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Synthesis, crystal structure, magnetic properties and electrochemical behaviour of the mixed valence compound [CuI(CN)3CuII(C3H10N2)2]
Inorganica Chimica Acta 358 (2005) 2558–2564 www.elsevier.com/locate/ica
Synthesis, crystal structure, magnetic properties and electrochemical behaviour of the mixed valence compound [CuI(CN)3CuII(C3H10N2)2] Thorsten Pretsch a, Jan Ostmann a, Constanze Donner b, Monika Nahorska c, Jerzy Mrozin´ski c, Hans Hartl a,* a
Institute of Chemistry/Inorganic and Analytical Chemistry, Free University of Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany b Institute of Chemistry/Physical and Theoretical Chemistry, Free University of Berlin, Takustrasse 3, D-14195 Berlin, Germany c Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie, 50-383 Wroclaw, Poland Received 21 January 2005; accepted 13 February 2005 Available online 25 March 2005
Abstract The binuclear mixed valence copper(I/II) compound [CuI(CN)3CuII(tn)2] (1) (tn = propane-1,3-diamine) and its acetonitrile ˚, adduct [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) have been synthesized. Complex 1 crystallizes triclinic, space group P 1, a = 8.117(2) A ˚ , c = 11.920(2) A ˚ , a = 108.728(3), b = 100.024(3), c = 104.888(4), Z = 2, and compound 2 monoclinic, space group b = 8.389(2) A ˚ , b = 13.243(3) A ˚ , c = 9.549(2) A ˚ , b = 114.678(4), Z = 2. In both crystal structures, the binuclear [CuI(CN)3 P21/m, a = 8.752(2) A II Cu (tn)2] complex with slightly different bonding geometries is formed. One of the three nitrogen atoms of a CuI(CN)3 moiety is coordinated to Cu(II) at the apex of a square-pyramid with two chelating ligands tn on its base. The shortest intramolecular ˚ . The EPR behaviour of 1 has been investigated at room temperature and at 77 K. The magCuII CuII distance in 1 is 5.640(7) A netic properties were measured in the temperature range 1.8–300 K. 2005 Elsevier B.V. All rights reserved. Keywords: Copper; Mixed valence compounds; Magnetic properties; N ligands; Hydrogen bonding; Crystal structure
1. Introduction There are many examples of mixed valence copper(I,II) complexes in the literature [1,2]. Such compounds are of interest since it is known that the metabolic roles of copper metalloenzymes involve transferring electrons from a substrate molecule to molecular oxygen.
*
Corresponding author. Tel.: +49 30 838 54003; fax: +49 30 838 543310. E-mail addresses: [email protected] (C. Donner), [email protected] (J. Mrozin´ski), hartl@chemie. fu-berlin.de (H. Hartl). 0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.02.018
The chemistry of cyanide-based mixed valence copper(I/II) compounds can be divided into two groups; one group is composed of isolated copper(I) and copper(II) species (salt-like compounds), the other one contains bridging cyanide ligands between copper(I) and copper(II) centres (neutral complexes). The former type of compounds can be formulated as [CuI(CN)3][CuIILn] (L = neutral ligand; n = 2 for tridentate [3], n = 3 for bidentate ligands [4,5]). The second class of complexes contains compounds such as [CuI(CN)Ln][CuIILn + 1] (n = 1 for bidentate ligands [6]) or [CuI(CN)3CuIILn] (n = 1 for tridentate [7] and quadridentate ligands [8]), which may exhibit intervalence charge transfer [9]. In the present study, we report on the synthesis as well as the structural, magnetic, electrochemical and
T. Pretsch et al. / Inorganica Chimica Acta 358 (2005) 2558–2564
IR spectroscopic characterization of [CuI(CN)3CuII(tn)2] (1). The title compound is a new example in the class of cyanide-bridged mixed valence copper(I/II) compounds [CuI(CN)3CuIILn] (n = 2). The crystal structure analysis of the solvate [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) reveals that hydrogen bridged assemblies of the binuclear complexes serve as hosts for acetonitrile molecules.
2. Experimental 2.1. General Copper(I) cyanide was purchased from Avocado, propane-1,3-diamine from Aldrich and sodium cyanide, hydrochloric acid (32%) and acetonitrile from Merck and used as received. Elemental analysis was performed with a CHNS-Elementaranalysator Vario EL instrument. The infrared spectra were recorded on a FT-IR spectrometer Nicolet 5SXC using the KBr pellet technique. The UV–Vis absorption spectrum of 1 was recorded at 25 C, using a Specord 40 spectrometer from the Analytic Jena AG. CuICN has been used as starting material for the formation of 1 and 2. Although no copper(II) salt was added to the reaction mixtures, one half of the copper atoms occur in both compounds in the oxidation state +2. It is well-known that N-ligands influence the redox potential for the oxidation of Cu(I) to Cu(II). The strong r-donors tn stabilize Cu(II) and prevent the reduction by cyanide. The first product during the preparation of 1 is a blue oil, which is solved in water. After one day 68.5 mg of 1 crystallized from the aqueous solution, one week later further 41.8 mg of 1 could be obtained. The blue oil is also produced when the reactions are carried out under argon atmosphere. This reaction behaviour is an indication that disproportionation plays a decisive role for the generation of Cu(II) in 1 and 2. It could be shown by means of IR spectroscopy that the blue oil has nearly the same vibration modes as the crystalline sample of 1. The assignment of the oxidation states in the crystal structures of 1 and 2 was made by means of the coordination numbers and is supported by the spectroscopic data (IR, EPR and magnetic measurements). 2.2. Synthesis of the complexes 2.2.1. Synthesis of bis(propane-1,3-diamine-j2N,N 0 ) copper(II)tricyanomonocuprate(I), [CuI(CN)3 CuII(tn)2] (1) A mixture of CuCN (537.6 mg, 6 mmol), KCN (195.6 mg, 3 mmol), propane-1,3-diamine (7.5 ml, 94 mmol) and HCl (32%, 4.65 ml, 47 mmol) was heated in acetonitrile (150 ml) under reflux with stirring for 2 h.
