Reactions of niobium and tantalum hexanuclear halide clusters with cadmium(II) halides, diamagnetic and paramagnetic clusters with semiconducting properties

Pergamon

Materials Research Bulletin 36 (2001) 211–225

Reactions of niobium and tantalum hexanuclear halide clusters with cadmium(II) halides, diamagnetic and paramagnetic clusters with semiconducting properties Marija Vojnovic´a, Nevenka Brnicˇevic´a,*, Ivan Basˇica, Rudolf Trojkoa, Marko Miljakb, Ida Dunja Desnica-Frankovic´a a

Rudjer Bosˇkovic´ Institute, P.O. Box 180, 10002 Zagreb, Croatia b Institute of Physics, P.O. Box 304, 10000 Zagreb, Croatia (Refereed) Received 6 April 2000; accepted 30 June 2000

Abstract The diamagnetic [Ta6Cl12(H2O)6][CdCl2X2]䡠12H2O (X ⫽ Br, I) 1, 2 and [M6Br12(H2O)6][CdBr2X2]䡠12H2O (M ⫽ Nb, Ta; X ⫽ Cl, Br, I) 3– 8 clusters as well as paramagnetic [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O 9 have been isolated. The crystal structures have been determined for [Nb6Br12(H2O)6][CdBr4]䡠12H2O 4, [Ta6Br12(H2O)6][CdBr4]䡠12H2O 7 and [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O 9. Octahedral cluster cations [M6Br12(H2O)6]2⫹ in 4, 7 and [(Ta6Cl12)Cl(H2O)5]2⫹ in 9, together with [CdBr4]2⫺ anion as discrete units are main structural building blocks. The crystal packing of the cations and anions in a three-dimensional network in 4 and 7 is realized via hydrogen bonds involving crystal and coordinated water molecules and Br⫺ atoms from [CdBr4]2⫺ anions. The paramagnetic cluster 9 displays the quazionedimensional magnetic properties with intrachain antiferromagnetic interaction J ⫽ 68 K and the interchain interaction J’ ⫽ 21 K. All compounds show semiconducting properties. The respective activation energies, Ea1 ⫽ 0.23 eV and Ea2 ⫽ 0.19 eV (the same values for 7 and 9) have been found. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; A. Semiconductors; C. X-ray diffraction; D. Crystal structures; D. Magnetic properties

* Corresponding author. Tel.: ⫹385-1-4561-189; fax: ⫹385-1-4680-098. E-mail address: [email protected] (N. Brnicˇevic´). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 5 1 0 - 4

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1. Introduction Limited data are available on reactions of M6 (M ⫽ transition metal) clusters with transition metal halides. Some of these reactions are described for (Mo6Cl8)Cl4 ⫽ (Mo6Cl8i)Cl2aCl4/2a-a. This cluster reacts with HgX2 (X ⫽ Cl, Br, I) in a sealed quartz tube at 400°C giving Hg[(Mo6X8)X6] [1,2]. Similar reactions for the same cluster are described for AgCl [3], MCl2 (M ⫽ Cu, Pb) [4,5] or BiCl3 [6] where products of different structural and physical properties were formed. Unlike (Mo6Cl8)Cl4, the niobium and tantalum halide clusters [(M6X12)X2(H2O)4]䡠4H2O have not been found to react with metal halides until recently. The diamagnetic [M6Br12(H2O)6][HgBr2X2]䡠12H2O [7] and paramagnetic [(Ta6Cl12)Cl(H2O)5][HgX4]䡠9H2O [8] compounds were obtained by the reaction of mercury(II) halides with [(M6Br12)Br2(H2O)4]䡠4H2O and [(Ta6Cl12)Cl2(H2O)4]䡠4H2O, respectively. Both series are semiconducting. The [M6X12(EtOH)6][(Mo6Cl8)Cl6]䡠nEtOH䡠mEt2O compounds consisting of the cluster cation and the cluster anion as constitutional entities of the same molecule, are the reaction products of (M6X12)X2䡠6EtOH with (Mo6Cl8)Cl4 [9]. Being particularly interested in semiconducting properties of M6 halide clusters, we have prepared and investigated a new series of dia- and paramagnetic cluster compounds exhibiting this behavior.

