Syntheses and structures of four antimony complexes with planar tridentate pyridine ligands

Inorganica Chimica Acta 360 (2007) 3642–3646 www.elsevier.com/locate/ica

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Syntheses and structures of four antimony complexes with planar tridentate pyridine ligands Khalil A. Abboud, Ruth C. Palenik, Gus J. Palenik *, Richard M. Wood Department of Chemistry, The University of Florida, P.O. Box 117200 Gainesville, FL 32611-7200, United States Received 18 December 2006; received in revised form 27 April 2007; accepted 6 May 2007 Available online 22 May 2007

Abstract The syntheses and structures of four antimony chloride complexes with tridentate N,N,O Schiff base ligands are reported. The tridentate ligands derived from 2-acetylpyridine and various acid hydrazides all lost a proton upon coordination. The ligand was either negative or zwitterionic depending on the acid hydrazide. The complexes are water soluble although a hydrolysis reaction can occur. The appearance of the l-dichloro-l-oxo-tetrachlorodiantimonate(III) anion in one of the complexes was unexpected but appears to be related to the temperatures used in the synthesis. The variation in the distances in the various complexes and the anion are discussed using bond valence sum calculations.  2007 Elsevier B.V. All rights reserved. Keywords: Water soluble Sb(III) complexes; Planar tridentate ligands; Bond valence sum analysis; l-Dichloro-l-oxo-tetrachlorodiantimonate(III)

1. Introduction The coordination chemistry of antimony has both a practical and theoretical interest [1]. The medicinal and cosmetic use of antimony complexes goes back at least to the Egyptians [2,3]. Potassium antimony tartrate or tartar emetic was widely used until the early 1900s despite the somewhat toxic nature of the material [4]. Antimony complexes, such as sodium antimony(V) gluconate, are still being studied as possible drugs for the treatment of various parasitic diseases despite the question of whether Sb(V) or Sb(III) is the active form of the drug [5–9]. More recently, the use of antimony complexes in cancer chemotherapy has become a topic of interest [4,10]. Our interest has been in synthesizing and studying water soluble Sb(III) complexes because of their possible use as therapeutic agents and also because of the interesting stereochemical and bond valence sum problems involving the lone pair of electrons on the Sb(III) ion [11]. This report presents the syntheses and

*

Corresponding author. E-mail address: [email protected]fl.edu (G.J. Palenik).

0020-1693/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.05.007

structural studies of four Sb(III) complexes with planar tridentate N,N,O Schiff base type ligands derived from pyridine 2-aldehydes. 2. Experimental All reagents and solvents were purchased from commercial sources and, unless otherwise noted, were used as received. 2.1. Bis[(2-acetyl-2 0 -picoliniumhyrazinatoaquachloroantimony(III))-6-acetylpyridine] hydrazone (Sb1) Antimony trioxide (Sb2O3; 0.40 g; 2.74 mmol Sb) was dissolved in 1 mL of concentrated HCl, and then 10 mL of dioxane was added. After heating for 2 h, 1.12 g (2.73 mmol) of the ligand, 2,6-diacetylpyridine bis(2-picolinic acid hydrazone), was added together with 40 mL of ethanol. After stirring for about 24 h at room temperature, 50 mL of water acidified to pH 1 with HCl was added. The mixture was then heated for 1 h after which time the solution was clear orange. Seven days later, 0.38 g of orange crystals (dec. 215) of Sb1 suitable for an X-ray study were

K.A. Abboud et al. / Inorganica Chimica Acta 360 (2007) 3642–3646

obtained. Anal. Calc. for C15H19N5O3.5SbCl4: C, 30.59; H, 3.25; N, 11.89. Found: C, 30.49; H, 3.01; N, 11.59%. After a day, 0.11 g of yellow crystals were also obtained. Found: C, 30.53; H, 3.63; N, 11.73%.

