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.