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Title: A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Towards Catalyst Deactivation During Ketone Hydrogenation

Abstract

The initial reaction observed between the N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO){sub 2}(PPh{sub 3})H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]{sup +}[CpM(CO){sub 2}(PPh{sub 3})]{sup -}. At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO){sub 2}(PR{sub 3})H (M = Mo, W; R = Me, Ph) produces CpM(CO){sub 2}(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph{sub 3}C{sub +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, which was crystallographically characterized using X-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H{sub 2}, THF, ketones, or alcohols. Reaction of CpW(CO){sub 2}(IMes){sup +} with H{sub 2} gives the dihydride complex [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +}. Addition of Et{sub 2}CH-OH to CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives the alcohol complex [CpW(CO){sub 2}(IMes)(Et{sub 2}CH-OH)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}, which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OHmore » group. Addition of H{sub 2} to the ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -} produces an equilibrium with the dihydride [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +} (K{sub eq} = 1.1 x 10{sup 3} at 25 {sup o}C). The tungsten ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}- serves as a modest catalyst for hydrogenation of Et{sub 2}C=O to Et{sub 2}CH-OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]{sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, indicating that these catalysts with N-heterocyclic carbene ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
930034
Report Number(s):
BNL-80652-2008-JA
Journal ID: ISSN 0276-7333; ORGND7; TRN: US200822%%1268
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Organometallics; Journal Volume: 26
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; ALCOHOLS; BONDING; CARBENES; CARBON; CATALYSTS; COMPLEXES; CRYSTALLOGRAPHY; DEACTIVATION; DECOMPOSITION; EQUILIBRIUM; HYDRIDES; HYDROGEN; HYDROGENATION; KETONES; LIGANDS; METALS; MOLYBDENUM; RADIATIONS; TETRAHYDROFURAN; TUNGSTEN; TUNGSTEN HYDRIDES; VULNERABILITY; national synchrotron light source

