Ultrafast ring closure energetics and dynamics of cyclopentadienyl manganese tricarbonyl derivatives


Ring closure following flash photolysis in alkane solvents has been detected for several complexes in the series (η5-C5H4R)Mn(CO)3 where R = COCH3 (1), COCH2SCH3 (2), CO(CH2)2SCH3 (3), COCH2- OCH3 (4), (CH2)2CO2CH3 (5), and CH2CO2CH3 (6). In each case where ring closure occurs, a metal CO is ultimately substituted by the side chain functional group. Photoacoustic calorimetry studies reveal that ring closures occur with rate constants faster than 107 s-1 or between 106 and 107 s-1, or in some cases the ring closure is biphasic with rate constants in both ranges. The enthalpies of CO dissociation followed by ring closure for 2 and 3 are the same (12 kcal/mol) and more favorable than those for 4-6 (25-15 kcal/mol). Studies of 1-3 by transient picosecond to microsecond for infrared spectroscopy confirm biphasic dynamics 2 and 3: ring closure occurs immediately (k; > 5 x 109 s-1) and at slower rates (k = 108-106 s-1). We propose that some ring closure occurs before solvent coordination and that the remaining ring closure, resulting in the displacement of solvent, is much slower. The relationships of the rates and energetics of ring closure to structure and quantum yields are discussed.

Publication Title

Journal of the American Chemical Society