2. Experimental 2.1. Materials and preparations Clusters [(M6X12)X2(H2O)4]䡠4H2O (M ⫽ Nb, Ta; X ⫽ Cl, Br) were prepared by the reduction of MX5 (puriss, Alfa) with niobium or tantalum metal powders (99.9%, Aldrich) [10]. Other p.a. grade chemicals, CdCl2䡠2.5H2O, CdBr2䡠4H2O (Aldrich), CdI2 (Polskie Odczynniki Chem.), CH3OH, AgNO3 and NaBr (Kemika) were used as received. 2.1a. [Ta6Cl12(H2O)6][CdCl2X2]䡠12H2O (X ⫽ Br 1, I 2). [(Ta6Cl12)Cl2(H2O)4]䡠4H2O (0.300 g; 0.174 mmol) was dissolved in methanol (7 ml) with stirring when methanolic solutions (4 ml) of CdBr2䡠4H2O (0.0599g; 0.174 mmol) or CdI2 (0.0637 g; 0.174 mmol) were added for 1 and 2, respectively. The clear solutions were filtered by means of fine-porosity filter paper and left under ambient conditions. Octahedra-like single crystals were grown after a week. For the analytical data collection the crystals were washed with carbon tetrachloride and air-dried shortly; yield ⬃70%. Anal. Calc. for H36O18Cl14Br2CdTa6: Ta, 49.84; Cl, 22.78; Br, 7.34. Found: Ta, 49.63; Cl, 23.27; Br, 7.56%. Calc. for H36O18Cl14I2CdTa6: Ta, 47.78; Cl, 21.84; I, 11.17. Found: Ta, 46.72; Cl, 22.62; I, 11.37%. 2.1b. [Nb6Br12(H2O)6][CdBr2X2]䡠12H2O (X ⫽ Cl 3, Br 4, I 5). [(Nb6Br12)Br2(H2O)4]䡠4H2O (0.600 g; 0.330 mmol) was dissolved in water (10 ml) by stirring when water solutions (4 ml) of CdCl2䡠2.5 H2O (0.0754 g; 0.330 mmol) or CdBr2䡠4H2O (0.1136 g; 0.330 mmol) or CdI2 (0.1208 g; 0.330 mmol) for 3, 4 and 5, respectively, were added. Further as described for 1

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and 2; yield ⬃70%. Anal. Calc. for H36O18Br14Cl2CdNb6: Nb, 25.53; Br, 51.23; Cl, 3.24. Found: Nb, 26.35; Br, 52.05; Cl, 2.89%. Calc. for H36O18Br16CdNb6: Nb, 24.53; Br, 56.26. Found: Nb, 25.15; Br, 57.16%. Calc. for H36O18Br14I2CdNb6: Nb, 23.55; Br, 47.27; I, 10.72. Found: Nb, 23.90; Br, 46.96; I, 11.51%. 2.1c. [Ta6Br12(H2O)6][CdBr2X2]䡠12H2O (X ⫽ Cl 6, Br 7, I 8). [(Ta6Br12)Br2(H2O)4]䡠4H2O (0.600 g; 0.255 mmol) was dissolved in water (10 ml) by stirring when water solutions (4 ml) of CdCl2䡠2.5 H2O (0.0582 g; 0.255 mmol) or CdBr2䡠4H2O (0.0878 g; 0.255 mmol) or CdI2 (0.0934 g; 0.255 mmol) for 6, 7 and 8, respectively, were added. Further procedure was as described for 1 and 2, yield ⬃70%. Anal. Calc. for H36O18Br14Cl2CdTa6: Ta, 40.03; Br, 41.25; Cl, 2.61. Found: Ta, 40.39; Br, 41.41; Cl, 2.09%. Calc. for H36O18Br16CdTa6: Ta, 38.76; Br, 45.65. Found: Ta, 39.23; Br, 46.11%. Calc. for H36O18Br14I2CdTa6: Ta, 37.50; Br, 38.64; I, 8.77. Found: Ta, 36.96; Br, 38.07; I, 8.36%. 2.1d. [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O 9. [(Ta6Cl12)Cl2(H2O)4]䡠4H2O (0.500 g; 0.290 mmol) was dissolved in methanol (8 ml) by stirring. Water solution (8 ml) of AgNO3 (0.0985 g; 0.580 mmol) was added dropwise. The emerald-green solution became gradually olivegreen indicating the oxidation of [Ta6Cl12]2⫹ to [Ta6Cl12]3⫹ with simultaneous precipitation of AgCl ⫹ Ag. The suspension was left over 24 h and then filtered by means of fine-porosity filter paper. The water solutions (4 ml) of NaBr (0.0597 g; 0.580 mmol) and CdBr2䡠4H2O (0.0998 g; 0.290 mmol) were added. The solution was filtered 24 h later and left under ambient conditions. Octahedral crystals were formed in two weeks; yield ⬃30%. Anal. Calc. for H22O11Cl13Br4CdTa6: Ta, 49.87; Cl, 21.17; Br, 14.68. Found: Ta, 48.72; Cl, 20.96; Br, 14.16%. Electronic spectra (nm): 227.7, 336.5, 339.2, 410.9 sh, 724.0, 827.2. 2.2. Analytical determinations Niobium and tantalum were determined as M2O5 after clusters decomposition in the mixture of sulfuric and nitric acid and precipitation with cupferron [11]. Halogens were determined by potentiometric titration with standard silver nitrate solution after decomposition of the clusters with KOH and H2O2. 2.3. Magnetic and electric measurements Magnetic susceptibility, ␹, of the [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O 9 single crystals was measured in the temperature range of 4.2 to 280 K using Faraday magnetometer [12] with an applied magnetic field of H ⫽ 9 kOe. The usual magnetization versus H, checked at different temperatures, showed no ferromagnetic contamination of the measured sample. A special procedure was applied in order to protect the sample due to the high vacuum exposure at ambient temperature. Before measurement the crystals were taken from the mother liquid, shortly dried on the filter paper, weighed and mounted on the balance within 10 –15 minutes.

