising energy conversion system.² As an important component in PEMFC, the proton exchange membrane (PEM) directly affects the transmission efficiency of protons between electrodes.³ Currently, Nafion has been widely used as a PEM in commerce, and shows a conductivity higher than 10⁻¹ S cm⁻¹.⁴ However, the large-scale applications of Nafion are limited due to their high costs, narrow working conditions (low temperature and high relative humidity), amorphous nature, etc.⁵ To overcome these limitations, several types of proton-conducting materials have been explored over the past decade.⁶ Among them, MOF materials were employed as ideal platforms to regulate proton conductivity owing to their high crystallinity, tunable structure and tailorable functionality. The crystallographically defined structure is also conductive to the deeply analysis of proton transport path and mechanism,⁷ furthermore, due to the visual structure of MOF, Density Functional Theory (DFT) is recently used to analyse the factors that affect proton conduction from a theoretical perspective, thus providing strong support for the experimental results.⁸ Multi-carboxylate ligands usually exhibit versatile coordination modes and strong complexing ability to metal ions. Moreover, the hydrophilic –COOH groups not only donate protons but also facilitate the formation of continuous hydrogen bond channel with water molecules. Simultaneously, selecting lanthanide metal ions as nodes in the construction of MOF, more water molecules tend to be bound by Ln(iii) ions, leading to an increase in the concentration of proton carrier, which would be beneficial for the effective proton transport. Therefore, the carboxylate-bridged Ln(iii)-MOF are good candidates for proton conduction.⁹ Currently, there are several proton conductive MOF materials, such as {H[(N(Me)₄)₂][Gd₃(NIPA)₆]}·3H₂O (σ = 7.17 × 10⁻² S cm⁻¹, 75 °C, 98% RH),¹⁰ Na₂[Eu(SBBA)₂(FA)]·0.375DMF·0.4H₂O (σ = 2.91 × 10⁻² S cm⁻¹, 90 °C, 90% RH)¹¹ and {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n (σ = 2.57 × 10⁻² S cm⁻¹, 60 °C, 98% RH)¹² that showing ultra-high conductivities (>10⁻² S cm⁻¹). These superprotonic conductors provided advantageous supports for the assembly strategies involved Ln(iii) ion and carboxylate ligand. In this work, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (H₆TTHA) and Eu(NO₃)₃·6H₂O were assembled at 140 °C for 72 h through solvothermal reaction to afforded colourless crystals, namely {[Eu₂(TTHA)(H₂O)₄]·9H₂O}_n (1). This complex has been previously reported by Wu and co-workers.¹³ In their work, the thermal stability and fluorescence properties of 1 were mainly focused. Research suggested that the complex 1 maintained structural stability until 400 °C and demonstrated strong fluorescent emission with high quantum yields (Φ > 70%), treating as a good candidate for light applications. To the best of our knowledge, MOFs usually exhibit a variety of potential applications for their structural diversity.¹⁴ For different researchers, their concerns about the applications of MOF may vary, but it is the continuously exploration and excavation of different performance that will enrich their potentials and meet them in different fields of the applications. Through careful structural analysis, we found that there is a rare infinite water cluster ((H₂O)_n) existing in the crystal structure of 1 (Fig. S1^†), (H₂O)_n further interacts with –COO⁻ groups to form an abundant hydrogen bond network (Fig. S2 and Table S1^†). The stability of (H₂O)_n as well as more complex hydrogen bond formed between (H₂O)_n and –COO⁻ groups has been confirmed by the density functional theory (DFT) calculations. The advantageous structural features including high concentration of water molecules and stable hydration channel provide the possibility to realize high proton conductivity of 1. Therefore, the proton conductivities of 1 under varying conditions were investigated in detail.Complex 1 crystallizes in the monoclinic space group C2/c, with the asymmetric building unit composed of two Eu(iii) ions, one [TTHA]⁶⁻ anion, four coordination water molecules and nine lattice water molecules. The Eu(iii) atom is distorted enneahedron coordinated by seven carboxylate oxygen atoms and two water molecules (Fig. 1a and Table S2^†). The bond length of Eu–O is in the range of 2.374(5)–2.606(5) Å (Table S3^†), comparable to that of the Eu(iii) complex reported in the literature.