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Title: Thermodynamic and activation parameters for dissociation of [CpCr(CO){sub 3}]{sub 2} and [Cp*Cr(CO){sub 3}]{sub 2} into paramagnetic monomers from {sup 1}H NMR shift and line width measurements

Abstract

Substitution of pentamethyl cyclopentadienide (Cp*) for cyclopentadienide (Cp) is commonly used as an approach for increasing ligand steric requirements that can enhance dissociation of M-M bonded complexes. {sup 1}H NMR spectra for solutions prepared by dissolution of [CpCr(CO){sub 3}]{sub 2} and [Cp*Cr(CO){sub 3}]{sub 2} in toluene in the temperature range 190--390 K are interpreted in terms of thermodynamic and kinetic parameters for dissociation of the diamagnetic dimers into the paramagnetic monomers CpCr(CO){sub 3} and Cp*Cr(CO){sub 3}. There is no evidence in this temperature range for thermally populated excited states or non-Curie magnetic behavior of the monomers making a significant contribution to the NMR. An expression for the temperature dependence of the NMR chemical shift at limiting fast interchange of monomer and dimer in terms of the {Delta}H{degree} and {Delta}S{degree} for dimer dissociation is applied in determining the thermodynamic parameters for Cr-Cr bond homolysis of [CpCr(CO){sub 3}]{sub 2} ({Delta}H{sub 1}{degree} = 15.3 {+-} 0.6 kcal mol{sup {minus}1}; {Delta}S{sub 1}{degree} = 39 {+-} 2 cal K{sup {minus}1} mol{sup {minus}1}) and [Cp*Cr(CO){sub 3}]{sub 2} ({Delta}H{sub 2}{degree} = 14.2 {+-} 0.4 kcal mol{sup {minus}1}; {Delta}S{sub 2}{degree} = 47 {+-} 2 cal K{sup {minus}1}mol{sup {minus}1}). Rate constants and activation parameters have been evaluated from {supmore » 1}H NMR line broadening in the region of slow dimer-monomer interchange for dissociation of [CpCr(CO){sub 3}]{sub 2} ({kappa}{sub 1} (240 K) {approx} 59 s{sup {minus}1}; {Delta}H{sub 1}{double{underscore}dagger} = 17 {+-} 2 kcal mol{sup {minus}1}; {Delta}S{sub 1}{double{underscore}dagger} = 21 {+-} 6 cal K{sup {minus}1} mol{sup {minus}1}) and [Cp*Cr(CO){sub 3}]{sub 2} ({kappa}{sub 2} (240 K) {approx} 1.4 x 10{sup 4} s{sup {minus}1}; {Delta}H{sub 2}{double{underscore}dagger} = 16 {+-} 1 kcal mol{sup {minus}1}; {Delta}S{sub 2}{double{underscore}dagger} = 30 {+-} 6 cal K{sup {minus}1} mol{sup {minus}1}). Paramagnetic shifts also were used in deriving electron-proton coupling constants (A{sub H}) for CpCr(CO){sub 3}(9.22 x 10{sup 5} Hz) and Cp*Cr(CO){sub 3} (1.33 x 10{sup 6} Hz).« less

Authors:
; ;
Publication Date:
Research Org.:
Univ. of Pennsylvania, Philadelphia, PA (US)
Sponsoring Org.:
National Science Foundation (NSF); USDOE
OSTI Identifier:
20000074
DOE Contract Number:  
FG02-86ER13615
Resource Type:
Journal Article
Journal Name:
Inorganic Chemistry
Additional Journal Information:
Journal Volume: 38; Journal Issue: 18; Other Information: PBD: 6 Sep 1999; Journal ID: ISSN 0020-1669
Country of Publication:
United States
Language:
English
Subject:
40 CHEMISTRY; CYCLOPENTADIENE; THERMODYNAMICS; DISSOCIATION; RADICALS; CHEMICAL BONDS; CHROMIUM COMPOUNDS; CARBONYLS; ENTHALPY; ENTROPY

