Trans effect in water-exchange reactions of chromium(III) complexes

Bracken, Donald E.; Baldwin, H. W.

doi:10.1021/ic50136a015

Cited by 13 publications

(13 citation statements)

References 21 publications

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“…Several hours were often not sufficient to reach full equilibrium, placing unreasonable demands on instrument time. As an alternative strategy using the pseudo-first-order approximation, the time-dependent plots of the concentration of Pd−olefin species were fitted to first-order exponential functions and the equilibrium concentrations and exchange rate constants thereby derived. , Details of the data analysis are provided above in the Experimental Section, along with our rationale in applying the pseudo-first-order approximation. The concentrations of species present in the NMR experiments are also tabulated in the Supporting Information.…”

Section: Resultsmentioning

confidence: 99%

“…Following the introduction of the second olefin, the concentrations of all species were monitored over a period of up to eight hours at low temperature by 1 H NMR. The concentrations of the complexes were plotted as functions of time and the curves fitted to a first-order exponential function , in the form of eqs or by the Origin software program. In this manner, the exchange rate constants and the equilibrium concentrations could be calculated.…”

Section: Methodsmentioning

confidence: 99%

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A Mechanistic Investigation on Copolymerization of Ethylene with Polar Monomers Using a Cyclophane-Based Pd(II) α-Diimine Catalyst

Popeney

Guan

2009

J. Am. Chem. Soc.

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A detailed mechanistic investigation of the copolymerization of ethylene and methyl acrylate (MA) by a Pd(II) cyclophane-based alpha-diimine catalyst is reported. Our previous observations of unusually high incorporations of acrylates in copolymerization using this catalyst (J. Am. Chem. Soc. 2007, 129, 10062) prompted us to conduct a full mechanistic study on ethylene/MA copolymerization, which indicates a dramatic departure from normal Curtin-Hammett kinetic behavior as observed in copolymerization using the normal Brookhart type of Pd(II) alpha-diimine catalysts. Further investigation reveals that this contrasting behavior originates from the axial blocking effect of the cyclophane ligand hindering olefin substitution and equilibration. In equilibrium studies of ethylene with nitriles, the cyclophane catalyst was found to more strongly favor the linearly binding nitrile ligands as compared to the standard acyclic Pd(II) alpha-diimine catalysts. Ethylene exchange rates in the complexes [(N--N)PdMe(C(2)H(4))](+) (N--N = diimine) were measured by 2D EXSY NMR spectroscopy and found to be over 100 times slower in the cyclophane case. Measurement of the slow equilibration of ethylene, methyl acrylate, and 4-methoxystyrene in cyclophane-based Pd(II) olefin complexes by (1)H NMR and fitting of the obtained kinetic plots allowed for the estimation of exchange rates and equilibrium constants of the olefins. After extrapolation to typical polymerization temperature, DeltaG(double dagger) = 20.6 and 16.4 kcal/mol for ethylene-methyl acrylate exchange in the forward (ethylene displacement by methyl acrylate) and reverse directions, respectively. These values are of similar magnitude to the previously determined migratory insertion barriers of ethylene (DeltaG(double dagger) = 18.9 kcal/mol) and methyl acrylate (DeltaG(double dagger) = 16.3 kcal/mol) under equivalent conditions, but contrast strongly to the rapid olefin exchange seen in the Brookhart acyclic catalyst. The large barrier to olefin exchange hinders olefin pre-equilibrium, decreasing the cyclophane catalyst's ability to preferentially incorporate one monomer (in this case ethylene) over the other, thus giving rise to high comonomer incorporations.

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

A Mechanistic Investigation on Copolymerization of Ethylene with Polar Monomers Using a Cyclophane-Based Pd(II) α-Diimine Catalyst

Popeney

Guan

2009

J. Am. Chem. Soc.

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“…147 Support for the conjugate-base effect as being electronic rather than due merely to charge reduction reaction on [M(H2O)5OH] 2+ M ∆H q , kJ mol -1 ∆V q , cm 3 mol -1 ∆H q , kJ mol -1 ∆V q , cm 3 comes from several other key observations. First, Baldwin 148 found that within the series of chromium complexes [Cr III (H 2 O) 5 X] 2+ (X ) various monoanionic ligands) one of the water molecules is more labile than the other four, a result that cannot be explained by a mere charge reduction effect. Second, the water exchange rate constant ( 25 In the Rh III dimer, the waters cis and trans exchange at the same rate (k H 2 O (25 °C) ) ca.…”

Section: Lability Of Hydroxoaqua Ionsmentioning

confidence: 99%

Ligand Substitution Reactions at Inorganic Centers

2005

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“…9 The speciation of chromium aqua and chloro complexes in an equilibrium state has been investigated for over a century. [10][11][12][13][14][15][16][17][18][19][20][21] Table 1 shows a list of investigations. Most of the investigations were based on the reports by N. Bjerrum 10 or M. J. Bjerrum and J.…”

Section: Introductionmentioning

confidence: 99%