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

Citation
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
Citations number
49
Categorie Soggetti
Chemistry
ISSN journal
00027863
Volume
119
Issue
18
Year of publication
1997
Pages
4160 - 4171
Database
ISI
SICI code
0002-7863(1997)119:18<4160:UC-ANF>2.0.ZU;2-Z
Abstract
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.