Intermediate States on the Way to Edge-Sharing BO4 Tetrahedra in M6B22O39H2O (M=Fe, Co)

The new borates FeII6B22O39H2O (colourless) and CoII6B22O39H2O (dichroic: red/bluish) were synthesised under the high-pressure/high-temperature conditions of 6GPa and 880°C (Fe)/950°C (Co) in a Walker-type multi-anvil apparatus. The compounds crystallise in the orthorhombic space group Pmn21 (Z=2) with the lattice parameters a=771.9(2), b=823.4(2), c=1768.0(4)pm, V=1.1237(4)nm3, R1=0.0476, wR2=0.0902 (all data) for Fe6B22O39H2O and a=770.1(2), b=817.6(2), c=1746.9(4)pm, V=1.0999(4)nm3, R1=0.0513, wR2=0.0939 (all data) for Co6B22O39H2O. The new structure type of M6B22O39H2O (M=Fe, Co) is built up from corner-sharing BO4 tetrahedra and BO3 groups, the latter being distorted and close to BO4 tetrahedra if additional oxygen atoms of the neighbouring BO4 tetrahedra are considered in the coordination sphere. This situation can be regarded as an intermediate state in the formation of edge-sharing tetrahedra. The structure consists of corrugated multiple layers interconnected by BO3/BO4 groups to form Z-shaped channels. Inside these channels, iron and cobalt show octahedral (M1, M3, M4, M5) and strongly distorted tetrahedral (M2, M6) coordination by oxygen atoms. CoII6B22O39H2O is dichroic and the low symmetry of the chromophore [CoIIO4] is reflected by the polarised absorption spectra ([Delta]t=4650cm-1, B=878cm-1).