Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270110027149/mx3031sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270110027149/mx3031Isup2.hkl |
CCDC reference: 790633
Dimethyldiphenylphosphonium iodide (3.98 mmol), iodine (5.03 mmol) and copper powder (20.02 mmol) were mixed and heated under reflux in hydroxyacetone (50 ml) under a nitrogen atmosphere. After 4 h, the solution became pale yellow, indicating the transformation of iodine to iodide. The mixture was filtered while hot and the solution kept in a refrigerator. Colourless crystals formed over the course of several days.
The structures were solved by charge flipping, giving the I-, Cu-, P- and a major part of the C-atom positions. Subsequently, the remaining C-atom positions were found using difference Fourier analysis. All non-H-atom positions were refined using full-matrix least-squares. Refinement was performed against F2. The H atoms were located by geometric methods and were allowed to ride, with methyl C—H = 1.00 Å [Uiso(H) = 1.5Ueq(C)] and phenyl C—H = 0.95 Å [Uiso(H) = 1.2Ueq(C)]. The Flack parameter (Flack, 1983) was calculated, yielding a value close to 1/2, viz. 0.43 (4). Refining the compound as a single crystal in either of the two possible absolute structures, or as a racemic twin, leads to indistinguishable results. In the final refinement, a racemic twin model was used leading to the twin fraction 0.43 (4). The main electron residuals are around I atoms.
Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: Superflip (Oszlányi & Sütő, 2004); program(s) used to refine structure: JANA2000 (Petricek et al., 2000); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: JANA2000 (Petricek et al., 2000).
(C14H16P)[Cu5I6] | F(000) = 2312 |
Mr = 1294.4 | Dx = 3.423 Mg m−3 |
Orthorhombic, Ama2 | Mo Kα radiation, λ = 0.71069 Å |
Hall symbol: A 2 -2a | Cell parameters from 9161 reflections |
a = 18.2110 (5) Å | θ = 3.6–34.5° |
b = 23.6861 (4) Å | µ = 11.61 mm−1 |
c = 5.8206 (1) Å | T = 100 K |
V = 2510.70 (9) Å3 | Flake, colourless |
Z = 4 | 0.16 × 0.11 × 0.05 mm |
Oxford Diffraction Excalibur3 diffractometer with a Sapphire-3 CCD detector | 5260 independent reflections |
Radiation source: Enhance (Mo) X-ray source | 3765 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.070 |
Detector resolution: 16.5467 pixels mm-1 | θmax = 34.6°, θmin = 3.6° |
ω scans | h = −28→28 |
Absorption correction: gaussian (CrysAlis RED; Oxford Diffraction, 2008) | k = −37→37 |
Tmin = 0.274, Tmax = 0.593 | l = −9→9 |
45244 measured reflections |
Refinement on F2 | H-atom parameters constrained |
