Intermediate States on the Way to Edge‐Sharing BO 4 Tetrahedra in M 6 B 22 O 39 ⋅H 2 O (M=Fe, Co)

• Published in: Chemistry : a European journal Vol. 16; no. 46; pp. 13659 - 13670
• Main Authors: Neumair, Stephanie C., Knyrim, Johanna S., Oeckler, Oliver, Glaum, Robert, Kaindl, Reinhard, Stalder, Roland, Huppertz, Hubert
• Format: Journal Article
• Language: English
• Published: 10.12.2010
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201001611

Abstract The new borates Fe II 6 B 22 O 39 ⋅ H 2 O (colourless) and Co II 6 B 22 O 39 ⋅ H 2 O (dichroic: red/bluish) were synthesised under the high‐pressure/high‐temperature conditions of 6 GPa and 880 °C (Fe)/950 °C (Co) in a Walker‐type multi‐anvil apparatus. The compounds crystallise in the orthorhombic space group Pmn 2 1 ( Z =2) with the lattice parameters a =771.9(2), b =823.4(2), c =1768.0(4) pm, V =1.1237(4) nm 3 , R 1 =0.0476, wR 2 =0.0902 (all data) for Fe 6 B 22 O 39 ⋅ H 2 O and a =770.1(2), b =817.6(2), c =1746.9(4) pm, V =1.0999(4) nm 3 , R 1 =0.0513, wR 2 =0.0939 (all data) for Co 6 B 22 O 39 ⋅ H 2 O. The new structure type of M 6 B 22 O 39 ⋅ H 2 O (M=Fe, Co) is built up from corner‐sharing BO 4 tetrahedra and BO 3 groups, the latter being distorted and close to BO 4 tetrahedra if additional oxygen atoms of the neighbouring BO 4 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 BO 3 /BO 4 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. Co II 6 B 22 O 39 ⋅ H 2 O is dichroic and the low symmetry of the chromophore [Co II O 4 ] is reflected by the polarised absorption spectra ( Δ t =4650 cm −1 , B =878 cm −1 ).