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

Abstract

The synthesis, properties and crystal structure of the cage complex (1-hydroxy-8-methyl-3,6,10,13,15,18-hexaazabicyclo[6.6.5] nonadecane )cobalt(III) chloride hydrate ([Co( Me,OH-absar )]Cl3.H2O) are reported. The mechanism of the formation of this contracted cavity cage from a nitro-capped hexaazabicycloicosane type cage has been investigated. Treatment of (1- methyl-8-nitro-3,6,10,13,16,19-hexaazabicyclo[6.6.6] icosane )cobalt(III) chloride ([Co(Me,NO2- sar )]3+) with excess base in aqueous solution leads initially to rapid (t½. < 1 ms) and reversible deprotonation of one coordinated secondary amine. This species undergoes a retro-Mannich type reaction and imine hydrolysis (t½ ≈ 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, resulting from the loss of a methylene unit from one of the arms of the cap. Heating either isomer in aqueous solution gives the new cage compound with the contracted cap. It is postulated that this occurs through a Nef reaction, resulting in the formation of a ketone which then condenses with the coordinated primary amine. A comparison with the corresponding bicycloicosane analogue indicates a reduced chromophoric cavity size 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 transitions are both shifted to higher energy, corresponding to a stronger ligand field.

Cobalt Cage Complexes with N3S3 Donor Sets and Differing Cavity Sizes: A Novel Macrobicyclic Cage with a Contracted Cap

“…This is a consequence of the higher ligand field imposed by the tighter cavity. A similar (but smaller) hypsochromic shift has been reported for the corresponding N 6 cages. ,

3 Absorption spectra of [Co III (Me 2 -N 3 S 3 sar)] 3+ () and [Co III (Me 2 -N 3 S 3 absar)] 3+ (- - -) in H 2 O.

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“…One notable feature that is present in all three structurally characterized “absar” based cage complexes is the lel 2 ob conformation of the three S(N)−C−C−N five-membered chelate rings. , Furthermore, in each case, the chelate ring having the ob conformation is not in the strand that is missing the methylene unit.…”

“…It seems likely that one of the N−H amines is deprotonated, and that there is rapid exchange of the remaining two protons between the three equivalent sites. When base was added to solutions of the corresponding N 6 cage complexes, an intense purple color was also observed, , although in those cases the resonances in the NMR spectra did not show significant broadening. It is possible that the increased accessibility of the cobalt(II) state in the presence of the more polarizable thioether donors leads to some net reduction and rapid electron transfer between traces of the low-spin cobalt(II) complex and the cobalt(III) complex.…”

“…However, the contracted cavity cages were only a minor product of the diazotization reaction, and their separation from the other complexes was tedious. Attempts to prepare a contracted cavity cage by analogy with the base-catalyzed reaction observed for the [Co(Me,NO 2 -sar)] 3+ complex did not yield the expected contracted cavity cage, but instead it gave rise to a novel pendant arm macrocycle resulting from a dehydrogenation reaction …”