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The resulting solution contains blue oil, which was separated after cooling to room temperature and redissolved in water (25 ml). After 24 h at 4 C, 68.5 mg of the product (m.p. 187–189 C) was obtained in the form of blue needle-shaped crystals. Anal. Calc. for C9H20N7Cu2 (553.40): C, 30.60; H, 5.70; N, 27.74. Found: C, 30.52; H, 5.66; N, 27.71%. IR data (KBr): ~m ½cm1 ¼ 1314 w, 1392 w, 1403 m, 1437 m, 1472 m, 1580 m, 1602 m, 2082 s, 2099 s, 2114 s, 2877 m, 2929 m, 2949 m, 2962 m, 3147 m, 3234 m, 3273 s, 3290 s, 3328 m. FIR data (KBr): ~m ½cm1 ¼ 147 s, 211 m, 311 m, 332 m, 356 m, 400 m, 492 s, 539 w, 552 w, 570 w, 611 w, 629 s, 638 s. 2.2.2. Synthesis of bis(propane-1,3-diamine-j2N,N 0 ) copper(II)tricyanomonocuprate(I)-diacetonitrilesolvate, [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) A mixture of CuCN (89.6 mg, 1 mmol), KCN (32.6 mg, 0.5 mmol), propane-1,3-diamine (0.06 ml, 0.75 mmol) and HCl (0.04 ml, 0.4 mmol) was heated under reflux with stirring in acetonitrile (35 ml) for 1 h. The resulting blue solution was filtered while hot to remove undissolved residue and the filtrate was gradually cooled to room temperature. After 24 h at 4 C, 13.1 mg of the product was obtained in the form of blue needleshaped crystals. As soon as the crystals are separated from the solution a loss of acetonitrile takes place, leading to results of the elemental analyses similar to those of 1. Found: C, 30.64; H, 6.22; N, 27.37%. 2.3. Crystal structure analysis Crystallographic data, data collection parameters and structure refinement details are given in Table 1. Single crystal X-ray data of compounds 1 and 2 were collected on a Bruker-XPS diffractometer (CCD area ˚ , graphite detector, Mo Ka radiation, k = 0.71073 A monochromator). A suitable crystal of 2 was selected in a dry nitrogen stream, because as soon as the crystals are separated from the mother liquor a loss of acetonitrile accompanied by the degradation of crystallinity occurs. The structures were solved and refined by applying SHELXS-97 [10] and SHELXL-97 [11] of the WinGX program system [12]. An empirical absorption correction was carried out, using symmetry-equivalent reflections (SADABS). It was possible to locate all H atoms of compounds 1 and 2 in the difference fourier maps. However, at the final stage of the refinement, the non-solvent H atoms of 2 were positioned with idealized geometry (N–H = 0.90 and C– ˚ ) and constrained to ride on their parent H = 0.97 A atoms, with Uiso(H) = 1.2Ueq(C) for the methylene H atoms as well as for the amine H atoms. All non-hydrogen atoms of 1 and 2 were refined with anisotropic displacement parameters. The cyanide C and N atoms
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Table 1 Crystal data and structure refinement of 1 and 2 Compound 1
Compound 2
Identification code Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group
[Cu (CN)3Cu (C3H10N2)2] C9H20N7Cu4 353.40 173(2) 0.71073 triclinic P 1
[CuI(CN)3CuII(C3H10N2)2] Æ 2MeCN C13H26N9Cu2 435.52 173(2) 0.71073 monoclinic P 21 =m
Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c ()
8.117(2) 8.389(2) 11.920(2) 108.728(3) 100.024(3) 104.888(4)
8.752(2) 13.243(3) 9.549(2)
˚ 3) Volume (A Z Dcalc (g/cm3) Absorption coefficient (mm1) Effective transmission min/max F (0 0 0) Crystal size (mm3) Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to hmax (%) Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)]a Final R indices (all data)b ˚ 3) Largest differential peak and hole (e A P P a R= iFoj jFci/ jFcj. P P b wR ¼ wðF 2o F 2c Þ2 = wðF 2o Þ.
I
II
114.678(4)
712.1(2) 2 1.646 2.977 0.69 362 0.47 · 0.09 · 0.09 1.88–30.50 11 6 h 6 11, 11 6 k 6 11, 17 6 l 6 17 8937 4293 (Rint = 0.0149) 98.6 full-matrix least-squares on F2 4293/0/223 0.628 R1 = 0.0201, wR2 = 0.0563 R1 = 0.0248, wR2 = 0.0630 0.479 and 0.370
were assigned by means of the hard-soft acid–base principle [13] and the displacement parameters. 2.4. EPR and magnetic susceptibility The EPR spectra of 1 were performed at room temperature and at 77 K in a continuous wave X-band EPR-spectrometer (Bruker E 600) with cylindrical cavity operating at 9.58 and 9.77 GHz. The magnetization of the powdered samples was measured over the temperature range 1.8–300 K using a Quantum Design SQUID-based MPMSXL-5-type magnetometer. The superconducting magnet generally operated at a field strength ranging from 0 to 5 T. Measurements of 1 were done in a magnetic field of 1 T. The SQUID magnetometer was calibrated with a palladium rod sample. Corrections are based on subtracting the sample holder signal and estimating the contribution vD from the Pascal constants [14].
1005.6(4) 2 2.606 4.239 0.78 812 0.14 · 0.14 · 0.02 2.35–30.59 12 6 h 6 12, 18 6 k 6 18, 13 6 l 6 13 12559 3191 (Rint = 0.0341) 99.4 full-matrix least-squares on F2 3191/0/130 0.795 R1 = 0.0261, wR2 = 0.0492 R1 = 0.0611, wR2 = 0.0698 0.454 and 0.274
2.5. Current–potential characteristics The current–potential curves were acquired in an aqueous solution of 1 M NaClO4 as electrolyte, using a Scanning Potentiostat (model 362) from EG&G Instruments. The samples of 1 were measured under nitrogen gas at a concentration of 3 mM in a single compartment cell with a rotating disk (glassy carbon electrode) as working electrode and a platinum net as counter electrode. The reference was Ag/Ag+, which has a potential of 0.74 against NHE. All potentials in the following are referred against NHE. The measurements were performed with a 5 mV s1 scan rate (I = 1 mA) at a rotating rate of 600 min1. The electrochemical cell was deaerated for at least 10 min by nitrogen before every experiment. The reference was used to avoid any contaminations by specifically adsorbed ions like SO4 2 and Cl, respectively.
