Stopped-flow mechanistic study of bromide substitution by an organonitrile at an iron(II) phosphinic centre; a π-electron driven process

Abstract

The kinetics of the displacement reactions of the bromide ligands of trans-[FeBr 2(depe) 2] (depe = Et 2PCH 2CH 2PEt 2) by the organonitrile NCCH 2C 6H 4OMe-4, in tetrahydrofuran (either in the absence or in the presence of added Br −), to give the corresponding mono- and dinitrile complexes trans-[FeBr(NCCH 2C 6H 4OMe-4)(depe) 2] + and trans-[Fe(NCCH 2C 6H 4OMe-4) 2(depe) 2] 2+, have been investigated by stopped-flow spectrophotometry. The substitution reaction occurs by a mechanism involving rate-limiting dissociation of bromo ligands to form the unsaturated intermediates [FeBr(depe) 2] + ( k 1 = 1.52 ± 0.02 s −1) and [Fe(NCR)(depe) 2] 2+ ( k 3 = 0.063 ± 0.008 s −1) which add the nitrile ligand to form those nitrile complexes. The competition between the nitrile and Br − for such metal centres has also been investigated and a stronger inhibiting effect of added Br − is observed for the substitution of the second bromo ligand relative to the first one. The kinetic data are rationalized in terms of π-electronic effects of these unsaturated metal centres and of the bromide and nitrile ligands.