Advanced

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)
Please use this url to cite or link to this publication:
author
organization
publishing date
type
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
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
2017-11-14 09:50:51
@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},
  keyword      = {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},
  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},
  volume       = {39},
  year         = {2000},
}