Citation Formats

Wu,F., Dioumaev, V., Szalda, D., Hanson, J., and Bullock, R.. A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Towards Catalyst Deactivation During Ketone Hydrogenation. United States: N. p., 2007. Web. doi:10.1021/om700694e.
Wu,F., Dioumaev, V., Szalda, D., Hanson, J., & Bullock, R.. A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Towards Catalyst Deactivation During Ketone Hydrogenation. United States. doi:10.1021/om700694e.
Wu,F., Dioumaev, V., Szalda, D., Hanson, J., and Bullock, R.. Mon . "A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Towards Catalyst Deactivation During Ketone Hydrogenation". United States. doi:10.1021/om700694e.
@article{osti_930034,
title = {A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Towards Catalyst Deactivation During Ketone Hydrogenation},
author = {Wu,F. and Dioumaev, V. and Szalda, D. and Hanson, J. and Bullock, R.},
abstractNote = {The initial reaction observed between the N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO){sub 2}(PPh{sub 3})H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]{sup +}[CpM(CO){sub 2}(PPh{sub 3})]{sup -}. At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO){sub 2}(PR{sub 3})H (M = Mo, W; R = Me, Ph) produces CpM(CO){sub 2}(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph{sub 3}C{sub +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, which was crystallographically characterized using X-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H{sub 2}, THF, ketones, or alcohols. Reaction of CpW(CO){sub 2}(IMes){sup +} with H{sub 2} gives the dihydride complex [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +}. Addition of Et{sub 2}CH-OH to CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives the alcohol complex [CpW(CO){sub 2}(IMes)(Et{sub 2}CH-OH)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}, which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H{sub 2} to the ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -} produces an equilibrium with the dihydride [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +} (K{sub eq} = 1.1 x 10{sup 3} at 25 {sup o}C). The tungsten ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}- serves as a modest catalyst for hydrogenation of Et{sub 2}C=O to Et{sub 2}CH-OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]{sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, indicating that these catalysts with N-heterocyclic carbene ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.},
doi = {10.1021/om700694e},
journal = {Organometallics},
number = ,
volume = 26,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • The initial reaction observed between N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO) 2(PPh 3)H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H] +[CpM(CO) 2(PPh 3)] . At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO) 2(PR 3)H (M = Mo, W; R = Me, Ph) produces CpM(CO) 2(IMes)H. Hydride transfer from CpW(CO) 2(IMes)H to Ph 3C +B(C 6F 5) 4 - gives CpW(CO) 2(IMes) +B(C 6F 5) 4 - which was crystallographically characterized using x-ray radiation from a synchrotron. The IMes is bondedmore » as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C =C bond of one mesityl ring. The weakly bound C =C ligand is hemilabile, being readily displaced by H 2, THF, ketones or alcohols. Reaction of CpW(CO) 2(IMes) + with H 2 gives the dihydride complex [CpW(CO) 2(IMes)(H) 2] +. Addition of Et 2CH–OH to CpW(CO) 2(IMes) +B(C 6F 5) 4 - gives the alcohol complex [CpM(CO) 2(IMes)(Et 2CH–OH)] +[B(C 6F 5) 4] which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H 2 to the ketone complex [CpW(CO) 2(IMes)(Et 2C =O)] +[B(C 6F 5) 4] produces an equilibrium with the dihydride [CpW(CO) 2(IMes)(H) 2] + (K eq = 1.1 x 103 at 25 °C). The tungsten ketone complex [CpW(CO) 2(IMes)(Et 2C =O)] +[B(C 6F 5) 4] serves as a modest catalyst for hydrogenation of Et 2C =O to Et 2CH–OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)] +B(C 6F 5) 4 -, indicating that these catalysts with N-heterocyclic carbenes ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.« less
  • The N?heterocyclic carbene ligand IMes was shown by synchrotron crystallography and DFT computations to adopt a hemilabile bidentate coordination mode in CpM(CO)2(IMes)+B(C6F5)4- (M = Mo, W), with a C=C bond of one mesityl weakly coordinated to the metal
  • The thermal W-W bond homolysis in [CpW(CO)2(IMe)]2 (IMe = 1,3-dimethylimidazol-2-ylidene) was investigated and was found to occur to a large extent compared to other tungsten dimers such as [CpW(CO)3]2. CpW(CO)2(IMe)H was prepared by heating a solution of [IMeH]+[CpW(CO)2(PMe3)]-, and exists in solution as a mixture of interconverting cis and trans isomers. The carbene rotation in CpW(CO)2(IMe)H was explored by DFT calculations, and low enthalpic barriers (< 3.5 kcal mol-1) are predicted. CpW(CO)2(IMe)H has pKaMeCN = 31.5(3) and deprotonation with KH gives K+[CpW(CO)2(IMe)]- (• MeCN). Hydride abstraction from CpW(CO)2(IMe)H with Ph3C+PF6- in the presence of a coordinating ligand L (MeCN ormore » THF) gives [CpW(CO)2(IMe)(L)]+PF6-. Electrochemical measurements on the anion [CpW(CO)2(IMe)]- in MeCN, together with digital simulations, give an E1/2 of -1.54(2) V vs Cp2Fe+/0 for the [CpW(CO)2(IMe)]•/- couple. A thermochemical cycle provides the solution bond dissociation free energy of the W-H bond of CpW(CO)2(IMe)H as 61.3(6) kcal mol-1. In the electrochemical oxidation of [CpW(CO)2(IMe)]-, reversible dimerization of the electrogenerated radical CpW(CO)2(IMe)• occurs, and digital simulation provides kinetic and thermodynamic parameters for the monomer-dimer equilibrium: kdimerization ~ 2.5 x 104 M-1 s-1, khomolysis ~ 0.5 s-1 (i.e., Kdim ~ 5 x 104 M-1). Reduction of [CpW(CO)2(IMe)(MeCN)]+PF6- with cobaltocene gives the dimer [CpW(CO)2(IMe)]2, which in solution exists as a mixture of anti and gauche rotomers. As expected from the electrochemical experiments, the dimer is in equilibrium with detectable amounts of CpW(CO)2(IMe)•. This species was observed by IR spectroscopy, and its presence in solution is also in accordance with the observed reactivity toward 2,6-di-tert-butyl-1,4-benzoquinone, chloroform and dihydrogen. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle. The EPR studies were performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory.« less
  • Hydride transfer from Cp(CO) 2(PPh 3)MoH to Ph 3C + BAr'4 [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO) 23 PPh 3)Mo] +[BAr'4] . Spectroscopic and crystallographic data indicate that one C=C of a Ph ring is weakly bound to the Mo, so that the PPh 3 ligand is four-electron donor ligand. Computations (DFT/B3LYP and MP2 on [Cp(CO) 23 PPh 3)Mo] + and [Cp(CO) 23 PH 2Ph)Mo] +, and DFT/B3LYP on [Cp(CO) 23 PHtBuPh)Mo] + and [Cp(CO) 23 PH2Ph)Nb]) provide further information on the bonding, and on the preference for bonding of the metal to the C=C bondmore » rather than an agostic C H interaction found in many related complexes. The hemilabile C=C bond is readily displaced by CH 3I or H 2O, and crystal structures are reported for [Cp(CO) 2(PPh 3)Mo(ICH 3)] + and [Cp(CO) 2(PPh 3)Mo(OH 2)] +. The equilibrium constant for [Cp(CO) 23 PPh 3)Mo] + + ICH 3 to give [Cp(CO) 2(PPh 3)Mo(ICH 3)] + is Keq = 5.2 x 10 2 M -1 in CD 2Cl 2 at 22 °C.« less