Evidence for a S sub(N)2-type pathway in the exchange of phosphines at a [PhSe] super(+) centre

• Published in: Dalton transactions : an international journal of inorganic chemistry 2014-12, Vol.44 (1), p.110-118
• Main Authors: far, Laura C, Green, Michael, Haddow, Mairi F, Hussein, Sharifa, Lynam, Jason M, Slattery, John M, Russell, Christopher A
• Format: Article
• Language: English
• Subjects: Associative; Cation exchanging; Cationic; Compatibility; Exchange; Mathematical analysis; Phosphines; Sulfur
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c4dt02253j

A range of thio- and seleno-phosphonium cationic complexes [RE(PR' sub(3))] super(+)[X] super(-) (R = Me, Ph; E = S, Se; X = GaCl sub(4), SbF sub(6)) have been synthesised and structurally characterised. Reaction of [PhSPPh sub(3)][GaCl sub(4)] and [PhSePPh sub(3)][GaCl sub(4)] with P super(t)Bu sub(3) results in the ready transfer of the "RS super(+)" and "RSe super(+)" fragments from PPh sub(3) to the stronger electron donor P super(t)Bu sub(3). NMR experiments combined with an Eyring analysis on the corresponding degenerate phosphine exchange reaction allowed the thermodynamic values for the phosphine exchange reaction of the sulfur cation ( Delta H super() 18.7 plus or minus 12.0 kJ mol super(-1); Delta S super() -99.3 plus or minus 36.3 J mol super(-1) K super(-1)) to be compared with the corresponding values ( Delta H super() 2.4 plus or minus 1.1 kJ mol super(-1) and Delta S super() -58.1 plus or minus 5.0 J mol super(-1) K super(-1)) for the [PhSePPh sub(3)] super(+) system. Importantly, the large negative entropy of activation and linear dependence on the rate of exchange are compatible with an S sub(N)2-type exchange process. This conclusion is supported by DFT calculations which confirm that the phosphine exchange process occurs via an associative mechanism. The rate of exchange was found to increase from sulfur to selenium and those with aryl substituents underwent exchange faster than those with alkyl substituents.