Douglas A. Vander Griend

Reduction of a Redox-Active Ligand Drives Switching in a Cu(I) Pseudorotaxane by a Bimolecular Mechanism

The reduction of a redox-active ligand is shown to drive reversible switching of a Cu(I) [2]pseudorotaxane ([2]PR(+)) into the reduced [3]pseudorotaxane ([3]PR(+)) by a bimolecular mechanism. The unreduced pseudorotaxanes [2]PR(+) and [3]PR(2+) are initially self-assembled from the binucleating ligand, 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine (Me(2)BPTZ), and a preformed copper-macrocycle moiety (Cu-M(+)) based on 1,10-phenanthroline. X-ray crystallography revealed a syn geometry of the [3]PR(2+). The UV-vis-NIR spectra show low-energy metal-to-ligand charge-transfer transitions that red shift from 808 nm for [2]PR(+) to 1088 nm for [3]PR(2+). Quantitative analysis of the UV-vis-NIR titration shows the stepwise formation constants to be K(1) = 8.9 x 10(8) M(-1) and K(2) = 3.1 x 10(6) M(-1), indicative of negative cooperativity. The cyclic voltammetry (CV) and coulometry of Me(2)BPTZ, [2]PR(+), and [3]PR(2+) shows the one-electron reductions at E(1/2) = -0.96, -0.65, and -0.285 V, respectively, to be stabilized in a stepwise manner by each Cu(+) ion. CVs of [2]PR(+) show changes with scan rate consistent with an EC mechanism of supramolecular disproportionation after reduction: [2]PR(0) + [2]PR(+) = [3]PR(+) + Me(2)BPTZ(0) (K(D)*, k(d)). UV-vis-NIR spectroelectrochemistry was used to confirm the 1:1 product stoichiometry for [3]PR(+):Me(2)BPTZ. The driving force (DeltaG(D)* = -5.1 kcal mol(-1)) for the reaction is based on the enhanced stability of the reduced [3]PR(+) over reduced [2]PR(0) by 365 mV (8.4 kcal mol(-1)). Digital simulations of the CVs are consistent with a bimolecular pathway (k(d) = 12 000 s(-1) M(-1)). Confirmation of the mechanism provides a basis to extend this new switching modality to molecular machines.

Modelling triazolophane–halide binding equilibria using Sivvu analysis of UV–vis titration data recorded under medium binding conditions

Two different models that calculate association constants from UV–vis titration data have been evaluated for the 1:1 binding between triazolophane receptor ([Host] = 1–13 μM) and various halides F− , Cl− , Br− and I− in CH2Cl2 (K a ≈ 103–106 M− 1). The Drago model fits the ΔA values at a single wavelength as a function of the added guest concentration. The new computer program Sivvu performs equilibrium-restricted factor analysis using all the wavelengths of the dataset simultaneously. Both models generate comparable K a values, and both provide a means to assess the accuracy of the binding constant determination: K a [host] < 12. Analysis with Sivvu (1) allows the number of unique chemical absorbers to be identified in an unbiased manner. This analysis allowed for a 2:1, triazolophane:F− intermediate to be identified and included in the model. Sivvu also (2) allows alternate models to be quickly evaluated, (3) generates more accurate binding constants, and (4) generates the extinction profiles for each absorbing species in solution.

A chiroptical molecular sensor for ferrocene

A homochiral, square-shaped, D2 symmetrical nanosized metal-linked macrocycle is able to form stable complexes with ferrocene in polar solvents, with detection achieved by means of multiple outputs (optical/chiroptical spectroscopies and cyclic voltammetry). Selective sensing using chiroptical spectroscopy in the presence of interfering analytes is demonstrated.

