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Kinetic and Computational Study of Dissociative Substitution and Phosphine Exchange at Tetrahedrally Distorted cis-[Pt(SiMePh2)2(PMe2Ph)2]

Wendt, Ola LU ; Deeth, Robert J. and Elding, Lars Ivar LU (2000) In Inorganic Chemistry 39(23). p.5271-5276
Abstract
The substitution kinetics of Me2PhP in cis-Pt(SiMePh2)2(PMe2Ph)2 (1) by the chelating ligand bis(diphenylphosphino)ethane has been followed at 25.0 °C in dichloromethane by stopped-flow spectrophotometry. Addition of the leaving ligand causes mass-law retardation compatible with a dissociative process via a three-coordinate transition state or intermediate. Exchange of Me2PhP in 1 has been studied by variable-temperature magnetization transfer 1H NMR in toluene-d8, giving kex326 = 1.76 ± 0.12 s-1, ΔH⧧ = 117.8 ± 2.1 kJ mol-1, and ΔS⧧ = 120 ± 7 J K-1 mol-1. An exchange rate constant independent of the concentrations of free phosphine, a strongly positive ΔS⧧, and nearly equal exchange and ligand dissociation rate constants also support a... (More)
The substitution kinetics of Me2PhP in cis-Pt(SiMePh2)2(PMe2Ph)2 (1) by the chelating ligand bis(diphenylphosphino)ethane has been followed at 25.0 °C in dichloromethane by stopped-flow spectrophotometry. Addition of the leaving ligand causes mass-law retardation compatible with a dissociative process via a three-coordinate transition state or intermediate. Exchange of Me2PhP in 1 has been studied by variable-temperature magnetization transfer 1H NMR in toluene-d8, giving kex326 = 1.76 ± 0.12 s-1, ΔH⧧ = 117.8 ± 2.1 kJ mol-1, and ΔS⧧ = 120 ± 7 J K-1 mol-1. An exchange rate constant independent of the concentrations of free phosphine, a strongly positive ΔS⧧, and nearly equal exchange and ligand dissociation rate constants also support a dissociative process. Density functional theory (DFT) calculations for a dissociative process give an estimate for the Pt−P bond energy of 98 kJ mol-1 for R = R‘ = Me, which is in reasonable agreement with the experimental activation energy given the differences between the substituents used in the calculation and those employed experimentally. DFT calculations on cis-Pt(PR3)2(SiR‘3)2 (R = H, CH3; R‘ = H, CH3) are consistent with the experimental molecular structure and show that methyl substituents on the Si donors are sufficient to induce the observed tetrahedral twist. The optimized Si−Pt−Si angle in cis-Pt(SiH3)2(PH3)2 is not significantly altered by changing the P−Pt−P angle from its equilibrium value of 104° to 80° or 120°. The origin of the tetrahedral twist is therefore not steric but electronic. The Si−Pt−Si angle is consistently less than 90°, but the Si−Si distance is still too long to support an incipient reductive elimination reaction with its attendant Si−Si bonding interaction. Instead, it appears that four tertiary ligands introduce a steric strain which can be decreased by a twist of two of the ligands out of the plane; this twist is only possible when two strong σ donors are cis to each other, causing a change in the metal's hybridization. (Less)
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Contribution to journal
publication status
published
subject
keywords
Dissociative substitution, DFT calculations, Phosphine exchange, Platinum(II), Kinetics and mechanism
in
Inorganic Chemistry
volume
39
issue
23
pages
6 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • scopus:0034645320
ISSN
1520-510X
DOI
10.1021/ic000492g
language
English
LU publication?
yes
id
13726bf1-d7cb-4358-817f-8f267269cd63
date added to LUP
2016-12-14 19:41:20
date last changed
2022-01-30 08:32:42
@article{13726bf1-d7cb-4358-817f-8f267269cd63,
  abstract     = {{The substitution kinetics of Me2PhP in cis-Pt(SiMePh2)2(PMe2Ph)2 (1) by the chelating ligand bis(diphenylphosphino)ethane has been followed at 25.0 °C in dichloromethane by stopped-flow spectrophotometry. Addition of the leaving ligand causes mass-law retardation compatible with a dissociative process via a three-coordinate transition state or intermediate. Exchange of Me2PhP in 1 has been studied by variable-temperature magnetization transfer 1H NMR in toluene-d8, giving kex326 = 1.76 ± 0.12 s-1, ΔH⧧ = 117.8 ± 2.1 kJ mol-1, and ΔS⧧ = 120 ± 7 J K-1 mol-1. An exchange rate constant independent of the concentrations of free phosphine, a strongly positive ΔS⧧, and nearly equal exchange and ligand dissociation rate constants also support a dissociative process. Density functional theory (DFT) calculations for a dissociative process give an estimate for the Pt−P bond energy of 98 kJ mol-1 for R = R‘ = Me, which is in reasonable agreement with the experimental activation energy given the differences between the substituents used in the calculation and those employed experimentally. DFT calculations on cis-Pt(PR3)2(SiR‘3)2 (R = H, CH3; R‘ = H, CH3) are consistent with the experimental molecular structure and show that methyl substituents on the Si donors are sufficient to induce the observed tetrahedral twist. The optimized Si−Pt−Si angle in cis-Pt(SiH3)2(PH3)2 is not significantly altered by changing the P−Pt−P angle from its equilibrium value of 104° to 80° or 120°. The origin of the tetrahedral twist is therefore not steric but electronic. The Si−Pt−Si angle is consistently less than 90°, but the Si−Si distance is still too long to support an incipient reductive elimination reaction with its attendant Si−Si bonding interaction. Instead, it appears that four tertiary ligands introduce a steric strain which can be decreased by a twist of two of the ligands out of the plane; this twist is only possible when two strong σ donors are cis to each other, causing a change in the metal's hybridization.}},
  author       = {{Wendt, Ola and Deeth, Robert J. and Elding, Lars Ivar}},
  issn         = {{1520-510X}},
  keywords     = {{Dissociative substitution; DFT calculations; Phosphine exchange; Platinum(II); Kinetics and mechanism}},
  language     = {{eng}},
  month        = {{10}},
  number       = {{23}},
  pages        = {{5271--5276}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Inorganic Chemistry}},
  title        = {{Kinetic and Computational Study of Dissociative Substitution and Phosphine Exchange at Tetrahedrally Distorted cis-[Pt(SiMePh2)2(PMe2Ph)2]}},
  url          = {{http://dx.doi.org/10.1021/ic000492g}},
  doi          = {{10.1021/ic000492g}},
  volume       = {{39}},
  year         = {{2000}},
}