Unusual structural motif in a zwitterionic Fe(ii) complex of a tetradentate phosphine

Abstract

The reaction of meso-DPPEPM (DPPEPM = bis(diphenylphosphino-ethylphenylphosphino)methane) with one equivalent of FeBr₂ in tetrahydrofuran generates a zwitterionic compound [FeBr(κ²-DPPEPM)(κ³-DPPEPM-FeBr₃)] (1). Compound 1 exhibits an unusual bonding arrangement with two meso-DPPEPM ligands and one bromide coordinated to a single Fe(II) center. One of the DPPEPM ligands binds to iron in a κ² mode via two central phosphorus atoms, leaving the terminal phosphines dangling. The second DPPEPM binds through three phosphines, whereas the fourth one coordinates to the iron center of an external FeBr₃⁻ unit. A 1 : 2 reaction of FeBr₂ and meso-DPPEPM in tetrahydrofuran generates [FeBr(κ²-DPPEPM)(κ³-DPPEPM)]Br ([2]Br) in which the positive charge on the pseudo-octahedral unit is balanced by free Br⁻ as opposed to the phosphine-bound FeBr₃⁻ in 1. The compound [2]PF₆6 was obtained from [2]Br and TlPF₆. Solution structures for 1, [2]Br and [2]PF₆6 were assigned on the basis of ³¹P NMR. For all three compounds the data are consistent with five phosphorus atoms bound to the metal.