Photochemical disproportionation dimers

Photochemical disproportionation of metal-metal bonded carbonyl dimers. A. E. Stiegman and D. R. Tyler, Coord. Chem. Rev., 1985, 63, 217 (59). 

The same substituted cations are obtained by the photochemical disproportionation of dimeric 7r-cyclopentadienylmolybdenum tricarbonyl, [CpMo(CO)3]2, with the following ligands PEt3, PPh3, PBu3n, and diphos U5)-... 

Lees, A. J. Luminescence Properties of Organometallic Complexes, Chem. Revs. 1987, 87,711-743. Stiegman, A. E. lyier, D. R. "Photochemical Disproportionation of Metal>Metal Bonded Carbonyl Dimers, Coord. Chem. Revs. 1985,63, 217-240. 

Studies on the photochemical reactions of dihydropyridines have proven to be interesting. There are a number of 1,4-dihydropyridines that are known to disproportionate when irradiated (equation 19) (B-76PH240). Analogous intramolecular reductions have also been observed by other workers (55JA447). In contrast to these results, the 1,4-dihydropyridine (59) rearranged to its 1,2-dihydro isomer (60). Further irradiation resulted in dimerization. Interestingly, the photodimer (61) cyclized to the cage compound (62). 

At first glance, all of the photochemical processes in Eqs. (8)-(10) are unique, with none showing any apparent stoichiometric relationship to the others. For example, the carbonylcobalt dimerization [Eq. (8)] and disproportionation [Eq. (9)] represent 1-electron oxidation processes of Co(CO)4 , whereas the formation of Co(CO)3L in Eq. (10) relates to a nonredox ligand substitution. The incorporation of the additive L into the carbonylmetal product, whether it be Co2(CO)6L2, Co(CO)3L2+, or Co(CO)3L in Eqs. (8), (9), and (10), respectively, is the sole feature that these photochemical processes have in common. Because the carbonylmetallates Co(CO)4 , V(CO)6-, and Mn(CO)5 are all thermally substitution-stable anions (52), the introduction of L into the carbonylmetallate moiety must occur in some reactive intermediate. 

Finally, care must be exercised in the photochemical activation of contact ion pairs to irradiate only the charge-transfer absorption bands, and not those of the (colored) products. For example, the irradiation of either Cp2Co+ Co(CO)4 or Q+ Co(CO)4 in the presence of PBu3 at wavelengths beyond 510 nm leads only to the dimeric Co2(CO)6(PBu3)2, despite the fact that the CT photochemistry of the same solution at wavelengths below 550 nm leads smoothly to only the disproportionation products. In fact, control experiments demonstrate that the carbonylcobalt dimer arises from a secondary process by the adventitious excitation of the disproportionation salt, namely,... 

This scheme of interrelated primary photochemical and subsequent radical reactions is comphcated by the back reaction of hydrogen atoms and hydroxyl radicals with formation of water (Fig. 7-16, reaction 2) or the dimerization of the latter with formation of hydrogen peroxide (Fig. 7-16, reaction 3). Furthermore, hydroxyl radicals are scavenged by hydroperoxyl radicals with formation of oxygen and water (Fig. 7-16, reaction 5) or by hydrogen peroxide to yield hydroperoxyl radicals and water (Fig. 7-16, reaction 4). In addition, hydroxymethyl radicals (HOCH ) formed by reaction 1 (Fig. 7-16) are able to dimerize with formation of 1,2-ethane-diole (Fig. 7-16, reaction 7) or they disproportionate to yield methanol and formaldehyde (Fig. 7-16, reaction 8). 

This reaction proceeds also photochemically as indicated by the concomitant spectral changes that are identical in both cases. When the irradiation is carried out in acidic solution (L = H20) comparable spectral variations are observed. Since the photolysis products are apparently stable under these conditions and interfering inner-filter effects are absent the photolysis can be performed to completion. The Os(III) dimer disappears with 4> = 8xl0 at lirr = 436nm. Disproportionations of this kind that take place as a consequence of the photochemical splitting of a metal-metal bond are an important reaction type in organo-metallic chemistry (17,18). 

The studies already described were done in relatively dilute solution (85 mM in P (OMe)3, however the photochemical studies of disproportionation were studied under relatively concentrated solution (1.6 M in P(OMe3)) as well.64 65 Under these conditions, disproportionation is observed to occur at a rate even faster than ligand substitution. The reason for this is that high concentrations were used to ensure that at least one P(OMe)3 was in the first solvation shell of the metal-metal-bonded dimer. In that case, reaction can occur prior to diffusion out of the solvent cage, as shown in Scheme 10.2. 

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