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-hydroxy-8-methyl-3,6,10,13,15,18-hexaazabicyclo[6.6.5]nonadecane)cobalt(III) chloride hydrate ([Co(Me,OH-absar)] C13.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 (t1/2 < 1 ms) and reversible deprotonation of one coordinated secondary amine. This species 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, 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

Electrochemistry of macrobicyclic mixed nitrogen-sulfur donor complexes of cobalt(III)

The redox chemistry of the mixed nitrogen-sulfur donor cobalt(III) complexes with the macrobicyclic ligands 8-methyl-6, 10, 19-trithia-l, 3, 14, 16-tetraazabicyclo[6.6.6]icosane (azacapten), l-methyl-8-nitro-3, 13, 16-trithia-6, 10, 19-triazabicyclo[6.6.6]icosane ((NO)capten), and 8-ammonio-1-methyl-3, 13, 16-trithia-6, 10, 19-triazabicyclo[6.6.6]icosane ((NH )capten) qnd of the hexadentate precursor ligand 5-(4-amino-2-thiabutyl)-5-methyl-3,7-dithia-l,9-nonanediamine (ten) has been investigated in aqueous and aprotic solutions. The macrobicyclic complexes exhibit a chemically and electrochemically reversible Co(III)/Co(II) couple near 0 V in aqueous (NHE) and aprotic solutions (Ag/AgCl) and a chemically irreversible but elctrochemically reversible 1-e couple near-1 V, nominally Co(II)/Co(I). In addition, a further multielectron couple is observed for the azacapten complex at more negative potentials. No evidence for oxidation processes was forthcoming. The low-spin d cobalt(II) complex of (NH )capten has been characterized by ESR spectroscopy at 4.2 K, and the nominally d cobalt(I) complex, at 100 K. The former compound exhibits g values at 1.94, 2.21, and 4.22, while the latter compound shows an eight-line pattern at g = 2.17 (A = 67 G) consistent with its formulation as d Co(I) and not as a d Co(II)-S radical state

Reactivity of coordinated nitriles

The base hydrolysis of coordinated acrylonitrile in [ (NH₃)₅CoN=CCH=CH₂]³⁺ to the acrylamide complex in carbonate buffers obeys the rate law k(obsd) = k(OH)OH⁻] + k(c)[C0₃²⁻] (k(OH) = 35 M⁻¹ s⁻¹; k(c) = 1 M⁻¹ s⁻¹; 25 °C, μ = l.O), and ¹⁸O tracer studies indicate the mechanism of hydrolsis by carbonate ion to involve direct nucleophilic attack at the nitrile group by C0₃²⁻ with subsequent elimination of C0₂. Coordination of malononitrile, cyanoacetic acid, and ethyl cyanoacetate to the (NH₃)₅CO(III) moiety leads to greatly increased acidity of the methylene protons (pK(a) of [(NH₃)₅CoN=CCH₂C=N]³⁺ is 5.7 ie 0.1 ( μ = 1.0,25 °C) compared with 11.3 for uncoordinated malononitrile). In basic aqueous solutions, these complexes equilibrate rapidly between the original complexes and the complexes containing the deprotonated activated methylene group. The protonated complexes then hydrolyze to the coordinated amides with both hydroxide ion and water acting as nucleophiles while the coordinated carbanions undergo an intramolecular electron transfer of a single electron to yield Co(II) and the ligand radical. The C-deprotonated complexes also function as nucleophiles for appropriate substrates (such as methyl iodide, methyl pyruvate, or bromine) to form the corresponding substituted species. In nonaqueous media, generation of the ligand radical initiates polymerizations of monomers methacrylate, styrene, acrylonitrile, and methacrylonitrile by a radical (rather than an ionic) mechanism