R[F2 > 2σ(F2)] = 0.041 | Weighting scheme based on measured s.u.'s w = 1/[σ2(I) + 0.0004I2] |
wR(F2) = 0.065 | (Δ/σ)max = 0.001 |
S = 1.02 | Δρmax = 1.87 e Å−3 |
5260 reflections | Δρmin = −2.73 e Å−3 |
126 parameters | Absolute structure: Flack (1983) |
0 restraints | Absolute structure parameter: 0.43 (4) |
(C14H16P)[Cu5I6] | V = 2510.70 (9) Å3 |
Mr = 1294.4 | Z = 4 |
Orthorhombic, Ama2 | Mo Kα radiation |
a = 18.2110 (5) Å | µ = 11.61 mm−1 |
b = 23.6861 (4) Å | T = 100 K |
c = 5.8206 (1) Å | 0.16 × 0.11 × 0.05 mm |
Oxford Diffraction Excalibur3 diffractometer with a Sapphire-3 CCD detector | 5260 independent reflections |
Absorption correction: gaussian (CrysAlis RED; Oxford Diffraction, 2008) | 3765 reflections with I > 2σ(I) |
Tmin = 0.274, Tmax = 0.593 | Rint = 0.070 |
45244 measured reflections |
R[F2 > 2σ(F2)] = 0.041 | H-atom parameters constrained |
wR(F2) = 0.065 | Δρmax = 1.87 e Å−3 |
S = 1.02 | Δρmin = −2.73 e Å−3 |
5260 reflections | Absolute structure: Flack (1983) |
126 parameters | Absolute structure parameter: 0.43 (4) |
0 restraints |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
I1 | 0.75 | 0.568480 (18) | 0.89346 (16) | 0.01258 (17) | |
I2 | 0.92261 (2) | 0.570810 (14) | 1.40745 (16) | 0.01377 (12) | |
I3 | 0.91865 (2) | 0.431453 (13) | 0.89358 (11) | 0.01282 (12) | |
I4 | 0.75 | 0.441964 (18) | 0.38605 (14) | 0.01168 (14) | |
Cu1 | 0.84638 (4) | 0.50594 (4) | 1.1367 (3) | 0.0198 (3) | |
Cu2 | 1 | 0.5 | 1.1508 (4) | 0.0165 (3) | |
Cu3 | 0.84237 (4) | 0.50221 (5) | 0.6508 (3) | 0.0200 (3) | |
P1 | 0.75 | 0.25993 (9) | 0.7803 (4) | 0.0132 (6) | |
C1 | 0.8295 (3) | 0.2519 (2) | 0.6042 (9) | 0.0118 (16) | |
C2 | 0.8587 (4) | 0.1981 (2) | 0.5698 (11) | 0.0169 (19) | |
C3 | 0.9161 (4) | 0.1895 (3) | 0.4182 (16) | 0.027 (2) | |
C4 | 0.9455 (4) | 0.2355 (3) | 0.2952 (11) | 0.021 (2) | |
C5 | 0.9157 (4) | 0.2892 (3) | 0.3302 (12) | 0.021 (2) | |
C6 | 0.8574 (4) | 0.2976 (2) | 0.4851 (11) | 0.0182 (19) | |
C11 | 0.75 | 0.2071 (4) | 1.0072 (17) | 0.024 (3) | |
C22 | 0.75 | 0.3275 (3) | 0.9172 (18) | 0.019 (3) | |
H2 | 0.838983 | 0.16691 | 0.651847 | 0.0203* | |
H3 | 0.935626 | 0.152709 | 0.396929 | 0.0319* | |
H4 | 0.984839 | 0.230068 | 0.190376 | 0.0249* | |
H5 | 0.935066 | 0.320492 | 0.248086 | 0.0253* | |
H6 | 0.837402 | 0.334233 | 0.507649 | 0.0219* | |
H11 | 0.705268 | 0.212013 | 1.104853 | 0.0361* | 0.5 |
H12 | 0.749795 | 0.168559 | 0.937419 | 0.0361* | |
H13 | 0.794938 | 0.211814 | 1.104088 | 0.0361* | 0.5 |
H21 | 0.705123 | 0.331295 | 1.015026 | 0.0284* | 0.5 |
H22 | 0.794793 | 0.331245 | 1.015434 | 0.0284* | 0.5 |
H23 | 0.750084 | 0.357867 | 0.797702 | 0.0284* |
U11 | U22 | U33 | U12 | U13 | U23 | |
