Enantiomeric resolution of helicochiral paddlewheel complexes and their infrared, Raman, UV-vis and X-ray optical activity

International audienceLinear polynuclear paddlewheel complexes-"extended metal atom chains" or "metal strings"-have provided attractive models for the study of metal-metal bonding, magnetism and conductivity since their discovery in the 1990s [1]. Their helicoidal chirality, arising from mutual steric hindrance of the 3-pyridyl protons, resulting in the twisting of the equatorial ligand around the metal axis (see figure), has been less studied. Nonetheless, in one of the few examples of chiral resolution, the obtained enantiomers of a trinickel complex showed a remarkably high specific rotation of 5000 deg•mL•g −1 •dm −1 [2], motivating us to seek a general technique for the chiral resolution of such racemates. We have developed a procedure based on anion exchange for the chiral resolution of [M3(dpa)4] 2+ salts (M = Co(II) or Ni(II), Hdpa = 2,2'-dipyridylamine). Homochiral arsenyl tartrate (AsT) salts promoted the selective crystallization of [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2, or [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2 in the P4212 space group. The enantiopure compounds demonstrated surprisingly large optical activities using UV-vis, Raman and infrared spectroscopy in solution and, for the cobalt derivatives, in the X-ray range at the Co K-edge in single crystals. An intense X-ray linear dichroism was observed in the orthoaxial crystal orientation, whereas it vanished in the axial confirmation, while the angular dependence of the circular dichroism spectra followed the expected (3cos 2 − 1) function, thus spectroscopically confirming the D4 crystal symmetry. X-ray magnetic circular dichroism and X-ray magnetochiral dichroism signals at the Co K-edge were not detected, likely due to a strongly delocalized spin density on the metal-metal bonded tricobalt core. Nevertheless, these results establish that chiral polynuclear paddlewheel complexes can be cleanly resolved using selective crystallization and demonstrate considerable optical activity in the infrared, UV-vis and X-ray energy ranges, thus potentially offering future perspectives in non-linear optics and asymmetric synthesis [3]

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

Enantiomeric resolution of helicochiral paddlewheel complexes and their infrared, Raman, UV-vis and X-ray optical activity

Linear polynuclear paddlewheel complexes-"extended metal atom chains" or "metal strings"-have provided attractive models for the study of metal-metal bonding, magnetism and conductivity since their discovery in the 1990s [1]. Their helicoidal chirality, arising from mutual steric hindrance of the 3-pyridyl protons, resulting in the twisting of the equatorial ligand around the metal axis (see figure), has been less studied. Nonetheless, in one of the few examples of chiral resolution, the obtained enantiomers of a trinickel complex showed a remarkably high specific rotation of 5000 deg•mL•g −1 •dm −1 [2], motivating us to seek a general technique for the chiral resolution of such racemates. We have developed a procedure based on anion exchange for the chiral resolution of [M3(dpa)4] 2+ salts (M = Co(II) or Ni(II), Hdpa = 2,2'-dipyridylamine). Homochiral arsenyl tartrate (AsT) salts promoted the selective crystallization of [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2, or [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2 in the P4212 space group. The enantiopure compounds demonstrated surprisingly large optical activities using UV-vis, Raman and infrared spectroscopy in solution and, for the cobalt derivatives, in the X-ray range at the Co K-edge in single crystals. An intense X-ray linear dichroism was observed in the orthoaxial crystal orientation, whereas it vanished in the axial confirmation, while the angular dependence of the circular dichroism spectra followed the expected (3cos 2 − 1) function, thus spectroscopically confirming the D4 crystal symmetry. X-ray magnetic circular dichroism and X-ray magnetochiral dichroism signals at the Co K-edge were not detected, likely due to a strongly delocalized spin density on the metal-metal bonded tricobalt core. Nevertheless, these results establish that chiral polynuclear paddlewheel complexes can be cleanly resolved using selective crystallization and demonstrate considerable optical activity in the infrared, UV-vis and X-ray energy ranges, thus potentially offering future perspectives in non-linear optics and asymmetric synthesis [3]

Enantiomeric resolution of helicochiral paddlewheel complexes and their infrared, Raman, UV-vis and X-ray optical activity

International audienceLinear polynuclear paddlewheel complexes-"extended metal atom chains" or "metal strings"-have provided attractive models for the study of metal-metal bonding, magnetism and conductivity since their discovery in the 1990s [1]. Their helicoidal chirality, arising from mutual steric hindrance of the 3-pyridyl protons, resulting in the twisting of the equatorial ligand around the metal axis (see figure), has been less studied. Nonetheless, in one of the few examples of chiral resolution, the obtained enantiomers of a trinickel complex showed a remarkably high specific rotation of 5000 deg•mL•g −1 •dm −1 [2], motivating us to seek a general technique for the chiral resolution of such racemates. We have developed a procedure based on anion exchange for the chiral resolution of [M3(dpa)4] 2+ salts (M = Co(II) or Ni(II), Hdpa = 2,2'-dipyridylamine). Homochiral arsenyl tartrate (AsT) salts promoted the selective crystallization of [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2, or [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2 in the P4212 space group. The enantiopure compounds demonstrated surprisingly large optical activities using UV-vis, Raman and infrared spectroscopy in solution and, for the cobalt derivatives, in the X-ray range at the Co K-edge in single crystals. An intense X-ray linear dichroism was observed in the orthoaxial crystal orientation, whereas it vanished in the axial confirmation, while the angular dependence of the circular dichroism spectra followed the expected (3cos 2 − 1) function, thus spectroscopically confirming the D4 crystal symmetry. X-ray magnetic circular dichroism and X-ray magnetochiral dichroism signals at the Co K-edge were not detected, likely due to a strongly delocalized spin density on the metal-metal bonded tricobalt core. Nevertheless, these results establish that chiral polynuclear paddlewheel complexes can be cleanly resolved using selective crystallization and demonstrate considerable optical activity in the infrared, UV-vis and X-ray energy ranges, thus potentially offering future perspectives in non-linear optics and asymmetric synthesis [3]