Organometallic thermochemistry. Metal hydrocarbyl, hydride, halide, carbonyl, amide, and alkoxide bond enthalpy relationships and their implications in pentamethylcyclopentadienyl and cyclopentadienyl complexes of zirconium and hafnium

Laurel E. Schock, Tobin J Marks

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298 Citations (Scopus)

Abstract

Metal-ligand bond disruption enthalpies (D) have been determined in the series Cp′2MX2, Cp2MX2, Cp′MX3 (M = Zr, Hf; Cp′ = η5-Me5C5; Cp = η5-C5H5; M = Zr, Hf; X = hydrocarbyl, hydride, alkoxide, amide, halide) and in Cp′2Zr(Cp)2 by anaerobic isoperibol batch-titration calorimetry. Heats of solution in toluene were measured followed by heats of reaction with HCl, I2, C6F5OH, C6H5OH, CF3CH2OH, or t-BuOH in toluene. Derived D(M-X) values decrease in the order OH ≈ Cl > alkoxide ≈ NH2 > phenoxide > I ≈ H > aryl > Me > alkyl, metallacyclopentane > η15-CH2C5Me4 M-C σ bond > CO. By using D(Cl3M-Cl) as a reference point, D(M-X) values are found to be rather large (e.g., for M = Zr(R): 78 (H), 73 (Ph), 67 (Me) kcal/mol) and not highly sensitive to ancillary η5-cyclopentadienyl ligation. D(Hf-X) - D(Zr-X) is estimated to be ca. 4 kcal/mol. Ancillary alkoxide ligands enhance D(Zr-H) in the Cp′2ZrH2/Cp′2Zr(OR)H series by ca. 5 kcal/mol. The metallacycle Cp′2ZrCH2(CHEt)2CH2 exhibits negligible ring strain while that in the zirconaindan Cp′2ZrCH2CH2-o-C6H4 is ca. -10 kcal/mol. A plot of D(Zr-X) vs D(H-X) is not linear but shows very substantial scatter. However, reasonably linear plots are observed within ligand subgroups such as hydrocarbyls, alkoxides, and halides. This behavior can be qualitatively explained on the basis of metal and ligand electronegativities. The quantities D(M-H) - D(M-Me) and D(M-I) - D(M-Me) vary considerably across the transition-metal series and are informative indices of metal-ligand bonding. The former is small for the present group 4 compounds and the latter large. The present data are used to semiquantitatively interpret a number of group 4 centered transformations. Among the conclusions drawn are that β-H elimination processes are usually endothermic; many C-H activating cyclometalation processes are endothermic, hence entropically driven (e.g., Cp′2ZrPh2 → Cp′Zr(Ph) (η15-CH2C5Me4) + PhH); Zr(II) → Zr(IV) oxidative additions are highly exothermic; D-(Cp′2Zr-benzyne) ≳ 120 kcal/mol; early transition metal/lanthanide/actinide M(η15-CH2C5Me4) species are energetically poised to serve as intermediates in a number of addition and elimination processes; and the large magnitude of D(M-OR) is one major driving force for the formation of alkoxide-like end products in group 4 centered CO activation chemistry.

Original languageEnglish
Pages (from-to)7701-7715
Number of pages15
JournalJournal of the American Chemical Society
Volume110
Issue number23
Publication statusPublished - 1988

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Hafnium
Thermochemistry
Organometallics
Amides
Zirconium
Hydrides
Enthalpy
Metals
Ligands
Toluene
Carbon Monoxide
Transition metals
Hot Temperature
Actinoid Series Elements
Lanthanoid Series Elements
Electronegativity
Calorimetry
Actinides
Rare earth elements
Titration

