The photochemistry of M(CO)6 and (n6-pyridne)Cr(CO)3 (M, Mo, or W) and related systems
Breheny, Ciara (1996) The photochemistry of M(CO)6 and (n6-pyridne)Cr(CO)3 (M, Mo, or W) and related systems. PhD thesis, Dublin City University.
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The reaction of M(CO)s(solvent) with CO has been investigated in a range of alkane solvents, where M = Cr, Mo, or W The kinetic and activation parameters have been determined for Reaction 1: M(CO)5(S) + CO - > M(CO)6 Reaction 1
For chromium hexacarbonyl the AH* is constant (22 ± 2 kJ mol'1), the rate of Reaction 1 increases with the lengthening as the alkane chain, this is reflected in the AS* term, which becomes less negative as the alkane chain length mcreases For the larger metals the variation in the kinetic and activation parameters is less significant. The kinetic and activation parameters indicate that solvent displacement by CO involves an interchange mechanism for the Cr(CO)6 system, while for the Mo(CO)6 and W(CO)e systems the mechanism is more associative in character.
The photochemistry of (rj6-2,6-X2C5H3N)Cr(CO)3 was investigated both in low-temperature matrices (X = H, (CH3)3Si) and in room temperature solution ( X = H, CH3, or (CH3)3Si) Room temperature photolysis (k^c > 410 nm) of (t]6-pyndine)Cr(CO)3 in CO saturated methanol yielded ('n1-pyndme)Cr(CO)5 which subsequently formed Cr(CO)6 in a secondary photochemical process The efficiency of this reaction is reduced in cyclohexane, or when X = CH3 rather than X = H Photolysis in low temperature matrices resulted in an i f to r\l haptotropic rearrangement of pyridine (Acxc - 460 nm, X = H) Visible irradiation in a CO-doped methane matrix produced (t|1-pyndine)Cr(CO)5 while in N2 matrix/ac-(Vi>yndine)(N2)2Cr(CO)3 is formed Irradiation with A*« =
308 nm produced both the nng-slip product and also the CO-loss product (rj6-pyndine)Cr(CO) 2 Time resolved infrared spectroscopy in cyclohexane revealed only the CO-loss product = 308 nm, X = H) The apparent difference in roomtemperature and low-temperature photochemistry is explamed by the rapid regeneration of the parent species from the r| ^intermediate This explanation is supported by the laser flash photolysis experiments = 355 nm) in CO-saturated cyclohexane (S), where the recovery of the (r|6-pyndine)Cr(CO) 3 absorption follows a biphasic time profile, whereby the faster process was assigned to the r|l to rf rearrangement and the slower to the reaction of (r|6-pyndine)Cr(CO)2(S) with CO When the matrix photochemistry was investigated in a CO-doped matrix, where X = (CH3)3Si, no pentacarbonyl or hexacarbonyl species were obtained Both the nng-slip and the CO-loss product were formed following irradiation with = 308 nm, and both were sensitive to photoreversal with white light, which may explain the lack of photoproducts following long wavelength photolysis.
Crystals of (ti6-2,6-X2C5H3N)Cr(CO)3 (X - H, CH3, or (CH3)3Si) were characterised by X-Ray diffraction. When X = H or CH3 an eclipsed conformation is adopted, where the carbonyl ligands eclipse the carbon atoms. When X = (CH3)3Si the more common conformation where the carbonyl groups are in a staggered conformation and eclipse the ring bond centres is adopted. In all the compounds the chromium atom is located directly below the arene ring, which is essentially planar.
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