THE REACTION OF A NITRO-CAPPED COBALT(III) CAGE COMPLEX WITH BASE - THE CRYSTAL-STRUCTURE OF A CONTRACTED CAGE COMPLEX, AND THE MECHANISM OF ITS FORMATION

The synthesis, properties and crystal structure of the cage complex ,1 5,18-hexaazabicyclo[6.6.5]nonadecane)cobalt(III) chloride hydrate ([Co (Me,OH-absar)] C13.H2O) are reported. The mechanism of the formation o f this contracted cavity cage from a nitro-capped hexaazabicycloicosan e type cage has been investigated. Treatment of ,13,16,19-hexaazabicyc lo[6.6.6]icosane)cobalt(III) chloride ([Co(Me,NO2-sar)] 3+) with exces s base in aqueous solution leads initially to rapid (t1/2 < 1 ms) and reversible deprotonation of one coordinated secondary amine. This spec ies undergoes a retro-Mannich type reaction and imine hydrolysis (t1/2 almost-equal-to 90 s). Quenching the reaction with acid gives rise to a pair of isomeric intermediate species which have been isolated and characterized. They have a pendant arm macrocyclic structure, resultin g from the loss of a methylene unit from one of the arms of the cap. H eating either isomer in aqueous solution gives the new cage compound w ith the contracted cap. It is postulated that this occurs through a Ne f reaction, resulting in the formation of a ketone which then condense s with the coordinated primary amine. A comparison with the correspond ing bicycloicosane analogue indicates a reduced chromophoric cavity si ze for the contracted cage. The reduction potential of the cobalt(III) /cobalt(II) couple is 170 mV more negative for the smaller cage, and, in the electronic spectrum of the cobalt(III) complex, the d-d transit ions are both shifted to higher energy, corresponding to a stronger li gand field.