Enantiomeric resolution and X-ray optical activity of a tricobalt extended metal atom chain

Crystal Engineering of Conglomerates: Dilution of Racemate-Forming Fe(II) and Ni(II) Congeners into Conglomerate-Forming [Zn(bpy)3](PF6)2

Conglomerate formation, where enantiomers within a racemic mixture self-segregate upon crystallization, is an advantageous property for obtaining chirally pure crystals and allows large-scale chiral resolution. However, the prevalence of conglomerates is low and difficult to predict. In this report, we describe our attempts to engineer conglomerates from racemate-forming compounds by integrating them into a conglomerate-forming matrix. In this regard, we found that Ni(II) and Fe(II) form molecular alloys with Zn(II) in [MxZn(1−x)(bpy)3](PF6)2 (where bpy = 2,2′-bipyridyl). Powder X-ray Diffraction (PXRD) and Energy-Dispersive X-ray spectroscopy (EDX) evidenced conglomerate crystallization with Ni(II) concentrations up to about 25%, while it was observed only for much lower concentrations of Fe(II). This can be attributed to the ability of [Ni(bpy)3](PF6)2 to access a metastable conglomerate phase, while no such phase has been detected in [Fe(bpy)3](PF6)2. Furthermore, the chiral phase appears to be favored in fast-growing precipitates, while the racemic phase is favored in slow re-crystallizations for both Ni(II) and Fe(II) molecular alloys. X-ray natural circular dichroism (XNCD) measurements on [Ni0.13Zn0.87(bpy)3](PF6)2 demonstrate the chirality of the nickel molecules within the zinc molecular matrix.

Molecular Chirality and Its Monitoring by Ultrafast X-ray Pulses

Major advances in X-ray sources including the development of circularly polarized and orbital angular momentum pulses make it possible to probe matter chirality at unprecedented energy regimes and with Ångström and femtosecond spatiotemporal resolutions. We survey the theory of stationary and time-resolved nonlinear chiral measurements that can be carried out in the X-ray regime using tabletop X-ray sources or large scale (XFEL, synchrotron) facilities. A variety of possible signals and their information content are discussed.

Resolution, structures, and vibrational circular dichroism of helicoidal trinickel and tricobalt paddlewheel complexes

It has been recently shown that enantiomers of the helicoidal paddlewheel complex [Co₃ (dpa)₄ (CH₃ CN)₂ ]²⁺ (dpa = the anion of 2,2'-dipyridylamine) can be resolved using the chiral [As₂ (tartrate)₂ ]^2- anion (AsT) and that these complexes demonstrate a strong chiroptical response in the ultraviolet-visible and X-ray energy regions. Here we report that the nickel congener, [Ni₃ (dpa)₄ (CH₃ CN)₂ ]²⁺ , can likewise be resolved using AsT. Depending on the stereochemistry of the enantiopure AsT anion, one or the other of the trinickel enantiomers crystallize from CH₃ CN and diethyl ether in space group P42₁ 2 as the (NBu₄ )₂ [Ni₃ (dpa)₄ (CH₃ CN)₂ ](AsT)₂ ·[solvent] salt. After resolution, the AsT salts were converted into the PF₆ ^- salts by anion exchange, with retention of the chirality of the trinickel complex. The enantiopure [Ni₃ (dpa)₄ (CH₃ CN)₂ ](PF₆ )₂ ·2CH₃ CN and [Co₃ (dpa)₄ (CH₃ CN)₂ ](PF₆ )₂ ·CH₃ CN·C₄ H₁₀ O compounds crystallize in space groups C2 and P2₁ , respectively. Both the Ni(II) and Co(II) complex cations are stable towards racemization in CH₃ CN. Vibrational circular dichroism (VCD) data obtained in CD₃ CN demonstrate the expected mirror image spectra for the enantiomers, the observed peaks arising from the dpa ligand. The VCD response is significant, with Δε values up to 6 Lmol^-1 cm^-1 and vibrational dissymmetry factors on the order of 10^-3 . Density functional theory calculations well reproduce the experimental spectra, showing little difference between the peak position, sign, and intensity in the VCD for the cobalt and nickel complexes. These results suggest that VCD enhancement of these peaks is unlikely, and their remarkable intensity may be due to their rigid helicoidal structure.

