The Very Low Barrier of CO Site Exchange in Tricarbonyl(4-1,5-cyclooctadiene)iron: Picosecond Kinetics in Solution Investig
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- 作者：Friedrich-Wilhelm Grevels ; Klaus Kerpen ; Werner E. Klotzbü ; cher ; R. E. D. McClung ; Graham Russell ; Manuella Viotte ; and Kurt Schaffner
- 刊名：Journal of the American Chemical Society
- 出版年：1998
- 出版时间：October 14, 1998
- 年：1998
- 卷：120
- 期：40
- 页码：10423 - 10433
- 全文大小：247K
- 年卷期：v.120,no.40(October 14, 1998)
- ISSN：1520-5126
文摘
Photochemical experiments with Fe(CO)3(
4-1,5-cyclooctadiene) (1) in a 13CO matrix at 10 K,monitored by means of IR spectroscopy, indicate the generation of stereoselectively labeled Fe(CO)2(13CO)(4-1,5-cyclooctadiene) (1-1a), with 13CO in the apical position of the square-pyramidal coordination geometry.The spectral changes occurring upon annealing the matrix to 28 K reveal the thermally activated conversioninto a mixture of the two possible stereoisotopomers, the species with 13CO in a basal position (1-1b) becomingpredominant. These findings characterize the carbonyl ligand site exchange in complex 1 as a chemical reactioninvolving a very small barrier. The variable-temperature IR spectra of 1 in hydrocarbon solution exhibitbroadening and coalescence of bands in the 
(CO) region, which is interpreted in terms of a CO site exchangeoccurring in the picosecond time domain. The theoretical approach to the simulation of these spectral changesinvolves a transfer of transition dipole moment between the (CO) modes. On the basis of this approach, therates of CO site exchange at the various temperatures could be evaluated by line shape simulation. They werefound to range from 0.15 × 1012 s-1 at 133 K to 1.54 × 1012 s-1 at 293 K, yielding 
H
= 0.7 kcal·mol-1(Eyring plot) and Ea = 1.1 kcal·mol-1 (Arrhenius plot) for the activation barrier of the underlying process.
4-1,5-cyclooctadiene) (1) in a 13CO matrix at 10 K,monitored by means of IR spectroscopy, indicate the generation of stereoselectively labeled Fe(CO)2(13CO)(4-1,5-cyclooctadiene) (1-1a), with 13CO in the apical position of the square-pyramidal coordination geometry.The spectral changes occurring upon annealing the matrix to 28 K reveal the thermally activated conversioninto a mixture of the two possible stereoisotopomers, the species with 13CO in a basal position (1-1b) becomingpredominant. These findings characterize the carbonyl ligand site exchange in complex 1 as a chemical reactioninvolving a very small barrier. The variable-temperature IR spectra of 1 in hydrocarbon solution exhibitbroadening and coalescence of bands in the (CO) region, which is interpreted in terms of a CO site exchangeoccurring in the picosecond time domain. The theoretical approach to the simulation of these spectral changesinvolves a transfer of transition dipole moment between the (CO) modes. On the basis of this approach, therates of CO site exchange at the various temperatures could be evaluated by line shape simulation. They werefound to range from 0.15 × 1012 s-1 at 133 K to 1.54 × 1012 s-1 at 293 K, yielding H = 0.7 kcal·mol-1(Eyring plot) and Ea = 1.1 kcal·mol-1 (Arrhenius plot) for the activation barrier of the underlying process.