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X-ray diffraction study had formed. Anal. Calc. for C26H26N8O3Sb2Cl6: C, 32.71; H, 2.74; N,11.74. Found: C, 32.66; H, 2.56; N, 11.53%. 2.4. Di[(2-acetylpyridineisonicotiniumhydrazinato-N,N 0 ,O) dichloroantimony(III)][di-l-chloro-l-oxobis(dichloroantimonate(III))] trihydrate (Sb4)

0

2.2. (2-Acetylpyridinesalicyloylhydrazinato-N,N ,O) dichloroantimony(III) hemihydrate (Sb2) Antimony trioxide (0.40 g; 2.74 mmol Sb) was dissolved in 1.5 mL of concentrated HCl, 20 mL of dioxane was added, and the solution was heated for 2 h. After cooling, 2-acetylpyridine (0.34 g; 2.74 mmol), salicylhydrazine (0.42 g; 2.74 mmol), and 40 mL of ethanol were added. The yellow solution was stirred at room temperature for 30 min, heated for 4 h, and then 30 mL of water acidified to pH 1 with HCl was added. Heating was continued for an additional 1.75 h. On cooling and filtering, 0.56 g of a bright yellow solid was obtained. Recrystallization of the solid from ethanol gave crystals suitable for an X-ray diffraction study. Anal. Calc. for C14H12N3O2SbCl2: C, 37.62; H, 2.70; N, 9.40. Found: C, 37.38; H, 2.52; N, 9.24%. 2.3. (2-Acetylpyridineisonicotiniumhydrazinato-N,N 0 ,O) trichloroantimony(III) hemihydrate (Sb3) Antimony trioxide (0.80 g; 5.48 mmol Sb) was dissolved in 2 mL of concentrated HCl, 20 mL of dioxane was added, and the solution was heated for 2 h. A solution of the ligand prepared by adding 2-acetylpyridine (0.67 g; 5.48 mmol), isoniazide (0.75 g; 5.48 mmol), and 1 drop of conc. HCl to 50 mL of ethanol was added to the antimony mixture together with an additional 100 mL of ethanol. The yellow slurry was stirred in an ice bath for 20 min and then refrigerated overnight. The next day 50 mL of water acidified to pH 1 with HCl was added to give a total volume of approximately 220 mL. The cloudy yellow solution was heated 45 min, filtered, and filtered again after 15 min. After about 1 week, red crystals suitable for an

Antimony trioxide (0.40 g; 2.74 mmol Sb) was dissolved in 1. mL of concentrated HCl, 10 mL of dioxane was added, and the mixture was heated for 2 h. A solution of the ligand was prepared by heating 2-acetylpyridine (0.34 g; 2.74 mmol), isoniazide (0.38 g; 2.78 mmol), and 1 drop of conc. HCl in 40 mL of ethanol for 1 h. The ligand and antimony solutions were mixed, and ethanol was added to give a total volume of 75 mL. The yellow slurry was stirred for 40 min and allowed to stand at room temperature overnight. An additional 30 mL of pH 1 water was added, and the clear solution was heated for 1 h and then filtered. The volume at this point was about 105 mL. The first crop of 0.10 g were X-ray quality yellow crystals. An additional 0.33 g of good quality yellow crystals were obtained before orange-red crystals appeared with the yellow crystals. Anal. Calc. for C26H30N8O6Sb4Cl10: C, 22.43; H, 2.17; N, 8 05. Found: C, 22.46; H, 1.50; N, 7.94%. 2.5. X-ray structure determinations The X-ray crystals for all four compounds were covered with oil, mounted, and cooled immediately after filtration. However, the analytical determinations from the same samples were carried out at a later date. Consequently, there are some discrepancies between the chemical analyses and the X-ray results which are consistent with the loss of water from the analytical sample. Intensity data for all four compounds were collected at 173 K on a Siemens Smart Platform equipped with a

Table 1 Crystal data and refinements for compounds Sb1, Sb2, Sb3, and Sb4

Formula FW Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) V (A Z Dcalc. (g/cm3) Crystal size (mm) l (Mo Ka)/mm R1 (I > 2rI) wR2 (I > 2rI) R1 (all data) wR2 (all data)

Sb1

Sb2

Sb3

Sb4

C15H22N5O5SbCl3 580.48 monoclinic P2/c 9.8190(18) 13.637(3) 17.625(5) 102.84(3) 2301.0(9) 4 1.676 0.22 · 0.21 · 0.18 1.582 0.0549 0.16230 0.0635 0.1706

