Tris(ethylenediamine) Cobalt(II) and Manganese(II) Nitrates: Synthesis, structure and crystal handedness mapping by X-ray circular dichroism

Octahedral tris(bidentate) 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 (en = ethylenediamine) 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. To extend the family of such compounds, we have investigated [Mn(en)₃](NO₃)₂ and [Co(en)₃](NO₃)₂, which were found to be isostructural to the Zn(II) and Ni(II) derivatives. The Mn(II) analogue undergoes the same phase transition centered at 149(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 diffraction and heat capacity measurements.¹

From the perspective of chirality, although conglomerate formation allows crystals of only one or the other enantiomer to be effortlessly obtained, spontaneous resolution can only truly be considered accomplished after the identification and triage of the two forms of crystals. For this, we have implemented an original method for determining the handedness of individual crystals in a mixture using a tightly-focused, circularly polarized X-ray beam.² The X-ray natural circular dichroism (XNCD) spectra measured at the metal K-edge on selected crystals of [Co(en)₃](NO₃)₂ and [Ni(en)₃](NO₃)₂ show maxima at the metal pre-edge. A mapping of a collection of crystals was performed, by setting the X-ray energy to the peak maximum and determining the sign of the difference in absorption for the two polarizations, which then directly yielded the handedness of the crystal. Furthermore, the mapping resolution (~150 mm) allowed us to evidence that while the majority of the crystals form a conglomerate as expected, a few individuals present opposite handedness domains, and in two cases we found unexpectedly fully racemic crystals.

1) M. Cortijo, A. Valentin-Perez, M. Rouzières, R. Clérac, P. Rosa, E.A. Hillard, Crystals, 2020, 10(6), 472 https://doi.org/10.3390/cryst10060472.

2) M. Cortijo, A. Valentin-Perez, A. Rogalev, F. Wilhelm, Ph. Sainctavit, P. Rosa, E.A. Hillard, Chem. Eur. J., 2020, https://doi.org/10.1002/chem.202001783