¹⁵ The coordination mode of [TTHA]⁶⁻ can be described as μ₆-η²η¹η¹η¹η¹η¹η¹η¹η¹η¹η¹η². In the complex 1, the adjacent metal ions were connected through O–C–O and μ–O bridging, forming a dimer, [Eu1]₂. The dimer acts as a linker and connects with four [TTHA]⁶⁻ (Fig. 1b). Furthermore, the [TTHA]⁶⁻ anions coordinate with [Eu1]₂ through six flexible arms in different directions, leading to the formation of a three-dimensional network structure, where the cavities with regular size of 8.356 × 10.678 Ų are left (Fig. 1c and Fig. S3^†). The topological representation of the network of 1 was analysed by using TOPOS software.¹⁶ As shown in Fig. 1d, the Eu(iii) ions are connected to four [TTHA]⁶⁻, which can be considered as 4-connected nodes. And the [TTHA]⁶⁻ anions were also viewed as 4-connected nodes for their connections with four Eu(iii) ions. So, the whole 3D structure was described as a 4,4-c net with an extended Schläfli symbol of {4²,8⁴}.Open in a separate windowFig. 1Coordination mode of the [TTHA]⁶⁻ in 1 showing [EuO₉] enneahedron (a). The dimer, [Eu1]₂, formed by O–C–O and μ–O bridging, connects with four [TTHA]⁶⁻ (b). The 3D structure of 1 formed by the coordination of Eu(iii) and [TTHA]⁶⁻ as well as water molecules (c). Topological representation of the network of 1 (d). Symmetry codes (i: 1.5 − x, 1.5 − y, 1 − z; ii: 0.5 + x, 1.5 − y, −0.5 + z; iii: x, 2 − y, −0.5 + z; iv: 2 − x, 2 − y, 1 − z; v: 2 − x, y, 0.5 − z; vi: 1 − x, y, 1.5 − z; vii: 0.5 + x, 0.5 + y, z).In 1, the theoretical hydrogen bonding calculations of (H₂O)_n and complex cluster were performed using the Gaussian 09 program. All the structures were obtained from the analysis of XRD results and the hydrogen atoms are optimized. We calculated the energy at DFT level by means of B3LYP-D3.^17a As polarity of molecule has great influence on intermolecular hydrogen bonding,^17b hydrogen bond-forming orbitals require larger space occupation.^17c Thus, diffuse and polarization functions augmented split valence 6-311+G(d,p) basis set is used. The binding energy (E_binding) is calculated as the difference between the energy of hydrogen-bonded cluster and the summation of the energies of each component monomer: E_binding = E_tol − ∑N_iE_iE_tol and E_i are energy of hydrogen-bonded cluster and each individual component monomer, respectively. A hydrogen-bonded cluster is more stable if interaction energy is more negative compared to other hydrogen-bonded configurations. With the help of density functional theory (DFT), we calculate the binding energies (E_binding) to compare the stability of systems. The binding energy of water cluster and complex cluster is −619.65 and −710.34 kcal mol⁻¹ (Fig. 2), respectively, indicating the complex cluster system is more stable.Open in a separate windowFig. 2The structures of water cluster and complex cluster. Oxygen, hydrogen, carbon, nitrogen atoms are marked by red, white, cyan, blue, respectively.The PXRD patterns of 1 were shown in Fig. S4.^† It was found that the diffraction peaks of powder sample are in good agreement with the simulated data from single-crystal diffraction, showing the high purity of the synthesized sample. The IR spectrum of 1 exhibits a strong peak at 3422 cm⁻¹, which corresponds to the stretching vibration of water molecules.^18a The absorption peaks appeared at 1551 cm⁻¹ and 1400 cm⁻¹ are attributed to the antisymmetric stretching of –COO⁻ groups^18b (Fig. S5^†). The water adsorption property of 1 was investigated at 25 °C by DVS Intrinsic Plus. Before the measurement, the sample was treated under 0% RH for 6 h (Fig. S6^†). Water adsorption and desorption isotherms of the fully dehydrated sample were shown in Fig. S7.