C14H14N3O3SbCl2 446.93 monoclinic P21/n 10.7767(2) 11.7484(2) 13.1318(2) 97.841(1) 1647.06(5) 4 1.875 0.28 · 0.18 · 0.15 2.016 0.0244 0.0585 0.0364 0.0612

C26H26N8O3Sb2Cl6 954.75 monoclinic C2/c 29.3733(3) 7.1831(1) 15.5901(2) 92.426(1) 3286.43(7) 4 1.930 0.32 · 0.30 · 0.15 2.176 0.0175 0.0431 0.0210 0.0437

C26H28N8O6Sb4Cl10 1390.06 monoclinic P21/c 11.8432(5) 28.5877(12) 13.6369(6) 105.888(2) 4440.7(3) 4 2.079 0.14 · 0.11 · 0.04 3.056 0.0541 0.1081 0.1016 0.1224

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CCD area detector and a graphite monochromator utiliz˚ ). A full sphere of ing Mo Ka radiation (k = 0.71073 A data, 1850 frames, was collected using the x-scan method with an 0.3 frame width. The first 50 frames were re-measured at the end of the data collection to monitor instrument and crystal stability. Absorption corrections by integration were applied based on the measured indexed crystal faces. The structure determinations by direct methods and the refinements by anisotropic least-squares on F2 were carried out using SHELXTL6 [12]. The relevant crystallographic data, refinement details, and the final R values are given in Table 1. In Sb1 the complex has a twofold axis of symmetry through the hydrazine group so that the asymmetric unit consists of one SbCl(H2O) unit coordinated to the zwitterionic ligand. The charge balance is achieved with uncoordinated and disordered Cl ions in the lattice. A closer look at the crystal packing diagram revealed that the disordered water and chloride ions occupied a channel along the a axis. In the case of Sb2 there appeared to be a disordered water molecule in the lattice which was not in reasonable agreement with the analytical results.

Fig. 1. A view of the cationic dimer in Sb1 showing the atomic numbering and 40%. probability thermal ellipsoids. The Sb–N5 distance is long, ˚ , and is discussed in the text. 2.629 A

Fig. 2. A view of the Sb complex in Sb2 showing the atomic numbering and the 40% probability thermal ellipsoids. The ligand is negative due to the loss of the proton from N3.

3. Results and discussion Our initial syntheses used the pentadentate ligand 2, 6-diacetylpyridine-bis(2-picolinic acid hydrazone) since pentadentate ligands of this type appeared to give water soluble complexes with a variety of metal ions. However, the final complex Sb1, shown in Fig. 1, revealed a potentially octadentate ligand formed from hydrolysis products of the starting materials. Since there is a twofold axis through the hydrazone N–N bond that relates the two halves of the complex, the ligand functions only as a planar tridentate to two different Sb(III) ions. Consequently, we decided to synthesize tridentate ligands using the reaction of 2-acetylpyridine with various acid hydrazides and then reacting these ligands with a solution of Sb2O3 dissolved in hydrochloric acid to yield water soluble Sb(III) complexes. As can be seen in Figs. 1–4, all four complexes have an Sb ion coordinated to the N,N 0 , and O of the appropriate ligand. The ligands in all four complexes have lost the proton on N3 making that N formally negative and, in the case of Sb2, an anionic ligand as well. In Sb1, Sb3, and Sb4, a proton is attached to the N on the uncoordinated pyridine ring making the ligand a zwitterion and formally neutral. The loss of the proton in similar Schiff base ligands has been

Fig. 3. A view of the Sb complex in Sb3 showing the atomic numbering and the 40% probability thermal ellipsoids. The ligand is formally neutral with the transfer of the proton from N3 to N4. The long Sb–N1 distance ˚ is shown with a dashed line. The complex is similar to the of 2.643 A cation in Sb4, see Fig. 4, if Cl3 is removed.

Fig. 4. A view of the one of the two crystallographically independent cations in Sb4 showing the atomic numbering and the 40% probability thermal ellipsoids. The cation is similar to Sb3, see Fig. 3, missing one of the Cl ions coordinated to the Sb(III).