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

@article{a7312fdf761a46b991c0b51822a116d0,
title = "Organometallic thermochemistry. Metal hydrocarbyl, hydride, halide, carbonyl, amide, and alkoxide bond enthalpy relationships and their implications in pentamethylcyclopentadienyl and cyclopentadienyl complexes of zirconium and hafnium",
abstract = "Metal-ligand bond disruption enthalpies (D) have been determined in the series Cp′2MX2, Cp2MX2, Cp′MX3 (M = Zr, Hf; Cp′ = η5-Me5C5; Cp = η5-C5H5; M = Zr, Hf; X = hydrocarbyl, hydride, alkoxide, amide, halide) and in Cp′2Zr(Cp)2 by anaerobic isoperibol batch-titration calorimetry. Heats of solution in toluene were measured followed by heats of reaction with HCl, I2, C6F5OH, C6H5OH, CF3CH2OH, or t-BuOH in toluene. Derived D(M-X) values decrease in the order OH ≈ Cl > alkoxide ≈ NH2 > phenoxide > I ≈ H > aryl > Me > alkyl, metallacyclopentane > η1:η5-CH2C5Me4 M-C σ bond > CO. By using D(Cl3M-Cl) as a reference point, D(M-X) values are found to be rather large (e.g., for M = Zr(R): 78 (H), 73 (Ph), 67 (Me) kcal/mol) and not highly sensitive to ancillary η5-cyclopentadienyl ligation. D(Hf-X) - D(Zr-X) is estimated to be ca. 4 kcal/mol. Ancillary alkoxide ligands enhance D(Zr-H) in the Cp′2ZrH2/Cp′2Zr(OR)H series by ca. 5 kcal/mol. The metallacycle Cp′2ZrCH2(CHEt)2CH2 exhibits negligible ring strain while that in the zirconaindan Cp′2ZrCH2CH2-o-C6H4 is ca. -10 kcal/mol. A plot of D(Zr-X) vs D(H-X) is not linear but shows very substantial scatter. However, reasonably linear plots are observed within ligand subgroups such as hydrocarbyls, alkoxides, and halides. This behavior can be qualitatively explained on the basis of metal and ligand electronegativities. The quantities D(M-H) - D(M-Me) and D(M-I) - D(M-Me) vary considerably across the transition-metal series and are informative indices of metal-ligand bonding. The former is small for the present group 4 compounds and the latter large. The present data are used to semiquantitatively interpret a number of group 4 centered transformations. Among the conclusions drawn are that β-H elimination processes are usually endothermic; many C-H activating cyclometalation processes are endothermic, hence entropically driven (e.g., Cp′2ZrPh2 → Cp′Zr(Ph) (η1:η5-CH2C5Me4) + PhH); Zr(II) → Zr(IV) oxidative additions are highly exothermic; D-(Cp′2Zr-benzyne) ≳ 120 kcal/mol; early transition metal/lanthanide/actinide M(η1:η5-CH2C5Me4) species are energetically poised to serve as intermediates in a number of addition and elimination processes; and the large magnitude of D(M-OR) is one major driving force for the formation of alkoxide-like end products in group 4 centered CO activation chemistry.",
author = "Schock, {Laurel E.} and Marks, {Tobin J}",
year = "1988",
language = "English",
volume = "110",
pages = "7701--7715",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
publisher = "American Chemical Society",
number = "23",

}

TY - JOUR

T1 - Organometallic thermochemistry. Metal hydrocarbyl, hydride, halide, carbonyl, amide, and alkoxide bond enthalpy relationships and their implications in pentamethylcyclopentadienyl and cyclopentadienyl complexes of zirconium and hafnium

AU - Schock, Laurel E.

