UNSUBSTITUTED CYCLIDENES - A NOVEL FAMILY OF LACUNAR DIOXYGEN CARRIERS WITH ENHANCED STABILITY TOWARD AUTOXIDATION - SYNTHESIS, CHARACTERIZATION, AND A REPRESENTATIVE X-RAY STRUCTURE
Ag. Kolchinski et al., UNSUBSTITUTED CYCLIDENES - A NOVEL FAMILY OF LACUNAR DIOXYGEN CARRIERS WITH ENHANCED STABILITY TOWARD AUTOXIDATION - SYNTHESIS, CHARACTERIZATION, AND A REPRESENTATIVE X-RAY STRUCTURE, Journal of the American Chemical Society, 119(18), 1997, pp. 4160-4171
A major advance has been made in the incremental molecular design of l
ong-lived cobalt(II) dioxygen carriers. Preceding mechanistic studies
revealed that ionizable methyl groups trigger the autoxidation of the
O-2 adduct of the cobalt(rr) 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 he
xamethylene bridge has been structurally characterized. in contrast to
previously studied cyclidenes, these materials bear no substituents o
n the chelated macrocyclic platform. As anticipated, the rates of auto
xidation of these unsubstituted cyclidene complexes were found to be 5
-8 times slower than those for the most stable previously known cyclid
ene derivatives. Because of the absence of Me-Me vicinal repulsion, th
e CG bridge assumes a zig-zag conformation directly across the cavity.
The accompanying, relatively low, dioxygen affinity is explained on t
he basis of electronic and steric factors. The rates of dioxygen bindi
ng to these newly prepared cobalt(II) unsubstituted cyclidenes are fas
t and approximately equal to the corresponding values for their Me-sub
stituted analogs. Consequently, differences in dissociation rates are
responsible for the differences in O-2 affinities. This isa clear exam
ple of an unusual steric effect for O-2 adducts.