Journal
JOURNAL OF MOLECULAR STRUCTURE
Volume 890, Issue 1-3, Pages 328-338Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.molstruc.2008.05.047
Keywords
19-Electron intermediates; Transition-metal dimers; Metal-centered radicals; Electron transfer; Cage effects; Step-scan FTIR spectroscopy
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Funding
- NSF's Division of Physical Chemistry
- US Department of Energy Office of Basic Energy Sciences, Chemical Sciences Division [DE-AC02-05CH11231]
- Alexander von Humboldt foundation
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The photochemical disproportionation mechanism of [CpW(CO)(3)](2) in the presence of Lewis bases PR3 was investigated on the nano- and microsecond time-scales with step-scan FTIR time-resolved infrared spectroscopy. Laser excitation (532 nm) was used to homolytically cleave the W-W bond, forming the 17-electron radicals CpW(CO)(3) and initiating the reaction. With the Lewis base PPh3, disproportionation to form the ionic products CpW(CO)(3)PPh3+ and CpW(CO)(3)(-) was directly monitored on the microsecond time-scale. Detailed examination of the kinetics and concentration dependence of this reaction indicates that disproportionation proceeds by electron transfer from the 19-electron species CpW(CO)(3)PPh3 to the 17-electron species CPW(CO)(3). This result is contrary to the currently accepted disproportionation mechanism which predicts electron transfer from the 19-electron species to the dimer [CPW(CO)(3)](2). With the Lewis base P(OMe)(3) on the other hand, ligand substitution to form the product [CpW(CO)(2)P(CMe)(3)](2) is the primary reaction on the microsecond time-scale. Density functional theory (DFT) calculations support the experimental results and suggest that the differences in the reactivity between P(OMe)(3) and PPh3 are due to steric effects. The results indicate that radical-to-radical electron transfer is a previously unknown but important process for the formation of ionic products with the organometallic dimer [CpW(CO)(3)](2) and may also be applicable to the entire class of organometallic dinners containing a single metal-metal bond. (C) 2008 Elsevier B.V. All rights reserved.
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