16
Optical Resolution of Facial and M e r i d i o n a l Tris(aminoacidato)cobalt(III) Chelates b y d-Tartrate a n d b y A n t i m o n y d-Tartrate
Stereochemistry of Optically Active Transition Metal Compounds Downloaded from pubs.acs.org by CORNELL UNIV on 06/10/17. For personal use only.
HAYAMI YONEDA, SHIGEO YAMAZAKI, and ΤΟΜΟΥΟ YUKIMOTO Department of Chemistry, Faculty of Science, Hiroshima University, Hiroshima, Japan It is not easy to obtain both enantiomers of non-charged chiral complexes in optically pure form. Chromatographic separa tion by the use of asymmetric adsorption on an optically active adsorbent(for example, quartz, starch and d-lactose etc.) has been regarded as almost the only means for this purpose. However, usu ally such procedures lead to partial separation of enantiomers. This is probably because asymmetric adsorption is very weak. To make matters worse, we do not know the stereochemistry of asymmet ric adsorption, nor how to improve the efficiency of separation. We attempted to attack this problem by extending the technique and the ion association model for ion-exchange chromatography. Here, we describe the mechanism of chromatographic separation of enan tiomers of facial and meridional tris(aminoacidato)cobalt(III) chelates (See Figure 1). Complete Resolution of
fac-[Co(β-ala) ] 3
The facial isomer of [Co(β-ala) ] has three carboxyl oxygen atoms in a triangular face of an octahedron and three amino groups in the opposite triangular face. The expected large dipole moment causes fairly large adsorption on the adsorbent. Thus, chromato graphic separation of enantiomers is expected with a suitable resolving agent. On the other hand, [Co(en) ] was reported to be resolved completely into enantiomers through a column packed with the Na form of SE-Sephadex cation-exchanger using an aqueous solution of Na d-tartrate (l). As a part of the structural studies of optical resolution, we have determined the crystal structures of a series of diastereomeric salts, A-[Co(en) ]Br·dt a r t · 5 H O (2), A - L i [ C r ( e n ) ] ( d - t a r t ) · 3 H O (l) andA-H[Co(en) ] (d-tart) ·3H O (4). Although these crystals are quite different in chemical formula, cell dimension, and space group symmetry, they have a remarkable resemblance to each other in the face-toface close contact mode of d-tart with A-[M(en)3] . This ionpair structure is formed by the contact of the four oxygen atoms of d-tart and the three NH groups of the complex. Therefore, 3
3+
3
2
3
2
3
2
2
2
3
2
3+
2
0-8412-0538-8/80/47-119-315$05.00/0 © 1980 American Chemical Society
STEREOCHEMISTRY OF TRANSITION METALS
316
only one h a l f of the complex i s i n v o l v e d i n t h i s a s s o c i a t i o n . Since fac-[Co( β-ala)3] has a s i m i l a r t r i a n g u l a r face of three NH groups and shows the AB-type PMR s i g n a l of the NH2 groups s i m i l a r to that of [Coien)^]^ *, quite s i m i l a r a s s o c i a t i o n of