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3. Results and discussion 3.1. Structural studies 3.1.1. Crystal structures of [CuI(CN)3CuII(tn)2] (1) and [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) The crystal structure analysis confirms that 1 and 2 are built up by binuclear [CuI(CN)3CuII(tn)2] complexes. In these molecules, one cyanide ion acts as a 1,2-l2 bridging ligand between the CuI ðCNÞ3 2 moiety and the copper(II) centre of the twofold positive CuII ðtnÞ2 2þ unit (Figs. 1 and 2). The two copper atoms ˚ in 1 and 5.166(4) A ˚ in 2, are separated by 5.183(14) A respectively. Both remaining cyanide ligands of the CuI ðCNÞ3 2 subunit are terminally bound to copper(I). The C–Cu– C bond angles of the trigonal planar anion of 1 and 2 differ only slightly from 120. The CuI–C and C„N bond lengths are nearly identical for the terminal and bridging cyano ligands. In both complexes, each copper(II) is coordinated by one cyanide N and by four N atoms, which belong to two propane-1,3-diamine r-donors, forming a slightly distorted square pyramid. The N atoms of the chelate ligands, which adopt an all-synclinal conformation, occupy the four coordination sites in the basal plane. The N atom of the bridging cyanide group takes up the remaining apical site. The copper(II) atom in 1 is displaced by ˚ [in 2: 0.22(1) A ˚ ] from the basal N4 plane to0.40(1) A wards the apical ligand. The average value in 1 for the
Fig. 1. ORTEP plot of the asymmetric unit of complex 1 with atom numbering scheme (50% probability level; H atoms are depicted as ˚ ] and angles small spheres of arbitrary radii). Selected bond lengths [A []: Cu1–C1, 1.944(6); Cu1–C2, 1.950(2); Cu1–C3, 1.938(5); C1–N1, 1.153(1); C2–N2, 1.153(3); C3–N3, 1.151(3); Cu2–N1, 2.117(5); Cu2– N4, 2.082(1); Cu2–N5, 2.002(3); Cu2–N6, 2.107(7); Cu2–N7, 2.008(3); C1–Cu1–C2, 119.6(1); C2–Cu1–C3, 118.5(1); C3–Cu1–C1, 121.8(1); Cu2–N1–C1, 167.3(1).
Fig. 2. ORTEP plot of the crystal structure of complex 2 with atom numbering scheme (50% probability level; H atoms are depicted as small spheres of arbitrary radii, with MeCN, symmetry code: 0 = x, ˚ ] and angles []: Cu1–C1, 1.936(3); 0.5 y, z). Selected bond lengths [A Cu1–C2, 1.936(1); C1–N1, 1.145(1); C2–N2, 1.145(1); Cu2–N1, 2.172(3); Cu2–N3, 2.040(3); Cu2–N4, 2.038(1); C1–Cu1–C2, 119.9(1); C2–Cu1–C2 0 , 120.0(1); C2 0 –Cu1–C1, 119.9(1); Cu2–N1–C1, 158.0(1).
˚ lower than that CuII–Ndiamine distance is 0.067(14) A II for Cu –Ncyanide, while the difference amounts to ˚ in 2. The elongation of the apical or axial 0.133(8) A bond lengths is an often-observed phenomenon in such cyanometallate compounds [8,15,16]. The decrease of the Cu2–N1–C1 bond angle from 167.3(1) in 1 to 158.0(1) in 2 is due to the influence of the solvent molecules (Fig. 2). The incorporated acetonitrile molecules ˚ (with reare encapsulated in the channels of 2.8 · 9.0 A ˚ and Cu: gard to the van der Waals radii of C: 1.70 A ˚ 1.40 A [17]), which run along the crystallographic a axis and are formed by the binuclear complexes (Fig. 3). The
Fig. 3. Projection of the crystal structure of 2 parallel [1 0 0], with MeCN molecules in a channel formed by stacked [CuI(CN)3CuII(tn)2] molecules (ORTEP plot, 50% probability level; H atoms are depicted as small spheres of arbitrary radii).
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fivefold coordination of copper(II) is in contrast to the sixfold coordination in the ethylene compound, which consists of discrete cations [CuII(en)3]2+ and isolated anions [CuI(CN)3]2 [4,5]. The different numbers of N–H N hydrogen bonds in both compounds result in various crystal packings. As can be seen in Fig. 4, each complex of 1 is connected with its two neighbours via six hydrogen bridges, which can be all classified as moderately weak. The shortest ˚. intramolecular CuII CuII distances are 5.640(7) A Another hydrogen bonding pattern was found in 2. Ten weak N–H N hydrogen bonds are formed between one binuclear complex and three neighbouring molecules. Two acetonitrile molecules are additionally bound via hydrogen bridges to each copper(I/II) complex. 3.2. Physical studies 3.2.1. EPR and magnetic susceptibility The X-band EPR spectrum of a powdered sample of 1 shows a rhombic EPR signal with the g^ region split into two features at 2.20 and 2.13. The gi value of 2.01 stays the same for the temperature range 77–293 K (Table 2). The magnetic measurements of 1 have been carried out in the temperature range 1.8–300 K. The value of
vM steadily increases with decreasing temperature (Fig. 5), which suggests paramagnetic properties of this compound. The magnetic moment, measured at room temperature, exceeds the spin only value of 1.73 B.M. due to mixing of some orbital angular momentum from excited states through spin–orbit coupling and reaches 1.80 B.M. at 300 K. The temperature dependence of vMT, obtained from the experiment, presents a stable value of vMT in the temperature range 100–300 K (Fig. 5). The magnetic data for the investigated compound follow the Curie–Weiss law in the 100–300 K temperature range with the C and H values equal to 0.402 cm3 K mol1 and 1.5 K, respectively. The vMT value of approximately 0.40 cm3 K mol1 at higher temperatures decreases upon cooling and reaches the value of 0.16 cm3 K mol1 at 1.8 K. The calculations of the magnetic parameters were done using the equation for the magnetic susceptibility with a spin value of S = 1/2 for the magnetic centre (Eq. (1)), and taking the influence of the molecular field into account (Eq. (2)) [18]. vM ¼
N b2 g2av sðs þ 1Þ; 3kT
vcorr M ¼
vM 0
vM 1 2zJ Ng2 b2
ð1Þ ð2Þ
;
av
0
where zJ is intermolecular exchange parameter, z is the number of the nearest neighbouring CuII centres, and N, b and k constants have their usual meaning. The value of the exchange parameter was determined by least-square procedure. The minimization of the following function was the criterion used to determine the best fit. vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uPn exp 2 exp 2 u ðv Þi ðvcalc M Þ i =ð vM Þ i : ð3Þ R ¼ t i¼1 M Pn exp 2 i¼1 1=ðvM Þi
77 293
0.06
-1
χM [cm mol ]
0.4
3
0.3 0.04
0.2
0.02
Spectroscopic splitting factors g1
g2
g3
gaverage
2.208 2.201
2.127 2.129
2.009 2.014
2.115 2.115
gaverage = 1/3(g1 + g2 + g3).
