The phenomenon of conglomerate crystallization.III [1]. Spontaneous resolution in coordination compounds.III [1]. The structures and absolute configuration of Λ(δλδ) [(-)589-cis-α-(triethylenetetraamine) Co(NO2)2]Cl· H2O (I), Λ(δλδ)[(-)589− cis-bis-ethylenediamine Co(NO)2)2]Cl (II) and of K[trans-Co(NH3)2(NO2)4] (III)stereoisomers displaying CLAVIC dissymmetry

Abstract

The crystal structures and absolute configurations of three cis-dinitro derivatives of Co(III) have been determined by single crystal X-ray diffraction data collected with a computer-controlled diffractometer.(I): Λ(δλδ)[(-)589-cIS-α-(triethylenetetraamine)Co(NO2) 2Cl·H2O crystallizes in the orthorhombic space group P212121 with lattice constants: a = 8.070(1) Å, b = 12.506(3) and c = 13.741(1); V = 1386.67 Å3. The calculated density (for z = 4 molecules/unit cell) is 1.679 g cm−3. A total of 3467 data were collected of which 2082 were considered observed (I> 3σ(I)) and refinement converged to R = 0.0436 and Rw = 0.0456. Compound (II): Λ(δλ)[(-)589 - cis-bis- ethylenediamine Co(NO)2]Cl crystallizes in the monoclinic system, space group P21. Cell constants are; a = 6.555(2) Å, b = 11.115(3), c = 8.254(2) and β = 106.59(2)δ; volume = 76.34 Å3 and density (calc) = 1.755 g cm−3, for z = 2 molecules/unit cell. For compound (II), a total of 2866 data were collected of which 2423, having I > 3σ(I), were considered observed. Using this set, the structure was refined to agreement indices of R = 0.0480 and Rw = 0.04666. Finally, (III): K[transCo(NH3)2(NO2)2] crystallizes in the orthorhombic space group P212121 with lattice constants of a = 11.075(3) Å, b = 12.806(5) and c = 6.696(3); cell volume = 316.118 Å3 and the calculated density, for z = 4, is 2.170 g cm−3. The data set collected for (III) consisted of 2749 measurements of which, by the same criterion as the other two, 2109 were found to be observed. Refinement of the atomic coordinates and thermal parameters converged, in this case, to R = 0.0367 and Rw = 0.0355. The absolute configurations of these substances, in the solid state, were established via the Bijvoet test and, for (I) and (II), correlations were made with other determinations. All three substances are self-resolving stereoisomers forming crystalline conglomerates when crystallized from water at room temperature. Thus, each crystal consists of a single, pure, stereoisomer even if other crystals from the same batch contain the opposite enantiomer. While it is true that (i) and (II) are chiral due to the presence of non-planar five-membered ring ligands which lend chirality to those molecules by their very nature, this is not the case with (III). Instead, all three substances share an interesting source of symmetry, not heretofore noticed, due to the arrangement of the pair of planar cis-dinitro ligands, which are held in position by hydrogen bonds with adjacent - NH2 or -NH3 ligands present in all three compounds. Due to steric hindrance effects, theses pairs of adjacent cis-dinitro ligands must, co-operatively, point in the same direction while each makes one short O⋯H and one long O⋯H bond, respectively, to a pair of hydrogens on the adjacent (trans amino ligands. It is this cooperative steric and hydrogen bonded arrangement which give rise to a form of atropisomerism which is reffered to in the text as CLAVIC (from the Latin word for tillers or paddles) dissymetry.