Novel Organometallic Building Blocks for Molecular Crystal

May 27, 2004 - Novel Organometallic Building Blocks for Molecular Crystal Engineering. 3. Synthesis, Characterization, and Hydrogen Bonding of the ...
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Novel Organometallic Building Blocks for Molecular Crystal Engineering. 3. Synthesis, Characterization, and Hydrogen Bonding of the Crystalline Mono- and Bis-Amide Derivatives of [CoIII(η5-C5H4-COOH)2]+ and of the Cationic Zwitterion [CoIII(η5-C5H4CONHC5H4NH)(η5-C5H4COO)]+

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 4 769-774

Dario Braga* and Marco Polito Dipartimento di Chimica G Ciamician, Universita` di Bologna, 40126 Bologna, Italy

Fabrizia Grepioni* Dipartimento di Chimica, Universita` di Sassari, Via Vienna 2, 07100, Sassari, Italy. Received February 6, 2004;

Revised Manuscript Received March 31, 2004

ABSTRACT: The synthesis and structural characterization of the molecular complexes [CoIII(η5-C5H4CONHC5H4N)2][PF6] (1), [CoIII(η5-C5H4CONHC5H4NH)(η5-C5H4COO)][PF6] (2), and of the supramolecular adducts [CoIII(η5-C5H4CONHC5H4N)2][Fe(η5-C5H4COOH)2][PF6] (3) and [CoIII(η5-C5H4COO)2](-)[(C16H9CH2NH3)](+) (4) are reported together with an investigation of the hydrogen-bonding interactions in the solid state. Compound [CoIII(η5-C5H4CONHC5H4NH)(η5-C5H4COO)](+) has been described as a cationic zwitterion in view of the charge separation arising from deprotonation of the carboxylic group and protonation of the pyridine moiety on the parent compound [CoIII(η5C5H4CONHC5H4N)(η5-C5H4COOH)]+. The complex forms a supramolecular dimer via charge-assisted N-H‚‚‚O hydrogen bonds, topologically analogous to the mixed metal supramolecular complex [CoIII(η5-C5H4CONHC5H4N)2](+)[Fe(η5-C5H4COOH)2], which has also been prepared and characterized. The differences in hydrogen bonding have been analyzed. Introduction A major issue in modern crystal engineering is that of exploiting the topological, electronic, and spin properties of metal atoms to construct molecular materials in which metal centers are joined by ligands1 and can communicate via coordination bonds or noncovalent interactions, such as hydrogen bonds.2 Organometallic molecules and ions combine the supramolecular bonding capacity of organic molecules with the presence of metal atoms, which bring about an almost combinatorial mixture of valence, spin, and charge states together with coordination geometries. Nowadays, the investigation of the interactions between molecules or ions involves all areas of chemistry, in particular, the thriving areas of supramolecular and materials chemistry.3 The goal is that of reaching an intelligent control of the recognition and assembly processes that lead from molecular or ionic components to superstructures, hence from individual to collective chemical and physical properties. All these ideas also apply to molecular crystal engineering,4 the area of supramolecular chemistry devoted to the design and bottom-up construction of functional crystalline materials. As noncovalent interactions are responsible for the existence and functioning of supermolecules, intermolecular and/or interionic interactions are responsible for cohesion and solid-state properties of molecular crystals.5 We are contributing,6 together with others,7 to the efforts to open an organometallic avenue to molecular * To whom correspondence should be addressed. [email protected] and [email protected].

