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Crystals 2012, 2(4), 1434-1440; doi:10.3390/cryst2041434
Published: 16 October 2012
Abstract: Piperidinium copper(I) bromide, (C5H12N)Cu2Br3, was obtained from a solution of CuBr2, piperidine, and HBr in ethanol. At 60 °C ethanol slowly reduces copper(II) to copper(I). Colorless plates of (C5H12N)Cu2Br3 crystallize in the triclinic space group P-1 with lattice parameters of a = 6.2948(10) Å, b = 8.2624(14) Å, c = 10.7612(17) Å, α = 75.964(19)°, β = 89.232(19)°, γ = 84.072(19)°, and Z = 2 at 173 K. [CuBr4] tetrahedra share edges and form [Cu2Br3]− ladders parallel to the a-axis. (C5H12N)+ ions adopt a chair conformation and connect the [Cu2Br3]− ladders via H-bonding. The (C5H12N)Cu2Br3 structure is related to the mineral rasvumite, KFe2S3, space group Cmcm, which is isostructural to several alkali copper(I) halides.
Piperidinium copper(I) bromide, (C5H12N)Cu2Br3, abbreviated as (HPip)Cu2Br3, has an interesting structure composed of [Cu2Br3]− ladders and (C5H12N)+ cations. The structural motif of ladders built from edge-sharing tetrahedra is known from alkali copper and silver halides, e.g., CsCu2Cl3, CsCu2Br3 , CsCu2I3 , CsAg2I3 , also from chalcogenides, e.g., the mineral rasvumite, KFe2S3 . Compared to those orthorhombic compounds the symmetry of (HPip)Cu2Br3 is strongly reduced due to the bulky (HPip)+ cations.
(HPip)Cu2Br3 was obtained during crystal growth experiments for (HPip)2CuBr4. Cu(II) compounds attract a lot of interest in solid state physics as quantum magnets. In compounds like SrCu2(BO3)2  and TlCuCl3  Cu2+ ions form dimers which are further linked to layers or ladders, respectively. An antiferromagnetic intra-dimer coupling results in a vanishing magnetic susceptibility at low temperatures. The reduced lattice dimensionality and weak inter-dimer interactions prevent three-dimensional magnetic order. But an external magnetic field can induce a quantum phase transition leading to exotic states of matter, e.g., a Luttinger liquid. For (HPip)2CuBr4 all quantum phases of a spin ladder were experimentally accessible for the first time and were investigated by inelastic neutron scattering on single crystals [7,8]. A prerequisite for the crystal growth of (HPip)2CuBr4 was an investigation of the HPipBr–CuBr2–ethanol system. (HPip)2CuBr4  was known in literature. As the isostructural chloride , it contains isolated [CuX4]2− tetrahedra. Also (HPip)CuCl3 is known with [CuCl4/2Cl]− chains . To our knowledge, no further structural data on piperidinium copper halides are published. Here we report on the synthesis and crystal structure of the new copper(I) halide (HPip)Cu2Br3.
2. Results and Discussion
(HPip)Cu2Br3 was synthesized from a solution of CuBr2 and HPipBr in ethanol. The slow reduction of Cu(II) to Cu(I) by ethanol at 60 °C offers a convenient method to obtain crystals of the rather insoluble (HPip)Cu2Br3. A direct synthesis from HPipBr and CuBr appears less facile. The low solubility of CuBr in ethanol hampers the reaction and prohibits the formation of sizeable crystals.
The crystal structure of (HPip)Cu2Br3 was determined by single crystal X-ray diffraction. The lattice parameters and experimental conditions are summarized in Table 1. The atomic positions are shown in Table 2 and selected distances and angles in Table 3. The structure has two Cu sites which are both tetrahedrally coordinated by Br− ions, see Figure 1. The Cu+ ions form a ladder centered at [x, 0.5, 0] with Cu–Cu distances of 2.889(2) Å and 2.903(2) Å along the rungs and longer distances of 3.015(2) Å and 3.283(2) Å along the legs. Br1 coordinates four Cu(I) ions alternatively in front and behind the ladder with longer Cu–Br distances between 2.53 Å and 2.58 Å. Br2 and Br3 are located at the side of the ladder. They coordinate only two Cu(I) ions at shorter distances around 2.41 Å. Accordingly, the Br2–Cu–Br3 angles of 124.8° are significantly wider than the other angles within the [CuBr4] tetrahedra. The [Cu2Br3]− ladder of (HPip)Cu2Br3, which also may be seen as a double chain of edge-sharing tetrahedra, is comparable to that of CsCu2Br3 . The Cs compound crystallizes in space group Cmcm and is therefore less distorted than the HPip one. The sequence of the Cu–Cu rung and leg distances of 3.094 Å and 2.909 Å, respectively, is inverted for CsCu2Br3. The Cu–Br distances of 2.425 Å and 2.587 Å are very similar, given that the structure of the Cs compound was determined at 298 K vs. that of the HPip one at 173 K. The widest Br–Cu–Br tetrahedral angle of 116.9° is significantly smaller than the respective Br2–Cu–Br3 angle of 124.8°.
