Evidence for a SN2-type pathway in the exchange of phosphines at a [PhSe]+ centre. Issue 1 (16th October 2014)

• Record Type: Journal Article
• Authors: Forfar, Laura C.; Green, Michael; Haddow, Mairi F.; Hussein, Sharifa; Lynam, Jason M.; Slattery, John M.; Russell, Christopher A.
• Is Part Of: Dalton transactions. Volume 44:Issue 1(2015)
• Page Start: 110
• Page End: 118
• Publication Date: 2014-10-16
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c4dt02253j
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : The phosphine unit of cationic thio- and seleno-phosphonium complexes exchange via a SN 2-type exchange process. Abstract : A range of thio- and seleno-phosphonium cationic complexes [RE(PR′3 )] + [X] − (R = Me, Ph; E = S, Se; X = GaCl4, SbF6 ) have been synthesised and structurally characterised. Reaction of [PhSPPh3 ][GaCl4 ] and [PhSePPh3 ][GaCl4 ] with P t Bu3 results in the ready transfer of the "RS + " and "RSe + " fragments from PPh3 to the stronger electron donor P t Bu3 . 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 (Δ H ‡ 18.7 ± 12.0 kJ mol −1 ; Δ S ‡ −99.3 ± 36.3 J mol −1 K −1 ) to be compared with the corresponding values (Δ H ‡ 2.4 ± 1.1 kJ mol −1 and Δ S ‡ −58.1 ± 5.0 J mol −1 K −1 ) for the [PhSePPh3 ] + system. Importantly, the large negative entropy of activation and linear dependence on the rate of exchange are compatible with an SN 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.