^† The adsorption process in the RH range of 0–95% can be divided into three stages. In the initial stage (0–10%), the adsorption of water molecules increased rapidly, which can be attributed to the hydrogen bond interaction between carboxylic acid oxygen atoms and water molecules. Then the water adsorption increased slowly at 10–70% RH, corresponding to the formation of water clusters. Another abrupt increase of water adsorption was found when the RH is above 70%, illustrating that enough energy is needed for the water clusters to exist in the cavity of the crystal.¹⁹ Clearly, large hysteresis was observed in the adsorption–desorption isotherms, this phenomenon was caused by the strong hydrophilic of –COO⁻ groups in 1.²⁰ Furthermore, the structural integrity of the sample after adsorption/desorption cycle was confirmed by PXRD (Fig. S4^†).Based on the previous structural analysis, the proton conduction of 1 was evaluated by the alternating-current (AC) impedance analyses. The Nyquist plots of 1 obtained at different temperature and relative humidity are shown in Fig. 3a and b and Fig. 3d. The resistance is estimated from the intercept of spikes or arcs on the Z′ axis, and the conductivity (σ) is calculated by the equation of σ = l/(A·R), where l, A and R represent the sample thickness, surface area and resistance, respectively. It was found that there are two different modes observed from the impedance spectroscopies under lower relative humidity (60–90% RH), a partial arc at high frequency component can be attributed to the grain interior contribution, while a characteristic spur at low frequency component illustrates that partial-blocking electrode response allows limited diffusion.²¹ So, the only spikes displayed in the Nyquist spectra at 98% RH and 293–353 K suggest that high temperature and high relative humidity are more favourable for the proton conduction. From the temperature-dependent measurements under 98% RH, significantly, the conductivity of 1 increases gradually from 1.34 × 10⁻⁴ S cm⁻¹ at 293 K to 3.5 × 10⁻³ S cm⁻¹ at 353 K (Fig. 3c and Table S4^†). The increasing conductivity can be attributed to the important role of water molecule. The high concentration of water molecules act as carriers and transmit in the form of H⁺(H₂O)_n, and the mobility of H⁺(H₂O)_n accelerates with the rising temperature. Moreover, the higher acidity of water molecules at higher temperature is more conducive to the improvement of proton conductivity. The relative humidity dependence measured at 298 K indicated that the conductivity of 1 presented significant positive correlations with the humidity changes. The conductivity is 1.42 × 10⁻⁵ S cm⁻¹ at 60% RH and increases to be 1.63 × 10⁻⁴ S cm⁻¹ at 98% RH (Fig. 3e and Table S5^†). This can be explained by the ability of (H₂O)_n to bind water molecules and strong hydrophilic of –COO⁻ group that has been confirmed by the water adsorption process, especially when the RH is above 60%. For water-mediated proton conductors, the lower RH usually results in the insufficient of transport media and further affects the diffusion of protons. At present, the theoretical simulations (e.g. aMS-EVB3)²² and activation energy (E_a)^23–27 are the main methods to analysis the proton conduction mechanism. Compared with the theoretical calculations, the judgment rule with E_a is more straightforward. Here, the E_a of 1 determined from the linear fit of ln(σT) vs. 1000/T is 0.44 eV (Fig. 3f), which reveals that the proton transfer in 1 follows a typical vehicle mechanism.¹² Further evaluate the long-term stability of 1, the time-dependent proton conductivity has been conducted, indicating negligible decline of proton conductivities even lasted 12 h (Fig. 4, S8 and Table S6^†). The sample of 1 after property measurements was collected and characterized by PXRD to examine any structural change, and the PXRD spectrum shows structural integrity even at high temperature and high relative humidity environment (Fig. S4^†). The long-term stable proton conductivities of 1 can be attributed to the robust hydrogen bonding channel that has been confirmed by the DFT calculations. In recent years, the proton conductive carboxylate-based MOF have been systematic reviewed by G. Li’ group,⁹ it was