Hard X-ray Magnetochiral Dichroism in a Paramagnetic Molecular 4f Complex, D. Mitcov (a), M. Platunov (b,c), C.D. Buch (a), A. Reinholdt (a,d), A.R. Døssing (a), F. Wilhelm (b), A. Rogalev (b) and
S. Piligkos (a), Chem. Sci. 11, 8306-8311 (2020); https://doi.org/10.1039/D0SC02709J. (a) Department of Chemistry, University of Copenhagen (Denmark)
(b) ESRF (c) Kirensky Institute of Physics, Federal Research Center KSC SB RAS (Russia) (d) Department of Chemistry, University of Pennsylvania, Philadelphia (USA)
 K.S. Pedersen et al., J. Am. Chem. Soc. 138, 5801-5804 (2016).  R. Hussain et al., J. Am. Chem. Soc. 140, 9814-9818 (2018).
PRINCIPAL PUBLICATION AND AUTHORS
To this purpose, investigations were carried out at beamline ID12, which is unique in its specificity for polarisation-dependent X-ray spectroscopy. The chosen studied system was Na5[Ho(ODA)3](BF4)2 × 6(H2O), (1), with ODA2- = oxydiacetate. In (1), the HoIII centre coordinates to three tridentate ODA ligands, resulting in the formation of two enantiomers (L, Δ) of the complex anion (Figure 100, top). Absorption of an X-ray photon at the L3-edge of Ho promotes a 2p3/2 core electron into empty 5d or 6s valence states via the allowed electric dipole transitions (∆l = ±1) or to 6p or 4f states via electric quadrupole transitions (∆l = 0, +2). Left (L) or right (R) circularly polarised X-rays whose wave vector, k, was parallel (+) or antiparallel (-) to the external magnetic field, B (Figure 100, bottom), were used. For each enantiomerically pure single crystal, four distinct X-ray absorption spectra were acquired, corresponding to the four different relative orientations of k and B and the polarisation of X-rays, namely, L+, L-, R+, R-. From these four experimental spectra, the three distinct dichroic contributions (X-ray natural circular dichroism: XNCD, X-ray magnetic circular dichroism: XMCD and X-ray magnetochiral dichroism: XMXD) could be obtained. The results are presented in Figure 101.
The observed XMXD response stems from electric dipole-quadrupole interference terms combined with the orbital toroidal current in the hybridised ground states. Clearly observable, albeit noisy, XMXD spectra of opposite sign for 1L and 1D were detected at the pre-edge (8069 eV) and the main (8078 eV) absorptions but not in the extended region. This suggests that nonmagnetic empty extended states like 6p or 6d do not contribute to XMXD. Conversely, orbitals that display a significant orbital angular momentum component, such as the hybridised 4f-5d orbitals, have a toroidal orbital moment and thus confer a sizeable XMXD contribution.
These observations established that X-ray spectroscopy is an excellent technique for the investigation of local chiral and magnetic properties in Ln-based coordination complexes. However, the detected XMXD signal at the L3-edge of Ho is rather small (i.e., an order of magnitude weaker than the XMCD or XNCD signals). This is due to the strong localisation and quasi-atomic character of the 4f states, which are only weakly affected by the chiral coordination environment. Furthermore, this also indicates that the 4f states are only weakly hybridised with more spatially extended 5d states, which are more sensitive to the local chiral structures. Thus, the high intrinsic magnetic moment of 4f states, combined with a large admixture of these with extended 5d states, emerges as a design criterion for the generation of large magnetochiral responses in lanthanoid- based chiral coordination complexes.
Fig. 101: XAS, XNCD, XMCD and XMXD
spectra of 1L and 1D.