0.08
-1
Temperature (K)
0.5
3
Table 2 EPR data of the powdered sample of 1
0.10
χMT [cm mol K]
Fig. 4. Linkage of [CuI(CN)3CuII(C3H10N2)2] units by N–H N hydrogen bridges in 1 (dashed lines indicate hydrogen bonding, symmetry codes: 0 = 1 x, 2 y, z; 00 = 2 x, 2 y, 1 z). Selected ˚ ], donor/acceptor distances [A ˚ ] and bond hydrogen bond lengths [A angles []: N200 H41, 2.321(3); N2 0 H51, 2.251(3); N200 H71, 2.141(6); N200 N4, 3.199(5); N2 0 N5, 3.065(2); N200 N7, 3.028(8); N4–H41 N200 , 165.2(3); N5–H51 N2 0 , 150.5(3); N7–H71 N200 , 168.3(4).
0.1
m (GHz) 0.0
0.00 0
9.593 9.771
50
100
150
200
250
300
T [K]
Fig. 5. Thermal dependence of vM () and vMT () in 1. The linear regression presents the theoretical value of vMT.
T. Pretsch et al. / Inorganica Chimica Acta 358 (2005) 2558–2564
The calculation of the spectroscopic splitting factor gave the value of gaverage = 2.11. The calculated negative value of zJ 0 = 3.10 cm1 (agreement factor R = 1.41 · 104, Eq. (3)) indicates the antiferromagnetic interactions between CuII centres in the crystal lattice of this complex. 3.2.2. Current–potential characteristics The electrochemical behaviour depends on the reverse potential at the negative potential region. When scanning potentials from +1.24 V towards +0.24 V, the first reduction step for 1 is obtained at about +0.82 V and was assigned to the reduction of CuII to CuI. The oxidation of the reduced species could be observed on the reverse scan at +0.85 V. The small discrepancy of these two potentials corresponds to an irreversible process, suggesting a change in the coordination sphere around the copper(II) centre upon oxidation. The closeness of the observed redox potential to the potential of +0.84 V, which exhibits a cyanide-bridged copper(I/II) aziridine complex in nitromethane [9], allows the assumption that the following one-electron oxidation and reduction takes place: þe
½CuðIIÞ CuðIIÞ ½CuðIÞ CuðIIÞ e
ð4Þ
In this connection, the stability of mixed valence copper complexes with aziridine [9] or propane-1,3-diamine is comparable, although these ligands are of totally different chemical nature. In order to find out if 1 also exhibits a complete reduction of CuII to CuI as known from the aziridine complex, a potential curve was recorded between +1.24 V and 0.14 V. Herein, at least two further reduction processes could be observed, leading to an irreversible passivation of the working electrode. This passivation layer inhibited the copper oxidation in the back scan according to Eq. (4). The inhibition layer grows with an increasing number of cycles. The passivation peaks on the electrode surface could not be well resolved. We assume that the reason for that is a combined pre-adsorption and passivation process. 3.2.3. Infrared and UV–Vis spectroscopy The infrared active CN stretching mode of the isolated [CuI(CN)3]2 complex ion (D3h symmetry) is observed at 2111 cm1 for solid Na2[CuI(CN)3] Æ 3H2O [19], at 2094 cm1 for an aqueous solution of K2[CuI(CN)3] [7] and at 2066 cm1 for crystalline [CuI(CN)3][CuII(en)3] [4,5]. The lower symmetry of the copper(II)-bound CuI ðCNÞ3 2 complex ion (C1) causes in complex 1 a splitting of the t(CN) band into a doublet at 2082/2099 cm1 and the appearance of a further mode at 2114 cm1. A similar splitting of the CN stretching mode was found for the binuclear complex [CuI(CN)3CuII(terpy)] (terpy = terpyridine) [7]. The room-temperature absorption spectrum for an aqueous solution of 1 (c = 2 mmol/l) exhibits a strong
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charge-transfer absorption band with a maximum centred around 570 nm, which can be assigned to the electron transfer from CuII to cyanide. 4. Conclusions A new binuclear mixed valence copper(I/II) compound (1) and its diacetonitrile adduct (2) have been prepared. In these complexes, cyanide acts as terminal or bridging ligand. Propane-1,3-diamine stabilizes copper(II) and prevents a three-dimensional linkage of the copper complexes. Weak hydrogen bonding interactions govern the structures of the two compounds, leading to various crystal packings. The magnetic data of 1 indicate properties due to weak antiferromagnetic couplings between CuII centres. Current–potential studies of 1 in an aqueous solution of 1 M NaClO4 reveal a one-electron oxidation-reduction process associated with the Cu(I)/Cu(II) couple. 5. Supplementary material CCDC-258622 (1) and CCDC-258623 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union road, Cambridge CB21EZ, UK (fax: (+44)1223-336033; or e-mail: [email protected]).
