Electrophilic attack on a series of dinuclear diphosphazane-bridged derivatives of iron by halogens. X-Ray crystal structures of [Fe2I(CO)5{µ-(MeO)2PN(Et)P(OMe)2}2]PF and [Fe2(µ-Br)(CO)4{µ-(PhO)2PN(Et)-P(OPh)2}2]PF6

Abstract

Treatment of [Fe₂(µ-CO)(CO)₄{µ-(RO)₂PN(Et)P(OR)₂}₂](R = CH₂, Me, Prⁱ, or Ph) with halogens results in the formation of [Fe₂X(CO)₅{µ-(RO)₂PN(Et)P(OR)₂}₂]⁺(X = Cl, Br, or I) which, on the basis of the structure established X-ray crystallographically for [Fe₂l(CO)₅{µ-(MeO)₂PN(Et)P(OMe)₂}₂] PF₆(A), contains an axially co-ordinated halogen. The iron atoms in these compounds are not only bridged by the two diphosphazane ligands but are also linked through a direct iron–iron bond [Fe–Fe = 2.787(3)Å in (A)]. Compound (A), and presumably the others, adopts a staggered conformation but the extent of twisting about the iron–iron bond is small as reflected by the P(1)–Fe(1)–Fe(2)–P(2) torsion angle of 28.8°. These pentacarbonyl derivatives readily decarbonylate in solution to produce the halogeno-bridged species [Fe₂(µ-X)(CO)₄{µ-(RO)₂PN(Et)-P(OR)₂}₂]⁺. On the basis of a single-crystal X-ray diffraction study on [Fe₂(µ-Br)(CO)₄{µ-(PhO)₂-PN(Et)P(OPh)₂)₂]PF₆(B), these tetracarbonyl compounds, in contrast to the pentacarbonyl species, adopt an eclipsed configuration with the plane containing the two iron and four phosphorus atoms being orthogonal to a plane containing the two iron atoms, the bridging halogen, and the four terminal carbonyl groups. Although these tetracarbonyl compounds do not add halide ions to produce [Fe₂X₂(CO)₄{µ-(RO)₂PN(Et)P(OR)₂}]₂ the tetramethoxydiphosphazane species [Fe₂X(CO)₅{µ-(MeO)₂PN(Et)P(OMe)₂}₂]⁺ and [Fe₂(µ-X)(CO)₄{µ-(MeO)₂PN(Et)-P(OMe)₂}₂]⁺ are very susceptible to halide ion attack affording the Michaelis–Arbuzov type rearrangement products [Fe₂X{µ-(MeO)₂PN(Et)P(O)(OMe)}(CO)₅(µ-(MeO)₂PN(Et)P(OMe)₂}] and [Fe₂(µ-X){µ-(MeO)₂PN(Et)P(O)(OMe)}(CO)₄{µ-(MeO)₂PN(Et)P(OMe)₂}] respectively. The reactions with other halogenation agents such as N-chloro- and N-bromo-succinimide, and mixed halogens (ICI) are also reported and a general mechanism for the halogenation of compounds of the type [Fe₂(µ-CO)(CO)₄{µ-(RO)₂PYP(OR)₂}₂][R = alkyl or aryl group; Y = CH, or N(Et)] is proposed.