I1 | 0.0141 (3) | 0.00763 (17) | 0.0160 (4) | 0 | 0 | 0.0015 (4) |
I2 | 0.0165 (2) | 0.00968 (12) | 0.0151 (3) | −0.00011 (13) | 0.0003 (3) | 0.0009 (2) |
I3 | 0.01413 (19) | 0.00834 (12) | 0.0160 (3) | 0.00002 (12) | −0.0007 (3) | −0.0017 (3) |
I4 | 0.0148 (3) | 0.00713 (16) | 0.0131 (3) | 0 | 0 | 0.0029 (3) |
Cu1 | 0.0204 (4) | 0.0145 (4) | 0.0245 (5) | 0.0015 (5) | −0.0007 (6) | −0.0031 (5) |
Cu2 | 0.0168 (5) | 0.0131 (4) | 0.0197 (5) | −0.0016 (5) | 0 | 0 |
Cu3 | 0.0223 (5) | 0.0144 (4) | 0.0234 (4) | −0.0022 (6) | −0.0014 (7) | 0.0031 (5) |
P1 | 0.0156 (12) | 0.0097 (9) | 0.0142 (11) | 0 | 0 | 0.0013 (8) |
C1 | 0.010 (3) | 0.017 (3) | 0.009 (3) | −0.001 (2) | −0.001 (2) | 0.002 (2) |
C2 | 0.019 (4) | 0.015 (3) | 0.017 (3) | −0.004 (2) | 0.000 (3) | 0.003 (2) |
C3 | 0.027 (4) | 0.011 (2) | 0.041 (5) | 0.004 (2) | 0.001 (5) | −0.002 (4) |
C4 | 0.013 (3) | 0.029 (3) | 0.020 (3) | 0.003 (3) | 0.004 (3) | −0.004 (3) |
C5 | 0.020 (4) | 0.024 (3) | 0.019 (3) | −0.001 (3) | −0.004 (3) | 0.010 (2) |
C6 | 0.023 (4) | 0.015 (3) | 0.017 (3) | 0.003 (3) | 0.001 (3) | 0.004 (2) |
C11 | 0.027 (7) | 0.029 (5) | 0.016 (5) | 0 | 0 | −0.002 (4) |
C22 | 0.022 (5) | 0.016 (3) | 0.019 (5) | 0 | 0 | −0.008 (4) |
I1—Cu1 | 2.6981 (13) | C1—C6 | 1.382 (8) |
I1—Cu3 | 2.6997 (13) | C2—C3 | 1.382 (10) |
I2—Cu1 | 2.6022 (13) | C2—H2 | 0.950 |
I2—Cu2 | 2.6516 (14) | C3—C4 | 1.408 (10) |
I2—Cu3i | 2.6042 (13) | C3—H3 | 0.950 |
I3—Cu1 | 2.6167 (13) | C4—C5 | 1.398 (9) |
I3—Cu2 | 2.6594 (14) | C4—H4 | 0.950 |
I3—Cu3 | 2.5952 (13) | C5—C6 | 1.407 (10) |
I4—Cu1ii | 2.7356 (13) | C5—H5 | 0.950 |
I4—Cu3 | 2.6908 (14) | C6—H6 | 0.950 |
Cu1—Cu2 | 2.8023 (8) | C11—H11 | 1.000 |
Cu1—Cu3 | 2.830 (3) | C11—H12 | 1.000 |
P1—C1 | 1.785 (6) | C11—H13 | 1.000 |
P1—C11 | 1.819 (10) | C22—H21 | 1.000 |
P1—C22 | 1.789 (8) | C22—H22 | 1.000 |
C1—C2 | 1.395 (8) | C22—H23 | 1.000 |
Cu1—I1—Cu1iii | 81.16 (4) | I2ii—Cu3—I4 | 111.73 (7) |
Cu1—I1—Cu3 | 63.25 (5) | I2ii—Cu3—Cu1ii | 54.86 (4) |
Cu1—I1—Cu3iii | 111.14 (4) | I2ii—Cu3—Cu1 | 120.63 (5) |
Cu1iii—I1—Cu1 | 81.16 (4) | I3—Cu3—I4 | 107.70 (4) |
Cu1iii—I1—Cu3 | 111.14 (4) | I3—Cu3—Cu1ii | 123.37 (5) |
Cu1iii—I1—Cu3iii | 63.25 (5) | I3—Cu3—Cu1 | 57.47 (4) |
Cu3—I1—Cu3iii | 77.08 (4) | I4—Cu3—Cu1ii | 57.22 (4) |
Cu3iii—I1—Cu3 | 77.08 (4) | I4—Cu3—Cu1 | 127.22 (5) |
Cu1—I2—Cu2 | 64.46 (3) | Cu1ii—Cu3—Cu1 | 175.49 (5) |
Cu1—I2—Cu3i | 70.22 (5) | Cu1—Cu3—Cu1ii | 175.49 (5) |
Cu2—I2—Cu3i | 102.12 (4) | C1—P1—C1iii | 108.5 (3) |
Cu1—I3—Cu2 | 64.16 (3) | C1—P1—C11 | 110.1 (3) |
Cu1—I3—Cu3 | 65.79 (5) | C1—P1—C22 | 110.6 (3) |
Cu2—I3—Cu3 | 102.15 (4) | C1iii—P1—C1 | 108.5 (3) |