Acknowledgements The authors express their gratitude to Mrs. Irene Bru¨dgam for collecting the X-ray diffraction data and to Mrs. Doris Plewinski for performing the elemental analysis.
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Synthesis, crystal structure, magnetic properties and electrochemical behaviour of the mixed valence compound [CuI(CN)3CuII(C3H10N2)2] Thorsten Pretsch a, Jan Ostmann a, Constanze Donner b, Monika Nahorska c, Jerzy Mrozin´ski c, Hans Hartl a,* a
Institute of Chemistry/Inorganic and Analytical Chemistry, Free University of Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany b Institute of Chemistry/Physical and Theoretical Chemistry, Free University of Berlin, Takustrasse 3, D-14195 Berlin, Germany c Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie, 50-383 Wroclaw, Poland Received 21 January 2005; accepted 13 February 2005 Available online 25 March 2005
Abstract The binuclear mixed valence copper(I/II) compound [CuI(CN)3CuII(tn)2] (1) (tn = propane-1,3-diamine) and its acetonitrile ˚, adduct [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) have been synthesized. Complex 1 crystallizes triclinic, space group P 1, a = 8.117(2) A ˚ , c = 11.920(2) A ˚ , a = 108.728(3), b = 100.024(3), c = 104.888(4), Z = 2, and compound 2 monoclinic, space group b = 8.389(2) A ˚ , b = 13.243(3) A ˚ , c = 9.549(2) A ˚ , b = 114.678(4), Z = 2. In both crystal structures, the binuclear [CuI(CN)3 P21/m, a = 8.752(2) A II Cu (tn)2] complex with slightly different bonding geometries is formed. One of the three nitrogen atoms of a CuI(CN)3 moiety is coordinated to Cu(II) at the apex of a square-pyramid with two chelating ligands tn on its base. The shortest intramolecular ˚ . The EPR behaviour of 1 has been investigated at room temperature and at 77 K. The magCuII CuII distance in 1 is 5.640(7) A netic properties were measured in the temperature range 1.8–300 K. 2005 Elsevier B.V. All rights reserved. Keywords: Copper; Mixed valence compounds; Magnetic properties; N ligands; Hydrogen bonding; Crystal structure
1. Introduction There are many examples of mixed valence copper(I,II) complexes in the literature [1,2]. Such compounds are of interest since it is known that the metabolic roles of copper metalloenzymes involve transferring electrons from a substrate molecule to molecular oxygen.
*
Corresponding author. Tel.: +49 30 838 54003; fax: +49 30 838 543310. E-mail addresses: [email protected] (C. Donner), [email protected] (J. Mrozin´ski), hartl@chemie. fu-berlin.de (H. Hartl). 0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.02.018
The chemistry of cyanide-based mixed valence copper(I/II) compounds can be divided into two groups; one group is composed of isolated copper(I) and copper(II) species (salt-like compounds), the other one contains bridging cyanide ligands between copper(I) and copper(II) centres (neutral complexes). The former type of compounds can be formulated as [CuI(CN)3][CuIILn] (L = neutral ligand; n = 2 for tridentate [3], n = 3 for bidentate ligands [4,5]). The second class of complexes contains compounds such as [CuI(CN)Ln][CuIILn + 1] (n = 1 for bidentate ligands [6]) or [CuI(CN)3CuIILn] (n = 1 for tridentate [7] and quadridentate ligands [8]), which may exhibit intervalence charge transfer [9]. In the present study, we report on the synthesis as well as the structural, magnetic, electrochemical and
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IR spectroscopic characterization of [CuI(CN)3CuII(tn)2] (1). The title compound is a new example in the class of cyanide-bridged mixed valence copper(I/II) compounds [CuI(CN)3CuIILn] (n = 2). The crystal structure analysis of the solvate [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) reveals that hydrogen bridged assemblies of the binuclear complexes serve as hosts for acetonitrile molecules.
2. Experimental 2.1. General Copper(I) cyanide was purchased from Avocado, propane-1,3-diamine from Aldrich and sodium cyanide, hydrochloric acid (32%) and acetonitrile from Merck and used as received. Elemental analysis was performed with a CHNS-Elementaranalysator Vario EL instrument. The infrared spectra were recorded on a FT-IR spectrometer Nicolet 5SXC using the KBr pellet technique. The UV–Vis absorption spectrum of 1 was recorded at 25 C, using a Specord 40 spectrometer from the Analytic Jena AG. CuICN has been used as starting material for the formation of 1 and 2. Although no copper(II) salt was added to the reaction mixtures, one half of the copper atoms occur in both compounds in the oxidation state +2. It is well-known that N-ligands influence the redox potential for the oxidation of Cu(I) to Cu(II). The strong r-donors tn stabilize Cu(II) and prevent the reduction by cyanide. The first product during the preparation of 1 is a blue oil, which is solved in water. After one day 68.5 mg of 1 crystallized from the aqueous solution, one week later further 41.8 mg of 1 could be obtained. The blue oil is also produced when the reactions are carried out under argon atmosphere. This reaction behaviour is an indication that disproportionation plays a decisive role for the generation of Cu(II) in 1 and 2. It could be shown by means of IR spectroscopy that the blue oil has nearly the same vibration modes as the crystalline sample of 1. The assignment of the oxidation states in the crystal structures of 1 and 2 was made by means of the coordination numbers and is supported by the spectroscopic data (IR, EPR and magnetic measurements). 2.2. Synthesis of the complexes 2.2.1. Synthesis of bis(propane-1,3-diamine-j2N,N 0 ) copper(II)tricyanomonocuprate(I), [CuI(CN)3 CuII(tn)2] (1) A mixture of CuCN (537.6 mg, 6 mmol), KCN (195.6 mg, 3 mmol), propane-1,3-diamine (7.5 ml, 94 mmol) and HCl (32%, 4.65 ml, 47 mmol) was heated in acetonitrile (150 ml) under reflux with stirring for 2 h.
