Substitutions selectivesde carbonyle dans des complexes polynucleaires: IV. Regioselectivite sous activation thermique ou electrochimique pour un complexe binucleaire du fer non-symetrique

Abstract

Controlled replacement of carbon monoxide by trimethylphosphite in μ-(alkoxymethanethione- C, S)-μ-(methylthio)1,1,1,2,2,2-hexacarbonyldi complexes (I) is studied using thremal-activation or electron-transfer catalysis. The thermal reaction is not selective and leads to a mixture of mono- and di-substituted complexes II and III, respectively. The monosubstitution is regiospecific with P(OMe), coordinated to Fe(1) that is also linked to the carbene ligand. Electrochemical studies show an enhanced reactivity for the ligand substitution catalyzed by electron transfer. Controlled potential electrolysis of I in the presence of P(OMe) 3 follows an ECE mechanism selectively affording monosubstituted products; the major complex II is accompanied by complex IV in which P(OMe) 3 is bound to Fe(1) but occupies the axial position in contrast to complex II where P(OMe) 3 is equatorial. The isomerization IV → II establishes that IV is not the primary product of the thermal reaction but is the kinetic product when the reaction is promoted by electron transfer. Under thermal activation the ligand substitution may operate via reaarangement of an intermediate between the phosphite and the carbene ligand.