Linkage Isomerization via Geminate Cage or Biomolecular Mechanisms: Time-Resolved Investigations of an Organometallic Photochrome
Edwin J. Heilweil, Kristy M. Dewitt, Tung T. To, Theodore J. Burkey
A metal center reversibly migrates from a pyridinyl group to a nitrile group in the light driven linkage isomerization of dicarbonyl(3-cyanomethylpyridine-N3)(eta-5-methylcyclopentadienyl)-manganese (4) to dicarbonyl(3-cyanomethylpyridine-CN)(eta-5-methylcyclopentadienyl)-manganese (5). We previously reported that a substantial part of the conversion between the isomers in benzene must be bimolecular reaction between a solvent coordinated dicarbonyl(eta-5-methylcyclopentadienyl)manganese (3). In this study, the path between 4 and 5 is followed by time resolved infrared spectroscopy (TRIR) on the picosecond to microsecond timescales. Assignments of transients are identified by comparison with those found in model reactions reported in the literature. Within one microsecond after photolysis of 4 in isooctane, no 5 is observed even in the presence of 9 mM 3-cyanomethylpyridine. Instead, the solvent coordinated 3 and a minor transient are observed within 25 ps after irradiation. The minor transient, similar to one previously reported for a triplet dicarbonyl(eta-5-cyclopentadienyl)manganese, decays in less than 200 ps as 3 continues to grow. The formation of 5 was observed 10-50 microseconds after irradiation of 4 but only in the presence of 9 mM 3-cyanomethylpyridine. Within the limits of detection, these results indicate the conversion of 4 to 5 occurs exclusively via a bimolecular reaction of 3-cyanomethylpyridine with solvent coordinated 3 and not a geminate cage reaction between 3-cyanomethylpyridine and the dicarbonyl(eta-5-methylcyclopentadienyl)manganese fragment.