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

Research output: Contribution to journalArticle

31 Citations (Scopus)

Abstract

The free energies interconnecting nine tungsten complexes have been determined from chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies 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]+ (where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), indicating that the bonds are the same within experimental uncertainty for the neutral hydride and the cationic dihydride. 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) and -16.9(5) kcal mol-1, respectively. Based on these two steps, the free energy change for the net conversion of [CpW(CO)2(IMes)H]•+ to [CpW(CO) 2(IMes)(MeCN)]+ in MeCN is 34(1) kcal mol-1, indicating a much lower bond strength for the 17-electron radical cation of the metal hydride compared to the 18-electron hydride or dihydride. The pK a 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 greatly 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 analogues. 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 an effective hydride donor ability of 57(1) kcal mol-1 in MeCN. Electrochemical oxidation of [CpW(CO)2(IMes)]- is fully reversible at all observed scan rates in cyclic voltammetry experiments (E° = -1.65 V vs Cp2Fe+/0 in MeCN), whereas CpW(CO) 2(IMes)H is reversibly oxidized (E° = -0.13(3) V) only at high scan rates (800 V s-1). For [CpW(CO)2(IMes)(MeCN)] +, high-pressure NMR experiments provide an estimate of δG° = 10.3(4) kcal mol-1 for the displacement of MeCN by H2 to give [CpW(CO)2(IMes)(H)2]+.

Original languageEnglish
Pages (from-to)14604-14613
Number of pages10
JournalJournal of the American Chemical Society
Volume133
Issue number37
DOIs
Publication statusPublished - Sep 21 2011

Fingerprint

Thermochemistry
Hydrides
Cations
Positive ions
Metals
Ligands
Hydrogen
Free energy
Electrons
carbene
phosphine

ASJC Scopus subject areas

  • Chemistry(all)
  • Catalysis
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

@article{b44f14030fd74f2eb7d332532e6e971f,
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 = "The free energies interconnecting nine tungsten complexes have been determined from chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies 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]+ (where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), indicating that the bonds are the same within experimental uncertainty for the neutral hydride and the cationic dihydride. 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) and -16.9(5) kcal mol-1, respectively. Based on these two steps, the free energy change for the net conversion of [CpW(CO)2(IMes)H]•+ to [CpW(CO) 2(IMes)(MeCN)]+ in MeCN is 34(1) kcal mol-1, indicating a much lower bond strength for the 17-electron radical cation of the metal hydride compared to the 18-electron hydride or dihydride. The pK a 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 greatly 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 analogues. 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 an effective hydride donor ability of 57(1) kcal mol-1 in MeCN. Electrochemical oxidation of [CpW(CO)2(IMes)]- is fully reversible at all observed scan rates in cyclic voltammetry experiments (E° = -1.65 V vs Cp2Fe+/0 in MeCN), whereas CpW(CO) 2(IMes)H is reversibly oxidized (E° = -0.13(3) V) only at high scan rates (800 V s-1). For [CpW(CO)2(IMes)(MeCN)] +, high-pressure NMR experiments provide an estimate of δG° = 10.3(4) kcal mol-1 for the displacement of MeCN by H2 to give [CpW(CO)2(IMes)(H)2]+.",
author = "John Roberts and Aaron Appel and DuBois, {Daniel L} and Bullock, {R Morris}",
year = "2011",
month = "9",
day = "21",
doi = "10.1021/ja202830w",
language = "English",
volume = "133",
pages = "14604--14613",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
number = "37",

}

TY - JOUR

T1 - 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

AU - Roberts, John

AU - Appel, Aaron

AU - DuBois, Daniel L

AU - Bullock, R Morris

PY - 2011/9/21

Y1 - 2011/9/21

N2 - The free energies interconnecting nine tungsten complexes have been determined from chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies 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]+ (where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), indicating that the bonds are the same within experimental uncertainty for the neutral hydride and the cationic dihydride. 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) and -16.9(5) kcal mol-1, respectively. Based on these two steps, the free energy change for the net conversion of [CpW(CO)2(IMes)H]•+ to [CpW(CO) 2(IMes)(MeCN)]+ in MeCN is 34(1) kcal mol-1, indicating a much lower bond strength for the 17-electron radical cation of the metal hydride compared to the 18-electron hydride or dihydride. The pK a 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 greatly 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 analogues. 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 an effective hydride donor ability of 57(1) kcal mol-1 in MeCN. Electrochemical oxidation of [CpW(CO)2(IMes)]- is fully reversible at all observed scan rates in cyclic voltammetry experiments (E° = -1.65 V vs Cp2Fe+/0 in MeCN), whereas CpW(CO) 2(IMes)H is reversibly oxidized (E° = -0.13(3) V) only at high scan rates (800 V s-1). For [CpW(CO)2(IMes)(MeCN)] +, high-pressure NMR experiments provide an estimate of δG° = 10.3(4) kcal mol-1 for the displacement of MeCN by H2 to give [CpW(CO)2(IMes)(H)2]+.

AB - The free energies interconnecting nine tungsten complexes have been determined from chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies 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]+ (where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), indicating that the bonds are the same within experimental uncertainty for the neutral hydride and the cationic dihydride. 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) and -16.9(5) kcal mol-1, respectively. Based on these two steps, the free energy change for the net conversion of [CpW(CO)2(IMes)H]•+ to [CpW(CO) 2(IMes)(MeCN)]+ in MeCN is 34(1) kcal mol-1, indicating a much lower bond strength for the 17-electron radical cation of the metal hydride compared to the 18-electron hydride or dihydride. The pK a 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 greatly 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 analogues. 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 an effective hydride donor ability of 57(1) kcal mol-1 in MeCN. Electrochemical oxidation of [CpW(CO)2(IMes)]- is fully reversible at all observed scan rates in cyclic voltammetry experiments (E° = -1.65 V vs Cp2Fe+/0 in MeCN), whereas CpW(CO) 2(IMes)H is reversibly oxidized (E° = -0.13(3) V) only at high scan rates (800 V s-1). For [CpW(CO)2(IMes)(MeCN)] +, high-pressure NMR experiments provide an estimate of δG° = 10.3(4) kcal mol-1 for the displacement of MeCN by H2 to give [CpW(CO)2(IMes)(H)2]+.

UR - http://www.scopus.com/inward/record.url?scp=80052814349&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=80052814349&partnerID=8YFLogxK

U2 - 10.1021/ja202830w

DO - 10.1021/ja202830w

M3 - Article

VL - 133

SP - 14604

EP - 14613

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 37

ER -