E4 Butterfly Complexes (E=P, As) as Chelating Ligands

The coordination properties of new types of bidentate phosphane and arsane ligands with a narrow bite angle are reported. The reactions of [{CpFe(CO)(2)}(2)(,(1:1)-P-4)] (1a) with the copper salt [Cu(CH3CN)(4)][BF4] leads, depending on the stoichiometry, to the formation of the spiro compound [{{CpFe(CO)(2)}(2)((3),(1:1:1:1)-P-4)}(2)Cu](+)[BF4](-) (2) or the monoadduct [{CpFe(CO)(2)}(2)((3),(1:1:2)-P-4){Cu(MeCN)}](+)[BF4](-) (3). Similarly, the arsane ligand [{CpFe(CO)(2)}(2)(,(1:1)-As-4)] (1b) reacts with [Cu(CH3CN)(4)][BF4] to give [{{CpFe(CO)(2)}(2)((3),(1:1:1:1)-As-4)}(2)Cu](+)[BF4](-) (5). Protonation of 1a occurs at the wing tip phosphorus atoms, which is in line with the results of DFT calculations. The compounds are characterized by spectroscopic methods (heteronuclear NMR spectroscopy and IR spectrometry) and by single-crystal X-ray diffraction studies.