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{η 5 -C 5 H 4 CH 2 COR}(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 bands 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.