Cu1ii—I4—Cu1iv | 79.82 (4) | C1iii—P1—C11 | 110.1 (3) |
Cu1ii—I4—Cu3 | 66.98 (5) | C1iii—P1—C22 | 110.6 (3) |
Cu1ii—I4—Cu3iii | 114.28 (4) | C11—P1—C22 | 107.0 (5) |
Cu1iv—I4—Cu1ii | 79.82 (4) | P1—C1—C2 | 119.3 (4) |
Cu1iv—I4—Cu3 | 114.28 (4) | P1—C1—C6 | 120.1 (5) |
Cu1iv—I4—Cu3iii | 66.98 (5) | C2—C1—C6 | 120.3 (5) |
Cu3—I4—Cu3iii | 77.38 (4) | C1—C2—C3 | 120.9 (5) |
Cu3iii—I4—Cu3 | 77.38 (4) | C1—C2—H2 | 119.5 |
I1—Cu1—I2 | 109.92 (4) | C3—C2—H2 | 119.5 |
I1—Cu1—I3 | 114.43 (7) | C2—C3—C4 | 119.9 (6) |
I1—Cu1—I4i | 99.51 (3) | C2—C3—H3 | 120.1 |
I1—Cu1—Cu2 | 133.82 (6) | C4—C3—H3 | 120.1 |
I1—Cu1—Cu3 | 58.40 (4) | C3—C4—C5 | 118.8 (6) |
I1—Cu1—Cu3i | 121.65 (5) | C3—C4—H4 | 120.6 |
I2—Cu1—I3 | 117.22 (3) | C5—C4—H4 | 120.6 |
I2—Cu1—I4i | 110.37 (7) | C4—C5—C6 | 121.0 (6) |
I2—Cu1—Cu2 | 58.62 (4) | C4—C5—H5 | 119.5 |
I2—Cu1—Cu3 | 129.59 (5) | C6—C5—H5 | 119.5 |
I2—Cu1—Cu3i | 54.92 (4) | C1—C6—C5 | 119.2 (5) |
I3—Cu1—I4i | 103.66 (4) | C1—C6—H6 | 120.4 |
I3—Cu1—Cu2 | 58.66 (4) | C5—C6—H6 | 120.4 |
I3—Cu1—Cu3 | 56.74 (4) | P1—C11—H11 | 109.5 |
I3—Cu1—Cu3i | 122.19 (5) | P1—C11—H11iii | 109.5 |
I4i—Cu1—Cu2 | 126.67 (6) | P1—C11—H12 | 109.5 |
I4i—Cu1—Cu3 | 119.75 (5) | P1—C11—H12iii | 109.5 |
I4i—Cu1—Cu3i | 55.80 (4) | P1—C11—H13 | 109.5 |
Cu2—Cu1—Cu3 | 93.06 (7) | P1—C11—H13iii | 109.5 |
Cu2—Cu1—Cu3i | 89.63 (6) | H11—C11—H12 | 109.5 |
Cu3—Cu1—Cu3i | 175.49 (5) | H11—C11—H12iii | 109.8 |
Cu3i—Cu1—Cu3 | 175.49 (5) | H11—C11—H13 | 109.5 |
I2—Cu2—I2v | 111.42 (9) | H11iii—C11—H12 | 109.8 |
I2—Cu2—I3 | 114.031 (12) | H11iii—C11—H12iii | 109.5 |
I2—Cu2—I3v | 103.127 (11) | H11iii—C11—H13iii | 109.5 |
I2—Cu2—Cu1 | 56.92 (3) | H12—C11—H13 | 109.5 |
I2—Cu2—Cu1v | 125.37 (6) | H12—C11—H13iii | 109.1 |
I2v—Cu2—I2 | 111.42 (9) | H12iii—C11—H13 | 109.1 |
I2v—Cu2—I3 | 103.127 (11) | H12iii—C11—H13iii | 109.5 |
I2v—Cu2—I3v | 114.031 (12) | P1—C22—H21 | 109.5 |
I2v—Cu2—Cu1 | 125.37 (6) | P1—C22—H21iii | 109.5 |
I2v—Cu2—Cu1v | 56.92 (3) | P1—C22—H22 | 109.5 |
I3—Cu2—I3v | 111.48 (9) | P1—C22—H22iii | 109.5 |
I3—Cu2—Cu1 | 57.18 (3) | P1—C22—H23 | 109.5 |
I3—Cu2—Cu1v | 120.60 (6) | P1—C22—H23iii | 109.5 |
I3v—Cu2—I3 | 111.48 (9) | H21—C22—H22 | 109.5 |
I3v—Cu2—Cu1 | 120.60 (6) | H21—C22—H23 | 109.5 |
I3v—Cu2—Cu1v | 57.18 (3) | H21—C22—H23iii | 109.3 |
Cu1—Cu2—Cu1v | 176.64 (11) | H21iii—C22—H22iii | 109.5 |
Cu1v—Cu2—Cu1 | 176.64 (11) | H21iii—C22—H23 | 109.3 |
I1—Cu3—I2ii | 105.75 (4) | H21iii—C22—H23iii | 109.5 |
I1—Cu3—I3 | 115.09 (7) | H22—C22—H23 | 109.5 |
I1—Cu3—I4 | 102.62 (3) | H22—C22—H23iii | 109.6 |
I1—Cu3—Cu1ii | 121.39 (5) | H22iii—C22—H23 | 109.6 |
I1—Cu3—Cu1 | 58.34 (4) | H22iii—C22—H23iii | 109.5 |
I2ii—Cu3—I3 | 113.50 (3) |
