Excited state reactions with Lewis bases

Reactions with Lewis Bases to Give Excited-State Complex, 84... 

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

In nonaqueous solvents, such treatment with dry hydrohalic acids is the only way to cleave the /r-oxo dimer nonoxidatively. However, the /r-oxo dimers of water-soluble porphyrins are readily cleaved by Lewis bases such as hydroxide, imidazole, histidine, and pyridine. Both the equilibria and kinetics of such reactions have been reported. In addition, /r-oxo dimers of water-insoluble Fe porphyrins in dichloromethane can be oxidatively cleaved to yield PFe F2 and (probably) an Fe species. Studies of the picosecond decay of the excited state of (TPPFe)20 in benzene following a 532- or 355-nm 25-ps pulse suggest that the intermediate state is a photodissociated pair, (TPP -)Fe + TPPFe — (0 ), and a small amount of disproportionation reaction products, TPPFe + TPPFe" = O. ... 

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