Magnetochiral dichroism (MChD), a fascinating manifestation of the light-matter interaction characteristic for chiral systems under magnetic fields, has become a well-established optical phenomenon reported for many different materials. However, its interpretation remains essentially phenomenological and qualitative, because the existing microscopic theory has not been quantitatively confirmed by confronting calculations based on this theory with experimental data. Here, we report the experimental low-temperature MChD spectra of two archetypal chiral paramagnetic crystals taken as model systems, tris(1,2-diaminoethane)nickel(II) and cobalt(II) nitrate, for light propagating parallel or perpendicular to the c axis of the crystals, and the calculation of the MChD spectra for the Ni(II) derivative by state-of-the-art quantum chemical calculations. By incorporating vibronic coupling, we find good agreement between experiment and theory, which opens the way for MChD to develop into a powerful chiral spectroscopic tool and provide fundamental insights for the chemical design of new magnetochiral materials for technological applications.

The dependence of nonlinear optical properties upon the spin state in molecular switches is still an unexplored area. Chiral [Fe( phen)] complexes are excellent candidates for those studies because they are expected to show nonlinear optical properties of interest and at the same time show photoconversion to a short-lived metastable high-Spin state by ultrafast optical pumping. Herein, we present the synthesis, crystallographic, and spectroscopic comparison of chiral [Fe( phen)] complexes obtained with chiral anions, a new lipophilic derivative of the D-symmetric (As(tartrate)), and D-symmetric tris(catechol)phosphate(V) (TRISCAT), tris(catechol)arsenate(V) (TRISCAS), and 3,4,5,6-tetrachlorocatechol phosphate(V) (TRISPHAT). Complexes [Fe( phen)]( rac-TRISCAT) (2) and [Fe( phen)](X-TRISCAS) (X = rac (3), Δ (4), Λ (5)) were found to be isomorphous in the R32 Sohncke space group with twinning by inversion correlated with the starting chiral anion optical purity. The structures show the [Fe( phen)] complex interacting strongly along its 3-fold axis with two anions. Only the structure of a [Fe( phen)]( rac-TRISPHAT) solvate (6) could be obtained, which showed no particular anion/cation interaction contrary to what was observed previously in solution. The [Fe( phen)](X-As(tartrate)) (X = Δ (7), Λ (8), and racemic mixture (9)) crystallizes in enantiomorphic space groups P321/ P321 with the same solid-state packing. Dichroic electronic absorption studies evidenced racemization for all chiral complexes in solution due to ion pair dissociation, whereas the asymmetric induction is conserved in the solid state in KBr pellets. We evidenced on chiral complexes 4 and 5 strong nonlinear second harmonic generation, the intensity of which could be correlated with the complex electronic absorption.

This tutorial review article discusses chirality determination in the solid state, both in single crystals and in crystal assemblies, with an emphasis on X-ray diffraction. The main principles of using X-ray diffraction to reliably determine absolute structure are summarized, and the complexity which can be encountered in chiral structures-kryptoracemates, scalemates, and inversion twinning-is illustrated with examples from our laboratories and the literature. We then address the problem of the bulk crystallization and discuss different techniques to determine chirality in a large assembly of crystal structures, with a special prominence given to an X-ray natural circular dichroism mapping technique that we recently reported.

Octahedral tris(ethylenediamine) coordination complexes demonstrate helicoidal chirality, due to the arrangement of the ligands around the metal core. The enantiomers of the nitrate salts [Ni(en)3](NO3)2 and [Zn(en)3](NO3)2 spontaneously resolve to form a mixture of conglomerate crystals, which present a reversible phase transition from space group P6322 to enantiomorphic P6522 or P6122, with the latter depending on the handedness of the enantiomer. We report here the synthesis and characterization of [Mn(en)3](NO3)2 and [Co(en)3](NO3)2, which are isostructural to the Zn(II) and Ni(II) derivatives. The Mn(II) analogue undergoes the same phase transition centered at 150(2) K, as determined by single-crystal X-ray diffraction, Raman spectroscopy, and differential scanning calorimetry. The Co(II) derivative does not demonstrate a phase transition down to 2 K, as evidenced by powder X-ray diffraction and heat capacity measurements. The phase transition does not impact the magnetic properties of the Ni(II) and Mn(II) analogues; these high spin compounds display Curie behavior that is consistent with S = 1 and 5/2, respectively, down to 20 K, while the temperature-dependent magnetic moment for the Co(II) compound reveals a significant orbital contribution.

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An original method for determiningt he handedness of individual non-centrosymmetric crystals in a mixtureu sing at ightly-focused, circularly polarized X-ray beam is presented. The X-ray natural circular dichroism (XNCD)s pectra recorded at the metalK-edge on selected crystalso f[ D-M(en) 3 ](NO 3) 2 and [L-M(en) 3 ](NO 3) 2 (M = Co II , Ni II)s how extrema at the metal pre-edge (7712 eV for Co, 8335 eV for Ni). Am apping of ac ollection of some 220 crystals was performed at the respective energies by using left and right circular polarizations.T he difference in absorption for the two polarizations,being either negative or positive, directly yielded the handedness of the crystal volumep robedb yt he beam.B yu sing this technique, it was found that the addition of l-ascorbic acid during the synthesis of [Co(en) 3 ](NO 3) 2 resultedi na ne nantiomeric enrichmento ft he L-isomer of 67 AE 13 %, whereas the Ni analogue was similarly,b ut conversely,e nriched in the Disomer (65 AE 22 %).

Herein, we report the preparation of chiral, one-dimensional coordination polymers based on trinuclear paddlewheel helices [M 3 (dpa) 4 ] 2+ (M = Co(II) and Ni(II); dpa = the anion of 2,2 -dipyridylamine). Enantiomeric resolution of a racemic mixture of [M 3 (dpa) 4 ] 2+ complexes was achieved by chiral recognition of the respective enantiomer by [∆-As 2 (tartrate) 2 ] 2− or [Λ-As 2 (tartrate) 2 ] 2− in N,N-dimethylformamide (DMF), affording crystalline coordination polymers formed from [(∆-Co 3 (dpa) 4 )(Λ-As 2 (tartrate) 2 )]·3DMF (∆-1), [(Λ-Co 3 (dpa) 4 )(∆-As 2 (tartrate) 2 )]·3DMF (Λ-1), [(∆-Ni 3 (dpa) 4 )(Λ-As 2 (tartrate) 2 )]·(4 − n)DMF·nEt 2 O (∆-2) or [(Λ-Ni 3 (dpa) 4 )(∆-As 2 (tartrate) 2 )]·(4 − n)DMF·nEt 2 O (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 [M 3 (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.

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