Vitamin B12-catalyzed reductive dechlorination of perchloroethylene (PCE) and trichloroethylene (TCE) is a potential strategy for cleanup of polluted environments. Presented are crystal structures of vinylcobalamin 2 and cis-chlorovinylcobalamin 1. They show a strong resistance toward photolysis. Reduction of 2 is difficult, but reduction of 1 occurs readily and produces 2. The mechanism of this latter reaction involves acetylene as an intermediate. These and other findings are discussed in the context of environmental studies on B12-catalyzed dechlorination of PCE and TCE and investigations of the haloalkene reductive dehalogenases that catalyze similar reactions.

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Two, bifunctional side-chain cyclopentadienylmanganese tricarbonyl complexes, 7 (pyridine and ketone side chain) and 14 (thioamide and pyridine side chain), were prepared and converted to chelates following CO dissociation by UV irradiation. Both pyridine (8) and carbonyl (9) chelates are observed following irradiation of 7. In contrast, only thioamide chelate ( 16) is observed following irradiation of 14 even though a pyridine group was available. Visible irradiation isomerizes the pyridine chelate 8 to the carbonyl chelate 9, which thermally isomerizes back to 8 at 25 °C in a few minutes, demonstrating a photochromic response from a cyclopentadienyl-manganese complex based on a linkage isomerization of a tethered functional group. DFT calculations predicted that the activation enthalpy of thermal linkage isomerization would be 20.8 kcal/mol and that the mechanism is likely an associative process that does not involve a simple dissociation of the manganese bond to the side-chain ketone. The DFT calculations were supported by subsequent dynamic NMR experiments that yielded an activation enthalpy and entropy of 21.4 ( 0.8 kcal/mol and 3.5 ( 0.1 eu, respectively. The studies indicate that a compound with a tethered, coordinated functional group, which is otherwise not labile, can isomerize by a low-energy pathway if there is an appropriate "conduit" to another functional group with a stronger metal-ligand bond. Thus, the preparation of photochromic organometallic complexes based on linkage isomerization will require a bridge that inhibits an associative walk between functional groups if they are to be bistable.