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Subsequently, the sample space with the ambient atmosphere was closed and cooled to the liquid nitrogen temperature within a few minutes, then pumped down and left in dynamic vacuum for at least 24 h keeping the temperature below 200 K. Long pumping time was required to remove oxygen from the sample space, which would otherwise give rise to the paramagnetic susceptibility peak in the temperature range of 40 to 60 K. At low temperatures (4.2 to 77 K), usually a small quantity of helium exchange gas was slowly added to the sample space. Following this procedure the magnetic properties showed high reproducibility. Electrical characterization of 7 and 9 has been carried out in the 90 to 300 K temperature range. A two-point contact DC method was employed to measure the samples resistivity. Contacts were made with silver paint on the opposite sides of the octahedra-like monocrystalline samples. We took special care that the time elapsed between the removal of the samples from the mother liquid and the start of the cooling process in cryostat, was not longer than 10 min. This ensured the stability of the samples and good reproducibility of the resistance measurements during several cooling-heating cycles. 2.4. Other physical measurements The X-ray powder diffraction patterns were recorded on a Philips X-ray diffractometer using graphite-monochromatized Cu K␣ radiation. Electronic spectra of methanolic solutions were obtained by the Pye-Unicam SP 8 –100 UV/VIS spectrometer in a quarz cell (⌽ ⫽ 1 cm). The thermogravimetric measurements were carried out on a Cahn RG microanalytical balance in air with a heating rate of 2 °C min⫺1. 2.5. X-ray structure determinations Octahedra-like crystals of 4, 7 and 9 were fixed with epoxy on top of a glass fiber and immediately inserted into the cold N2 stream of a low temperature device on an Enraf-Nonius CAD4. Graphite monochromated Mo K␣ radiation was used. Routine procedures were used to determine the lattice parameters, Laue group and data collections. Data were collected at 100 K using the ␻-2⌰ scan method. During data collection, the intensities of three standard reflections were re-measured every hour of the exposure to the X-rays. Decays of 2.2%, 5.8% and 2.4% were observed for 4, 7 and 9, respectively. The absorption corrections were made on the basis of ␺-scans, JANA-98 [13] for 4 and 7 and WINGX [14] for 9. Crystallographic data, details of data collection and refinement are listed in Table 1, and the positional and anisotropic thermal parameters are summarized in Tables 2, 3 and 4. The structures were solved by direct methods using SHELXS-97 [15] and refined by full-matrix least squares on F2. The non-hydrogen atoms were located and refined with anisotropic displacement parameters in succeeding Fourier synthesis using SHELXL-97 [16]. The disordered O1 (or Cl2) of coordinated water molecules in compound 9 were not anisotropically refined (Table 4). Namely, the charge neutrality requirement dictates the presence of additional negative charge in the cluster. The analytical data are consistent with the presence of 13 Cl⫺ per cluster. One chlorine atom, Cl2, shares statistically six terminal positions of the Ta6 entity with five oxygen atoms, O1, from coordinated water molecules. This explains the residual electron density in the difference Fourier map near

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Table 1 Crystal data and details of the structure determination for 4, 7 and 9 Compound

4

7

9

Formula weight Crystal system Space group a (Å) Volume, V (Å3) Z Density, ␳calc (gcm⫺3) Temperature (K) Wavelength (Å) F(000) Crystal size (mm)/color

2272.42 cubic Fd3m 21.004(1) 9266.3(8) 8 3.206 100(2) 0.71073 7984 0.100 ⫻ 0.100 ⫻ 0.190/dark-green 25 19.3–25.3 2.09–26.92

2836.95 cubic Fd3m 20.9658(6) 9215.8(5) 8 4.037 100(2) 0.71073 9808 0.09 ⫻ 0.09 ⫻ 0.200/ dark-green 25 20.4–26.3 2.75–27.92

2176.75 cubic Fd3m 19.974(1) 7968.8(7) 8 3.624 100(2) 0.71073 7632 0.108 ⫻ 0.108 ⫻ 0.240/ dark-green 25 19.8–25.8 2.88–27.91

