Why are Hexol Cluster Cations [Tris(cis-tetraaminedihydroxo)Co]+6 So Easily Resolved into Their Antipodes? Part 2. Preparation, Crystal Structural Studies and the Nine Sources of Chirality of Hexol Clusters

Abstract

Listen

Translate article

Heretofore, it had been impossible to determine the structure of the original Werner's hexol cation inasmuch as the synthetic procedure given by Werner produces crystals unsuitable for single crystal X-ray (XRD) diffraction studies. Thus, it was greatly satisfying that crystals obtained here, using a different method of preparation, produced useful crystals of three different derivatives. X-ray quality crystals of composition [Co(NH3)4CO3][Co{(HO)2Co(NH3)4}3](NO3)6(OH) · 2 H2O(III) were obtained while attempting to grow crystals of [Co(NH3)5CO3]NO3 from water solutions allowed to evaporate at room temperature (ca. 22°C). (III) crystallizes in space group P\(\overline 1\) (No. 2), with lattice constants: a = 11.767(1), b = 12.856(2), c = 17.040(1) A, α = 75.49(2), β = 72.59(2), γ = 68.53(7)°, V = 2259.84 A3, and d(calc; M.W. = 1148.2, Z = 2) = 1.687 g-cm−3. A total of 4428 data were collected using MoKα (λ = 0.71073 A radiation, over the range of 4° ≤ 2θ ≤ 40°; of these, 2469 [independent and with I ≥ 3σ(I)] were used in the structural analysis. Data were corrected for absorption (μ = 18.97 cm−1) and transmission coefficients ranged from 0.536 to 0.999. The final R(F) and RW(F) residuals were, respectively, 0.18 and 0.18, the high values of which are due to disorder of several of the nitrate counteranions. Nonetheless, the results are of interest since this double salt consists of two fully ordered cations: [Co(NH3)4CO3]+, containing a bidentate carbonato ligand, and Werner's hexol cation {Co[Co(OH)2(NH3)4]3}6+. The presence of the bidentate carbonato cation is important in view of the fact that the starting material was the monodentate pentaamine carbonato cation, a result which provides a guide to the mechanism of the reactions involved in the preparation of compound (III), in particular, and of the hexol cations, in general. Thus, this study is an amplification of our recent description [1] of Werner's hexol salts. Finally, we explain here the origin of the large optical rotatory power of these cations and enumerate the nine sources of chirality present in them. In our previous paper, we listed only seven [1]; however, we have recently come to realize there are two additional ones, described below.