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Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)
Published on: November 22, 2016
Rapid phosphorus(III) ligand evaluation utilising potassium selenocyanate
Alfred Muller1, Stefanus Otto, Andreas Roodt
1Department of Chemistry, University of Johannesburg, Johannesburg, 2006, South Africa.
Dalton Transactions (Cambridge, England : 2003)
|January 25, 2008
Summary
The study investigated the reaction rates of selenocyanate (SeCN-) with tertiary phosphine ligands, finding that reaction speed depends on phosphorus electron density and steric bulk. These findings support an SN2 mechanism for this oxidative addition reaction.
Area of Science:
- Organometallic Chemistry
- Reaction Kinetics
- Inorganic Chemistry
Background:
- Tertiary phosphines are versatile ligands in organometallic chemistry.
- Understanding the reactivity of phosphines with chalcogens is crucial for synthesis and catalysis.
- Oxidative addition reactions are fundamental transformations in inorganic and organometallic chemistry.
Purpose of the Study:
- To investigate the oxidative addition of selenocyanate (SeCN-) to various tertiary phosphine ligands.
- To elucidate the kinetic and mechanistic aspects of this reaction.
- To explore the influence of electronic and steric factors of phosphine ligands on reaction rates.
Main Methods:
- UV-Vis stopped-flow spectrophotometry was employed to study reaction kinetics at 298 K in methanol.
- Conventional spectrophotometry was used for complementary rate measurements.
- Kinetic data were analyzed to determine rate constants (k(obs)) and activation parameters (DeltaH‡, DeltaS‡).
Main Results:
- Reaction rates followed the rate expression k(obs) = k(1)[SeCN(-)], indicating a first-order dependence on selenocyanate concentration.
- Rate constants (k(1)) varied over five orders of magnitude, demonstrating significant sensitivity to the electronic properties of the phosphorus center.
- Activation parameters supported an SN2 mechanism, with the nucleophilic attack of phosphorus on selenium as the rate-determining step.
- Reaction rates increased with solvent polarity and showed correlations with phosphine electronic parameters (pK(a), ¹J(P-Se), χd).
- Steric bulk of substituents, particularly ortho-phenyl groups, significantly decreased reaction rates.
Conclusions:
- The oxidative addition of SeCN- to tertiary phosphines proceeds via an SN2 mechanism.
- Phosphine ligand electronics and sterics are critical determinants of reaction rates.
- The study provides valuable insights into the reactivity patterns of phosphines with selenium electrophiles.