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observed previously [13] and is related to the acidity of the metal ion which is related to the Z2/r ratio [14]. Presumably, a high positively charged metal ion increases the acidity of the proton on N making ionization more facile in spite of the fact that the metal ion is not bonded directly to the N atom. The coordination sphere of the Sb is completed by two chloride ions in the case of Sb2 and Sb4 and three chloride ions in the case of Sb3. The dimeric nature of Sb1 probably restricts the coordination sphere so that only one chlorine and one O, from H2O, can be accommodated in the Sb coordination sphere. The distances and angles around the Sb ions in each structure are summarized in Table 2 together with the bond valences around each Sb ion computed using the program VALENCE [15]. We see that the sum of the bond valences around each Sb atom is 3, as expected. The bond distances, bond angles, and geometry in Sb2 are almost identical to those in Sb4 despite the fact that one ligand is formally negative and the other neutral. This observation is not totally unexpected since the donor atoms in the two cases are identical. In the case of Sb1, the ˚ is longer but corresponds to Sb–N5 distance of 2.622 A 0.25 valence units. Since the BVS around the Sb is 3.33, the question of whether there is a ‘‘bond’’ between Sb and N5 is unresolved (see Table 3). The unusual [Sb2Cl6O]2 anion, Fig. 5, was first characterized in a communication [16] and a subsequent publication [17]. There are only three complete reports of this interesting anion [18–21]. The structure of the ion has been described as 2 SbOCl5 (lone pair) octahedra sharing a common face [18]. Indeed, the Cl3–Cl4–Cl5–Cl8 atoms are ˚ , with Sb2 being coplanar, mean deviation is 0.049 A ˚ from the plane. The Cl5–Cl6–Cl7–Cl8 atoms 0.391 A ˚ , with Sb2 being are coplanar, mean deviation is 0.041 A ˚ from the plane, and the two planes being at 0.330 A 81.2 to each other. The O2 atom is trans to the presumed lone pair on each Sb(III) ion. There are surprisingly large variations in the Sb–Cl and Sb–O bond distances found in the anion in the various publications [17–21] and Sb4. The terminal Sb–Cl distances vary ˚ , with an average of 2.452(40) A ˚ correfrom 2.408 to 2.554 A sponding to 0.80 vu. As expected, the bridging Sb–Cl bonds ˚ , with a mean of are longer and vary from 2.725 to 3.082 A ˚ , or 0.26 vu. The Sb–O bridge distances have a 2.874(103) A ˚ , with the average smaller variation, from 1.920 to 1.983 A ˚ or 1.02 vu. The large variation in the bridging of 1.949(18) A Sb–Cl distances may reflect the weakness of these bonds relative to the Sb–O and terminal Sb–Cl bonds and suggests that they are subject to variations because of packing considerations. In spite of the wide variations in distances, the BVS for the Sb ions only varies from 2.84 to 3.27 valence units, illustrating the usefulness of the BVS in estimating the reliability of a structure determination. ˚ correThe Sb–O distances of 1.964(5) and 1.983(2) A spond to bond valences of 0.98 and 0.93, respectively. The strong Sb–O link may explain the existence of the l-oxo bridge in a fairly acidic solution. Alternatively, the

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Table 2 ˚ ) and X–Sb–X angles (in ) in the Summary of the Sb–X distances (in A four complexes X Cl1 Cl2 Cl3 N1 N2 N5 O1 O2 Val 1-Sb-2 N1–Cl1 N1–Cl2 N2–Cl1 N2–Cl2 N2–Cl3 O1–Cl1 O2–C11 O1–Cl2 O1–Cl3 O1–O2 Cl1–Cl2 Cl1–Cl3 Cl2–Cl3 N1–N2 N1–O1 N1–O2 N2–O1 N2–O2

Sb1

Sb2

Distance 2.662(2)

2.426(2) 2.350(2) 2.629(2) 2.232(2) 1.945(2) 3.13 Angle 145.1(1) 144.4(1)

77.2(1) 85.1(2)

2.613(1) 2.526(1)

Sb3

Sb4

Sb4a

2.531(1) 2.564(1)

2.564(2) 2.571(2)

