NaCu5(C2)3: The First Alkali Metal Transition Metal Acetylide with a Three-dimensional Framework Structure

NaCu5(C2)3: The First Alkali Metal Transition Metal Acetylide with a Three-dimensional Framework Structure

Ulrich Cremer and Uwe Ruschewitz*

Köln, Institut für Anorganische Chemie der Universität

Received July 18th, 2003.

Dedicated to Professor Klaus-Jürgen Range on the Occasion of his 65th Birthday

Abstract. By heating the red residue of the reaction of NaC2H andCuI in liquid ammonia at 463 K for 30 min in a dynamic vacuumorange polycrystalline NaCu5(C2)3 is obtained. The crystal struc-ture was solved and refined from X-ray powder diffraction data(orthorhombic, Pnma, a � 732.80(2) pm, b � 1099.63(4) pm, c �

726.15(2) pm, Z � 4, RB � 0.030). It consists of a three-dimen-sional framework of CuI and C2

2� ions with small channels run-ning parallel to [100] and [001]. The sodium ions reside at the inter-

NaCu5(C2)3: das erste Alkalimetallübergangsmetallacetylid mit einerdreidimensionalen Gerüststruktur

Inhaltsübersicht. Durch 30-minütiges Erhitzen des roten Rück-stands der Reaktion von NaC2H mit CuI in flüssigem Ammoniakauf 463 K in einem dynamischen Vakuum wird orangefarbenes po-lykristallines NaCu5(C2)3 erhalten. Die Kristallstruktur wurde aufGrundlage von Röntgenpulverbeugungsuntersuchungen gelöst undverfeinert (orthorhombisch, Pnma, a � 732,80(2) pm, b �

1099,63(4) pm, c � 726,15(2) pm, Z � 4, RB � 0,030). Sie bestehtaus einem dreidimensionalen Netzwerk von CuI und C2

2�-Ionen

IntroductionTwo general synthetic routes for the synthesis of ternaryalkali metal transition metal acetylides have been described.In a first approach alkali metal acetylides of compositionA2C2 (A � Na-Cs) are reacted with palladium or platinumin a typical solid-state reaction yielding ternary acetylidesof composition A2M0C2 (M0 � Pd, Pt) [1�3]. The secondapproach utilizes the thermal decomposition of precursors,which have been synthesised by reactions in liquid ammonia[4] according to:

2 AC2H � MII ��liq. NH3 A[MI(C2H)2] � AI

A[MI(C2H)2] ��∆ AMIC2 � C2H2

with A � Li-Cs and MI � Cu (for A � Na) [5], Ag [6], Au [7].

* Prof. Dr. U. RuschewitzInstitut für Anorganische ChemieUniversität zu KölnGreinstraße 6D-50939 KölnFax: (49) 221 470 4899E-Mail: [email protected]

Z. Anorg. Allg. Chem. 2004, 630, 161�166 DOI: 10.1002/zaac.200300271 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 161

sections of these channels. The vibrational spectra of NaCu5(C2)3

show some similarities with the spectra of phenylethynylcopper(I),but they are different to those of Cu2C2. Therefore NaCu5(C2)3

does probably not represent a crystalline model compound for theunknown structure of amorphous Cu2C2.

Keywords: Acetylides; Copper; Sodium; X-ray powder diffraction

mit kleinen Kanälen, die parallel zur a- und c-Achse verlaufen. DieLagen an den Kreuzungspunkten dieser Kanäle werden durch dieNatriumionen besetzt. Die Schwingungsspektren von NaCu5(C2)3

zeigen eine gewisse Ähnlichkeit mit denen von Kupfer(I)-phenyl-acetylid, aber sie unterscheiden sich stark von denen des Cu2C2.Deshalb kann NaCu5(C2)3 höchstwahrscheinlich nicht als eine kri-stalline Modellsubstanz der bisher noch unbekannten Struktur vonamorphem Cu2C2 aufgefasst werden.

Although it has not been possible up to now to isolateand characterize any of the intermediate bisethynylometal-lates, the proposed reaction scheme is supported by the factthat the formation of alkali metal iodide and acetylenecould be confirmed. The crystal structures of the com-pounds A2M0C2 and AMIC2, which are the only knownternary alkali metal transition metal acetylides, are charac-terized by 1

�[M(C2)n�2/2] chains (M � Pd, Pt: n � 2; M � Cu,

Ag, Au: n � 1), which are separated by the alkali metalions.

