Tris(ethylenediamine) Cobalt(II) and Manganese(II) Nitrates

Miguel Cortijo,Patrick Rosa,Rodolphe Clérac,Elizabeth A Hillard,Mathieu Rouzières,Ángela Valentín-Pérez

doi:10.3390/cryst10060472

Abstract

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.

Highlights

The octahedral tris(bidentate) ligand geometry is a classic motif in coordination chemistry and it provides an archetypal model for chirality in metal complexes
The first demonstration of the spontaneous resolution of coordination complexes involved a molecule of this type, [CoIII2]Br3, which was resolved by Werner from a super-saturated aqueous solution slightly enriched in one of the enantiomers [1]
A Cambridge Structural Database (CSD) [3] search at the time of this writing of six-coordinate transition metals crystallizing in groups allowing for enantiopure substances (Sohncke groups) yielded almost 19,000 hits, when compared to ca. 204,000 hits when the Sohncke group constraint was removed

Summary

Introduction

The octahedral tris(bidentate) ligand geometry is a classic motif in coordination chemistry and it provides an archetypal model for chirality in metal complexes. The first demonstration of the spontaneous resolution of coordination complexes involved a molecule of this type, [CoIII (en) (ox)]Br3 , which was resolved by Werner from a super-saturated aqueous solution slightly enriched in one of the enantiomers [1]. Many such complex cations are known to form conglomerates, a mechanical mixture of non-centrosymmetric crystals containing only one enantiomer. A Cambridge Structural Database (CSD) [3] search at the time of this writing of six-coordinate transition metals crystallizing in groups allowing for enantiopure substances (Sohncke groups) yielded almost 19,000 hits, when compared to ca.

Methods

Results

Conclusion