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Title: Comprehensive Thermochemistry of W–H Bonding in the Metal Hydrides CpW(CO) 2 (IMes)H, [CpW(CO) 2 (IMes)H] •+ , and [CpW(CO) 2 (IMes)(H) 2 ] + . Influence of an N -Heterocyclic Carbene Ligand on Metal Hydride Bond Energies

Abstract

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)]+ 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 amore » 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

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1025078
Report Number(s):
PNNL-SA-78855
Journal ID: ISSN 0002-7863; KC0302010; TRN: US201120%%262
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 133; Journal Issue: 37; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; BINDING ENERGY; BONDING; CARBENES; CATIONS; DISSOCIATION; ELECTRONS; FREE ENERGY; HYDRIDES; OXIDATION; PHOSPHINES; RADICALS; TUNGSTEN COMPLEXES; VALENCE; metal hydride; bond dissociation energy

Citation Formats

Roberts, John A. S., Appel, Aaron M., DuBois, Daniel L., and Bullock, R. Morris. Comprehensive Thermochemistry of W–H Bonding in the Metal Hydrides CpW(CO) 2 (IMes)H, [CpW(CO) 2 (IMes)H] •+ , and [CpW(CO) 2 (IMes)(H) 2 ] + . Influence of an N -Heterocyclic Carbene Ligand on Metal Hydride Bond Energies. United States: N. p., 2011. Web. doi:10.1021/ja202830w.
Roberts, John A. S., Appel, Aaron M., DuBois, Daniel L., & Bullock, R. Morris. Comprehensive Thermochemistry of W–H Bonding in the Metal Hydrides CpW(CO) 2 (IMes)H, [CpW(CO) 2 (IMes)H] •+ , and [CpW(CO) 2 (IMes)(H) 2 ] + . Influence of an N -Heterocyclic Carbene Ligand on Metal Hydride Bond Energies. United States. doi:10.1021/ja202830w.
Roberts, John A. S., Appel, Aaron M., DuBois, Daniel L., and Bullock, R. Morris. Wed . "Comprehensive Thermochemistry of W–H Bonding in the Metal Hydrides CpW(CO) 2 (IMes)H, [CpW(CO) 2 (IMes)H] •+ , and [CpW(CO) 2 (IMes)(H) 2 ] + . Influence of an N -Heterocyclic Carbene Ligand on Metal Hydride Bond Energies". United States. doi:10.1021/ja202830w.
@article{osti_1025078,
title = {Comprehensive Thermochemistry of W–H Bonding in the Metal Hydrides CpW(CO) 2 (IMes)H, [CpW(CO) 2 (IMes)H] •+ , and [CpW(CO) 2 (IMes)(H) 2 ] + . Influence of an N -Heterocyclic Carbene Ligand on Metal Hydride Bond Energies},
author = {Roberts, John A. S. and Appel, Aaron M. and DuBois, Daniel L. and Bullock, R. Morris},
abstractNote = {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)]+ 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]+.},
doi = {10.1021/ja202830w},
journal = {Journal of the American Chemical Society},
issn = {0002-7863},
number = 37,
volume = 133,
place = {United States},
year = {2011},
month = {9}
}