When trying to synthesise NaCuC2 according to the se-cond approach [5], we found that on heating the precipitateof the reaction of NaC2H and CuI in liquid ammonia acrystalline intermediate with an orange colour is formed [8].The unprecedented crystal structure of this intermediate,which was solved from X-ray powder diffraction data, willbe presented as well as some aspects of its unusual syn-thesis.

Synthesis of NaCu5(C2)3

All preparations were carried out under an inert argon at-mosphere using Schlenk techniques. Sodium (Degussa,

U. Cremer, U. Ruschewitz

Fig. 1 X-ray powder diffraction pattern of NaCu5(C2)3 at ambienttemperature (Stoe Stadi P2, CuKα1 radiation). The observed (�)and calculated patterns (solid line) as well as the difference betweenthe two are shown. Vertical bars mark the positions of reflectionsof Cu and NaCu5(C2)3 (from above).

99 %), ammonia (Messer Griesheim, 99.99 %), and acety-lene (Linde, technical) were used as purchased. In a typicalexperiment 65 mg Na (2.83 mmol) were dissolved in liquidammonia (approx. 40 mL, cooled with a CO2/acetonebath). At 195 K acetylene was passed over the blue stirredsolution until it decolorized. After addition of 269 mg CuI(1.41 mmol; Riedel-de Haen, 99.5 %) the ammonia was re-moved by warming the solution to ambient temperature.The remaining red precipitate was dried in a dynamic vac-uum overnight. The residue was then heated at 463 K for30 min in a dynamic vacuum, which resulted in a significantpressure increase. After approx. 30 min the starting pressurewas reached and the yellow solid was cooled to ambienttemperature and washed three times with liquid ammoniato remove soluble by-products. After drying in a dynamicvacuum an orange product was obtained.

The resulting X-ray powder diffraction patterns (see nextchapter, Figure 1) mainly showed reflections of the productand of elemental copper probably due to decompositionduring the thermal treatment. Furthermore very weak re-flections of an unknown impurity and some broad back-ground features were observed, which point to the existenceof an amorphous impurity. Heating the red residue of thereaction of NaC2H and CuI in liquid ammonia (s. above)at 463 K for more than 30 min leads to the formation ofNaCuC2 [5] and an increasing ratio of elemental copper.

A possible reaction scheme for the synthesis ofNaCu5(C2)3 is:

6 NaC2H � 5 CuI � NaCu5(C2)3 � 5 NaI � 3 C2H2.

This hypothesis is supported by the fact that NaI is foundas a by-product of the synthesis. It is removed by washingwith liquid ammonia after the heat treatment. The evol-ution of acetylene could be the reason for the observedpressure increase during the heat treatment. But it is sur-

2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2004, 630, 161�166162

Table 1 Details and results of X-ray powder investigations onNaCu5(C2)3 at ambient temperature (Stoe Stadi P, CuKα1 radia-tion). The C�C distances are refined with constrained values of120 pm (soft constraints, see text).

Space group, Z Pnma (No. 62), 4Lattice constants a � 732.80(2) pm, b � 1099.63(4) pm, c � 726.15(2) pmData range 13° � 2θ � 90°, 7701 data pointsRefined parameters 33Refined reflections 253wRP, RP, RB 0.038, 0.029, 0.030

Atomic coordinates and isotropic thermal parameters

x y z Uiso/pm2

Na 4(c) 0.050(1) 0.75 0.316(1) 500(40)Cu1 4(c) 0.0978(4) 0.25 0.0767(4) 170(10)Cu2 8(d) 0.0565(3) 0.4583(2) 0.1862(3) 151(6)Cu3 8(d) 0.1938(3) 0.4472(2) 0.4944(4) 188(6)C1 8(d) 0.203(2) 0.056(1) 0.750(2) 200a)

C2 8(d) 0.145(2) 0.092(1) 0.895(2) 200a)

C3 8(d) 0.066(2) 0.1954(1) 0.364(2) 200a)

a) fixed

prising that a surplus of sodium is needed for the successfulsynthesis of NaCu5(C2)3 (molar ratio Na:Cu � 2:1). Usingthe stochiometric ratio (Na:Cu � 6:5) no NaCu5(C2)3 wasobtained. It must be assumed that the unknown crystallineand amorphous impurities contain surplus sodium, as heat-ing the resulting NaCu5(C2)3 sample at temperatures above463 K in sealed ampoules leads to the formation ofNaCuC2 and minor amounts of elemental copper.