found that the complex 1 shows higher conductivity of 3.5 × 10⁻³ S cm⁻¹ under 353 K and 98% RH when compared to the Ln(iii)-MOF materials, such as [Me₂NH₂][Eu(ox)₂(H₂O)]·3H₂O (σ = 2.73 × 10⁻³ S cm⁻¹, 95% RH, 55 °C),²³ {[Gd(ma)(ox)(H₂O)]_n·3H₂O} (σ = 4.7 × 10⁻⁴ S cm⁻¹, 95% RH, 80 °C),²⁴ (N₂H₅)[Nd₂(ox)₄(N₂H₅)]·4H₂O (σ = 2.7 × 10⁻³ S cm⁻¹, 100% RH, 25 °C),²⁵ {[SmK(BPDSDC)(DMF)(H₂O)]·x(solvent)}_n (σ = 1.11 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁶ [Nd(mpca)₂Nd(H₂O)₆Mo(CN)₈]·nH₂O (σ = 2.8 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁷ MFM-550(M) and MFM-555(M) (M = La, Ce, Nd, Sm, Gd, Ho) (σ = 1.46 × 10⁻⁶ to 2.97 × 10⁻⁴ S cm⁻¹, 99% RH, 20 °C)²⁸ as well as other conductive materials showing lower conductivities in the range of 10⁻⁹ to 10⁻⁵ S cm⁻¹.⁹ However, the conductivity of 1 is inferior to those Ln(iii)-MOFs with conductivities higher than 10⁻² S cm⁻¹ × ^10–12 In recent years, another two H₆TTHA-derived MOF and CP, {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n¹² and {[Co₃(H₃TTHA)₂(4,4′-bipy)₅(H₂O)₈]·12H₂O}_n^19b have been previously reported by our group, which show highest proton conductivities of 2.57 × 10⁻² S cm⁻¹ at 60 °C and 8.79 × 10⁻⁴ S cm⁻¹ at 80 °C under 98% RH, respectively. The noticeable performance difference between these two complexes and 1 was analysed based on the visual structures. The higher conductivity of 1 when compared to the Co(ii) complex can be attributed to the concentration of water molecules, 23.25% for 1 and 15.92% for the Co(ii) complex. The high concentration of proton carrier in 1 promotes the transfer of protons. Although the water molecular concentration of 1 is higher than that of the Tb(iii) complex, however, the coordination numbers of Ln(iii) ions in the two compounds are different, eight for the Tb(iii) ion and nine for the Eu(iii) ion, respectively. The coordination sites are obviously not satiated, especially for the Tb(iii) complex, the lower coordination number may prone to chelate more water molecules under high relative humidity, leading to the formation of more consecutive hydration channel with TTHA⁶⁻ anions and water molecules, thus accelerating the proton transport. In contrast, the molecular structure of the Eu(iii) compound contains nearly a quarter of water molecules, these water molecules have almost filled the pores, so the smaller pore structure is difficult to accommodate more adsorbed water molecules.Open in a separate windowFig. 3Nyquist plots for proton conductivity of 1 (98% RH) at 293–313 K (a) and 318–353 K (b). Plot of log(σ) vs. T for 1 in the temperature range of 293–353 K (c). Plots of the impedance plane for 1 at different relative humidities and 298 K (d). Humidity dependence of the proton conductivity at 298 K (e). Arrhenius plot of 1 at 98% RH (f).Open in a separate windowFig. 4Time-dependent proton conductivity of 1 at 343 K and 98% RH.In conclusions, a water-mediated proton-conducting Eu(iii)-MOF has been synthesized, displaying a 3D network structure with high concentration water molecules and –COO⁻ groups as well as abundant H-bond networks. Interesting, there is an infinite water cluster of (H₂O)_n existing in the crystal structure of Eu(iii)-MOF, which is rare in the H₆TTHA-derived complexes and even other reported MOF/CPs. Based on this, the density functional theory was conducted to evaluate the stability of water cluster and complex cluster. As expected, the calculated binding energies indicate that the more stable system was formed by (H₂O)_n and –COO⁻ groups, which provides a favourable guarantee for proton conduction. The advantageous structural features of Eu(iii)-MOF result in the realization of comparable proton conductivity of 3.5 × 10⁻³ S cm⁻¹ at 353 K and 98% RH and long-term stability at least 12 h. Additionally, the factors affecting the electrical conductivity of several H₆TTHA-derived MOF/CPs have been compared and analysed from the visual structures, and the structure-activity relationship of such compounds was also summarized, which will provide guidance to design novel crystalline superprotonic conductors assembled from multi-carboxylate.     