La3Cu2VO9: A surprising variation on the YAlO3 structure-type with 2D copper clusters of embedded triangles

Abstract La3Cu2VO9 crystallizes in a heretofore unknown superstructural variation of the rare-earth hexagonal structure (P63/mmc) for ABO3 stoichiometries as characterized by X-ray, neutron and electron diffraction. The alternating hexagonal layers of LaO6/3 and (Cu/V)O3/3 resemble those of YAlO3, except that the copper and vanadium order to create an a 13 ×a 13 ×c supercell with P63/m symmetry. The B-cation coordination environments accommodate copper and vanadium in a two to one molar ratio. The CuII lattice consists of nine atom clusters of triangles in triangles similar to a kagome lattice. The phase is paramagnetic from 5 to 370 K with three temperature regimes that correspond closely to 1/9, 4/9, and 9/9 spins per copper.

From red to blue shift: switching the binding affinity from the acceptor to the donor end by increasing the π-bridge in push–pull chromophores with coordinative ends

A series of homologous push–pull compounds, in which identical donor (a dimethylamino) and acceptor (a malonate ester) functionalities endcap crescent PPV fragments, exhibit striking differences in their supramolecular recognition of cations acting as Lewis acids. The shorter conjugated compound (one phenyl ring) coordinates a wide variety of lanthanide cations (Eu3+, Yb3+ and Er3+) in MeCN solutions to the 1,3-dicarbonyl acceptor end, resulting in an overall supramolecular polarization of the system (red shift of the intramolecular charge-transfer ICT band). With the “hard” cation Sc3+, recognition switches to the tertiary amine donor end, turning the conjugated system from D–π–A to A–π–A, and resulting in a blue shift of the ICT band upon complexation. Interestingly, increasing the conjugation by means of the insertion of sequential p-phenylenevinylene units into the ligand results in coordination to the donor end regardless of cation “hardness” (Sc3+, Eu3+ and Er3+), suggesting a relative change in the nucleophilicity of the two coordinating ends when increasing the length of the conjugated π-bridge. Such a hypothesis is supported by quantum chemical calculations on the ligands and subsequent atomic charges determination using two independent approaches (QTAIM and CHelpG). The characterization of the thermodynamic stabilities and the dimensionalities of the ligand–cation complexes in solution reveals striking differences from case to case, yet increasing affinities (from log Kav = 2.5 to log Kav = 4.9) are recorded with the increase of the π-conjugated bridge.

Crystal growth and co-substitution in (Mg1-xFex)(Mo2-xVx)O7 (0.13≤x≤0.47) with (V/Mo)=O oxo double bonds

Abstract New solid solution crystals of (Mg 1− x Fe x )(Mo 2− x V x )O 7 (0.13≤ x ≤0.47) were grown from MgO–Fe 2 O 3 –V 2 O 5 –MoO 3 melts. Single crystal X-ray diffraction studies reveal that the structure consists of (Mg/Fe) 2 O 10 octahedral dimers and [(Mo 2− x V x )O 7 ] (2+ x )− tetrahedral clusters. The structure contains (V/Mo)O oxo double bonds similar to MgMo 2 O 7 and FeVMoO 7 . Equimolar co-substitution of V 5+ for Mo 6+ and Fe 3+ for Mg 2+ avoids the creation of any ion vacancies or interstitials and generates disordered cation sites. Crystal data: for (Mg 0.87 Fe 0.13 )(Mo 1.87 V 0.13 )O 7 , monoclinic, space group P 2 1 / n (No. 14), with a =8.3356(6) A, b =9.1079(6) A, c =8.5086(6) A, β =111.927(1)° and Z =4; for (Mg 0.53 Fe 0.47 )(Mo 1.53 V 0.47 )O 7 , monoclinic, space group P 2 1 / n (No. 14), with a =8.2766(6) A, b =9.0208(7) A, c =8.4817(6) A, β =112.144(1)° and Z =4.

Self-assembly snapshots of a 2 × 2 copper(I) grid

Self-assembled 2 × 2 grids have been characterised as high-fidelity species produced when the correct stoichiometric ratios are met, but rarely are the individual steps leading to and from their formation characterised. Here, we present such a study using equilibrium-restricted factor analysis to model a set of UV–vis spectra starting from a bis-bidentate ligand to the assembly of a 2 × 2 grid complex upon titration with 1 equiv. of [Cu(MeCN)4](PF6) and to disassembly upon further titration. Intermediate species [CuL2]+, [Cu2L3]2+, [Cu3L2]3+ and [Cu2L]2+ are evidenced along the assembly and disassembly pathways. Complementary 1H NMR titrations are consistent with the rich set of complexes and equilibria involved. Given the nature of the assembly process, the assembly is entropy driven and likely enthalpy driven as well. The disassembly process is both enthalpy and entropy driven according to the standard free energy values derived from the modelling of the spectrophotometric titration data.

