Metal binding of polyalcohols. Part 2. (all-cis-1,3,5-Triaminocyclohexane)cobalt(III), a facially co-ordinating, diamagnetic probe exemplified by its complex formation with cis-inositol

Abstract

Interactions of cis-inositol (ino)(cyclohexane-1,2,3,4,5,6-hexol) and 1,3,5-triacetamido-1,3,5-trideoxy-cis-inositol (taino) with the Co(tach)³⁺ unit (tach =all-cis-1,3,5-triaminocyclohexane) have been examined. Reaction with cis-inositol resulted in the formation of [Co(tach)(H_–1ino)]²⁺, with taino in either [(tach)Co(µ-OH)₂(µ-NO₃)Co(tach)]³⁺ or [Co(taino)₂]²⁺. The crystal structures of [Co(tach)(H_–1-ino)][NO₃]₂1, [(tach)Co(µ-OH)₂(µ-NO₃)Co(tach)][NO₃]₃·0.75H₂O 2, [Co(taino)₂][NO₃]₂·4H₂O 3a and [Co(taino)₂][NO₃]₂·8H₂O 3b have been determined. The Co^III in solid 1 is bound to two hydroxyl groups and one alkoxo group of cis-inositol in a syn-1,3,5-triaxial arrangement. In aqueous solution, [Co(tach)(ino)]³⁺ reacted as a strong acid. The values pk₁ < 2.0, pk₂= 4.05(2) and pk₃= 6.68(2) have been established by potentiometric measurements (0.1 mol dm^–3 KNO₃, 25 °C). In addition, an isomerization reaction [binding of the Co(tach)³⁺ unit to three adjacent hydroxyl groups in an axial–equatorial–axial arrangement] as well as the reversible dissociation of cis-inositol have been observed by NMR measurements. Both reactions show a characteristic pH dependence for the equilibrium composition as well as for the reaction rate. The individual microscopic acidity constants of the two isomers were elucidated by ¹H NMR-monitored pD titrations. Molecular mechanics calculations were used to rationalize differences in energy between the two isomeric forms. The rather weak binding of neutral polyalcohols to metal ions was confirmed. However, it was also demonstrated that the deprotonated polyols can act as powerful ligands for metal ions.