Stereochemical factors affecting second-sphere co-ordination of ferrioxamine B with cis-syn-cis and cis-anti-cis isomers of dicyclohexano-18-crown-6 in chloroform and a comparison with alkali-metal and ammonium cations

Abstract

Second-sphere co-ordination of ferrioxamine B [Fe(HL)]⁺ by dicyclohexano-18-crown-6 (eicosahydrodibenzo[b,k][1,4,7,10,13,16]hexaoxacyclooctadecine) occurs through host–guest complex formation with the protonated amine side chain (CH₂)₅NH₃⁺. The influence of steric factors on the association constants (K_a) for both nitrate and perchlorate salts of ferrioxamine B with cis-syn-cis and cis-anti-cis isomers of dicyclohexano-18-crown-6 (CE) in CHCl₃ has been investigated. For comparison purposes, association of the perchlorate salt of the indium(III) analogue of ferrioxamine B [In(HL)]⁺ and the picrate salts of NH₄⁺ and alkali-metal cations are included in this study (picrate = 2,4,6-trinitrophenolate). Although the absolute values of K_a for [M(HL)]X association with CE are strongly dependent on the anion identity, the selectivity ratio expressed for the syn crown ether isomer is 3 : 1 and is independent of anion variations. Comparison with alkali-metal and ammonium cations shows that the discrimination between syn and anti isomers of cis-dicyclohexano-18-crown-6 is dependent upon cation size and is the least for K⁺ where the ionic radius best matches the cavity size. While syn isomer discrimination in CHCl₃ is closely related to the ionic radius, in water it is related to the hydrated radius. The syn over anti isomer preference exhibited by alkali-metal and ammonium cations is much more pronounced in water than in chloroform, which is likely due to the steric hindrance imposed by the large effective radius of the cations due to their hydration shell. Discrimination between the syn and anti isomers by [M(HL)]⁺ in CHCl₃ is greater than expected based on comparison with the parent NH₄⁺ ion. This is likely due to the significant steric requirements of the bulky metal complex molecule. The selectivity of the syn over the anti isomer is dominated by the preference of the bulky [M(HL)]⁺ complex for the less hindered face of the syn isomer.