REVERSIBLE OXYGEN CARRIERS - SYNTHESIS AND LOW-TEMPERATURE (-171 DEGREE) STRUCTURE OF AN UNSTABLE MONOMERIC DIOXYGEN ADDUCT OF N,N'-(1,1,2,2-TETRAMETHYL)-ETHYLENEBIS(3-FLUOROSALICYLIDENIMINATO)-(1-METHYLIMIDAZOLE)COBALT(II), CO(3-F-SALTMEN)(1-ME-IMID)(O2).2(CH3)2C

The structure at -171.degree. of N,N''-(1,1,2,2-tetramethyl)ethylenebis(3-fluorosalicylideniminato)(1-methylimidazole)superoxocobalt(III) diacetone solvate, Co(3-F-Saltmen)(1-Me-Imid)(O2).cntdot.2(CH3)2CO, was determined from 3-dimensional X-ray data collected by counter methods and was refined on F2 using 6078 reflections, including those with negative intensities. The conventional RF factor based on the portion of the data with F2 greater than 3.sigma.(F2) is 6.0%. The monomeric adduct (Co:O2 = 1:1) of Co(II) crystallizes in the monoclinic space group C52h-P21/c (No. 14), with 4 formula units in a cell with the low-temperature dimensions a = 11.934 (6) .ANG., b = 13.864 (5) .ANG., c = 18.018 (8) .ANG. and .beta. = 92.35 (2).degree.. The superoxide ligand, O2-, is bonded end-on to the Co atom of the chelate, with a Co.sbd.O.sbd.O angle of 117.4 (2).degree. and an O.sbd.O bond length of 1.302 (3) .ANG.. The dioxygen group lies nearly in the plane defined by the methylimidazole trans to it. The Co.sbd.O (dioxygen) and Co.sbd.N (imidazole) bond distances are 1.881 (2) and 2.004 (3) .ANG., respectively. The other coordination bonds average 1.893 (Co.sbd.N) and 1.901 .ANG. (Co.sbd.O). The molecular packing in the vicinity of the coordinated dioxygen ligand is similar to that found in the hydrophobic distal imidazole pocket of myoglobin. Crystals of the Co.sbd.O adduct are not stable at ambient conditions and evolution of O2 may be readily observed, as the crystals change from dark burgundy red to golden yellow upon decomposition.