0,23;⫺23,0;⫺26,⫺12 939 413 (R(int) ⫽ 0.0746) 309 I ⬎ 2␴(I) 15.725

⫺27,0;⫺27,0;0,27 2771 571 (R(int) ⫽ 0.1326) 449 I ⬎ 2␴(I) 28.558

⫺26,0;0,26;0,26 2404 502 (R(int) ⫽ 0.0786) 379 I ⬎ 2␴(I) 21.837

1.041 and 0.864

0.384 and 0.183

0.995 and 0.254

full-matrix least-squares on F2 413/0/25 1.027 R1 ⫽ 0.0367, wR2 ⫽ 0.0967 w⫽1/ [␴2(F02) ⫹ (0.0671P)2 ⫹0.000P] where P ⫽ (F02 ⫹ 2Fc2)/3

full-matrix leastsquares on F2 571/0/26 1.059 R1 ⫽ 0.0424, wR2 ⫽ 0.0835 w⫽1/[␴2(Fo2) ⫹(0.0533P)2 ⫹0.000P] where P ⫽ (Fo2 ⫹ 2Fc2)/3 0.000 0.000057(12) 2.44 (1.03 Å from Ta) ⫺3.46 (0.35 Å from Ta)

full-matrix least-squares on F2 502/0/23 1.120 R1 ⫽ 0.0414, wR2 ⫽ 0.0996 w⫽1/[␴2(F02)⫹ 0.0405P)2⫹905.5807P] where P ⫽ (F02 ⫹ 2Fc2)/3

No. reflections (lattice) ⌰ Range (lattice) (°) ⌰ Range for data collection (°) Range of h, k, l Reflections collected Independent Reflections Reflections observed Criterion for observation Absorption coefficient (mm⫺1) Max. and min. transmission Refinement method Data/restraints/parameters S (goodness of fit on F2) Final R indices (I ⬎ 2␴(I))

Maximal (⌬/␴) Extinction coefficient Largest difference peak and hole (eÅ⫺3)

0.000 none 0.97 and ⫺0.85

0.000 none 4.89 (0.51 Å from O1) ⫺4.44 (0.00 Å from Cd)

O1 and Cl2 mixed-atom sites. In the final structure refinement best values on R, wR2 and S were obtained when the occupancy factor for O2 was 1/2 (6H2O). The positions of the hydrogen atoms of water molecules were not located. The locations of the atoms refer to the origin at the center (3m) of the space group Fd3m. The atoms of the cluster units are placed in special positions. The molecular geometries were calculated by the PLATON-99 programme and the drawings were prepared by ORTEP-3 [17]. Further

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Table 2 Positional and anisotropic thermal parameters for [Nb6Br12(H2O)6][CdBr4]䡠12H2O Atom

Site

Multiplicity

x

y

Nb Br1 Cd Br2 O1 O2

48f 96g 8b 32e 48f 96g

0.25 0.5 0.04167 0.16667 0.25 0.5

0.02590(5) 1/8 0.00331(3) 0.00331(3) 3/8 3/8 0.30348(5) 0.30348(5) ⫺0.0808(5) 1/8 0.0431(3) 0.0431(3) Ueq ⫽ (1/3)兺i兺jUijai*aj*ai䡠aj

z

Ueq(Å2)

1/8 0.12507(5) 3/8 0.30348(5) 1/8 0.3891(4)

0.0076(5) 0.0108(4) 0.0134(6) 0.0196(5) 0.021(2) 0.023(2)

Atom

U11

U22

U33

U23

U13

U12

Nb Br1 Cd Br2 O1 O2

0.0070(6) 0.0098(5) 0.0134(6) 0.0196(5) 0.017(5) 0.025(3)

0.0079(5) 0.0098(5) 0.0134(6) 0.0196(5) 0.023(3) 0.025(3)

0.0079(5) 0.0127(6) 0.0134(6) 0.0196(5) 0.023(3) 0.019(4)

⫺0.0002(4) ⫺0.0002(2) 0 ⫺0.0033(4) ⫺0.002(5) 0.005(2)

0 ⫺0.0002(2) 0 ⫺0.0033(4) 0 0.005(2)

0 ⫺0.0016(3) 0 ⫺0.0033(4) 0 0.003(3)

details of the crystal structure determination can be ordered from FACHINFORMATIONSZENTRUM KARLSRUHE, 76344 Eggenstein-Leopoldshafen, under the depository number CSD-411006 for 4, CSD-411007 for 7 and CSD-411008 for 9.

3. Results and discussion Diamagnetic compounds 1⫺8 were prepared by the reaction of the equimolar solutions of [(Ta6Cl12)Cl2(H2O)4]䡠4H2O with CdBr2 and CdI2 or [(M6Br12)Br2(H2O)4]䡠4H2O (M ⫽ Nb, Table 3 Positional and anisotropic thermal parameters for [Ta6Br12(H2O)6][CdBr4]䡠12H2O y

z

Ueq(Å2)

1/8 0.12509(4) 3/8 0.30346(5) 1/8 0.3892(4)

0.0113(3) 0.0142(3) 0.0165(5) 0.0236(4) 0.020(2) 0.026(2)

