METAL-BINDING OF POLYALCOHOLS .2. (ALL-CIS-1,3,5-TRIAMINOCYCLOHEXANE)COBALT(III), A FACIALLY COORDINATING, DIAMAGNETIC PROBE EXEMPLIFIED BYITS COMPLEX-FORMATION WITH CIS-INOSITOL

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(ta ch)3+ unit (tach = all-cis-1,3,5-triaminocyclohexane) have been examin ed. Reaction with cis-inositol resulted in the formation of [Co(tach)( H-1ino)]2+, with taino in either [(tach)Co(mu-OH)2(mu-NO,)Co(tach)]3or [Co(taino)2]2+. The crystal structures of [Co(tach)(H-1-ino)][NO3]2 1, [(tach)Co(mu-OH)2(mu-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 gro up of cis-inositol in a syn-1,3,5-triaxial arrangement. In aqueous sol ution, [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 potentiome tric measurements (0.1 mol dm-3 KNO3, 25-degrees-C). In addition, an i somerization reaction [binding of the Co(tach)3+ unit to three adjacen t 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 indi vidual microscopic acidity constants of the two isomers were elucidate d by H-1 NMR-monitored pD titrations. Molecular mechanics calculations were used to rationalize differences in energy between the two isomer ic forms. The rather weak binding of neutral polyalcohols to metal ion s was confirmed. However, it was also demonstrated that the deprotonat ed polyols can act as powerful ligands for metal ions.