Crystal Structure Analysis

A diffraction pattern of a sample of NaCu5(C2)3 synthesized ac-cording to the procedures described above was recorded on a StoeStadi P X-ray powder diffractometer (Debye-Scherrer geometry,CuKα1 radiation, Ge(111) monochromator, � 0.3 mm capillary,linear position sensitive detector) at 298 K. The resulting pattern isshown in Figure 1 (13° � 2θ � 90°; step size 0.01°). It was analysedusing the WinXPOW software [9]. Reflections were indexed withan orthorhombic unit cell [10], the systematic absences of reflec-tions led to the possible space groups Pnma (No. 62) and Pn21a(No. 33). It must be mentioned that for a long time this step ofthe structural solution was hampered by the occurrence of weakadditional reflections of an unknown impurity. Le Bail extraction[11�13] within space group Pnma and a subsequent structure de-termination using direct methods [14] resulted in the determinationof the positions of the copper and sodium atoms. They were intro-duced into a Rietveld refinement and the remaining carbon atomswere found in difference Fourier maps [15]. In the final refinementthe C�C distances were constrained to 120 pm, the value expectedfor a C�C triple bond [16]. A free refinement of the positional andthermal parameters of the carbon atoms was not possible. The lat-ter had to be fixed to a reasonable value. Details and results of theX-ray powder investigations on NaCu5(C2)3 are summarized inTable 1, selected interatomic distances are given in Table 2. Thequality of the refinement can be estimated from Figure 1. A broadbackground feature up to 2θ � 40° is probably due to an amorph-ous impurity (see below).

Attempts to get better refinement results by using synchrotronpowder diffraction data were not successful, as sample inherentlarge reflection widths did not lead to an improved diffraction pat-

NaCu5(C2)3: The First Alkali Metal Transition Metal Acetylide

Table 2 NaCu5(C2)3: selected interatomic distances (pm).

Na�C1 283(2) 2x Cu2�C2 217(1)287(1) 2x 228(1)

Na�C2 272(2) 2x Cu2�C3 212.9(7)288(2) 2x

Na�C3 255(2) 2x Cu3�C1 186(1)227(1)

Cu1�Cu2 244.4(2) 2x Cu3�C2 211(1)Cu2�Cu3 245.7(3) Cu3�C3 205.7(8)

252.3(4)Cu1�C2 221(1) 2x C1�C2 120a)

Cu1�C3 218(1) 2x C3�C3 120a)

Cu2�C1 211(1)224(1)

a) fixed using soft constraints

Table 3 Vibrational spectroscopy data

Cu2C2 [17] NaCu5(C2)3 [this work] CuC2C6H5 [18, 19]ν̃IR/cm�1 a) ν̃IR/cm�1 ν̃IR/cm�1 (excerpt)

1899, 1805b), 1753, 1722 1957, 19301610, 1400 1580, 1446 1594, 1571, 1481, 1440b)

1220 1385 1380b), 1328b), 1280b)

1100-1000 1192, 1173, 1156, 1070(several signals)970 1026, 999, 985, 960915, 850 881 915, 905760, 730 730 779, 746

700 685600 615 (broad) 623, 525

484 (broad) 515, 424, 403

a) estimated from the IR spectrum given in [17]b) from Raman spectra, only Raman active

tern. A high temperature X-ray powder diffraction investigationwas performed in the range 293 K to 513 K (Stoe Stadi P, CuKα1

radiation, � 0.3 mm capillary in a Stoe capillary furnace 0.65.1,Stoe IP-PSD detector). Up to 463 K the expected increase of thelattice parameters was observed. Above 463 K NaCu5(C2)3 decom-posed with elemental copper being the only crystalline decompo-sition product that could be found in the diffraction patterns. Sur-prisingly, no NaCuC2 could be detected, which is usually syn-thesised by heating NaCu5(C2)3 in sealed ampoules above 463 K[5].

Vibrational Spectroscopy

IR (KBr pellets, Bruker IFS 66v/S) and Raman spectra (� 1.0 mmcapillary, Bruker IFS 66v/S with FRA 106/S, Nd:YAG laser, λ �

1064 nm) of NaCu5(C2)3 were recorded to compare it to other com-pounds with Cu-C�C units known from the literature [17�19].This comparison is given in Table 3. The range of the C-C stretch-ing vibrations of NaCu5(C2)3 of both spectra is shown in Figure 2.The spectra will be discussed in more detail below.

Crystal Structure of NaCu5(C2)3

The crystal structure of NaCu5(C2)3 is shown in Figure 3.The projection along [100] shows a complicated three di-mensional framework of CuI ions and C2

2� dumbbells.Small channels are found running parallel to [100] and

Z. Anorg. Allg. Chem. 2004, 630, 161�166 zaac.wiley-vch.de 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 163

Fig. 2 Raman and IR spectra of NaCu5(C2)3. Only the region ofthe C-C stretching vibration is shown.

