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Solid State Sciences 13 (2011) 768e772

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Solid State Sciences

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Bis(m3-iodo)-pentakis(m2-iodo)-penta-copper(I) e A fully ordered, isolated[Cu5I7]2� cluster

Ehsan Jalilian a, Sven Lidin b,*

aDepartment of Materials and Environmental Chemistry, Arrhenius Laboratory for Chemistry, Stockholm University, 106 91 Stockholm, Swedenb Polymer and Materials Chemistry, Chemical Centre, Lund University, PO Box 124, 221 00 Lund, Sweden

a r t i c l e i n f o

Article history:Received 24 January 2010Received in revised form28 March 2010Accepted 30 March 2010Available online 14 April 2010

Keywords:Copper(I) iodide clusterStructure

* Corresponding author. Tel.: þ46 8 161256; fax: þE-mail addresses: Ehsan.Jalilian@MMK.SU.Se (E. J

lth.se (S. Lidin).

1293-2558/$ e see front matter � 2010 Elsevier Masdoi:10.1016/j.solidstatesciences.2010.03.019

a b s t r a c t

Two new compounds, [(C4H9)4P]2[Cu5I7] and [(C4H9)4P]2[Cu5I7]$CH3COCH2OH were synthesized bysolvolysis and their crystal structures were solved. The first compound is disordered, but the solvaterepresents the first example of a fully ordered [Cu5I7]2� cluster. It displays a molecular symmetry veryclose to C2. Both compounds crystallize in space group P21/c.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

The great flexibility of coordination in silver and copper halidesleads to a plethora of cluster compounds. For compounds formed inthe absence of counter ions that are more strongly coordinatingthan the halide, the clusters are essentially naked, and presenta pure halide surface to the exterior. Even under these restrictiveconditions, the variety of cluster geometries is bewildering, but inhomologous series of compounds, formed under similar conditions,there is a simple relation between the size of the counter ion andthe size of the cluster formed. On the other hand, differentsynthesis routes will produce dramatically different compoundsfrom the same constituents.

The isolated cluster [Cu5X7]2� was first reported in 1984 [1] inthe compound [(C3H7)4N]2[Cu5I7]. The general geometry of thiscluster is that of a pentagonal bipyramid of iodide with Cu atomsfilling all the tetrahedral interstices, but the Cu atoms are displacedfrom the central positions towards the faces of the tetrahedra toavoid unphysically short CueCu distances. The external shape ofthe cluster is still very close C5 symmetry (conf. Fig.1), and theweakinteraction between the cluster and the counter ions allowsmultiple orientations of the cluster. The cluster is indeed disor-dered, and in the model presented, each of the five Cu atoms isdistributed over two positions, each between 0.7 and 1.0�A apart. In

46 8 152187.alilian), Sven.Lidin@polymat.

son SAS. All rights reserved.

another report from 1988 [2], the corresponding bromine cluster isgiven is the compound [(C4H9)3(CH3)N]2[Cu5Br7]. Here the puck-ering of the Cu5 ring is more severe, four out of five Cu positionshave a trigonal rather than a tetrahedral environment, and thepentagonal bipyramid of the halide is distorted far away from theC5 symmetry adopting a pseudo C2 symmetry, and there is nodisorder reported (conf. Fig. 2). Presumably, the shorter BreBrdistances (compared to IeI) forces this behavior.