Atom

Site

Multiplicity

x

Ta Br1 Cd Br2 O1 O2

48f 96g 8b 32e 48f 96g

0.25 0.5 0.04167 0.16667 0.25 0.5

0.02725(2) 1/8 0.00312(3) 0.00312(3) 3/8 3/8 0.30346(5) 0.30346(5) ⫺0.0797(5) 1/8 0.0439(3) 0.0439(3) Ueq ⫽ (1/3)兺i兺jUijai*aj*ai䡠aj

Atom

U11

U22

U33

U23

U13

U12

Ta Br1 Cd Br2 O1 O2

0.0107(3) 0.0129(3) 0.0165(5) 0.0236(4) 0.009(4) 0.030(3)

0.0116(3) 0.0129(3) 0.0165(5) 0.0236(4) 0.026(3) 0.030(3)

0.0116(3) 0.0167(5) 0.0165(5) 0.0236(4) 0.026(3) 0.019(4)

0.0000(2) 0.0001(2) 0 ⫺0.0042(4) 0.001(4) 0.002(2)

0 0.0001(2) 0 ⫺0.0042(4) 0 0.002(2)

0 ⫺0.0019(3) 0 ⫺0.0042(4) 0 ⫺0.002(3)

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Table 4 Positional and anisotropic thermal parameters for [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O y

z

Ueq(Å2)

1/8 0.1251(2) 3/8 0.30115(13) 0.396(2) 1/8 1/8

0.0130(3) 0.0180(6) 0.0260(8) 0.094(2) 0.114(18) 0.039(3) 0.039(3)

Atom

Site

Multiplicity

x

Ta1 Cl1 Cd Br O2 Cl2 O1

48f 96g 8b 32e 96g 48f 48f

0.25 0.5 0.04167 0.16667 0.25 0.042 0.208

0.02214(3) 1/8 0.00423(11) 0.00423(11) 3/8 3/8 0.30115(13) 0.30115(13) 0.0414(14) 0.0414(14) ⫺0.0993(7) 1/8 ⫺0.0993(7) 1/8 Ueq ⫽ (1/3)兺i兺jUijai*aj*ai䡠aj

Atom

U11

U22

U33

U23

U13

U12

Ta1 Cl1 Cd Br O2

0.0108(4) 0.0155(9) 0.0260(8) 0.094(2) 0.15(3)

0.0141(3) 0.0155(9) 0.0260(8) 0.094(2) 0.15(3)

0.0141(3) 0.023(2) 0.0260(8) 0.094(2) 0.03(2)

⫺0.0005(3) ⫺0.0003(10) 0 ⫺0.034(2) 0.03(1)

0 ⫺0.0003(10) 0 ⫺0.034(2) 0.03(1)

0 ⫺0.006(1) 0 ⫺0.034(2) ⫺0.09(3)

Ta) with CdX2 (X ⫽ Cl, Br, I). Under the same experimental conditions no reaction occurred for [(Nb6Cl12)Cl2(H2O)4]䡠4H2O. The paramagnetic compound 9 was obtained after oxidation of [Ta6Cl12]2⫹ to [Ta6Cl12]3⫹ by water solution of AgNO3 in the presence of CdBr2 and NaBr. In fact the cluster was oxidized by the Ag⫹ ions and only one terminally bonded chlorine atom in [(Ta6Cl12)Cl2(H2O)4]䡠4H2O [18] was precipitated as AgCl. The electronic spectra for compound 9 recorded in methanol confirmed the presence of [Ta6Cl12]3⫹ [19] (see Experimental part). All substances are crystalline materials soluble in both methanol and ethanol. As revealed by the X-ray powder diffraction patterns and crystal structure determination for 4, 7 and 9, the diamagnetic and paramagnetic substances are isostructural and crystallize in the cubic space group Fd3m (Table 1). They are also isostructural to [M6Br12(H2O)6][HgBr4]䡠12H2O [7] and [(Ta6Cl12)Cl(H2O)5][HgBr4]䡠9H2O [8]. 3.1. Molecular and crystal structures 3.1.1. Structures of 4 and 7 The basic units of the two crystal structures are regular [M6Br12(H2O)6]2⫹ octahedra with bromine atoms as ␮2-ligands of the central M6 octahedron, and [CdBr4]2⫺ tetrahedra (Fig. 1). Both polyhedra are defined by the single bond distances per atom type pair (Tables 5 and 6). The Nb–Nb bond distance of 2.944 (2) Å in 4 is approximately 0.04 Å shorter than in some other compounds with [Nb6Br12]2⫹ [20,23]. The Ta–Ta bond length of 2.8982 (7) Å in 7 is comparable to 2.9000 (8) Å and 2.898(2) Å found for [Ta6Br12(H2O)6][HgBr4]䡠12H2O [7] and CsEr[(Ta6Br12)Br6] [24], respectively, but is shorter than the Nb–Nb bond distance in 4. The reduction of the unit cell volume of the tantalum derivative has been found as well (Table 1). This phenomenon (lanthanoide contraction) is usually observed when the Nb6 or Ta6 octahedra are in the same ligand coordination sphere [7,9,24] and is recently theoretically discussed [25]. The M–Br (bridging) distances of 2.5997 (8) and 2.6049(8) Å for 4 and

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Fig. 1. ORTEP drawing (50% probability level) of the complex units: [Nb6Br12(H2O)6]2⫹ and [CdBr4]2⫺ with the atom numbering scheme.