Fig. 3 Crystal structure of NaCu5(C2)3 in a projection along [100].

[001]. At the intersection of these channels the sodium ionsreside with a surprisingly high coordination number of 10.The coordination sphere of the sodium ions is depicted inFigure 4. Na-C distances range from 255(2) to 288(2) pm(Na2C2: Na-C 261.7 � 279.8, 6x [20]). The three dimen-sional copper-carbon framework consists of three crystallo-graphically distinct copper ions and three crystallographi-cally distinct carbon atoms, which lead to two crystallo-graphically distinct C2 dumbbells. The coordination spheresof the copper ions and C2 dumbbells are given in Figures5 and 6. Cu-C distances range from 186(1) to 228(1) pm(NaCuC2: Cu-C 188.0(1) pm [5]). It must be mentioned thatthese distances have to be considered with some care, as thelow quality of the powder diffraction data did not allow a


Fig. 4 NaCu5(C2)3: Coordination sphere around a sodium ion ta-king interatomic distances up to 290 pm into account.

free refinement of the atomic parameters of the carbonatoms (see above). Nonetheless, the obtained distancesagree quite well with the values found for comparable com-pounds. An intriguing aspect of the crystal structure ofNaCu5(C2)3 are short Cu-Cu distances ranging from244.4(2) to 252.3(4) pm. Similar CuI-CuI distances werefound in some ternary alkaline earth copper nitrides(SrCuN: 247.7 pm [21], Sr6Cu3N5: 244.8 pm [21], BaCuN:249.0 pm [22]). The coordination spheres of Cu1 and Cu3are quite similar. The central copper atom is coordinated tothree C2 dumbbells (one side-on, two end-on) and two cop-per atoms. Cu2, however, is coordinated to four C2 dumb-bells (three of them end-on and one side-on) and three cop-per atoms (see Figure 5). The C1-C2 dumbbell is sur-rounded by six copper and two sodium atoms (Figure 6).This polyhedron shows some similarities with the poly-hedron around the C2 dumbbells in binary acetylides A2C2

(A � Li, Na, K; [20, 23]), which crystallize in distortedfluorite structures with an eight-fold coordination. Thepolyhedron around the (C3)2 dumbbell however is formedby five copper and one sodium atoms (Figure 6). This coor-dination is comparable to the one found in metastable CaC2

[24] with a distorted rocksalt structure, where C2 dumbbellsoccupy octahedral holes formed by six Ca atoms.

NaCu5C6 is the first ternary alkali metal transition ace-tylide with a three dimensional framework structure. Asmentioned in the introduction all hitherto known com-pounds are characterized by polymeric transition metal car-bon chains with a linear coordination of the transition me-

Fig. 5 NaCu5(C2)3: Coordination sphere around the Cu(I) ions taking interatomic distances up to 255 pm into account.


tal. This linear coordination is in agreement with the d10

electron configuration of the transition metals in the knownacetylides (Pd0, Pt0, Cu�, Ag�, Au�), but in the title com-pound no longer a linear coordination of CuI is observed.

Discussion

NaCu5(C2)3 is a compound with an unusual synthesis. Itsdiffraction pattern mainly shows reflections of the productand copper as well as a very broad background feature atlow θ angles. It must be assumed that this is due to anamorphous impurity containing sodium, as NaCu5(C2)3

can be converted to NaCuC2 by heating. Possible impuritiesare amorphous NaC2H and Na2C2, but the Raman spec-trum of a NaCu5(C2)3 sample does not show the significantsignals for the C-C stretching vibrations of NaC2H (ν̃C�C �1871 m�1 [25]) or Na2C2 (ν̃C�C � 1845 m�1 [20]), see Table3. Therefore, the composition of the amorphous impurityremains unknown.