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The resulting solution contains blue oil, which was separated after cooling to room temperature and redissolved in water (25 ml). After 24 h at 4 C, 68.5 mg of the product (m.p. 187–189 C) was obtained in the form of blue needle-shaped crystals. Anal. Calc. for C9H20N7Cu2 (553.40): C, 30.60; H, 5.70; N, 27.74. Found: C, 30.52; H, 5.66; N, 27.71%. IR data (KBr): ~m ½cm1 ¼ 1314 w, 1392 w, 1403 m, 1437 m, 1472 m, 1580 m, 1602 m, 2082 s, 2099 s, 2114 s, 2877 m, 2929 m, 2949 m, 2962 m, 3147 m, 3234 m, 3273 s, 3290 s, 3328 m. FIR data (KBr): ~m ½cm1 ¼ 147 s, 211 m, 311 m, 332 m, 356 m, 400 m, 492 s, 539 w, 552 w, 570 w, 611 w, 629 s, 638 s. 2.2.2. Synthesis of bis(propane-1,3-diamine-j2N,N 0 ) copper(II)tricyanomonocuprate(I)-diacetonitrilesolvate, [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) A mixture of CuCN (89.6 mg, 1 mmol), KCN (32.6 mg, 0.5 mmol), propane-1,3-diamine (0.06 ml, 0.75 mmol) and HCl (0.04 ml, 0.4 mmol) was heated under reflux with stirring in acetonitrile (35 ml) for 1 h. The resulting blue solution was filtered while hot to remove undissolved residue and the filtrate was gradually cooled to room temperature. After 24 h at 4 C, 13.1 mg of the product was obtained in the form of blue needleshaped crystals. As soon as the crystals are separated from the solution a loss of acetonitrile takes place, leading to results of the elemental analyses similar to those of 1. Found: C, 30.64; H, 6.22; N, 27.37%. 2.3. Crystal structure analysis Crystallographic data, data collection parameters and structure refinement details are given in Table 1. Single crystal X-ray data of compounds 1 and 2 were collected on a Bruker-XPS diffractometer (CCD area ˚ , graphite detector, Mo Ka radiation, k = 0.71073 A monochromator). A suitable crystal of 2 was selected in a dry nitrogen stream, because as soon as the crystals are separated from the mother liquor a loss of acetonitrile accompanied by the degradation of crystallinity occurs. The structures were solved and refined by applying SHELXS-97 [10] and SHELXL-97 [11] of the WinGX program system [12]. An empirical absorption correction was carried out, using symmetry-equivalent reflections (SADABS). It was possible to locate all H atoms of compounds 1 and 2 in the difference fourier maps. However, at the final stage of the refinement, the non-solvent H atoms of 2 were positioned with idealized geometry (N–H = 0.90 and C– ˚ ) and constrained to ride on their parent H = 0.97 A atoms, with Uiso(H) = 1.2Ueq(C) for the methylene H atoms as well as for the amine H atoms. All non-hydrogen atoms of 1 and 2 were refined with anisotropic displacement parameters. The cyanide C and N atoms
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Table 1 Crystal data and structure refinement of 1 and 2 Compound 1
Compound 2
Identification code Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group
[Cu (CN)3Cu (C3H10N2)2] C9H20N7Cu4 353.40 173(2) 0.71073 triclinic P 1
[CuI(CN)3CuII(C3H10N2)2] Æ 2MeCN C13H26N9Cu2 435.52 173(2) 0.71073 monoclinic P 21 =m
Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c ()
8.117(2) 8.389(2) 11.920(2) 108.728(3) 100.024(3) 104.888(4)
8.752(2) 13.243(3) 9.549(2)
˚ 3) Volume (A Z Dcalc (g/cm3) Absorption coefficient (mm1) Effective transmission min/max F (0 0 0) Crystal size (mm3) Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to hmax (%) Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)]a Final R indices (all data)b ˚ 3) Largest differential peak and hole (e A P P a R= iFoj jFci/ jFcj. P P b wR ¼ wðF 2o F 2c Þ2 = wðF 2o Þ.
I
II
114.678(4)
712.1(2) 2 1.646 2.977 0.69 362 0.47 · 0.09 · 0.09 1.88–30.50 11 6 h 6 11, 11 6 k 6 11, 17 6 l 6 17 8937 4293 (Rint = 0.0149) 98.6 full-matrix least-squares on F2 4293/0/223 0.628 R1 = 0.0201, wR2 = 0.0563 R1 = 0.0248, wR2 = 0.0630 0.479 and 0.370
were assigned by means of the hard-soft acid–base principle [13] and the displacement parameters. 2.4. EPR and magnetic susceptibility The EPR spectra of 1 were performed at room temperature and at 77 K in a continuous wave X-band EPR-spectrometer (Bruker E 600) with cylindrical cavity operating at 9.58 and 9.77 GHz. The magnetization of the powdered samples was measured over the temperature range 1.8–300 K using a Quantum Design SQUID-based MPMSXL-5-type magnetometer. The superconducting magnet generally operated at a field strength ranging from 0 to 5 T. Measurements of 1 were done in a magnetic field of 1 T. The SQUID magnetometer was calibrated with a palladium rod sample. Corrections are based on subtracting the sample holder signal and estimating the contribution vD from the Pascal constants [14].
1005.6(4) 2 2.606 4.239 0.78 812 0.14 · 0.14 · 0.02 2.35–30.59 12 6 h 6 12, 18 6 k 6 18, 13 6 l 6 13 12559 3191 (Rint = 0.0341) 99.4 full-matrix least-squares on F2 3191/0/130 0.795 R1 = 0.0261, wR2 = 0.0492 R1 = 0.0611, wR2 = 0.0698 0.454 and 0.274
2.5. Current–potential characteristics The current–potential curves were acquired in an aqueous solution of 1 M NaClO4 as electrolyte, using a Scanning Potentiostat (model 362) from EG&G Instruments. The samples of 1 were measured under nitrogen gas at a concentration of 3 mM in a single compartment cell with a rotating disk (glassy carbon electrode) as working electrode and a platinum net as counter electrode. The reference was Ag/Ag+, which has a potential of 0.74 against NHE. All potentials in the following are referred against NHE. The measurements were performed with a 5 mV s1 scan rate (I = 1 mA) at a rotating rate of 600 min1. The electrochemical cell was deaerated for at least 10 min by nitrogen before every experiment. The reference was used to avoid any contaminations by specifically adsorbed ions like SO4 2 and Cl, respectively.