|Table 1. Crystal data, data collection, and refinement details at 173 K.|
|Formula weight||452.97 g/mol|
|Space group, Z||P-1 (no. 2), 2|
|Lattice parameters||a = 6.2948(10) Å|
|b = 8.2624(14) Å|
|c = 10.7612(17) Å|
|α = 75.964(19)°|
|β = 89.232(19)°|
|γ = 84.072(19)°|
|Crystal size||0.45 × 0.15 × 0.1 mm3|
|MoKα radiation, λ||0.71073 Å|
|Absorption coefficient||14.978 mm−1|
|Angle range||1.95° < θ < 25.96°|
|Index ranges||−7 ≤ h ≤ 7|
|−10 ≤ k ≤ 10|
|−13 ≤ l ≤ 13|
|Independent reflections, Rint||1967, 0.0687|
|No. of parameters||101|
|Transmission, max., min.||0.566, 0.102|
|R1[F2 > 2σ(F2)]||0.0534|
|e-density, min., max.||−1.679, 1.552 e/Å3|
|Deposition no.||CCDC 873300|
|Table 2. Atomic coordinates and displacement factors Ueq/pm2.|
a The anisotropic Uij are available from .
|Table 3. Atomic distances/Å and angles/° at 173 K.|
|Cu1–Cu1 (rung)||2.889(2)||Cu2–Cu2 (rung)||2.903(2)|
|Cu1–Cu2 (leg)||3.015(2)||Cu1–Cu2 (leg)||3.283(2)|
The piperidinium ions in (HPip)Cu2Br3 have a chair conformation, see Figure 2. Their geometry is close to that in (HPip)2CuBr4  and the respective chlorides [10,11]. They connect the ladders via H-bonding, see Figure 3. The shortest distances of 3.384(6) Å and 3.370(7) Å are observed between N1 and Br2 and Br3, respectively, followed by C–Br distances in the range of 3.67 Å to 4 Å. Each [Cu2Br3]− ladder is surrounded by six stacks of HPip+ ions. Compared to CsCu2Br3, the bulky HPip+ ions widen the structure and reduce the symmetry from Cmcm to P-1.
3. Experimental Section
(HPip)Cu2Br3 was obtained from a solution of CuBr2 and HPipBr in absolute ethanol. CuBr2 (99,999%, Aldrich) is very soluble in ethanol with a deep violet color. The HPipBr solution was prepared by adding a slight excess of aqueous HBr (47%, suprapur, Merck) to a solution of piperidine (≥ 99% p.a., Sigma-Aldrich) in ethanol. The concentrated solutions with a HPip to Cu ratio of 2:1 were sealed in a flask under argon and heated to 50–60 °C. The slow reduction of Cu(II) to Cu(I) by ethanol resulted after several days in the growth of big, colorless, plate-like crystals of (HPip)Cu2Br3. They were separated from the solution and dried in vacuum. (HPip)Cu2Br3 is rather insoluble in ethanol. Under dry conditions (HPip)Cu2Br3 crystals are stable. In contact with ethanol or water the crystals are oxidized by air.
A suitable single crystal of (HPip)Cu2Br3 was selected for X-ray structure determination and mounted in a 0.3 mm capillary. Data were collected on a STOE IPDS diffractometer  at 173 K using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods using the program SHELXS-97  and refined by full matrix least squares on F2 with SHELXL-97 . All hydrogen atoms were included at calculated positions (d(C–H) = 97 pm) and treated as riding atoms using SHELXL-97 default parameters. An empirical absorption correction was applied using DELrefABS (PLATON ). The results of the structure determination as well as selected atomic distances and angles are summarized in Table 1, Table 2 and Table 3. Further details may be obtained from the Cambridge structure database under reference CCDC 873300 .
The reduction of Cu(II) halides by ethanol provides a convenient way for the synthesis of the respective Cu(I) compounds. The structure of (HPip)Cu2Br3 resembles those of the alkali homologues AM2X3, but its symmetry is lower due to the bulky HPip+ ions. The [Cu2Br3]− ladders, which also may be seen as double chains of edge-sharing tetrahedra, occur as a remarkably stable structural feature.
The financial support by the Swiss National Science Foundation is gratefully acknowledged.
Conflict of Interest
The authors declare no conflict of interest.
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