Open-framework iron borophosphates have received much attention for their potential magnetic properties. A broad spectrum of borophosphates with various dimensionalities and stoichiometries has been prepared using hydro-/solvothermal, as well as boric acid flux methods with or without the employment of organic amines as templates [1-7]. So far, the ionothermal synthesis of iron borophosphates has not been explored [8-11].

The known borophosphate anionic frameworks can be derived from a few typical borophosphate fundamental building units (FBUs) with B/P ratios of 6/1, 5/1, 3/1, 3/2, 1/1, 3/4, 2/3, 1/2, 2/5, 1/3 and 1/4 [12,13]. Among them, nonameric FBUs, [B₃P₆X₂₆] [14] and [BP₂X₈] [15] (X = O, OH) with a B/P ratio of 1/2, form the largest group. Borophosphates of the general formula AM[BP₂O₈(OH)] (A = K, Rb, Cs or NH₄, M = V, Fe, Al, Ga, Sc) have been described in the past several years [12]. Here we report on a new member of this family, KFe[B(PO₄)₂OH], obtained by ionothermal synthesis.

KFe[BP₂O₈(OH)] was prepared by reacting ionothermally FeCl₃·4H₂O, KH₂PO₄, H₃BO₃ and 1-butyl-1-methylpyrrolidinium bromide, [C₄mpy]Br, in a ratio of 3:3:1;1.5 at 200 °C for 5 days. Upon cooling crystals of KFe[BP₂O₈(OH)] formed. The crystal structure of KFe[BP₂O₈(OH)] (monoclinic, P2₁/c, a = 9.372(2) Å, b = 8.146(2) Å, c = 9.587(2) Å, β = 101.18(3)°, V = 718.0(2) ų and Z = 4) is built up by a 3-D framework of {Fe(III)O₆} octahedra, {BO₃(OH)} and {PO₄} tetrahedra. It is isotypic with KAl[BP₂O₈(OH)] [16], (NH₄)M[BP₂O₈(OH)] (M, Al, Ga, Fe, V) [17-20], RbM[BP₂O₈(OH)] (M = Al, Ga, V, Fe) [21-25], Cs[BP₂O₈(OH)] (M = Al, Ga, Fe) [26-28].

The asymmetric unit of KFe[BP₂O₈(OH)] contains one crystallographically distinct Fe³⁺ cation, one B³⁺ cation, one K⁺ cation, two crystallographically distinct P⁵⁺ cations, which are all coordinated by oxygen (Figure 1).

In KFe[BP₂O₈(OH)], the Fe(1) atom connects three O atoms to three P(1), two O atoms to two P(2) atoms, and one O(9)H to the B(1) atom. The Fe–O internuclear distances vary in the range of 1.954(4)–2.143(4) Å. All boron and phosphorus atoms are tetrahedrally coordinated by oxygen atoms. The P(1) atom connects via three oxygen atoms to Fe(1) and one O atom to B(1) atom; the P(2) atom connects two Fe(1) atoms and two B(1) atoms via four oxygen atoms, respectively. The P–O bond lengths vary in the range of 1.505(4)–1.562(3) Å. The B(1) atom connects via one oxygen atom to a P(1) atom, two oxygen atoms to P(2) atoms and one O(9)H to Fe(1) atom, respectively. The B–O bond lengths vary in the range of 1.457(6)–1.475(7) Å.

KFe[BP₂O₈(OH)] features an unbranched trimer, 3□:3□, as the basic building unit (BBU) (Figure 2a). These BBUs are connected to an infinite open-branched (oB) vierer ¹_∞{[BP₂Φ₉]₂ (Φ = O, OH) chain (Figure 2b). The condensation of this borophosphate FBU with {Fe(III)O₅(OH)} octahedra by sharing common oxygen atoms with five phosphate tetrahedra and an OH group with a hydrogenborate group link to a 3-D framework (Figure 2c). It features 8-ring channels along the [100] direction enclosed by two FeO₅(OH) octahedra, two BO₃(OH) and four PO₄ tetrahedra.

KFe[BP₂O₈(OH)] is prepared under ionothermal synthesis conditions using the ionic liquid 1-butyl-1-methylpyrrolidinium bromide, [C₄mpyr]Br, as the solvent. A mixture of FeCl₃·4H₂O (99%, Fluka), KH₂PO₄ (99%, J.T. Baker), H₃BO₃ (99.8%, Appl. Chem.) and [C₄mpy]Br (99%, Merck) was reacted in a 3 mL Teflon-lined stainless steel container at 200 °C for 5 days followed by cooling to room temperature. The products were filtered off, washed with deionized water and acetone, filtered by suction, and dried at 60 °C for one day.

A suitable single crystal of KFe[BP₂O₈(OH)] was selected for single-crystal X-ray diffraction (XRD) analysis. The data were collected at ambient temperature using graphite-monochromated Mo-Kα radiation on an Image Plate Diffraction System, IPDS I, (Stoe, Darmstadt, Germany). The data were corrected for Lorentz and polarization effects. Data correction was carried out with the program X-RED [29]. A face-indexed numerical absorption correction (X-SHAPE) was applied [30]. The structure was solved by direct methods and refined by full-matrix least-squares techniques with the SHELXTL crystallographic software package [31]. The Fe, B, P, and O atoms could be unambiguously located. The K⁺ ions were subsequently located from a difference Fourier map. In the final refinement cycles H atoms associated with the hydroxyl groups were added computationally.

Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.hmtl) on quoting ICSD-422900.

Crystal data for KFe[BP₂O₈(OH)], (312.71 gmol⁻¹); diffractometer IPDS-I, Stoe, Darmstadt; Mo-K_α (graphite monochromator, λ = 71.073 pm); T = 293(2) K; 2θ_max = 56.18°; 100 images, 0° ≤ φ ≤ 200°; Δφ = 2°; indices: -12 ≤ h ≤12, -10 ≤ k ≤ 10, -12 ≤ l ≤12; transmission (min, max) = 0.0307, 0.0929; ρ_calc = 2.893 g cm⁻³; 8321 reflection intensities measured of which 1726 were symmetrically independent, R_int = 0.0653, F(000) = 612, μ = 27.547 mm⁻¹. Monoclinic, P2₁/c (no. 62), a = 9.372(2) Å, b = 8.146(2) Å, c = 9.587(2) Å, β = 101.18(3)°, V = 718.0(2) ų and Z = 4. R values: R₁/wR₂ for final indices with [I_o > 2σ(I_o)]: 0.0400/ 0.1095 and for all data: 0.0541/0.1122; S_all = 1.071.

A new open-framework iron borophosphate KFe[BP₂O₈(OH)], has been prepared under ionothermal conditions using the ionic liquid 1-butyl-1-methyl pyrrolidinium bromide, [C₄mpy]Br, as the solvent. The successful preparation of the new iron borophosphate in an ionic liquid demonstrates not only that many more open-framework borophosphates could be obtained, but also that the ionothermal method is a promising method to synthesize new kinds of open-framework materials.