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Issue 5, 2008
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Rapid phosphorus(III) ligand evaluation utilising potassium selenocyanate

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Oxidative addition of SeCN to tertiary phosphine ligands has been investigated in methanol at 298 K by use of UV-Vis stopped-flow and conventional spectrophotometry. In most cases kobsvs. [SeCN] plots were linear with zero intercepts corresponding to a rate expression of kobs = k1[SeCN]. Reactions rates are dependent on the electron density of the phosphorus centre with k1 varying by five orders of magnitude from 1.34 ± 0.02 × 10−3 to 51 ± 3 mol−1 dm3 s−1 for P(2-OMe-C6H4)3 to PCy3 respectively. Activation parameters range from 27 ± 1 to 49.0 ± 1.3 kJ mol−1 for ΔH and −112 ± 9 to −140 ± 3 J K−1 mol−1 for ΔS supporting a SN2 mechanism in which the initial nucleophilic attack of P on Se is rate determining. Reaction rates are promoted by more polar solvents supporting the mechanistic assignment. Reasonable linear correlations were observed between log k1vs. pKa, 1JP–Se and χd values of the phosphines. The reaction rates are remarkably sensitive to the steric bulk of the substituents, and substitution of phenyl rings in the 2 position resulted in a decrease in the reaction rate. The crystal structures of SePPh2Cy and SePPhCy2 have been determined displaying Se–P bond distances of 2.111(2) and 2.1260(8) Å respectively.

Graphical abstract: Rapid phosphorus(iii) ligand evaluation utilising potassium selenocyanate

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The article was received on 20 Aug 2007, accepted on 31 Oct 2007 and first published on 16 Nov 2007

Article type: Paper
DOI: 10.1039/B712782K
Citation: Dalton Trans., 2008,0, 650-657
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    Rapid phosphorus(III) ligand evaluation utilising potassium selenocyanate

    A. Muller, S. Otto and A. Roodt, Dalton Trans., 2008, 0, 650
    DOI: 10.1039/B712782K

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