AU - Marks, Tobin J

PY - 1988

Y1 - 1988

N2 - Metal-ligand bond disruption enthalpies (D) have been determined in the series Cp′2MX2, Cp2MX2, Cp′MX3 (M = Zr, Hf; Cp′ = η5-Me5C5; Cp = η5-C5H5; M = Zr, Hf; X = hydrocarbyl, hydride, alkoxide, amide, halide) and in Cp′2Zr(Cp)2 by anaerobic isoperibol batch-titration calorimetry. Heats of solution in toluene were measured followed by heats of reaction with HCl, I2, C6F5OH, C6H5OH, CF3CH2OH, or t-BuOH in toluene. Derived D(M-X) values decrease in the order OH ≈ Cl > alkoxide ≈ NH2 > phenoxide > I ≈ H > aryl > Me > alkyl, metallacyclopentane > η1:η5-CH2C5Me4 M-C σ bond > CO. By using D(Cl3M-Cl) as a reference point, D(M-X) values are found to be rather large (e.g., for M = Zr(R): 78 (H), 73 (Ph), 67 (Me) kcal/mol) and not highly sensitive to ancillary η5-cyclopentadienyl ligation. D(Hf-X) - D(Zr-X) is estimated to be ca. 4 kcal/mol. Ancillary alkoxide ligands enhance D(Zr-H) in the Cp′2ZrH2/Cp′2Zr(OR)H series by ca. 5 kcal/mol. The metallacycle Cp′2ZrCH2(CHEt)2CH2 exhibits negligible ring strain while that in the zirconaindan Cp′2ZrCH2CH2-o-C6H4 is ca. -10 kcal/mol. A plot of D(Zr-X) vs D(H-X) is not linear but shows very substantial scatter. However, reasonably linear plots are observed within ligand subgroups such as hydrocarbyls, alkoxides, and halides. This behavior can be qualitatively explained on the basis of metal and ligand electronegativities. The quantities D(M-H) - D(M-Me) and D(M-I) - D(M-Me) vary considerably across the transition-metal series and are informative indices of metal-ligand bonding. The former is small for the present group 4 compounds and the latter large. The present data are used to semiquantitatively interpret a number of group 4 centered transformations. Among the conclusions drawn are that β-H elimination processes are usually endothermic; many C-H activating cyclometalation processes are endothermic, hence entropically driven (e.g., Cp′2ZrPh2 → Cp′Zr(Ph) (η1:η5-CH2C5Me4) + PhH); Zr(II) → Zr(IV) oxidative additions are highly exothermic; D-(Cp′2Zr-benzyne) ≳ 120 kcal/mol; early transition metal/lanthanide/actinide M(η1:η5-CH2C5Me4) species are energetically poised to serve as intermediates in a number of addition and elimination processes; and the large magnitude of D(M-OR) is one major driving force for the formation of alkoxide-like end products in group 4 centered CO activation chemistry.

AB - Metal-ligand bond disruption enthalpies (D) have been determined in the series Cp′2MX2, Cp2MX2, Cp′MX3 (M = Zr, Hf; Cp′ = η5-Me5C5; Cp = η5-C5H5; M = Zr, Hf; X = hydrocarbyl, hydride, alkoxide, amide, halide) and in Cp′2Zr(Cp)2 by anaerobic isoperibol batch-titration calorimetry. Heats of solution in toluene were measured followed by heats of reaction with HCl, I2, C6F5OH, C6H5OH, CF3CH2OH, or t-BuOH in toluene. Derived D(M-X) values decrease in the order OH ≈ Cl > alkoxide ≈ NH2 > phenoxide > I ≈ H > aryl > Me > alkyl, metallacyclopentane > η1:η5-CH2C5Me4 M-C σ bond > CO. By using D(Cl3M-Cl) as a reference point, D(M-X) values are found to be rather large (e.g., for M = Zr(R): 78 (H), 73 (Ph), 67 (Me) kcal/mol) and not highly sensitive to ancillary η5-cyclopentadienyl ligation. D(Hf-X) - D(Zr-X) is estimated to be ca. 4 kcal/mol. Ancillary alkoxide ligands enhance D(Zr-H) in the Cp′2ZrH2/Cp′2Zr(OR)H series by ca. 5 kcal/mol. The metallacycle Cp′2ZrCH2(CHEt)2CH2 exhibits negligible ring strain while that in the zirconaindan Cp′2ZrCH2CH2-o-C6H4 is ca. -10 kcal/mol. A plot of D(Zr-X) vs D(H-X) is not linear but shows very substantial scatter. However, reasonably linear plots are observed within ligand subgroups such as hydrocarbyls, alkoxides, and halides. This behavior can be qualitatively explained on the basis of metal and ligand electronegativities. The quantities D(M-H) - D(M-Me) and D(M-I) - D(M-Me) vary considerably across the transition-metal series and are informative indices of metal-ligand bonding. The former is small for the present group 4 compounds and the latter large. The present data are used to semiquantitatively interpret a number of group 4 centered transformations. Among the conclusions drawn are that β-H elimination processes are usually endothermic; many C-H activating cyclometalation processes are endothermic, hence entropically driven (e.g., Cp′2ZrPh2 → Cp′Zr(Ph) (η1:η5-CH2C5Me4) + PhH); Zr(II) → Zr(IV) oxidative additions are highly exothermic; D-(Cp′2Zr-benzyne) ≳ 120 kcal/mol; early transition metal/lanthanide/actinide M(η1:η5-CH2C5Me4) species are energetically poised to serve as intermediates in a number of addition and elimination processes; and the large magnitude of D(M-OR) is one major driving force for the formation of alkoxide-like end products in group 4 centered CO activation chemistry.

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