SrTi(IO3)6·2H2O and SrSn(IO3)6: distinct arrangements of lone pair electrons leading to large birefringences

Three new iodates SrTi(IO₃)₆·2H₂O, (H₃O)₂Ti(IO₃)₆, and SrSn(IO₃)₆ have been synthesized via a facile hydrothermal method. The three compounds have zero-dimensional crystal structures composed of one [MO₆]⁸⁻ (M = Ti, Sn) octahedron connected with six [IO₃]⁻ trigonal pyramids. However, the particular coordination of Sr²⁺ cations results in distinct arrangements of lone pair electrons in an [IO₃]⁻ trigonal pyramid, which leads to large birefringences. More importantly, this work enriches the species crystal chemistry for [M(IO₃)₆]²⁻ (M = Ti, Sn) clusters-containing iodates.

The distinct arrangements of [IO₃]⁻ trigonal pyramids lead to larger birefringences in SrTi(IO₃)₆·2H₂O and SrSn(IO₃)₆ than that in (H₃O)₂Ti(IO₃)₆.     

Salt bridges govern the structural heterogeneity of heme protein interactions and porphyrin networks: microperoxidase-11

In this work, a proteolytic digest of cytochrome c (microperoxidase 11, MP-11) was used as a model to study the structural aspects of heme protein interactions and porphyrin networks. The MP-11 structural heterogeneity was studied as a function of the starting pH (e.g., pH 3.1–6.1) and concentration (e.g., 1–50 μM) conditions and adduct coordination. Trapped ion mobility spectrometry coupled to mass spectrometry (TIMS-MS) showed the MP-11 structural dependence of the charge state distribution and molecular ion forms with the starting pH conditions. The singly charged (e.g., [M]⁺, [M − 2H + NH₄]⁺, [M − H + Na]⁺ and [M − H + K]⁺) and doubly charged (e.g., [M + H]²⁺, [M − H + NH₄]²⁺, [M + Na]²⁺ and [M + K]²⁺) molecular ion forms were observed for all solvent conditions, although the structural heterogeneity (e.g., number of mobility bands) significantly varied with the pH value and ion form. The MP-11 dimer formation as a model for heme-protein protein interactions showed that dimer formation is favored toward more neutral pH and favored when assisted by salt bridges (e.g., NH₄⁺, Na⁺ and K⁺vs. H⁺). Inspection of the dimer mobility profiles (2+ and 3+ charge states) showed a high degree of structural heterogeneity as a function of the solution pH and ion form; the observation of common mobility bands suggest that the different salt bridges can stabilize similar structural motifs. In addition, the salt bridge influence on the MP-11 dimer formations was measured using collision induced dissociation and showed a strong dependence with the type of salt bridge (i.e., a CE₅₀ of 10.0, 11.5, 11.8 and 13.0 eV was observed for [2M + H]³⁺, [2M − H + NH₄]³⁺, [2M + Na]³⁺ and [2M + K]³⁺, respectively). Measurements of the dimer equilibrium constant showed that the salt bridge interactions increase the binding strength of the dimeric species.

In this work, a proteolytic digest of cytochrome c (microperoxidase 11, MP-11) was used as a model to study the structural aspects of heme protein interactions and porphyrin networks.     

Branched-chain Amino Acid Nitrogen Transfer to Alanine In Vivo in Dogs: DIRECT ISOTOPIC DETERMINATION WITH [15N]LEUCINE

To investigate the contribution of branched-chain amino acids as a nitrogen source for alanine in vivo, dogs were infused with l-[¹⁵N]leucine, l-[U-¹⁴C]leucine, l-[2,3,3,3-²H₄]alanine, and d-[6,6-²H₂]-glucose. ¹⁴C and ¹⁵N isotopic equilibrium in plasma leucine, and deuterium enrichment in arterial and femoral plasma glucose and alanine were achieved within 3 h of initiation of the respective isotope infusion in all animals. The average flux of leucine determined by [¹⁵N]leucine was 5.4 μmol·kg⁻¹·min⁻¹, whereas using [¹⁴C]leucine it was 3.7 μmol·kg⁻¹·min⁻¹. Turnover rates for alanine and glucose were 11.0 and 17.2 μmol·kg⁻¹·min⁻¹, respectively.     