2. Experimental

Elemental copper powder in excess (20 mmol) was immersed ina solution of 3.72 mmol [(C4H9)4P]I in 50 ml of acetol, and 5 mmolof iodine was added. The mixture was refluxed under stirring untilthe solution paled to colourless to indicate the complete reductionof iodine to iodide. The solution was filtered while hot and kept at280 K. The Cu5I7 cluster was found in two different compoundsformed by the solvolysis reaction of Cu with iodine in acetol con-taining [(C4H9)4P]þ as counter ion. Initially, a disordered phase,[(C4H9)4P]2[Cu5I7] (I) crystallizes, to be replaced over the course ofa few days by aging in the mother liquor at 4 �C by the fully orderedcompound [(C4H9)4P]2[Cu5I7]$CH3COCH2OH (II). Since the structureof the original cluster compound, [(C3H7)4N]2[Cu5I7], was deter-mined at ambient conditions, this compound was resynthesized,and its structure was redetermined. Single crystals suitable for theX-ray diffraction experiment were chosen based on shape andappearance, and mounted on glass fibers. Data collection was per-formed at 100 K on an Oxford diffraction XCalibur3 system using

Fig. 2. The Cu5Br7 cluster from the work of Jagner [2]. On top (a) the cluster is vieweddown the pseudo five fold direction. At the bottom (b), the cluster is turned 90� . Notehow the puckering of the Br and Cu arrangements both strongly violate the five foldsymmetry, and how instead a pseudo two fold, parallel to the line of sight is apparent.

Fig. 1. The Cu5I7 cluster from [(C3H7)4N]2[Cu5I7]. On top (a) the cluster is viewed downthe pseudo five fold direction. At the bottom (b), the image shows a view perpen-dicular to the pseudo five fold. Note the relatively planar arrangement of equatorialiodides.

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MoKa radiation. Data reduction was performed using the softwarepackage CrysAlis Red [3]. The structures were solved by chargeflipping [4] as implemented in the program Superflip [5]. All figuresin the paper are produced using Diamond 2.1 [6].

3. Results and discussion

The structure of [(C3H7)4N]2[Cu5I7] was redetermined at 100 K,and it agrees well the solution previously published. The iodinepositions for a nearly perfect pentagonal bipyramid, and allcopper positions are split, and displaced away from the centres ofthe tetrahedral interstices of that bipyramid to reside closer to theexterior faces (conf. Fig. 1), thereby increasing CueCu distances.Disregarding the unphysically short distances created by the splitpositions, CueCu contacts are between 2.39 �A and 2.66 �A.Compound I is problematic. On cooling from ambient temperatureto 100 K, there is a weak doubling of the a-axis. The structure ofthe compound was first determined from the ambient tempera-ture data. This structure suffers from extensive disorder not onlyon the cluster sites, but the majority of the carbon positions of thecounter ion were not possible to locate. Using the 100 K data, butignoring the weak doubling produced a structure that refines well

but the model has split positions for several Cu and I positions.This ordering on cooling suggests that the disorder has a dynamiccomponent. There are significant problems in assigning a defini-tive shape to the cluster due to the disorder. The most significantresidual electron densities were found around the heavy atoms,and introducing additional split positions lowered the R valueincrementally, but it was decided to remove any split positionthat refined to an occupancy of less than 10%. Comparing themodels with additional split positions with the reduced modelthus produced, the difference in agreement was deemed insig-nificant (less then 0.5% on the R value based on F). A furthercomplication in the structural refinement was the location of thecounter ions. Since so much of the scattering power of the crys-tals come from the cluster, the disorder in this entity results indifficulties in locating and refining all the lighter atomic positionsin the counter ion. To reduce the number of parameters, the twocounter ions where modeled using constrained CeC distances andCeCeC angles. One of the two independent counter ions is fullyordered while the other shows disorder that was possible tomodel using split positions for the side chains. Hydrogen atomic

Fig. 3. The disordered cluster from compound I. The top left (a) shows the cluster projected down the pseudo five fold direction. The top right view (b) shows the same clusterturned by 90� . The split positions in the top images can be interpreted as two distinct clusters. One, bottom left (c) is similar to that of the Cu5Br7 cluster by Jagner, while the other(bottom right, d) adopts a different Cu configuration. It is particularly notable that the Cu5 ring features two neighboring Cu atoms that are displaced in the same direction out of theequatorial plane, creating a short (but not unreasonably so) CueCu distance. This is the behavior noted by Hartl.