7, respectively, are comparable. The Nb–O1 and Ta–O1 bond distances of 2.242(11) and 2.243(10) Å for 4 and 7, respectively, are identical and close to their analogues [M6Br12(H2O)6][HgBr4]䡠12H2O (2.27 (1) and 2.22(2) Å, for M ⫽ Nb and Ta, respectively) [7]. The Cd2⫹ atom is located at 43m site symmetry. The Cd–Br2 bond distances of 2.602 (2) and 2.598 (2) Å found for 4 and 7, respectively, are close to the values found for other compounds with [CdBr4]2⫺ anion [26,27]. Crystal and coordinated water molecules connect the [M6Br12(H2O)6]2⫹ cations and [CdBr4]2⫺ anions into a three-dimensional network by the hydrogen bonding system. The coordinated water molecules are proton donors to crystal water molecules in the hydrogen bonds [O1–H䡠䡠䡠O2] of 2.724(8) Å and 2.709(8) Å for 4 and 7, respectively. The crystal water molecules donate protons to bromine from [CdBr4]2⫺ anions with the [O2–H䡠䡠䡠Br2] distance

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Table 5 Bond distances (Å) and angles (°) with esd’s for 4 Bond distances (Å) Nb–Nbi Nb–Br1

2.944(2) 2.5997(8)

Nb–O1 Cd–Br2

2.242(11) 2.602(2)

Bond angles (°) Nbiv–Nb–Nbi Nbi–Nb–Nbv Br1–Nb–Nbi Br1i–Nb–Nbiv Br1i–Nb–Nbi Br1i–Nb–Nbv Nb–Br1–Nbvi

Br1i–Nb–Br1iii Br1i–Nb–Br1 Br1i–Nb–Br1ii O1–Nb–Nbi O1–Nb–Br1 Br2–Cd–Br2vii

60.0 90.0 97.44(3) 97.39(3) 55.52(3) 145.52(3) 68.97(6)

158.97(6) 88.15(5) 88.03(5) 135.0 79.48(3) 109.5

Symmetry codes: i ⫽ z, x, y ii ⫽ x, ⫺y ⫹ 1/4, ⫺z ⫹ 1/4 iii ⫽ z, ⫺x ⫹ 1/4, ⫺y ⫹ 1/4 iv ⫽ ⫺y ⫹ 1/4, z, ⫺x ⫹ 1/4 v ⫽ ⫺z ⫹ 1/4, ⫺x ⫹ 1/4, y vi ⫽ y, z, x vii ⫽ ⫺x ⫹ 3/4, y, ⫺z ⫹ 3/4

of 3.386(7) Å for 4 and 3.401(7) Å for 7 (Table 7). The unit cell of [Nb6Br12(H2O)6][CdBr4]䡠12H2O is given in Fig. 2. Table 6 Bond distances (Å) and angles (°) with esd’s for 7 Bond distances (Å) i

2.8982(7) 2.6049(8)

Ta–Ta Ta–Br1

Ta–O1 Cd–Br2

2.243(10) 2.598(2)

Bond angles (°) iv

i

Ta –Ta–Ta Tai–Ta–Tav Br1–Ta–Tai Br1i–Ta–Taiv Br1i–Ta–Tai Br1i–Ta–Tav Ta–Br1–Tavi Symmetry codes: i ⫽ z, x, y ii ⫽ x, ⫺y ⫹ 1/4, ⫺z ⫹ 1/4 iii ⫽ z, ⫺x ⫹ 1/4, ⫺y ⫹ 1/4 iv ⫽ ⫺y ⫹ 1/4, z, ⫺x ⫹ 1/4 v ⫽ ⫺z ⫹ 1/4, ⫺x ⫹ 1/4, y vi ⫽ y, z, x vii ⫽ ⫺x ⫹ 3/4, y, ⫺z ⫹ 3/4

60.0 90.0 97.92(2) 97.86(2) 56.20(2) 146.20(2) 67.60(3)

Br1i–Ta–Br1iii Br1i–Ta–Br1 Br1i–Ta–Br1ii O1–Ta–Tai O1–Ta–Br1 Br2–Cd–Br2vii

157.60(3) 87.92(4) 87.76(4) 135.0 78.80(2) 109.5

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Table 7 Contact distances (Å) in the crystal structures of 4 and 7 4 Br1 . . . O2 O2 . . . Br2 O2 . . . O2 O1 . . . O2

7 3.550(8) 3.386(7) 2.708(10) 2.724(8)