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crystal engineering. While many organic compounds utilized by the crystal engineer are commercially available and can be used directly in the supramolecular assembly (cocrystallization, charge-transfer, inclusion, host-guest, etc.) this is not so with organometallic species, which need, most often, to be synthesized. With this idea in mind, we have begun to prepare, also in collaboration with others, novel organometallic building blocks with adequate supramolecular bonding functionalities for the construction of desired architectures.8 We have focused our strategy on the possibility of adding hydrogen bonding donor/acceptor groups, such as -COOH, -OH, and -CHO, to robust sandwich complexes. The rationale for this choice is that the hydrogen bond is the strongest of the noncovalent interactions and best combines strength and directionality.9 Strength is synonym of cohesion and stability, while directionality implies topological control and selectivity which are fundamental prerequisites for a successful control of the aggregation processes. In previous studies, we have investigated the participation in intermolecular interactions between organometallic and coordination complexes carrying -COOH and -OH groups showing that they form essentially the same type of hydrogen-bonding interactions whether as part of organic molecules or as metal coordinated ligands.10 This is not surprising, as hydrogen bonds formed by such strong donor and acceptor groups are at least 1 order of magnitude stronger than most noncovalent interactions. Dicarboxylic acid molecules, for example, offer the possibility of extended supramolecular networking because of the twin hydrogen

10.1021/cg049942w CCC: $27.50 © 2004 American Chemical Society Published on Web 05/27/2004

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Table 1. Relevant Hydrogen Bonding Distances and Intramolecular Parameters (Å) for Compounds 1-4 cpd

O(H)‚‚‚O

N(H)‚‚‚O

1

O(2)‚‚‚O(3) 2.96(2)

2

O(1)‚‚‚O(5) 2.81(2) O(2)‚‚‚O(4) 2.73(2) O(5)‚‚‚O(4) 2.71(2) O(5)‚‚‚O(5) 2.86(2) O(1)‚‚‚O(3) 2.75(3) O(2)‚‚‚O(3) 2.73(3) O(1)‚‚‚O(6) 2.75(5) O(2)‚‚‚O(8) 2.81(3) O(3)‚‚‚O(5) 2.75(2) O(4)‚‚‚O(7) 2.74(3) O(4)‚‚‚O(8) 2.71(2) O(5)‚‚‚O(6) 2.74(2) O(5)‚‚‚O(7) 2.75(3) O(6)‚‚‚O(7) 2.77(2)

N(1)‚‚‚O(4) 2.91(3) N(2)‚‚‚O(3) 2.72(2) N(3)‚‚‚O(3) 2.89(2) N(4)‚‚‚O(4) 2.75(3) N(1)‚‚‚O(4) 2.86(2) N(2)‚‚‚O(1) 2.84(2) N(2)‚‚‚O(2) 2.88(2)

3 4

N(1)‚‚‚O(3) 2.92(3) N(2)‚‚‚O(2) 2.52(3) N(1)‚‚‚O(1) 2.676(5) N(1)‚‚‚O(8) 2.839(5) N(1)‚‚‚O(2) 2.849(5)

C-OCOOH/COO-

C(12)-O(1) 1.23(2) C(12)-O(2) 1.24(2) C(6)-O(1) 1.19(3) C(6)-O(2) 1.31(3) C(28)-O(3) 1.224(5) C(28)-O(4) 1.243(5) C(29)-O(1) 1.247(5) C(29)-O(2) 1.254(5)