E. Jalilian, S. Lidin / Solid State Sciences 13 (2011) 768e772770

positions were added in a riding model for the fully orderedcounter ion only. In Fig. 3 the final model for the cluster is shown.Three Cu positions and one I position are split. Using the majoritypositions only produces a fully consistent model with reasonableCueCu and CueI distances (Fig. 3c) reminiscent of the Cu5Br7cluster in [(C4H9)3(CH3)N]2[Cu5Br7] having a pseudo C2 axis.Attempts to use the weak super structure reflections to producea well ordered model failed. The obvious model generated byproducing two distinct, ordered sites for the cluster or for thedisordered counter ion failed, as did all attempts to phase thestructure de novo. The final model for compound I is therefore onethat does not take the super structure reflections into account.

The ordered compound (II) produced after aging affordsa much more dependable basis for analysis of the cluster geom-etry. This compound is a solvate, containing one molecule of acetolper cluster. All entities in the structure are fully ordered, thecluster, the counter ions and the solvent molecule. The refinementwas substantially easier than for the disordered compound, andthe final agreement between data and model was excellent. Theonly constraints used were for hydrogen positions that were fixedin a riding model. The cluster is shown in Fig. 4. It is notable thatthe exterior shape from the seven iodide positions is close to anideal pentagonal bipyramid, while the Cu arrangement within thisunit adheres to the pseudo C2 symmetry adopted by the Brcompound. This pseudo symmetry goes a lot deeper than super-ficial appearances, and it is instructive to study the bond distancesin the cluster closely (Fig. 4c). The five equatorial iodides are eachbound to two coppers. The bond distances are highly homoge-neous, varying between 2.520 �A and 2.578 �A, while the two apicaliodides bond to three coppers, two at short distances(2.599e2.658 �A) and one longer distance 2.802 �A and 2.858 �A

respectively. The next CueI contact is at 3.36 �A. The CueCucontacts are also all very similar, varying between 2.548 �A and2.693 �A. Comparing to the disordered cluster in the previouslypublished compound, [(C3H7)4N]2[Cu5I7], the similarities arestriking for the equatorial iodides. The distances between theseiodides and copper are limited to the range 2.52e2.59 �A. Thedistances between the apical iodides and copper differ subtlybetween the two compounds: They range continuously from 2.61to 2.75 �A. This absence of distances between 2.75 �A and 3.0 �Asignals the main difference between the two clusters (apart fromone being ordered and the other disordered). In compound II, oneCu position, Cu5, resides in the plane defined by the equatorialiodides and this copper has a distinctly tetrahedral environment.In [(C3H7)4N]2[Cu5I7] on the other hand, all copper positions aresplit out of this plane. This produces a cluster where all copperatoms have triangular coordination. The consequences for the five-membered ring of Cu is that in compound II this has a nearlyperfect C2 symmetry, with the 2 fold axis passing through theunique tetrahedrally coordinated Cu5. As discussed in the originalpaper by Hartl, in [(C3H7)4N]2[Cu5I7] the displacement of allcopper atoms of the five-membered ring away from the equatorialplane leads to a frustration of the zig-zag arrangement, forcingtwo adjacent Cu positions to displace in the same direction rela-tive to that plane. The local symmetry for this arrangement is bestdescribed by local mirror plane through both apical and oneequatorial iodide and through on split Cu position. There is noresidual electron density in the equatorial plane to suggest thattetrahedrally coordinated Cu has been overlooked in the analysisof the structure of [(C3H7)4N]2[Cu5I7]. This is in perfect agreementwith the work of Hartl. Crystallographic data including FoFc tableshave been deposited as supplementary material.