Citation Formats

Woska, D.C., Ni, Y., and Wayland, B.B. Thermodynamic and activation parameters for dissociation of [CpCr(CO){sub 3}]{sub 2} and [Cp*Cr(CO){sub 3}]{sub 2} into paramagnetic monomers from {sup 1}H NMR shift and line width measurements. United States: N. p., 1999. Web. doi:10.1021/ic990342+.
Woska, D.C., Ni, Y., & Wayland, B.B. Thermodynamic and activation parameters for dissociation of [CpCr(CO){sub 3}]{sub 2} and [Cp*Cr(CO){sub 3}]{sub 2} into paramagnetic monomers from {sup 1}H NMR shift and line width measurements. United States. doi:10.1021/ic990342+.
Woska, D.C., Ni, Y., and Wayland, B.B. Mon . "Thermodynamic and activation parameters for dissociation of [CpCr(CO){sub 3}]{sub 2} and [Cp*Cr(CO){sub 3}]{sub 2} into paramagnetic monomers from {sup 1}H NMR shift and line width measurements". United States. doi:10.1021/ic990342+.
@article{osti_20000074,
title = {Thermodynamic and activation parameters for dissociation of [CpCr(CO){sub 3}]{sub 2} and [Cp*Cr(CO){sub 3}]{sub 2} into paramagnetic monomers from {sup 1}H NMR shift and line width measurements},
author = {Woska, D.C. and Ni, Y. and Wayland, B.B.},
abstractNote = {Substitution of pentamethyl cyclopentadienide (Cp*) for cyclopentadienide (Cp) is commonly used as an approach for increasing ligand steric requirements that can enhance dissociation of M-M bonded complexes. {sup 1}H NMR spectra for solutions prepared by dissolution of [CpCr(CO){sub 3}]{sub 2} and [Cp*Cr(CO){sub 3}]{sub 2} in toluene in the temperature range 190--390 K are interpreted in terms of thermodynamic and kinetic parameters for dissociation of the diamagnetic dimers into the paramagnetic monomers CpCr(CO){sub 3} and Cp*Cr(CO){sub 3}. There is no evidence in this temperature range for thermally populated excited states or non-Curie magnetic behavior of the monomers making a significant contribution to the NMR. An expression for the temperature dependence of the NMR chemical shift at limiting fast interchange of monomer and dimer in terms of the {Delta}H{degree} and {Delta}S{degree} for dimer dissociation is applied in determining the thermodynamic parameters for Cr-Cr bond homolysis of [CpCr(CO){sub 3}]{sub 2} ({Delta}H{sub 1}{degree} = 15.3 {+-} 0.6 kcal mol{sup {minus}1}; {Delta}S{sub 1}{degree} = 39 {+-} 2 cal K{sup {minus}1} mol{sup {minus}1}) and [Cp*Cr(CO){sub 3}]{sub 2} ({Delta}H{sub 2}{degree} = 14.2 {+-} 0.4 kcal mol{sup {minus}1}; {Delta}S{sub 2}{degree} = 47 {+-} 2 cal K{sup {minus}1}mol{sup {minus}1}). Rate constants and activation parameters have been evaluated from {sup 1}H NMR line broadening in the region of slow dimer-monomer interchange for dissociation of [CpCr(CO){sub 3}]{sub 2} ({kappa}{sub 1} (240 K) {approx} 59 s{sup {minus}1}; {Delta}H{sub 1}{double{underscore}dagger} = 17 {+-} 2 kcal mol{sup {minus}1}; {Delta}S{sub 1}{double{underscore}dagger} = 21 {+-} 6 cal K{sup {minus}1} mol{sup {minus}1}) and [Cp*Cr(CO){sub 3}]{sub 2} ({kappa}{sub 2} (240 K) {approx} 1.4 x 10{sup 4} s{sup {minus}1}; {Delta}H{sub 2}{double{underscore}dagger} = 16 {+-} 1 kcal mol{sup {minus}1}; {Delta}S{sub 2}{double{underscore}dagger} = 30 {+-} 6 cal K{sup {minus}1} mol{sup {minus}1}). Paramagnetic shifts also were used in deriving electron-proton coupling constants (A{sub H}) for CpCr(CO){sub 3}(9.22 x 10{sup 5} Hz) and Cp*Cr(CO){sub 3} (1.33 x 10{sup 6} Hz).},
doi = {10.1021/ic990342+},
journal = {Inorganic Chemistry},
issn = {0020-1669},
number = 18,
volume = 38,
place = {United States},
year = {1999},
month = {9}
}