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

Abstract

The photochemistry of (η6-2,6-X2C5H3N)Cr(CO)3 was investigated both in low-temperature matrices (X = H or (CH3)3Si) and in room-temperature solution (X = H, CH3, or (CH3)3 Si). Room-temperature photolysis (λexc > 410 nm) in CO-saturated methanol or acetonitrile produced (η1-2,6-X2C5H3N)Cr(CO)5 which subsequently formed Cr(CO)6 in a secondary photochemical process (X = H or CH3). The efficiency of pentacarbonyl formation is lower in CO-saturated cyclohexane and follows the order X = H > X = CH3. Photolysis in low-temperature matrices resulted in an η6 to η1 pyridine ring-slippage (λexc = 460 nm; X = H). Visible irradiation in a CO-doped methane matrix produced (η1-C5H5N)Cr(CO)5, while in an N2 matrix fac-(η1-C5H5N)(N2)2Cr(CO)3 is formed. Irradiation with λexc = 308 nm produced both the ring-slippage product and also the CO-loss product (η6-C5H5)Cr(CO)2, which in a N2 matrix is trapped as (η6- C5H5N)Cr(CO)2(N2). Time-resolved infrared spectroscopy in cyclohexane revealed only the CO-loss product (λexc = 308 nm; X = H). The apparent difference in room-temperature and low-temperature photochemistry is explained by a rapid regeneration of (η6-C5H5N)Cr(CO)3 from the η1-intermediate. This explanation was supported by laser flash photolysis experiments (λexc = 355 nm) in CO-saturated cyclohexane (Sol), where the recovery of the (η6-C5H5N)Cr(CO)3 absorption follows a biphasic time profile, whereby the faster process was assigned to the η1 to η6 transformation and the slower to the reaction of (η6-C5H5N)Cr(CO)2(Sol) with CO. Crystals of (η6-2,6-(CH3)2C5H3N)Cr(CO)3 and (η6-2,6-((CH3)3Si)2C5H3N)Cr(CO)3 were characterized by X-ray diffraction.