Symmetry codes: (i) x, y, z+1; (ii) x, y, z−1; (iii) −x+3/2, y, z; (iv) −x+3/2, y, z−1; (v) −x+2, −y+1, z. |
Experimental details
Crystal data | |
Chemical formula | (C14H16P)[Cu5I6] |
Mr | 1294.4 |
Crystal system, space group | Orthorhombic, Ama2 |
Temperature (K) | 100 |
a, b, c (Å) | 18.2110 (5), 23.6861 (4), 5.8206 (1) |
V (Å3) | 2510.70 (9) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 11.61 |
Crystal size (mm) | 0.16 × 0.11 × 0.05 |
Data collection | |
Diffractometer | Oxford Diffraction Excalibur3 diffractometer with a Sapphire-3 CCD detector |
Absorption correction | Gaussian (CrysAlis RED; Oxford Diffraction, 2008) |
Tmin, Tmax | 0.274, 0.593 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 45244, 5260, 3765 |
Rint | 0.070 |
(sin θ/λ)max (Å−1) | 0.798 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.041, 0.065, 1.02 |
No. of reflections | 5260 |
No. of parameters | 126 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 1.87, −2.73 |
Absolute structure | Flack (1983) |
Absolute structure parameter | 0.43 (4) |
Computer programs: CrysAlis CCD (Oxford Diffraction, 2008), CrysAlis RED (Oxford Diffraction, 2008), Superflip (Oszlányi & Sütő, 2004), JANA2000 (Petricek et al., 2000), DIAMOND (Brandenburg, 1999).
Copper(I) halide complexes are well known for their structural variations. They exist in several different oligo- and polymeric forms from discrete anion complexes to infinite chains and sheets. It is also well known that this wide variation is due to the ability of copper to adopt different local geometries, primarily trigonal planar and tetrahedral coordination. The type of the counter-ion is crucial for stabilizing different complexes. Typically, small counter-ions give rise to infinite chains or two- or three-dimensional Cu–I complexes, but discrete Cu–I anions are formed when a large or bulky counter-ion is employed. Other important facts that influence the type of compound formed is the synthesis temperature and the solvent employed.
The title compound, (I) (Fig. 1), crystallizes in the space group Ama2, but there is a pronounced pseudosymmetry, indicating further centring (Fmm2). This is caused by the peculiar nature of the two rather well separated parts of the structure. The title compound is layered, consisting of anionic nets of the composition {[Cu5I6]-}2∞ interleaved with organic layers hosting the dimethyldiphenylphosphonium (DMDPP) counter-ions. Unlike in the pure iodide, the organic moieties form a dense herringbone pattern, as might be expected from their shape. The {[Cu5I6]-}2∞ entity appears to be a new addition to the zoo of Cu–I species.