Br1 . . . O2 O2 . . . Br2 O2 . . . O2 O1 . . . O2

3.538(8) 3.401(7) 2.706(10) 2.709(8)

3.1.2. Structure of 9 Compound [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O 9 is isostructural to 4 and 7 (Table 1) with the same regular octahedral architecture regardless of mixed-ligand coordination (O1 and Cl2) in terminal positions of the Ta6 entity or the valence electron count (VEC) of 15e⫺ as compared to the 16e⫺ available for the Ta–Ta bonding in 7. The interatomic distances (Table 8) are generally in agreement with the [(Ta6Cl12)L6]3⫹ (L ⫽ H2O, CH3OH) [28,29]. Yet, it is interesting to compare the interatomic distances in this compound with those in [(Ta6Cl12)Cl(H2O)5][HgBr4]䡠9H2O [8]. In spite of an entirely

Fig. 2. The unit cell of [Nb6Br12(H2O)6][CdBr4]䡠12H2O; crystal water molecules have been omitted for reasons of clarity.

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Table 8 Bond distances (Å) and angles (°) with esd’s for 9 Bond distances (Å) i

2.906(1) 2.439(3)

Ta1–Ta1 Ta1–Cl1

Ta1–L Cd–Br

2.43(1) 2.555(5)

Cl1i–Ta1–Cl1iii Cl1i–Ta1–Cl1 Cl1i–Ta1–Cl1ii L–Ta1–Cl1 L–Ta1–Ta1i Br–Cd–Brvii

163.13(9) 88.9(2) 88.7(2) 81.57(5) 135.0 109.5

Bond angles (°) iv

i

Ta1 –Ta1–Ta1 Ta1i–Ta1–Ta1v Cl1–Ta1–Ta1i Cl1i–Ta1–Ta1iv Cl1i–Ta1–Ta1i Cl1i–Ta1–Ta1v Ta1–Cl1–Ta1vi

60.0 90.0 95.98(7) 95.92(8) 53.43(5) 143.43(5) 73.13(9)

L ⫽ O1, Cl2 Symmetry codes: i ⫽ z, x, y ii ⫽ x, ⫺y ⫹ 1/4, ⫺z ⫹ 1/4 iii ⫽ z, ⫺x ⫹ 1/4, ⫺y ⫹ 1/4 iv ⫽ ⫺y ⫹ 1/4, z, ⫺x ⫹ 1/4 v ⫽ ⫺z ⫹ 1/4, ⫺x ⫹ 1/4, y vi ⫽ y, z, x vii ⫽ ⫺x ⫹ 3/4, y, ⫺z ⫹ 3/4

identical ligand coordination sphere of the Ta6 entity, the difference in the geometry of the [(Ta6Cl12)Cl(H2O)5]2⫹ cation in the two compounds is obvious. Slight reduction of

Fig. 3. Temperature dependence of ␹spin for [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O as a function of temperature: one-dimensional empirical function using equation (2) (dashed line); equation (3) which includes interchain interaction (solid line).

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the Ta–Ta bond length is found in [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O (2.906(1) Å) if compared to that in [(Ta6Cl12)Cl(H2O)5][HgBr4]䡠9H2O (2.911(1) Å). A more pronounced difference is observed for the terminal mixed-ligand coordination sites within which five water molecules (O1) and one chlorine atom (Cl2) are statistically distributed over six terminal octahedral positions averaging the Ta–L (L ⫽ O1, Cl2) bond distances to 2.43(1) Å. The values of 2.43(1) Å and 2.32(2) Å for [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O and [(Ta6Cl12)Cl(H2O)5][HgBr4]䡠9H2O [8], respectively, well illustrate the influence of the environment, especially hydrogen bonding system involved. Usually, the shorter M–M bonds are associated with weaker and longer M–O terminal bonds. The Cd 150 Br bond distance of 2.555(5) Å in regular tetrahedral [CdBr4]2⫺ anion (Table 8) is shorter than for 4 and 7 and belongs to the shortest observed in the chemistry of [CdBr4]2⫺. This is probably a consequence of the 43m symmetry of the anion. It is assumed that the [(Ta6Cl12)Cl(H2O)5]2⫹ cations and [CdBr4]2⫺ anions are connected by hydrogen bonds in a three-dimensional network. Unfortunately, due to the disordered positions of the oxygen atoms from coordinated (O1 or Cl2 atoms) and crystal (O2 atoms) water molecules, the system of hydrogen bonds remains unsettled. 3.2. Magnetic and electric properties The temperature dependence of the molar spin susceptibility, ␹spin, for [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O is shown in Fig. 3 and is given by the expression: ␹ spin ⫽ ␹ TOT ⫺ ␹ corr. ⫽ ␹ TOT ⫺ ␹ DiaPascal ⫺ ␹ Diadeloc ⫺ ␹ ParaV.Vleck