Molybdenum imidazole citrate and bipyridine homocitrate in different oxidation states – balance between coordinated α-hydroxy and α-alkoxy groups

Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K₂{Mo^IV₃O₄(im)₃[Mo^VIO₃(Hcit)]₂}·3im·4H₂O (1), (Him)₂{Mo^IV₃SO₃(im)₃[Mo^VIO₃(Hcit)]₂}·im·6H₂O (2), molybdenum(v) bipyridine homocitrate trans-[(Mo^VO)₂O(H₂homocit)₂(bpy)₂]·4H₂O (3) and molybdenum(vi) citrate (Et₄N)[Mo^VIO₂Cl(H₂cit)]·H₂O (4) (H₄cit = citric acid, H₄homocit = homocitric acid, im = imidazole and bpy = 2,2′-bipyridine) with different oxidation states were prepared. 1 and 2 are the coupling products of [Mo^VIO₃(Hcit)]³⁻ anions and incomplete cubane units [Mo^IV₃O₄]⁴⁺ ([Mo^IV₃SO₃]⁴⁺) with monodentate imidazoles, respectively, where tridentate citrates coordinate with α-hydroxy, α-carboxy and β-carboxy groups, forming pentanuclear skeleton structures. The molybdenum atoms in 1 and 2 show unusual +4 and +6 valences based on charge balances, theoretical bond valence calculations and Mo XPS spectrum. The coordinated citrates in 1 and 2 are protonated with α-hydroxy groups, while 3 and 4 with higher oxidation states of +5 and +6 are deprotonated with α-alkoxy group even under strong acidic condition, respectively. This shows the relationship between the oxidation state and protonation of the α-alkoxy group in citrate or homocitrate, which is related to the protonation state of homocitrate in FeMo-cofactor of nitrogenase. The homocitrate in 3 chelates to molybdenum(v) with bidentate α-alkoxy and monodentate α-carboxy groups. Molybdenum(vi) citrate 4 is only protonated with coordinated and uncoordinated β-carboxy groups. The solution behaviours of 1 and 2 are discussed based on ¹H and ¹³C NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.

Oxo and thiomolybdenum(iv/vi) citrates, molybdenum(v) homocitrate and molybdenum(vi) citrate were obtained, showing the influence of coordinated α-hydroxy and α-alkoxy groups with different oxidation states.     

Synthesis and study of C-substituted methylthio derivatives of cobalt bis(dicarbollide)

The C-methylthio derivatives of cobalt bis(dicarbollide) were synthesized by reaction of anhydrous CoCl₂ with nido-carborane [7-MeS-7,8-C₂B₉H₁₁]⁻ and isolated as a mixture of rac-[1,1′-(MeS)₂-3,3′-Co(1,2-C₂B₉H₁₀)₂]⁻ and meso-[1,2′-(MeS)₂-3,3′-Co(1,2-C₂B₉H₁₀)₂]⁻ isomers. The structures of both isomers were studied using DFT quantum chemical calculations. The most preferable geometry of rotamers and the stabilization energy of C-methylthio derivatives of cobalt bis(dicarbolide) were calculated. The (BEDT-TTF)[1,1′-(MeS)₂-3,3′-Co(1,2-C₂B₉H₁₀)₂] salt was prepared and its structure was determined by single crystal X-ray diffraction. The cisoid conformation of the rac-[1,1′-(MeS)₂-3,3′-Co(1,2-C₂B₉H₁₀)₂]⁻ anion is stabilized by short intramolecular CH⋯S hydrogen and BH⋯S chalcogen bonds between the dicarbollide ligands, that is in good agreement with the data of quantum chemical calculations.

The C-methylthio derivatives of cobalt bis(dicarbollide) rac-[1,1′-(MeS)₂-3,3′-Co(1,2-C₂B₉H₁₀)₂]⁻ and meso-[1,2′-(MeS)₂-3,3′-Co(1,2-C₂B₉H₁₀)₂]⁻ were synthesized and studied by DFT calculations and X-ray diffraction.