Resonance Raman spectra of dioxygen adducts of pillared dicobalt cofacial diporphyrins

Abstract

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.

Dioxygen Reduction at Dicobalt Complexes of a Schiff Base Calixpyrrole Ligand

“…The isotropic g value and the magnitude of the hyperfine splitting obtained by simulation of the observed spectra are comparable to those of diporphyrinic analogues, [4,10,11] and the addition of pyridine to the EPR sample results in a spectrum that supports axial pyridine coordination in 2-py 2 + . However, quantitative calibration by double integration of the EPR signal of 2 + to a [Cu(acac) 2 ] standard reveals that this former complex is only a minor component of the mixture (< 10 %); presumably, 2 + is formed by initial oxidation of 1 to 1 + followed by dioxygen uptake.…”

“…[4,10,11] Plausible mechanistic pathways for both two-and four-electron reductions of dioxygen have also been elucidated. [1,2,5] [1,5] Significantly, neither of these modes has been corroborated by solidstate structural data for any dioxygen-containing cobalt diporphyrin complexes or porphyrinic analogues.…”

Metalloporphyrin Catalysts of Oxygen Reduction

“…Resonance Raman studies of dioxygenated cofacial porphyrins similar to (FTF4)Co 2 and of [(DPA)Co 2 O 2 ] [Proniewicz et al, 1989] …”

On the mechanism of dioxygen electroreduction at transition metal meso‐tetrakis pyridyl porphyrins—a comparative spectroelectrochemical study

“…The dioxygen stretching mode was found at 1144 cm À1 by Odo et al in the case of a picket-fencetype Co porphyrin [86]. Pillared dicobalt cofacial diporphyrins have been studied by Proniewicz et al [57]. In the case of a dioxygen-bridged dimer the Co-O mode was found at 625 cm À1 and the dioxygen mode at 1098 cm À1 ; the none-bridging dioxygencobalt adduct showed these bands at 514 and 1139 cm À1 .…”

“…Electron-donating or -withdrawing properties of peripheral substituents present in the up to four meso positions of the chelate skeleton can affect the interaction significantly. The molecular structure of the metal chelates, in particular this interaction, has been studied especially with resonance Raman spectroscopy [54][55][56][57][58][59][60][61][62].…”