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Title: 19-electron intermediates in the Ligand Substitution of CpW(CO)3with a Lewis Base

Abstract

Odd electron species are important intermediates in organometallic chemistry, participating in a variety of catalytic and electron-transfer reactions which produce stable even-electron products. While electron deficient 17-electron (17e) radicals have been well characterized, the possible existence of short-lived 19-electron (19e) radicals has been a subject of continuing investigation. 19e radicals have been postulated as intermediates in the photochemical ligand substitution and disproportionation reactions of organometallic dimers containing a single metal-metal bond, yet the reactions of these intermediates on diffusion-limited time scales (ns-{micro}s) have never been directly observed. This study resolves the 19e dynamics in the ligand substitution of 17e radicals CpW(CO){sub 3}{sup {sm_bullet}} (Cp = C{sub 5}H{sub 5}) with the Lewis base P(OMe){sub 3}, providing the first complete description 19e reactivity.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
Sponsoring Org.:
USDOE Director. Office of Science. Office of Basic EnergySciences. Chemical Sciences Geosciences and Biosciences Division; National Science Foundation (NSF); Alexander von HumboldtFoundation
OSTI Identifier:
891214
Report Number(s):
LBNL-59215
Journal ID: ISSN 0002-7863; JACSAT; R&D Project: 4005; BnR: KC0301020; TRN: US200621%%675
DOE Contract Number:
DE-AC02-05CH11231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the American Chemical Society; Journal Volume: 128; Journal Issue: 10; Related Information: Journal Publication Date: 03/15/2006
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CHEMISTRY; DIMERS; ELECTRON TRANSFER; ELECTRONS; LEWIS BASES; OXIDATION; RADICALS; REDUCTION; electronligandLewis base