2.332(2) 2.194(2)

2.489(1) 2.736(1) 2.698(1) 2.643(1) 2.317(1)

2.340(5) 2.189(5)

2.385(6) 2.208(5)

2.128(2)

2.079(1)

2.161(4)

2.138(5)

3.20

3.11

3.20

3.08

86.07(5) 85.25(5) 78.77(5) 82.25(5) 87.30(5)

83.91(4) 80.1(2) 147.86(4) 87.38(4)

87.4(2) 80.8(2) 80.6(1) 78.6(15)

86.5(2) 87.1(2) 81.6(15) 81.34(4)

86.6(2)

86.8(2)

89.13(5)

83.23(4) 76.71(4)

92.7(2)

86.6(2)

160.87(2)

164.45(2) 92.83(2) 97.01(2)

159.8(6)

160.2(7)

70.6(2) 142.3(2)

69.2(2) 141.2(2)

85.6(2)

66.4(2) 134.5(2) 83.9(2) 68.4(2) 83.2(2)

69.99(7) 142.09(13) 72.11(7)

71.7(2)

72.0(2)

71.21(5)

Table 3 Summary of the Sb–X distances and angles in the Sb2Cl6O2 anion in Sb4 Bond

Distance

Atoms

Angle

Sb2–O2 Sb2–Cl3 Sb2–Cl4 Sb2–Cl5 Sb2–Cl8 Val

1.964(5) 2.461(3) 2.425(2) 2.775(3) 3.054(2) 3.11

O2–Sb2–Cl3 O2–Sb2–Cl4 O2–Sb2–Cl5 O2–Sb2–Cl8 Cl3–Sb2–Cl5 Cl6–Sb2–Cl8

86.4(2) 89.5(2) 77.9(2) 78.7(2) 164.2(1) 158.5(1)

Sb3–O2 Sb3–Cl5 Sb3–Cl6 Sb3–Cl7 Sb3–Cl8 Val

1.983(5) 1.790(2) 2.495(2) 2.487(2) 2.735(2) 3.05

O2–Sb3–Cl5 O2–Sb3–Cl6 O2–Sb3–Cl7 O2–Sb3–Cl8 Cl5–Sb3–Cl7 Cl6–Sb3–Cl8

77.2(1) 86.7(2) 87.8(2) 78.6(2) 164.3(1) 165.3(1)

oxo group may be shielded by the large Cl ions so that the stability is a kinetic effect. There also appears to be a strong N–H  O hydrogen bond from the protonated pyridinium ring to the O of the [Sb2Cl6O]2 ion, a feature found in two of the other reports [18,19]. The strong hydrogen bond may also contribute to the variations in Sb–Cl and Sb–O distances. The origin of the ion is intriguing, particularly since Sb3 and Sb4 used the same ratios of starting material, and the total volumes were only slightly different so that the concen-

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5. Summary and conclusions We have shown that tridentate Schiff base ligands derived from 2-acetylpyridine and various acid hydrazides give water soluble Sb complexes. Because of the size and charge of the Sb(III) ion, the resulting ligands can be formally neutral or zwitterionic depending on the nature of the acid hydrazide used in the synthesis. The water solubility of these Sb(III) complexes could be useful in the treatment of various health problems. We have also seen that the unusual anion [Sb2Cl6O]2 can undergo distortions of the weaker bridge bonds to accommodate hydrogen bonding and/or optimize crystal packing. Finally, we see that the use of the BVS method in analyzing structural studies can be extremely useful. Fig. 5. A view of the Sb2Cl6O2 anion in Sb4 showing the atomic numbering and the 40% probability thermal ellipsoids.

