The resonance Raman spectra of dioxygen adducts of anthracene pillared cofacial dicobalt(II) diporphyrin (Co-Co complex) have been measured in methylene chloride at ~190 K (457.9-nm excitation). In the absence of a base, the Co-Co complex forms a superoxo adduct in which dioxygen is bridged between the two porphyrin planes intramolecularly. This adduct exhibits the (02) and vs(Co-0) at 1081 and 628 cm"1, respectively. Further support of this structure is provided by an ESR spectrum which exhibits a symmetrical 15-peak hyperfine structure. In the presence of a large base such as 4-(dimethylamino)pyridine, the Co-Co complex forms a mixture of the bridging and nonbridging dioxygen adducts in which the base ligands are coordinated to the Co atoms from outside the interporphyrin cavity. The former exhibits the v(02) and vs(Co-0) at 1098 and 625 cm"1, respectively, of the bridging adduct whereas the latter shows the v(02) and v(Co-02) at 1139 and 514 cm"1, respectively, which are typical of nonbridging, six-coordinate dioxygen adducts. Upon raising the temperature, the latter decomposes, and only the former remains at room temperature. If the Co-Co complex solution containing a small base (e.g., -picoline) is oxygenated, only the bands characteristic of nonbridging adducts are observed at 1138 (v(02)) and 514 cm"1 (v(Co-02)). On the other hand, the bands characteristic of both bridging and nonbridging dioxygen adducts are observed when the Co-Co complex is oxygenated prior to the addition of the base ligand. These results suggest that a small base can enter inside the interporphyrin cavity, thus blocking the formation of the Co-O-O-Co bridge. Using such spectral patterns as the criteria, we classify 4-phenylpyridine, 4-(dimethylamino)pyridine, 3,5-lutidine, and 3,5-dichloropyridine as "large bases" and pyridine, -picoline, 4-ethylpyridine, and 3-ethyl-4-methylpyridine as "small bases". Model building studies show that even "small bases" cannot coordinate to the Co atom from inside the interporphyrin cavity unless the two porphyrin rings are slipped laterally and tilted to take an "open-end" configuration. The subtle difference between 4-(dimethylamino)pyridine (large base) and 4-ethylpyridine (small base) may be explained in terms of steric repulsion between the bifurcated N(CH3)2 group and the porphyrin plane that exists only in the former.
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