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3. Results and discussion 3.1. Structural studies 3.1.1. Crystal structures of [CuI(CN)3CuII(tn)2] (1) and [CuI(CN)3CuII(tn)2] Æ 2MeCN (2) The crystal structure analysis confirms that 1 and 2 are built up by binuclear [CuI(CN)3CuII(tn)2] complexes. In these molecules, one cyanide ion acts as a 1,2-l2 bridging ligand between the CuI ðCNÞ3 2 moiety and the copper(II) centre of the twofold positive CuII ðtnÞ2 2þ unit (Figs. 1 and 2). The two copper atoms ˚ in 1 and 5.166(4) A ˚ in 2, are separated by 5.183(14) A respectively. Both remaining cyanide ligands of the CuI ðCNÞ3 2 subunit are terminally bound to copper(I). The C–Cu– C bond angles of the trigonal planar anion of 1 and 2 differ only slightly from 120. The CuI–C and C„N bond lengths are nearly identical for the terminal and bridging cyano ligands. In both complexes, each copper(II) is coordinated by one cyanide N and by four N atoms, which belong to two propane-1,3-diamine r-donors, forming a slightly distorted square pyramid. The N atoms of the chelate ligands, which adopt an all-synclinal conformation, occupy the four coordination sites in the basal plane. The N atom of the bridging cyanide group takes up the remaining apical site. The copper(II) atom in 1 is displaced by ˚ [in 2: 0.22(1) A ˚ ] from the basal N4 plane to0.40(1) A wards the apical ligand. The average value in 1 for the
Fig. 1. ORTEP plot of the asymmetric unit of complex 1 with atom numbering scheme (50% probability level; H atoms are depicted as ˚ ] and angles small spheres of arbitrary radii). Selected bond lengths [A []: Cu1–C1, 1.944(6); Cu1–C2, 1.950(2); Cu1–C3, 1.938(5); C1–N1, 1.153(1); C2–N2, 1.153(3); C3–N3, 1.151(3); Cu2–N1, 2.117(5); Cu2– N4, 2.082(1); Cu2–N5, 2.002(3); Cu2–N6, 2.107(7); Cu2–N7, 2.008(3); C1–Cu1–C2, 119.6(1); C2–Cu1–C3, 118.5(1); C3–Cu1–C1, 121.8(1); Cu2–N1–C1, 167.3(1).
Fig. 2. ORTEP plot of the crystal structure of complex 2 with atom numbering scheme (50% probability level; H atoms are depicted as small spheres of arbitrary radii, with MeCN, symmetry code: 0 = x, ˚ ] and angles []: Cu1–C1, 1.936(3); 0.5 y, z). Selected bond lengths [A Cu1–C2, 1.936(1); C1–N1, 1.145(1); C2–N2, 1.145(1); Cu2–N1, 2.172(3); Cu2–N3, 2.040(3); Cu2–N4, 2.038(1); C1–Cu1–C2, 119.9(1); C2–Cu1–C2 0 , 120.0(1); C2 0 –Cu1–C1, 119.9(1); Cu2–N1–C1, 158.0(1).
˚ lower than that CuII–Ndiamine distance is 0.067(14) A II for Cu –Ncyanide, while the difference amounts to ˚ in 2. The elongation of the apical or axial 0.133(8) A bond lengths is an often-observed phenomenon in such cyanometallate compounds [8,15,16]. The decrease of the Cu2–N1–C1 bond angle from 167.3(1) in 1 to 158.0(1) in 2 is due to the influence of the solvent molecules (Fig. 2). The incorporated acetonitrile molecules ˚ (with reare encapsulated in the channels of 2.8 · 9.0 A ˚ and Cu: gard to the van der Waals radii of C: 1.70 A ˚ 1.40 A [17]), which run along the crystallographic a axis and are formed by the binuclear complexes (Fig. 3). The
Fig. 3. Projection of the crystal structure of 2 parallel [1 0 0], with MeCN molecules in a channel formed by stacked [CuI(CN)3CuII(tn)2] molecules (ORTEP plot, 50% probability level; H atoms are depicted as small spheres of arbitrary radii).
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fivefold coordination of copper(II) is in contrast to the sixfold coordination in the ethylene compound, which consists of discrete cations [CuII(en)3]2+ and isolated anions [CuI(CN)3]2 [4,5]. The different numbers of N–H N hydrogen bonds in both compounds result in various crystal packings. As can be seen in Fig. 4, each complex of 1 is connected with its two neighbours via six hydrogen bridges, which can be all classified as moderately weak. The shortest ˚. intramolecular CuII CuII distances are 5.640(7) A Another hydrogen bonding pattern was found in 2. Ten weak N–H N hydrogen bonds are formed between one binuclear complex and three neighbouring molecules. Two acetonitrile molecules are additionally bound via hydrogen bridges to each copper(I/II) complex. 3.2. Physical studies 3.2.1. EPR and magnetic susceptibility The X-band EPR spectrum of a powdered sample of 1 shows a rhombic EPR signal with the g^ region split into two features at 2.20 and 2.13. The gi value of 2.01 stays the same for the temperature range 77–293 K (Table 2). The magnetic measurements of 1 have been carried out in the temperature range 1.8–300 K. The value of
vM steadily increases with decreasing temperature (Fig. 5), which suggests paramagnetic properties of this compound. The magnetic moment, measured at room temperature, exceeds the spin only value of 1.73 B.M. due to mixing of some orbital angular momentum from excited states through spin–orbit coupling and reaches 1.80 B.M. at 300 K. The temperature dependence of vMT, obtained from the experiment, presents a stable value of vMT in the temperature range 100–300 K (Fig. 5). The magnetic data for the investigated compound follow the Curie–Weiss law in the 100–300 K temperature range with the C and H values equal to 0.402 cm3 K mol1 and 1.5 K, respectively. The vMT value of approximately 0.40 cm3 K mol1 at higher temperatures decreases upon cooling and reaches the value of 0.16 cm3 K mol1 at 1.8 K. The calculations of the magnetic parameters were done using the equation for the magnetic susceptibility with a spin value of S = 1/2 for the magnetic centre (Eq. (1)), and taking the influence of the molecular field into account (Eq. (2)) [18]. vM ¼
N b2 g2av sðs þ 1Þ; 3kT
vcorr M ¼
vM 0
vM 1 2zJ Ng2 b2
ð1Þ ð2Þ
;
av
0
where zJ is intermolecular exchange parameter, z is the number of the nearest neighbouring CuII centres, and N, b and k constants have their usual meaning. The value of the exchange parameter was determined by least-square procedure. The minimization of the following function was the criterion used to determine the best fit. vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uPn exp 2 exp 2 u ðv Þi ðvcalc M Þ i =ð vM Þ i : ð3Þ R ¼ t i¼1 M Pn exp 2 i¼1 1=ðvM Þi
77 293
0.06
-1
χM [cm mol ]
0.4
3
0.3 0.04
0.2
0.02
Spectroscopic splitting factors g1
g2
g3
gaverage
2.208 2.201
2.127 2.129
2.009 2.014
2.115 2.115
gaverage = 1/3(g1 + g2 + g3).
