Magnetic exchange interaction via HF2--bridges? Structure and magnetism ofpipzH(2)[MnF4(HF2)]

pipzH(2)[MnF4(HF2)] (pipz = piperazine) has been prepared from an aqueous h ydrofluoric acid solution of Mn(III) acetate and its crystal structure was determined by single-crystal X-ray analysis: triclinic, space group P (1) o ver bar, Z = 1, a = 5.636(1), b = 6.151(1), c = 6.498(1) Angstrom, alpha = 99.52(1), beta = 90.62(1), gamma = 107.43(1)degrees, wR(2) = 0.063, R = 0.0 23. The structure consists of hitherto unique anionic chains where [MnF4] u nits are bridged by HF2- anions. The resulting [MnF6] octahedra are strongl y elongated, mainly due to the Jahn-Teller effect (< Mn-F-eq> 1.84 Angstrom , Mn-F-ax 2.19 Angstrom). The geometry of the centrosymmetrical bifluoride anion is close to that of KHF2 (H-F 1.14 Angstrom), the angles at the angul ar bridge Mn-F-H are 111.7 degrees. Surprisingly, magnetic investigations r evealed pure paramagnetic behavior. Thus, in contrast to several examples w here antiferromagnetic coupling was observed via O-H . F hydrogen bonds, th e very strong hydrogen bonds in the symmetric F . H . F- anion are obviousl y not able to mediate magnetic exchange interactions. (C) 2000 Editions sci entifiques et medicales Elsevier SAS. All rights reserved.