Naked two-dimensional Cu–I polymeric species are relatively rare. The archetype is found in the trigonal and rhombohedral forms of CuI itself (Keen & Hull, 1995; Shan et al., 2009), where electroneutral double layers display iodide surfaces that are separated by van der Waals gaps. There are also several examples of intercalation compounds with this basic motif (Cariati et al., 2001; Shibaeva & Lobkovskaya, 1988; Wu et al., 2008; Hartl & Brudgam, 1989; Mishra et al., 2007). In the title compound, however, the anionic net has an underlying tetragonal rather than trigonal symmetry. Cu atoms form a defect, almost square-planar arrangement, with iodide above and below, in an arrangement reminiscent of that formed by BiO in BiOCl (Keramidas et al., 1993). Defect-free, the layer would be charge neutral, but every sixth Cu position is missing in a regular manner giving rise to a superstructure in the layer. Along the a direction, the unit cell is trebled by the alternation of defect-free rows of Cu atoms and rows where every second Cu atom is missing. As a consequence, along the c direction the unit cell is doubled due to the alternation of defect-free rows and rows with every third Cu position missing. The missing 1/6 of the Cu positions thus generate a sixfold, 3 × 2 superstructure in the ac plane. To complicate matters further, the DMDPP counter-ions are stacked in a herringbone pattern along the c direction, and the relative positions of these rows are out of step along the a direction, generating a further doubling of the unit cell along a. Thus, overall the result is a 6 × 2 superstructure as seen in both the direct-space image and the reciprocal-space image in Fig. 2.
Since the scattering from the Cu5I6 layer completely dominates the diffraction pattern of the compound, this superstructuring is evident in the reconstructed reciprocal-lattice layer perpendicular to [010] in Fig. 2. The symmetry allowed by the defect-free CuI layer is 4/mmm, and the defects lower the local symmetry to mmm. Further, a slight tilting of the DMDPP molecules with respect to the normal of the plane defined by CuI violates the reflection perpendicular to c, and the space-group symmetry is reduced to Ama2. It is notable that the Cu positions are described by larger displacement ellipsoids than the I atoms. This reflects a common phenomenon in Cu–I cluster systems. The size mismatch between Cu and I leads to a large configurational freedom for Cu within the convex hull defined by the I positions. At room temperature, many polymeric species are ionic conductors. The Cu5I6 layer is subject to relaxation around the vacancies, and this together with the tendency of the DMDPP counter-ion to form dense, flat layers is probably responsible for the stabilization of this unique form of a bidimensional Cu–I anion. In Fig. 3, the unit-cell packing is shown (projected along c). Cu2 is located at the intersection of a mirror plane perpendicular to a and a pseudo-mirror plane perpendicular to c, facing two defects. There is room for the surrounding iodides to relax into, and since they are all related by symmetry (or pseudosymmetry), Cu2 displays the most homogeneous tetragonal coordination of all the three independent Cu-atom positions in the structure, with two bond distances of 2.652 (1) Å and two of 2.659 (1) Å. The position Cu3 faces Cu atoms in three directions and a defect on one side. The Cu3 position moves towards the defect and, as a consequence, the Cu3I4 tetrahedron is highly irregular with two short bonds [2.604 (1) and 2.596 (1) Å] and two long bonds [2.691 (1) and 2.699 (1) Å]. Cu1 finally displays the most severe distortions of the tetrahedron. Like Cu3, it is displaced from the mirror plane, avoiding short Cu3···Cu3 contacts. It is the Cu the furthest away from the defects. Like for Cu3, there are two groups of distances, two short and two long, but the spread is larger: 2.603 (1), 2.617 (1), 2.698 (1) and 2.734 (1) Å. The distances from the defect to the surrounding iodides are 2.611 Å, well below the average Cu—I contacts for each of the Cu positions (2.663 Å for Cu1, 2.656 Å for Cu2 and 2.648 Å for Cu3). This indicates that the defects are a necessary condition for the stability of the layer at ambient conditions. It would be interesting to attempt the synthesis of similar compounds with larger, layer-forming counter-ions to dilute the cationic charge to create chemical pressure to reduce the number of defects. The limiting factor in determining the stability of the Cu–I layer may well be Cu—Cu contacts rather than Cu—I contacts. In the title compound, there is a number of short (below 3 Å) Cu···Cu distances. Cu2 and Cu3 each have two such contacts [2.803 (1) and 2.994 (1) Å, respectively] and, as a consequence, Cu1 has three. It may prove hard to increase these numbers significantly.