0.08
-1
Temperature (K)
0.5
3
Table 2 EPR data of the powdered sample of 1
0.10
χMT [cm mol K]
Fig. 4. Linkage of [CuI(CN)3CuII(C3H10N2)2] units by N–H N hydrogen bridges in 1 (dashed lines indicate hydrogen bonding, symmetry codes: 0 = 1 x, 2 y, z; 00 = 2 x, 2 y, 1 z). Selected ˚ ], donor/acceptor distances [A ˚ ] and bond hydrogen bond lengths [A angles []: N200 H41, 2.321(3); N2 0 H51, 2.251(3); N200 H71, 2.141(6); N200 N4, 3.199(5); N2 0 N5, 3.065(2); N200 N7, 3.028(8); N4–H41 N200 , 165.2(3); N5–H51 N2 0 , 150.5(3); N7–H71 N200 , 168.3(4).
0.1
m (GHz) 0.0
0.00 0
9.593 9.771
50
100
150
200
250
300
T [K]
Fig. 5. Thermal dependence of vM () and vMT () in 1. The linear regression presents the theoretical value of vMT.
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The calculation of the spectroscopic splitting factor gave the value of gaverage = 2.11. The calculated negative value of zJ 0 = 3.10 cm1 (agreement factor R = 1.41 · 104, Eq. (3)) indicates the antiferromagnetic interactions between CuII centres in the crystal lattice of this complex. 3.2.2. Current–potential characteristics The electrochemical behaviour depends on the reverse potential at the negative potential region. When scanning potentials from +1.24 V towards +0.24 V, the first reduction step for 1 is obtained at about +0.82 V and was assigned to the reduction of CuII to CuI. The oxidation of the reduced species could be observed on the reverse scan at +0.85 V. The small discrepancy of these two potentials corresponds to an irreversible process, suggesting a change in the coordination sphere around the copper(II) centre upon oxidation. The closeness of the observed redox potential to the potential of +0.84 V, which exhibits a cyanide-bridged copper(I/II) aziridine complex in nitromethane [9], allows the assumption that the following one-electron oxidation and reduction takes place: þe
½CuðIIÞ CuðIIÞ ½CuðIÞ CuðIIÞ e
ð4Þ
In this connection, the stability of mixed valence copper complexes with aziridine [9] or propane-1,3-diamine is comparable, although these ligands are of totally different chemical nature. In order to find out if 1 also exhibits a complete reduction of CuII to CuI as known from the aziridine complex, a potential curve was recorded between +1.24 V and 0.14 V. Herein, at least two further reduction processes could be observed, leading to an irreversible passivation of the working electrode. This passivation layer inhibited the copper oxidation in the back scan according to Eq. (4). The inhibition layer grows with an increasing number of cycles. The passivation peaks on the electrode surface could not be well resolved. We assume that the reason for that is a combined pre-adsorption and passivation process. 3.2.3. Infrared and UV–Vis spectroscopy The infrared active CN stretching mode of the isolated [CuI(CN)3]2 complex ion (D3h symmetry) is observed at 2111 cm1 for solid Na2[CuI(CN)3] Æ 3H2O [19], at 2094 cm1 for an aqueous solution of K2[CuI(CN)3] [7] and at 2066 cm1 for crystalline [CuI(CN)3][CuII(en)3] [4,5]. The lower symmetry of the copper(II)-bound CuI ðCNÞ3 2 complex ion (C1) causes in complex 1 a splitting of the t(CN) band into a doublet at 2082/2099 cm1 and the appearance of a further mode at 2114 cm1. A similar splitting of the CN stretching mode was found for the binuclear complex [CuI(CN)3CuII(terpy)] (terpy = terpyridine) [7]. The room-temperature absorption spectrum for an aqueous solution of 1 (c = 2 mmol/l) exhibits a strong
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charge-transfer absorption band with a maximum centred around 570 nm, which can be assigned to the electron transfer from CuII to cyanide. 4. Conclusions A new binuclear mixed valence copper(I/II) compound (1) and its diacetonitrile adduct (2) have been prepared. In these complexes, cyanide acts as terminal or bridging ligand. Propane-1,3-diamine stabilizes copper(II) and prevents a three-dimensional linkage of the copper complexes. Weak hydrogen bonding interactions govern the structures of the two compounds, leading to various crystal packings. The magnetic data of 1 indicate properties due to weak antiferromagnetic couplings between CuII centres. Current–potential studies of 1 in an aqueous solution of 1 M NaClO4 reveal a one-electron oxidation-reduction process associated with the Cu(I)/Cu(II) couple. 5. Supplementary material CCDC-258622 (1) and CCDC-258623 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union road, Cambridge CB21EZ, UK (fax: (+44)1223-336033; or e-mail: [email protected]).
Acknowledgements The authors express their gratitude to Mrs. Irene Bru¨dgam for collecting the X-ray diffraction data and to Mrs. Doris Plewinski for performing the elemental analysis.
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