Photochemistry of (η6-2,6-X2C5H3N)Cr(CO)3 (X = H, CH3, (CH3)3Si). First Example of a Photoinduced Ring-Slip at an (η6-Arene)M(CO)3 Center. Molecular Structures of (η6-2,6-(CH3)2C5H3N)Cr(CO)3 and (η6-2,6-((CH3)3Si)2C5H3N)Cr(CO)3

The photochemistry of (η 6 -2,6-X 2 C 5 H 3 N)Cr(CO) 3 was investigated both in low-temperature matrices (X = H or (CH 3 ) 3 Si) and in room-temperature solution (X = H, CH 3, or (CH 3 ) 3 Si). Room-temperature photolysis (λ exc > 410 nm) in CO-saturated methanol or acetonitrile produced (η 1 -2,6-X 2 C 5 H 3 N)Cr(CO) 5 which subsequently formed Cr(CO) 6 in a secondary photochemical process (X = H or CH 3 ). The efficiency of pentacarbonyl formation is lower in CO-saturated cyclohexane and follows the order X = H > X = CH 3 . Photolysis in low-temperature matrices resulted in an η 6 to η 1 pyridine ring-slippage (λ exc = 460 nm; X = H). Visible irradiation in a CO-doped methane matrix produced (η 1 -C 5 H 5 N)Cr(CO) 5, while in an N 2 matrix fac -(η 1 -C 5 H 5 N)(N 2 ) 2 Cr(CO) 3 is formed. Irradiation with λ exc = 308 nm produced both the ring-slippage product and also the CO-loss product (η 6 -C 5 H 5 )Cr(CO) 2, which in a N 2 matrix is trapped as (η 6 - C 5 H 5 N)Cr(CO) 2 (N 2 ). Time-resolved infrared spectroscopy in cyclohexane revealed only the CO-loss product (λ exc = 308 nm; X = H). The apparent difference in room-temperature and low-temperature photochemistry is explained by a rapid regeneration of (η 6 -C 5 H 5 N)Cr(CO) 3 from the η 1 -intermediate. This explanation was supported by laser flash photolysis experiments (λ exc = 355 nm) in CO-saturated cyclohexane (Sol), where the recovery of the (η 6 -C 5 H 5 N)Cr(CO) 3 absorption follows a biphasic time profile, whereby the faster process was assigned to the η 1 to η 6 transformation and the slower to the reaction of (η 6 -C 5 H 5 N)Cr(CO) 2 (Sol) with CO. Crystals of (η 6 -2,6-(CH 3 ) 2 C 5 H 3 N)Cr(CO) 3 and (η 6 -2,6-((CH 3 ) 3 Si) 2 C 5 H 3 N)Cr(CO) 3 were characterized by X-ray diffraction.