J . Am. Chem. SOC.1983, 105, 2639-2641 Scheme 1 MgO
(CaO behaves
400-1000 'C
MgOX
vacuum
thermally activated
x: 0
similarly but not identically)
0
a possibility
0 II
0 7P light red
\
225 'CI-CO
$.:;\
i
\ no change
\\
heat or chemical reagents
0
0
P
a possibility as the monoanion also
various adsorbed conformers of C606J-* with varying degrees of magnetic interactions of I3C with the unpaired spin density P
yellow
Adsorption of C O onto thermally activated MgO or C a O also leads to the formation of several adsorbed, paramagnetic radical anions. Although these species may be colored, their concentrations are much lower than the dianion species discussed above, and therefore color is not indicative of their presence (2-10% of the adsorbed C O yield paramagnetic species). The formation of the anion radicals is preceded by formation of the colored dianaions. A variety of considerations, including ESR spectral evidence, spectral computer simulations, and chemical treatments, lead us to the conclusion that the most common radical anion is the C606,-. species, which is probably formed from C,O6,-. The surface adsorption state of this species is such that only two carbons strongly interact with the unpaired spin density. Upon treatment of this trianion radical with heat, oxygen (slow addition), CO,, H,O, HCl, or N H , it is converted to a different adsorbed state where different magnetic interactions occurs but in which no gross chemical changes have occurred (new bond formations or bond cleavages). It is also possible that some smaller ring species form. These ideas are illustrated in Scheme I. Apparently the added reagents adsorb on the MgO or CaO surface near the C606,-. species and cause a change in its conformational state, either by changing the electronic properties of the basic surface or simply by crowding the C6O6)-. species.
2639
Neither the dianions or trianions are affected by adding relatively nonreactive substances (at 25 "C) such as Hz, CH4, N,, C2H4,or C6H,CH3. In fact, the activated MgO can be placed in a bath of pure toluene followed by C O addition, and both colored dianions as well as paramagnetic trianions form without toluene interference. Experimental Section Materials. The preparation of MgO samples has been described p r e v i o ~ s l y . ~A* ~single ~ ~ source of calcium oxide was obtained from ROC/RIC listed as 99.99% pure. CO (Linde, 99.999%) was passed through a 77 K cold trap just prior to adsorption on the oxide samples. "CO (Prochem Isotopes, 98% I3CO,2% '*Co) was passed over a freshly regenerated BASF-R3-11 deoxygenation catalyst and then through a -196 OC trap just prior to adsorption. EPR Spectroscopy. Two spectrometers were used, a Bruker ER420/10V and a Varian V-4502. The Bruker instrument was used to obtain all "C data, and experimental parameters have been reported previously.28 Sample Preparations. The oxide samples were washed with boiling distilled water for 1 h, recovered by hot filtration, and dried in air at 110 OC for -6 h, and the samples were stored in vials awaiting thermal activation. Each oxide sample was chopped (not ground since grinding produced higher concentrations of paramagnetic defects) into small pieces
