Engineering femtosecond organometallic chemistry: Photochemistry and dynamics of ultrafast chelation of cyclopentadienylmanganese tricarbonyl derivatives with pendant benzenecarbonyl and pyridinecarbonyl groups


The chelation following photodissociation of CO for cyclopentadienyl manganese tricarbonyl derivatives with a bifunctional side chain has been investigated. Previous studies show that steady-state irradiation of 1 (Mn{μ5-C5H4CH2COR}-(CO) 3, R = 2-pyridyl) leads to CO dissociation and formation of O-chelate 2 with smaller amounts of N-chelate 3. Subsequently, 2 rearranges thermally to 3. A new preparation for 1 is reported, while analogues 4 (R = phenyl) and 5 (R = 4-pyridyl) are prepared for the first time. Steady-state UV-vis, FTIR, and NMR studies of 4 and 5 in heptane demonstrate that O-chelates 6 and 7, respectively, are formed with the side-chain oxygen but decay on the minute time scale. The linkage isomerization of O-chelate 2 to 3 is faster than the decay observed for the O-chelate 6 (R = 2-pyridyl versus Ph), even in the presence of 0.1 M pyridine for the latter. Following irradiation of 4 during time-resolved infrared studies in heptane, ultrafast O-chelation is observed but not ultrafast solvent coordination. Ultrafast O-chelation is also observed for 5 along with an unidentified transient. Following irradiation of 1, ultrafast O-and N-chelation are observed, to the exclusion of ultrafast solvent coordination. This result suggests that chelate formation is a subpicosecond process and that both chelates are formed independently. A split in the otherwise degenerate stretching band s for 4 and 5 in FTIR spectra suggests that there is significant electronic communication between the side chain and the metal carbonyl groups. The results suggest that ultrafast chelation is favored by side-chain conformations that position a functional group near the metal center. © 2011 American Chemical Society.

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