The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains

Chromium(II)-based extended metal atom chains have been the focus of considerable discussion regarding their symmetric versus unsymmetric structure and magnetism. We have now investigated four complexes of this class, namely, [Cr₃(dpa)₄X₂] and [Cr₅(tpda)₄X₂] with X = Cl^- and SCN^- [Hdpa = dipyridin-2-yl-amine; H₂tpda = N²,N⁶-di(pyridin-2-yl)pyridine-2,6-diamine]. By dc/ac magnetic techniques and EPR spectroscopy, we found that all these complexes have easy-axis anisotropies of comparable magnitude in their S = 2 ground state (|D| = 1.5-1.8 cm^-1) and behave as single-molecule magnets at low T. Ligand-field and DFT/CASSCF calculations were used to explain the similar magnetic properties of tri- versus pentachromium(II) strings, in spite of their different geometrical preferences and electronic structure. For both X ligands, the ground structure is unsymmetric in the pentachromium(II) species (i.e., with an alternation of long and short Cr-Cr distances) but is symmetric in their shorter congeners. Analysis of the electronic structure using quasi-restricted molecular orbitals (QROs) showed that the four unpaired electrons in Cr₅ species are largely localized in four 3d-like QROs centered on the terminal, "isolated" Cr²⁺ ion. In Cr₃ complexes, they occupy four nonbonding combinations of 3d-like orbitals centered only on the two terminal metals. In both cases, then, QRO eigenvalues closely mirror the 3d-level pattern of the terminal ions, whose coordination environment remains quite similar irrespective of chain length. We conclude that the extent of unpaired-electron delocalization has little impact on the magnetic anisotropy of these wire-like molecular species.

Advances in optical and electrochemical techniques for biomedical imaging

Yi-Tao Long and Thomas J. Meade introduce the Chemical Science retrospective themed collection on advances in optical and electrochemical techniques for biomedical imaging.

Enantiopure Chiral Coordination Polymers Based on Polynuclear Paddlewheel Helices and Arsenyl Tartrate

Herein, we report the preparation of chiral, one-dimensional coordination polymers based on trinuclear paddlewheel helices [M₃(dpa)₄]2+ (M = Co(II) and Ni(II); dpa = the anion of 2,2'-dipyridylamine). Enantiomeric resolution of a racemic mixture of [M₃(dpa)₄]2+ complexes was achieved by chiral recognition of the respective enantiomer by [Δ-As₂(tartrate)₂]2- or [Λ-As₂(tartrate)₂]2- in N,N-dimethylformamide (DMF), affording crystalline coordination polymers formed from [(Δ-Co₃(dpa)₄)(Λ-As₂(tartrate)₂)]·3DMF (Δ-1), [(Λ-Co₃(dpa)₄)(Δ-As₂(tartrate)₂)]·3DMF (Λ-1), [(Δ-Ni₃(dpa)₄)(Λ-As₂(tartrate)₂)]·(4 - n)DMF∙nEt₂O (Δ-2) or [(Λ-Ni₃(dpa)₄)(Δ-As₂(tartrate)₂)]·(4 - n)DMF∙nEt₂O (Λ-2) repeating units. UV-visible circular dichroism spectra of the complexes in DMF solutions demonstrate the efficient isolation of optically active species. The helicoidal [M₃(dpa)₄]2+ units that were obtained display high stability towards racemization as shown by the absence of an evolution of the dichroic signals after several days at room temperature and only a small decrease of the signal after 3 h at 80 °C.