Magnetic exchange interaction via HF2-bridges ? Structure and magnetism of pipzH2[MnF4(HF2)]

Abstract pipzH 2 [MnF 4 (HF 2 )] (pipz=piperazine) has been prepared from an aqueous hydrofluoric acid solution of Mn(III) acetate and its crystal structure was determined by single-crystal X-ray analysis: triclinic, space group P 1 1, Z =1, a =5.636(1), b =6.151(1), c =6.498(1) A, α =99.52(1), β =90.62(1), γ =107.43(1)°, wR 2 =0.063, R =0.023. The structure consists of hitherto unique anionic chains where [MnF 4 ] units are bridged by HF 2 − anions. The resulting [MnF 6 ] octahedra are strongly elongated, mainly due to the Jahn–Teller effect (〈MnF eq 〉 1.84 A, MnF ax 2.19 A). The geometry of the centrosymmetrical bifluoride anion is close to that of KHF 2 (HF 1.14 A), the angles at the angular bridge MnFH are 111.7°. Surprisingly, magnetic investigations revealed pure paramagnetic behavior. Thus, in contrast to several examples where antiferromagnetic coupling was observed via OH⋯F hydrogen bonds, the very strong hydrogen bonds in the symmetric F⋯H⋯F − anion are obviously not able to mediate magnetic exchange interactions.