trations differed by about 5%. In the case of Sb4, the solution was at room temperature overnight and also was heated for a slightly longer time, while Sb3 was cooled and maintained in the refrigerator overnight. Since the [Sb2Cl6O]2 anion is the result of a hydrolysis reaction involving an SbCl3 species, if the activation energy for the reaction is small, then the slightly different temperatures could account for the different products in Sb3 and Sb4. 4. Antimony coordination The coordination of the antimony, as well as other main group elements, is not driven by crystal field effects. Consequently, the final complex is a function of both the ligand and synthetic pathway. The four Sb complexes in this report have both similarities and differences that point out the complexity of the coordination chemistry of Sb(III). One aspect of Sb(III) coordination which is generally considered is whether the lone pair is stereochemically active which is inferred from the geometry but difficult to prove. The five coordinate complexes observed in Sb2 and Sb4 could be interpreted as evidence for the stereochemical activity of the lone pair. The fact that Sb in Sb3 can be viewed as five or six coordinate depending on the acceptance of the long Sb  N1 interaction as a bond adds to the complexity of antimony coordination. A similar situation occurs in Sb1 where there is a long Sb  N5 interaction which again can be viewed as a weak bond or not depending on one’s point of view. The Sb1 complex is also unusual in that the Cl ion is in the plane of the ligand and O2 is perpendicular to the plane which may be a steric effect resulting from the dimeric complex. When we compare Sb3 and Sb4 that had identical starting materials but slightly different reaction conditions, we see that the coordination of the Sb(III) ion also depends on the synthesis. However, in all the complexes that we have studied the one constant factor is that the bond valence sum always equals the oxidation state of the Sb ion.

Appendix A. Supplementary material CCDC 630375, 630376, 630377 and 630378 contain the supplementary crystallographic data for Sb1, Sb2, Sb3 and Sb4. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2007.05.007. References [1] W. Levason, G. Reid, Comprehensive Coordination Chemistry II, vol. 3, Elsevier, Oxford, 2004, pp. 465–544. [2] R.E. Krebs, The History and Use of our Earth’s Chemical Elements, Greenwood Press, Westport, Conn, 2004, pp. 219–242. [3] N. Ulrich, Chem. Eng. News 81 (2003) 126. [4] J. Duffin, B.G. Campling, J. Hist. Med. Allied Sci. 57 (2002) 61. [5] N. Singh, Indian J. Med. Res. 123 (2006) 411. [6] C. Demicheli, L.S. Santos, C.S. Ferreira, N. Bouchemal, E. Hantz, M.N. Eberlin, F. Frezard, Inorg. Chim. Acta 359 (2006) 159. [7] C. Brochu, J. Wang, G. Roy, N. Messier, X.-Y. Wang, N.C. Saravia, M. Ouellette, Antimicrob. Agents Chemother. 47 (2003) 3073. [8] S. Rais, A. Perianin, M. Lenoir, A. Sadak, D. Rivollet, M. Paul, M. Deniau, Antimicrob. Agents Chemother. 44 (2000) 2406. [9] R. Ge, H. Sun, Accts. Chem. Res. 40 (2007) 267. [10] E.R.T. Tiekink, Crit. Rev. Oncol./Hematol. 41 (2002) 217. [11] R.C. Palenik, K.A. Abboud, G.J. Palenik, Inorg. Chim. Acta 358 (2005) 1034. [12] G.M. Sheldrick, SHELXTL 5 Bruker-AXS, Madison, WI, 1995. [13] L.P. Battaglia, A.B. Corradi, C. Pelizzi, G. Pelosi, P. Tarasconi, J. Chem. Soc., Dalton Trans. (1990) 3857, and references therein. [14] G. Wulfsberg, Inorganic Chemistry, University Science Books, Sausalito, CA, 2000, pp. 56–67. [15] I.D. Brown, J. Appl. Crystallogr. 29 (1996) 479. [16] M. Hall, D.B. Sowerby, J. Chem. Soc., Chem. Comm. (1979) 1134. [17] M.J. Bagley, M. Hall, M. Nunn, D.B. Sowerby, J. Chem. Soc., Dalton Trans. (1986) 1735. [18] G.R. Willey, A. Asab, M.T. Lakin, N.W. Alcock, J.C.S. Dalton Trans. (1993) 365. [19] G.R. Willey, L.T. Daly, P.R. Meehan, M.G.B. Drew, J.C.S. Dalton Trans. (1996) 4045. [20] B. Jaschinski, R. Blachnik, R. Pawlak, H. Reuter, Z. Kristallogr. 213 (1998) 543. [21] T. Steiner, Angew. Chem., Int. Ed. 41 (2002) 49.