Fig. 4. The fully ordered cluster of compound II viewed along the pseudo five fold (top a)and the pseudo two fold (centre b) directions. Note the almost perfect pseudo two foldprojection in b. At the bottom (c), the cluster is shown at an oblique angle to allowlabeling of the bonds. Note how the pseudo two fold axis that runs through Cu5 and I4relates pair wise similar bonds.

Fig. 5. The polymeric cluster unit from the compound [C16H19N2þ]2n n[Cu5I7]. [8]. On

top (a) the pentagonal units are viewed along the five fold direction, dotted linesindicate bonds between formal sub-cluster units. At the bottom (b), the samecompound is viewed along the monoclinic b-axis. Note how the pseudo C2 axis fromcompound II is realized as a bona fide symmetry operation in this compound.

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It is also interesting to compare the isolated [Cu5I7] clusters withpolymeric species that may be dissected to yield these units. In[C16H19N2

þ]2n n[Cu5I72�] [8] clusters very similar to those in compoundII have been fused by to infinite chains. As shown in Fig. 5a, anequatorial iodide fromone cluster bonds to a copper fromanadjacentcluster and symmetrically vice versa. This leads to three out of five Cupositions having tetrahedral coordination. The approximate C2 axis incompound II is realized in [C16H19N2

þ]2n n[Cu5I7]. A more complex

example of fusion of [Cu5I7] clusters is found in the compounds[C12H30O9M2þ]n n[Cu5I72�] M2þ ¼ Zn,Cd,Sr [9]. Here, the fusion isasymmetric, which leads to a stronger distorsion of the [Cu5I7] sub-clusters and to disorder of the copper positions.

4. Conclusion

The cluster in the fully ordered compound (II) is rather differentfrom that in the work of Hartl. Both clusters display an iodidearrangement that is very close to a perfect pentagonal bipyramid.The arrangement of Cu however, appears to be quite distinct in thetwo compounds. Formally, this means that while the cluster in[(C3H7)4N]2[Cu5I7] is best described as (m3-iodo)-hexakis(m2-iodo)-penta-copper(I), the corresponding systematic name for the clusterin [(C4H9)4P]2[Cu5I7]$CH3COCH2OH (compound II) should be bis(m3-iodo)-pentakis(m2-iodo)-penta-copper(I) to highlight thedifferences in local bonding. The cluster in the disorderedcompound [(C4H9)4P]2[Cu5I7] (I) cannot be rationalized as a simplerotational disorder of either of these two cluster arrangements, butperhaps as a combination of the two.

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The BVS [7] for all clusters are remarkably consistent, and thislends further credence to that the clusters may indeed by distinctfor the three compounds. While a word of caution may be in placefor the over-interpretation of structural data for disorderedcompounds, the experimental evidence for distinct differencesbetween the three Cu5I7 clusters is compelling, and it certainly addsto the allure of the remarkable flexible structural chemistry of theCu(I) halides. It is also clear that high quality data such as forcompound II is an important basis for the analysis of bonding inthese systems.

References

[1] H. Hartl, F. Mahdjourhassanabadi, Angew. Chem., Int. Ed. Engl. 23 (5) (1984)378e379.

[2] S. Andersson, S. Jagner, J. Crystallogr. Spectrosc. Res. 18 (5) (1988) 591e600.[3] CrysAlis CCD Data Reduction. Oxford Diffraction Ltd., Abingdon, 2008.[4] G. Oszlanyi, A. Süto, Acta Crystallogr. A(60) (2004) 134e141.[5] L. Palatinus, G. Chapuis, J. Appl. Crystallogr. 40 (2007) 786e790.[6] K. Brandenburg, Diamond (2001).[7] I.D. Brown, D. Altermatt, Acta Crystallogr. A41 (1985) 244e247.[8] H.-G. Zhu, Z. Yu, H. Cai, X.-Z. You, J.J. Vittal, G.-K. Tan, Chem. Lett. (1999) 289.[9] A. Nurtaeva, E.M. Holt, J. Chem. Crystallogr. 32 (2002) 337.