(1)

were ␹DiaPascal represents a core diamagnetic contribution [30], ␹Diadeloc is a diamagnetic contribution of the 15 delocalized electrons from six Ta atoms and ␹ParaV.Vleck represents paramagnetic orbital contribution [31]. The observed temperature dependent paramagnetism is in accord with the assumed presence of the [Ta6Cl12]3⫹ unit, having one unpaired spin. The data display a broad maximum around 88 K, decreasing smoothly and then again gradually rising at the lowest temperatures. The maximum in susceptibility suggests a short-range order, typical of low dimensional magnetic system, which is somewhat surprising for a cubic crystal symmetry. As illustrated in Fig. 3, the Curie-Weiss plot gives poor data description, except for the temperature range 180 to 260 K. It could be only used for crude estimation of the Curie constant, the strength and the sign of the interaction, which gives one mole of spins s ⫽ 1/2 and Curie-Weiss constant ⌰ ⫽ ⫺150 K. The data were analyzed using the Heisenberg model with antiferromagnetic intrachain interaction between cluster unit moments, assuming uniform spaced moments arranged in the chains. We used empirical function [32]: ␹ 1D

冋

Ng2␮02 0.25x ⫹ 0.14995x2 ⫹ 0.30094x3 ⫽ 兩J兩 1⫹1.9862x ⫹ 0.66854x2 ⫹ 6.0626x3

册

(2)

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Fig. 4. Temperature dependence of the electrical resistance for [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O (䊐) and [Ta6Br12(H2O)6][CdBr4]䡠12H2O (o) (upper part). Activation energies (Ea) plot (lower part): linear fits yielding Ea1 (solid lines) and Ea2 (dashed lines).

with N ⫽ NAvogadro, g is an average g-factor, ␮0 is the Bohr magneton and x ⫽ 兩J兩/kBT, aimed to represent the numerical calculations performed by Bonner and Fisher [33].

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In order to reproduce the experimental spin susceptibility maximum, ␹max ⫽ 1.33䡠10⫺3 emu/mol located at Tmax ⫽ 88 K using g ⫽ 2.0, the value of intrachain interaction J/kB should be close to 65 K. Calculated ␹1D with the J ⫽ 68 K, displayed in Fig. 3 as dashed line, shows general behavior of the data above 70 K but the calculated values are shifted above the measured data. Introducing interchain interaction in a mean field approximation, using [34] ␹ ⫽

␹ 1D 1 ⫺ 2zJ⬘␹1D/Ng2␮o2

(3)

where z ⫽ 4 is the number of nearest neighbors in adjacent chains and antiferromagnetic interchain interaction J’ ⫽ 21 K, the solid curve in Fig. 3 was produced. The agreement of this curve with the experimental data (above 70 K), considering interchain interactions only, is rather satisfactory. At temperature close to room temperature, the data deviation from this curve, which is also marked in Curie-Weiss plot, is caused by few effects all originating from the loss of the crystal water molecules. The decrease in the sample mass up to 2% is causing the change in the ␹DiaPascal(H2O) and ␹ParaV.Vleck [7] and finally introduces the defects in Heisenberg regular chains causing segmentation of the chains. The temperature dependence of the electrical resistance for [Ta6Br12(H2O)6][CdBr4]䡠12H2O and [(Ta6Cl12)Cl(H2O)5][CdBr4]䡠6H2O is shown in Fig. 4 (upper part). The resistivity exhibits typical semiconducting behavior and increases for eight orders in magnitude in the temperature range 300 to 100 K. The same data are presented in the ln(1/R) vs 103/T representation. For both compounds, two linear slopes related to the temperature ranges of 150 to 300 K and 105 to 150 K, could be clearly distinguished. The corresponding activation energies, Ea1 ⫽ 0.23 eV and Ea2 ⫽ 0.19 eV for both compounds are the same, within the margin of the experimental error (Fig. 4, lower part). This behavior is analogous to the temperature dependence of the resistance of [(Ta6Cl12)Cl(H2O)5][HgBr4]䡠9H2O [8] in which two very similarly spaced values for activation energies, Ea1 ⫽ 0.24 eV (2800 K) and Ea2 ⫽ 0.17 eV (1990 K), have been obtained. The origin of this conductivity and semiconducting behavior is not fully understood. However, the conductivity is not affected by the difference in the charge of the cluster entities: [Ta6Br12]2⫹ and [Ta6Cl12]3⫹ in the respective compounds, nor by the exchange of mercury(II) for cadmium(II) atoms or niobium for tantalum like in [Nb6Br12(H2O)6][HgBr4]䡠12H2O for which Ea ⫽ 0.20 eV was obtained [7].

Acknowledgments The support of this research by the Ministry of Science and Technology of the Republic of Croatia under the Project 980908 is gratefully acknowledged. We are grateful to Aleksandar Visˇnjevac for the crystallographic data collection and to Prof. Branko Kaitner for the diffraction data discussion.

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