Citation Formats

Cahoon, James F., Kling, Matthias F., Sawyer, Karma R., Frei,Heinz, and Harris, Charles B. 19-electron intermediates in the Ligand Substitution of CpW(CO)3with a Lewis Base. United States: N. p., 2005. Web.
Cahoon, James F., Kling, Matthias F., Sawyer, Karma R., Frei,Heinz, & Harris, Charles B. 19-electron intermediates in the Ligand Substitution of CpW(CO)3with a Lewis Base. United States.
Cahoon, James F., Kling, Matthias F., Sawyer, Karma R., Frei,Heinz, and Harris, Charles B. Wed . "19-electron intermediates in the Ligand Substitution of CpW(CO)3with a Lewis Base". United States. doi:. https://www.osti.gov/servlets/purl/891214.
@article{osti_891214,
title = {19-electron intermediates in the Ligand Substitution of CpW(CO)3with a Lewis Base},
author = {Cahoon, James F. and Kling, Matthias F. and Sawyer, Karma R. and Frei,Heinz and Harris, Charles B.},
abstractNote = {Odd electron species are important intermediates in organometallic chemistry, participating in a variety of catalytic and electron-transfer reactions which produce stable even-electron products. While electron deficient 17-electron (17e) radicals have been well characterized, the possible existence of short-lived 19-electron (19e) radicals has been a subject of continuing investigation. 19e radicals have been postulated as intermediates in the photochemical ligand substitution and disproportionation reactions of organometallic dimers containing a single metal-metal bond, yet the reactions of these intermediates on diffusion-limited time scales (ns-{micro}s) have never been directly observed. This study resolves the 19e dynamics in the ligand substitution of 17e radicals CpW(CO){sub 3}{sup {sm_bullet}} (Cp = C{sub 5}H{sub 5}) with the Lewis base P(OMe){sub 3}, providing the first complete description 19e reactivity.},
doi = {},
journal = {Journal of the American Chemical Society},
number = 10,
volume = 128,
place = {United States},
year = {Wed Dec 14 00:00:00 EST 2005},
month = {Wed Dec 14 00:00:00 EST 2005}
}
  • The role of 19-electron intermediates is investigated.
  • Laser flash photolysis of [CpW(CO)[sub 3]][sub 2] was carried out at 490 nm, where the primary photoprocess gives a 17-electron radical, CpW(CO)[sub 3]. The metal radical acts as both an electron donor and acceptor. The radical reduces ferrocenium ions (k = [1.89(4)] [times] 10[sup 7] L mol[sup [minus]1] s[sup [minus]1]) and benzoquinone (k = [2.7(1)] [times] 10[sup 7] L mol[sup [minus]1] s[sup [minus]1]) at 23[degrees]C in acetonitrile. Triphenylphosphine accelerates the reaction of the tungsten radical with ferrocenium ions, suggesting the formation of the more strongly reducing 19-electron radical CpW(CO)[sub 3]PPh[sub 3]. It reacts with ferrocenium ions; k = 3 [times]more » 10[sup 9] L mol[sup [minus]1] s[sup [minus]1]. The base-induced disproportionation of CpW(CO)[sub 3] was used to evaluate binding constants for CpW(CO)[sub 3] and PPh[sub 3] (K = 6 [+-] 1 L mol[sup [minus]1]) and pyridine (K = 0.16 [+-] 0.04 L mol[sup [minus]1]). Also, the CpW(CO)[sub 3] radical oxidizes decamethylferrocene (2.2 [times] 10[sup 8] L mol[sup [minus]1] s[sup [minus]1]) and N,N,N[prime],N[prime]-tetramethylphenylenediamine (2.6 [times] 10[sup 7] L mol[sup [minus]1] s[sup [minus]1]). In this system there is a secondary process responsible for the re-reduction of TMPD[sup [center dot]+]. We suggest the rapid formation of the radical anion [CpW(CO)[sub 3]][sub 2][sup [center dot][minus]], presumed to be a powerful electron donor, to account for back-electron transfer, but this has not been proved. 36 refs., 4 figs.« less
  • The tungsten(IV) dication CpW(CO){sub 2}(NCMe){sub 2}Me{sup 2+}, obtained from the 2-electron oxidation of CpW-(CO){sub 3}Me, serves as the precursor to a number of tungsten(IV) complexes via a series of CO insertion and substitution reactions. In acetonitrile, the dication undergoes smooth CO insertion ({Delta}H* = 89.5 {sup +}{sub -} 2.5 kJ/mol, {Delta}S* = -43.9 {sup +}{sub -} 8.4J/(K{lg_bullet}mol), k(20{degrees}C) = 3.3 x 10{sup -6} 8{sup -1}) and substitution to give the {eta}{sup 2}-acetyl complex CpW(NCMe){sub 3}({eta}{sup 2}-COMe){sup 2+}. The oxidation of CpW(CO){sub 3}Et provides CpW(CO)(NCMe){sub 2}({eta}{sup 2}-COEt){sup 2+} via the intermediacy of CpW(CO){sub 2}(NCMe){sub 2}Et{sup 2+}. CpW(CO)(NCMe){sub 2}({eta}-COEt){sup 2+} in turnmore » undergoes CO substitution to yield CpW(CO){sub 3}(NCMe)Me{sup 2+}. Analogous CO insertion reactions were not observed for the compounds CpW(CO){sub 3}(NCMe)Me{sup 2+} and CpW(CO){sub 3}(NCMe)Et{sup 2+}. 21 refs., 3 figs., 3 tabs.« less
  • A free energy map interconnecting nine tungsten complexes has been determined based on chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies (BDFEs) are 59.3(3) kcal mol -1 for CpW(CO)2(IMes)H and 59(1) kcal mol -1 for the dihydride [CpW(CO)2(IMes)(H) 2]+. For the radical cation, [CpW(CO)2(IMes)H]•+, W-H bond homolysis to generate the 16-electron cation [CpW(CO)2(IMes)]+ is followed by MeCN uptake, with free energies for these steps being 51(1) kcal mol -1 and -16.9(5) kcal mol -1 respectively. Based on these two steps, the free energy change for conversion of [CpW(CO)2(IMes)H]•+ to [CpW(CO)2(IMes)(MeCN)]+more » in MeCN is 34(1) kcal mol -1. The pKa of CpW(CO)2(IMes)H in MeCN was determined to be 31.9(1), significantly higher than the 26.6 reported for the related phosphine complex, CpW(CO)2(PMe3)H. This difference is attributed to the electron donor strength of IMes far exceeding that of PMe3. The pKa values for [CpW(CO)2(IMes)H]•+ and [CpW(CO)2(IMes)(H)2]+ were determined to be 6.3(5) and 6.3(8), much closer to the pKa values reported for the PMe3 analogs. The free energy of hydride abstraction from CpW(CO)2(IMes)H is 74(1) kcal mol -1, and the resultant [CpW(CO)2(IMes)]+ cation is significantly stabilized by binding MeCN to form [CpW(CO)2(IMes)(MeCN)]+, giving a hydride donor ability of 57(1) kcal mol -1. Electrochemical oxidation of [CpW(CO)2(IMes)] - shows a fully reversible wave (E° = -1.65 V vs. Cp2Fe+/0 in MeCN), and CpW(CO)2(IMes)H is reversibly oxidized (E° = -0.13(3) V) at high scan rates (800 V s -1). High pressure NMR experiments provide an estimate of ΔG° = 10.3(4) kcal mol -1 for the displacement of MeCN by H2 for [CpW(CO)2(IMes)(MeCN)]+ to give [CpW(CO)2(IMes)(H)2]+.« less
  • A series consisting of a tungsten anion, radical and cation, supported by the N-heterocyclic carbene IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and spanning formal oxidation states W(0), W(I) and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)2(IMes)H with KH and 18 crown 6 gives the tungsten anion [CpW(CO)2(IMes)]-[K(18 crown 6)]+. The crystal structure of this complex shows that the K+ interacts not only with the oxygen atoms in the crown ether, but also with the carbonyl oxygens. The electrochemical oxidation of [CpW(CO)2(IMes)]- in acetonitrile is fully reversible (E½ = -1.65 V vs Cp2Fe+•/0) at all scan rates, indicating that CpW(CO)2(IMes)•more » is a persistent radical. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+PF6 affords [cis-CpW(CO)2(IMes)(MeCN)]+PF6 . Comproportionation of [CpW(CO)2(IMes)]- with [CpW(CO)2(IMes)(MeCN)]+ gives the 17-electron tungsten radical CpW(CO)2(IMes)•. This complex shows paramagnetically shifted resonances in 1H NMR spectra and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)2(IMes)• is very stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)2(IMes)• and [CpW(CO)2(IMes)]- are reported. DFT calculations were carried out on CpW(CO)2(IMes)H, as well as on related complexes bearing NHC ligands with N,N´ substituents Me [CpW(CO)2(IMe)H] or H [CpW(CO)2(IH)H] to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal W H homolytic bond dissociation energies (BDEs) to decrease with increasing steric bulk of the NHC ligand, from 67 for CpW(CO)2(IH)H to 64 for CpW(CO)2(IMe)H to 63 kcal/mol for CpW(CO)2(IMes)H. The calculated spin density at W for CpW(CO)2(IMes)• is 0.63. The W radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)• are calculated to form weak W W bonds. The weakly bonded complexes [CpW(CO)2(IMe)]2 and [CpW(CO)2(IH)]2, are predicted to have W-W BDEs of 6 and 18 kcal/mol, respectively, and to dissociate readily to the W-centered radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)•. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less