Crystal symmetry of La3Cu2VO9 and La4Cu3MoO12 derived from the YAlO3 hexagonal structure by transmission electron microscopy

Abstract La4Cu3MoO12 and La3Cu2VO9, homeotypes of YAlO3, an ABO3 hexagonal phase, have been investigated using a variety of transmission electron microscopy techniques. Both compounds possess the same hexagonal subcell but exhibit different supercells. The unit cell parameters and symmetry have been determined using selected area and convergent beam electron diffraction techniques. For La3Cu2VO9, electron energy loss spectroscopy and energy dispersive X-ray spectroscopy were used to determine the oxidation-state of vanadium and the cation stoichiometry, respectively, which conforms to La3Cu2VO9. This paper highlights the need for multitechnique approaches to complex oxide structures.

Functional characterization of a cadmium resistance operon in Staphylococcus aureus ATCC12600: CadC does not function as a repressor

Sequencing of a cadmium resistance operon from a Staphylococcus aureus ATCC12600 plasmid revealed that it is identical to a cadCA operon found in MRSA strains. Compared to plasmid‐cured and cadC‐mutant strains, cadC‐positive ATCC12600 cells had increased resistance to cadmium (1 mg ml−1 cadmium sulfate) and zinc (4 mg ml−1 zinc sulfate), but not to other metal ions. After growth in media containing 20 µg ml−1 cadmium sulfate, cadC‐mutant cells contained more intracellular cadmium than cadC‐positive ATCC12600 cells, suggesting that cadC absence results in impaired cadmium efflux. Electrophoretic mobility shift assays were performed with CadC proteins encoded by the S. aureus ATCC12600 plasmid and by the cadC gene of pI258, which is known to act as a transcriptional repressor and shares only 47% protein sequence identity with ATCC12600 CadC. Mobility shifts occurred when pI258 CadC protein was incubated with the promoter DNA‐regions from the pI258 and S. aureus ATCC12600 cadCA operons, but did not occur with S. aureus ATCC12600 CadC protein, indicating that the ATCC12600 CadC protein does not interact with promoter region DNA. This cadCA operon, found in MRSA strains and previously functionally uncharacterized, increases resistance to cadmium and zinc by an efflux mechanism, and CadC does not function as a transcriptional repressor.

Methyl-β-cyclodextrin directly binds methylene blue and blocks both its cell staining and glucose uptake stimulatory effects

GLUT1, the most ubiquitously expressed member of the GLUT family of glucose transporters, can be acutely activated by a variety of cell stresses. Methylene blue activates glucose transport activity of GLUT1 in L929 fibroblast cells presumably by a redox cycling of MB, which generates an oxidative stress. Data shown here reveal that methyl-beta-cyclodextrin (MCD) blocks both the staining of cells and activation of glucose uptake by directly binding to MB. MCD binding to MB was qualitatively demonstrated by a significantly slower dialysis rate of MB in the presence of MCD. Analysis of the complete spectra of aqueous MB solutions and MB plus MCD solutions by a factor analysis program called SIVVU indicated that these equilibria can be modeled by three species: MB monomer, MB dimer, and MCD-MB inclusion complex. The molar extinction coefficients for each species from 500 to 700nm were determined. The equilibrium association constant (K(a)) for MB dimer formation was measured at 5846+/-30M(-1) and the K(a) for formation of the MCD-MB complex was 310+/-10M(-1). MCD also dramatically enhances the destaining rate of MB-stained cells. The loss of MB from the cell is tightly correlated with the loss of activated glucose uptake. This suggests that the MB activation of glucose uptake is likely not caused by its redox cycling, but more likely the result of a specific interaction between MB and a protein directly involved in the activation of GLUT1.