Abstract:The cobalt(II) cyclidene complexes are known to bind dioxygen reversibly under ambient conditions; however, the lifetime of the dioxygen adduct is limited by autoxidation. This paper describes a study of the autoxidation reactions of the cobalt(II) cyclidenes in nonaqueous media. The kinetics of the autoxidation reaction of the cobalt(II) complex are dependent on the nature of the ligand substituents and dioxygen concentration and on the nature and concentration of added base. Mechanistic details have been exp… Show more
“…Autoxidation. Previous studies on the kinetics and mechanism of autoxidation of cobalt(II) cyclidene−dioxygen adduct , revealed a conjugate base mechanism in which deprotonation may occur either in the rate determining step or in a preequilibrium step. In previous studies, substitution for relatively acidic protons at the R 2 and R 3 positions had given orders or magnitude decreases in rates of autoxidation, but the same conjugate mechanism persisted, albeit at much retarded rates.…”
Section: Resultsmentioning
confidence: 99%
“…Replacement of the R 3 methyl group by phenyl results in a 20-fold retardation of the rate of autoxidation. However, the complexes do still undergo autoxidation, and the rates respond to base in a manner suggestive of the conjugate base mechanism . The methyl groups at the R 4 positions are common to all structures and the most likely sites for deprotonation in those cases where NH and ionizable R 3 groups have been eliminated.…”
Section: Introductionmentioning
confidence: 99%
“…Earlier studies have provided vital mechanistic insight into the autoxidation of cobalt(II) cyclidenes …”
A major advance has been made in the incremental
molecular design of long-lived cobalt(II) dioxygen
carriers. Preceding mechanistic studies revealed that ionizable
methyl groups trigger the autoxidation of the
O2
adduct of the cobalt(II) cyclidene. Two members of a new
family of unsubstituted (no methyl groups) lacunar
cyclidene dioxygen carriers have been prepared in an eight-step
synthesis, and a complex with a hexamethylene
bridge has been structurally characterized. In contrast to
previously studied cyclidenes, these materials bear no
substituents on the chelated macrocyclic platform. As anticipated,
the rates of autoxidation of these unsubstituted
cyclidene complexes were found to be 5−8 times slower than those for
the most stable previously known cyclidene
derivatives. Because of the absence of Me−Me vicinal repulsion,
the C6 bridge assumes a zig-zag conformation
directly across the cavity. The accompanying, relatively low,
dioxygen affinity is explained on the basis of electronic
and steric factors. The rates of dioxygen binding to these newly
prepared cobalt(II) unsubstituted cyclidenes are fast
and approximately equal to the corresponding values for their
Me-substituted analogs. Consequently, differences in
dissociation rates are responsible for the differences in
O2 affinities. This is a clear example of an unusual
steric
effect for O2 adducts.
“…Autoxidation. Previous studies on the kinetics and mechanism of autoxidation of cobalt(II) cyclidene−dioxygen adduct , revealed a conjugate base mechanism in which deprotonation may occur either in the rate determining step or in a preequilibrium step. In previous studies, substitution for relatively acidic protons at the R 2 and R 3 positions had given orders or magnitude decreases in rates of autoxidation, but the same conjugate mechanism persisted, albeit at much retarded rates.…”
Section: Resultsmentioning
confidence: 99%
“…Replacement of the R 3 methyl group by phenyl results in a 20-fold retardation of the rate of autoxidation. However, the complexes do still undergo autoxidation, and the rates respond to base in a manner suggestive of the conjugate base mechanism . The methyl groups at the R 4 positions are common to all structures and the most likely sites for deprotonation in those cases where NH and ionizable R 3 groups have been eliminated.…”
Section: Introductionmentioning
confidence: 99%
“…Earlier studies have provided vital mechanistic insight into the autoxidation of cobalt(II) cyclidenes …”
A major advance has been made in the incremental
molecular design of long-lived cobalt(II) dioxygen
carriers. Preceding mechanistic studies revealed that ionizable
methyl groups trigger the autoxidation of the
O2
adduct of the cobalt(II) cyclidene. Two members of a new
family of unsubstituted (no methyl groups) lacunar
cyclidene dioxygen carriers have been prepared in an eight-step
synthesis, and a complex with a hexamethylene
bridge has been structurally characterized. In contrast to
previously studied cyclidenes, these materials bear no
substituents on the chelated macrocyclic platform. As anticipated,
the rates of autoxidation of these unsubstituted
cyclidene complexes were found to be 5−8 times slower than those for
the most stable previously known cyclidene
derivatives. Because of the absence of Me−Me vicinal repulsion,
the C6 bridge assumes a zig-zag conformation
directly across the cavity. The accompanying, relatively low,
dioxygen affinity is explained on the basis of electronic
and steric factors. The rates of dioxygen binding to these newly
prepared cobalt(II) unsubstituted cyclidenes are fast
and approximately equal to the corresponding values for their
Me-substituted analogs. Consequently, differences in
dissociation rates are responsible for the differences in
O2 affinities. This is a clear example of an unusual
steric
effect for O2 adducts.
“…Many of those reactions are known (32)(33)(34) and are initiated by protons contained in the solvent (Scheme 7, (12) and (13)), by the solvent itself (Scheme 7, (14)) or by autooxidation of the adduct (Scheme 7, (15) and (16)) (34). The first studies initiated by our industrial partner (5,6), showed that when no oxygenated complex remained in solution, the ligand would still be intact.…”
“…These totally synthetic materials provide an almost ideal environment for dioxygen binding to the central atoms, with an additional advantage in the ease with which structural modifications may be carried out on the ligand. This, in turn, allows systematic studies of the effect of structural modifications on dioxygen affinities, rates of binding, dissociation, and autoxidation, and relative efficacies of different conformers. − 1 Planar (a) and three-dimensional (b) representations of cyclidene complexes. These complexes are denoted below as M(R 1 R 2 R 3 R 4 cyclidene), where M = Ni or Co.…”
A new type of isomerism has been detected in the cyclidene family of lacunar dioxygen carriers, providing an additional structural variable for the control of their oxygen affinity. In those rare complexes that do not have methyl substituents on the primary macrocycle, NMR and X-ray crystallographic data indicate that, in addition to their usual cis orientation, the bridges can also adopt a trans orientation. In the crystal structure of [Co(C(8)MeHH[16]cyclidene)](PF(6))(2).3CH(3)OH, the bridge has this trans orientation with one end in the "lid-on" configuration while the other end is "lid-off". The trans orientation of the bridge is identified as the principal cause of the decreased dioxygen affinity of such unsubstituted cyclidenes.
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