Jahn-Teller-distorted dimeric anions in (cat)[Mn2F8(H2O)2]•2H2O (cat = pipzH2, dabcoH2) and (dabcoH2)2[Mn2F8(H2PO4)2]

Using biprotonated dabco (1,4-diazabicyclo[2.2.2]octane) or pipz (piperazine) as counter cations, mixed-ligand fluoromanganates(III) with dimeric anions could be prepared from hydrofluoric acid solutions. The crystal structures were determined by X-ray diffraction on single crystals: dabcoH(2)[Mn2F8(H2O)(2)]center dot 2H(2)O (1), space group P2(1), Z=2, a = 6.944(1), b = 14.689(3), c = 7.307(1) angstrom, beta = 93.75(3)degrees, R-1 = 0.0240; pipzH(2)[Mn2F8(H2O)(2)]center dot 2H(2)O (2), space group P (1) over bar, Z=2, a = 6.977(1), b = 8.760(2), c= 12.584(3) angstrom, alpha = 83.79(3). beta = 74.25(3), gamma = 71.20(3), R-1 = 0.0451; (dabcoH(2))(2)[Mn2F8(H2PO4)(2)] (3), space group P2(1)/n, Z = 4, a = 9.3447(4), b = 12.5208(4), c = 9.7591(6) angstrom, beta = 94.392(8)degrees, R-1=0.0280. All three compounds show dimeric anions formed by [MnF5O] octahedra (0 from oxo ligands) sharing a common edge, with strongly asymmetric double fluorine bridges. In contrast to analogous dimeric anions of Al or Fe(III), the oxo ligands (H2O (1,2) or phosphate (3)) are in equatorial trans-positions within the bridging plane. The strong pseudo-Jahn-Teller effect of octahedral Mn(III) complexes is documented in a huge elongation of an octahedral axis, namely that including the long bridging Mn-F bond and the Mn-O bond. In spite of different charge of the anion in the fluoride phosphate, the octahedral geometry is almost the same as in the aqua-fluoro compounds. The strong distortion is reflected also in the ligand field spectra. (C) 2011 Elsevier B.V. All rights reserved.