The role of ion-association in the chromatographic separation of isomeric cationic cobalt(III)-amine complexes on cation-exchange resins, particularly SP-Sephadex

Abstract

Chromatographic separations of a set of closely similar or isomeric cobalt(III) complexes, of the kinds [Co(dien)2]3+, [Co(dien)(medien)]3+ and [Co(medien)2]3+ [dien = diethylenetriamine; medien = 4-methyldiethylenetriamine or 2,2'-methyliminodi(ethylamine)] on SP-Sephadex C-25 and Dowex 50W-X2 cation-exchange resins have been examined with different eluents as sodium salts. The separations obtained appear to be due primarily to differences in the association constants for ion-pairs formed betwen the complex cations and the eluent anions, rather than due to differences in affinities of the free cations for the resins. The strengths of the associations depend on: (1) the availability on the complex cations of appropriately disposed N-H bonds for hydrogen-bonding to oxyanions, and probably on the acidities of these hydrogens (acidity sec-NH > NH2), rather than on polarity of charge distribution, and (2) the basicities of anions, in the order PO43- > SeO32- > (+)-tartrate2- > SO42- > (+)-Sb2(C,4H2O6)22- > Cl- ≈ NO3- > ClO4-. The effects of association with PO43-, SeO32- and (+)-tartrate2- may be so great that complex cations of different charges are not necessarily eluted in the order of their increasing charges. Determinations of charges and polarities (cis/trans) of cationic complexes by chromatographic procedures must therefore be carried out under conditions where association is minimized, as with sodium perchlorate as eluent. Under these conditions, separations are determined largely by cation-resin interactions which are less specific and less discriminating than cation-anion interactions. SP-Sephadex is a particularly sensitive medium for separating different cationic complexes, and tri- sodium orthophosphate and sodium (+)-tartrate are the most effective and generally useful eluents.

Chromatographic separation of stereoisomers of ligand-bridged diruthenium polypyridyl species

“…The association between the SP Sephadex C-25 support and the cationic substrate should be essentially temperature independent, being between a solid and solute phase. 30 Since coulombic forces dominate the rate of passage of species down the SP Sephadex C-25 support, 14 it can be assumed that the observed temperature dependence can be attributed to changes in the effective charges of the species travelling down the column. Tris(bipyridine)ruthenium()-type species associate with organic anions, and it is the differences in relative associations between the anion of the eluent and the various stereoisomers that lead to their separation by cation-exchange chromatography (the nature of these associations will be the subject of a subsequent publication).…”

“…In these studies, SP Sephadex C-25 was chosen as the cationexchange support, rather than the more commonly used polystyrene-based resins for which we had previously observed significant absorptions by some complexes containing polypyridyl ligands. This material has been extensively used in the separation of stereoisomers of various octahedral cobalt() species, [14][15][16] but surprisingly it appears not to have been widely applied to ruthenium() complexes containing polypyridyl ligands until our recent studies. SP Sephadex is composed of a cross-linked dextran matrix, functionalised with strongly acidic propanesulfonate groups.…”

Structures of the cis and trans isomers of the bis(1-thia-4,7-diazacyclononane)cobalt(III) cation

“…The elution of the complex down SP-Sephadex columns using the eluents Na2SO4/H 4 and Na2(+)tartrate was examined in comparison with some related charged (3+) cobalt(Ill) complexes. These included the three geometric isomers of ICo(dien)213~ (dien = diethylenetriamine) whose relative elutions symfac>>unsym-fac>mer are reasonably understood on the basis of ion association (Searle, 1977), unsym- (Searle, 1977(Searle, , 1987, and the results are shown in Table 6.…”

“…Chromatography of the ICo(tasn)zl 3~ complex by cation exchange has given unusually poor discrimination between the isomers (Gahan, Hambley, Searle, Bjerrum & Larsen, 1988) and this can now be understood in terms of the detailed geometries found for the isomers, There is substantial evidence that chromatographic discrimination results from 'polydentate' interactions between a tetrahedral anion such as SO42 or PO 3-and two or three H(amine) atoms of the complex cation (Keene & Searle, 1974;Searle, 1977;Sakagushi, Tsuge & Yoneda, 1983). In the case of the trans isomer (Fig.…”

Anion interactions with (polypyridyl)ruthenium complexes, and their importance in the cation-exchange chromatographic separation of stereoisomers of dinuclear species †

“…[14][15][16] In our stereochemical investigations of mono-and oligonuclear ruthenium complexes with α,αЈ-diimine ligands, the cation-exchange support SP Sephadex C-25 has proved remarkably successful in the separation of individual geometric isomers, diastereoisomers and enantiomers of mixtures typically produced in structurally-uncontrolled syntheses. [15][16][17][18][19][20][21][22][23] This material had been extensively used in the separation of stereoisomers of various octahedral cobalt() species, [24][25][26] but surprisingly it appears not to have been widely applied to ruthenium() complexes containing polypyridyl ligands until our recent studies. SP Sephadex TM is composed of a cross-linked dextran matrix functionalised with strongly acidic propylsulfonate groups.…”