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

The reaction of meso-DPPEPM (DPPEPM = bis(diphenylphosphino-ethylphenylphosphino)methane) with one equivalent of FeBr(2) in tetrahydrofuran generates a zwitterionic compound [FeBr(κ(2)-DPPEPM)(κ(3)-DPPEPM-FeBr(3))] (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 κ(2) 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 FeBr(3)(-) unit. A 1 : 2 reaction of FeBr(2) and meso-DPPEPM in tetrahydrofuran generates [FeBr(κ(2)-DPPEPM)(κ(3)-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 FeBr(3)(-) in 1. The compound [2]PF(6) was obtained from [2]Br and TlPF(6). Solution structures for 1, [2]Br and [2]PF(6) were assigned on the basis of (31)P NMR. For all three compounds the data are consistent with five phosphorus atoms bound to the metal.