UNSUBSTITUTED CYCLIDENES - A NOVEL FAMILY OF LACUNAR DIOXYGEN CARRIERS WITH ENHANCED STABILITY TOWARD AUTOXIDATION - SYNTHESIS, CHARACTERIZATION, AND A REPRESENTATIVE X-RAY STRUCTURE

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.