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 [1], CsCu₂I₃ [2], CsAg₂I₃ [3], also from chalcogenides, e.g., the mineral rasvumite, KFe₂S₃ [4]. 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 [5] and TlCuCl₃ [6] 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 [9] was known in literature. As the isostructural chloride [10], it contains isolated [CuX₄]²⁻ tetrahedra. Also (HPip)CuCl₃ is known with [CuCl_4/2Cl]⁻ chains [11]. 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.

(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 [1]. 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°.

The piperidinium ions in (HPip)Cu₂Br₃ have a chair conformation, see Figure 2. Their geometry is close to that in (HPip)₂CuBr₄ [9] 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.

(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 [13] 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 [14] and refined by full matrix least squares on F² with SHELXL-97 [15]. 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 [16]). 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 [12].

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.