The composition of NaCu5(C2)3 is very close to Cu2C2

(� CuCu5(C2)3). The crystal structure of the later is un-known. In our own experiments only amorphous redsamples with broad diffraction features around 2θ � 30°and 2θ � 42° (CuKα1 radiation) could be obtained [26].Therefore it was interesting to compare the vibrationalspectra of NaCu5(C2)3 with the reported IR spectrum ofCu2C2 [17] to check for any similarities, which could be in-terpreted as a possible similar structural arrangement inboth compounds. But as can be seen from Table 3 the re-spective spectra are quite different. For Cu2C2 no signals ofthe C-C stretching vibration are found in the IR spectrumpointing to a symmetrical surrounding of the C2 dumbbell.In the IR spectrum of NaCu5(C2)3 three signals are visiblein the range for C-C stretching vibrations. From Figure 6it can be seen that the surrounding of the C2 units is notsymmetrical, but replacing Na by Cu a symmetrical situ-ation can be assumed for the proposed compoundCuCu5(C2)3. Thus, this difference in the IR spectra ofCu2C2 and NaCu5(C2)3 does not exclude a possible struc-tural similarity between both compounds, but as there arealso big differences in other ranges of the IR spectra, it isvery unlikely that NaCu5(C2)3 represents are crystallinemodel compound for the unknown structure of Cu2C2.More similarities are found in the spectra of NaCu5(C2)3

and phenylethynylcopper(I) ([18, 19], see Table 3). For the

NaCu5(C2)3: The First Alkali Metal Transition Metal Acetylide

Fig. 6 NaCu5(C2)3: Coordination sphere around the C22� dumb-

bells taking interatomic distances up to 290 pm into account.

later only a vague structural model has been given [27], butthe general structural features are similar to those found inNaCu5(C2)3, i.e. short Cu-Cu distances and side-on andend-on co-ordinations of the C2 dumbbells so that the simi-lar vibrational spectra are reasonable. Frequencies in therange 950 � 1200 m�1 in the spectrum of phenylethynyl-copper(I) are due to vibrations, which include the phenylring, and are therefore not visible in the spectrum ofNaCu5(C2)3. The C-C stretching vibrations in NaCu5(C2)3

occur at lower wave numbers compared to phenylethynyl-copper(I) (see Table 3) and KCuC2 (ν̃C�C � 1959 m�1 [5]).In KCuC2 each C2 dumbbell is coordinated end-on by twocopper atoms and side-on by four sodium atoms. In pheny-lethynylcopper(I) there are two different types of C2 units,but both are coordinated side-on by one and end-on by twocopper atoms. For both dumbbells a signal for theC-C stretching vibration is found in the IR spectrum. Therespective wave numbers (ν̃C�C � 1957 and 1930 m�1, seeTable 3) are slightly smaller than that in KCuC2 displayingthe influence of a side-on coordinating copper atom.Finally, in NaCu5(C2)3 the C-C stretching vibrations areshifted to even smaller wave numbers (ν̃C�C � 1899 �1722 m�1, see Table 3), as here the C2 dumbbells are coordi-nated side-on by two copper and two sodium atoms(C1-C2, see Figure 6) or one copper and one sodium atom(C3-C3, see Figure 6). Thus a side-on coordinating copperatom seems to weaken the C-C triple bond, which shouldresult in an elongated C-C bond length. This was found in[Cu4(µ-dppm)4(µ4-η1,η2-C�C)](BF4)2 (dppm � Bis(di-phenylphosphino)methane), where a C2 dumbbell residesinside a Cu4 rectangle with two copper atoms coordinatingside-on and two end-on [28]. The C-C distance, whose de-termination is based on a single-crystal X-ray analysis, is126(2) pm and thus 6 pm longer than the expected value fora C-C triple bond [16]. No data are given for the frequenciesof the C-C stretching vibrations. In this respect it is veryunfortunate that due to the low quality of the diffractionpattern we have not been able to get reasonable positionalparameters for the carbons atoms in NaCu5(C2)3 to deter-mine the C-C distances in this compound.

Another interesting structural feature of NaCu5(C2)3 areshort Cu-Cu distances so that weak d10-d10 interactions canbe proposed. Therefore we checked for luminescence, butdown to low temperatures we did not find any significanteffects.

Z. Anorg. Allg. Chem. 2004, 630, 161�166 zaac.wiley-vch.de 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 165

The sodium ions in NaCu5(C2)3 are only weakly bondedas displayed by the high coordination number and highthermal parameters (see Table 1). Therefore we tried to ex-change sodium by other alkali metal ions using typical solu-tion based ion exchange techniques. For the resulting solidof lithium exchange we found a similar diffraction patternwith slightly shifted reflections compared to the pattern ofNaCu5(C2)3, but large reflection widths hampered a struc-tural solution up to now.

Acknowledgements. We thank PD Dr. Angela Möller for help withthe vibrational spectra. The financial support by the Deutsche For-schungsgemeinschaft (RU 546/2-1 and 2-2) is greatly acknow-ledged.

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NaCu5(C2)3: The First Alkali Metal Transition Metal Acetylide with a Three-dimensional Framework Structure