bonding function. We have extensively exploited this feature in the construction and/or utilization of the sandwich acids [Fe(η5-C5H4COOH)2],11 [Co(η5-C5H4COOH)2]+, 5a and [Cr(η6-C6H5COOH)2].8 The cobalt cationic complex, in particular, has proved to be extremely versatile for the selective trapping of alkali ions and for use in solid-gas reactions with vapors of acid and bases.12 With this paper, we first expand our chemistry and crystal engineering efforts toward different supramolecular bonding functionalities, namely, the monoand bis-amido functionalities derived from the dicarboxylic acid cation [CoIII(η5-C5H4COOH)2][PF6]. Second we utilize these building blocks in intermolecular bonding with other organic or organometallic molecules and investigate the supramolecular bonding. In particular, we will report synthesis, characterization, and hydrogen bonding analysis for cobaltocinium-1,1′-bis-amide N-4pyridine hexafluorophosphate, [CoIII(η5-C5H4CONHC5H4N)2][PF6] (1), cobaltocinium-1-(amide N-4-pyridine)-1′-carboxylate, [CoIII(η5-C5H4CONHC5H4NH)(η5C5H4COO)][PF6] (2), the adducts obtained by reacting ferrocene dicarboxylic acid with the complex 1, viz. [CoIII(η5-C5H4CONHC5H4N)2] [Fe(η5-C5H4COOH)2][PF6] (3) and the adducts obtained by reacting the dicarboxylic acid cation [CoIII(η5-C5H4COOH)2](+) with 2-methylamino pyrene, viz. [CoIII(η5-C5H4COO)2](-) [(C16H9CH2NH3)](+) (4). Results and Discussion The focus of this paper is on the intermolecular interactions established by the building blocks; the main intermolecular hydrogen bonding parameters are collected together in Table 1. Relevant molecular features, including bond lengths and angles, will be discussed throughout when relevant. The structure of compound 1 was unknown, while the synthesis had been reported previously by Beer.14 The mixed amido-carboxylic acid complex 2, as well as the supramolecular compounds 3 and 4, have been synthesized by us (see Experimental Section) and are reported for the first time. The cationic complex cobalticinium-1,1′-bis-amide-N4-pyridine, [CoIII(η5-C5H4CONHC5H4N)2] [PF6]‚2H2O (1) has been obtained as described in ref 14. The complex crystallizes in the monoclinic space group P21/c. In the

Figure 1. The structure of [CoIII(η5-C5H4CONHC5H4N)2]+ (1): the molecule adopts an eclipsed conformation with the two pyridine groups slightly tilted with respect to the cyclopentadienyl planes. [HCH atoms in this and in the following figures are not shown for clarity].

Chart 1. Four Structural Types that Can Be Obtained by Different Extents of Protonation of the Complex [CoIII(η5-C5H4CONHC5H4N)(η5-C5H4COOH)](+), 1

solid state, the molecule adopts an eclipsed conformation with the two pyridine groups slightly tilted with respect to the cyclopentadienyl planes (Figure 1). The complex crystallizes with two water molecules in the asymmetric unit. These water molecules interact via hydrogen bonds with the N atoms of the pyridyl [N‚‚‚O 2.72(2) and 2.75(3) Å] and amido groups [N‚‚‚O 2.91(3) and 2.89(3) Å]. No hydrogen bonds are observed between the water molecules, while one of them interacts with the O atom of an amido group [O‚‚‚O 2.96(2) Å]. The carboxylate-monoamido complex 2 also shows an almost eclipsed conformation of the ligands in the solid state. It is interesting to say a few words about this complex. As its relative 1, the cobalticinium complex is cationic. X-ray diffraction, however, clearly shows that the carboxylic group is deprotonated, while the N atom of the pyridine group is protonated. As a consequence of the formal proton transfer from the carboxylic group to the pyridine, the global ionic charge of the complex does not change, but the complex acquires a zwitterionic nature. The relationship between the four structural types that can be obtained by different extent of protonation of 1 is shown in Chart 1. The availability of charge separation within a cationic complex is not a common situation. To the best of the authors’ knowledge, the formation of a cationic-zwitterionic organometallic complex has been discussed only in the case of the zirconocene system [Zr(C5H4CMe2C6H4Me-p)(MeB(C6F5)3]+[MeB(C6F5)3],15 while other examples are known in the case of amino acid derivatives.16 In summary, the complex [CoIII(η5-C5H4CONHC5H4NH)(η5-C5H4COO)]+ can represent a useful starting material in a number of reactions, since it can behave both as a proton donor acid and as a proton acceptor base, showing an amphoteric behavior analogous to that shown by the zwitterionic form of cobalticinium dicar-

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Figure 2. The dimers of [CoIII(η5-C5H4CONHC5H4NH)(η5C5H4COO)][PF6] (2) formed via bifurcate N-H(+)‚‚‚O(-) hydrogenbonding interactions [N‚‚‚O 2.84(2) and 2.88(2) Å].

Figure 4. The tetrameric unit formed by the protonated 2-methylamino pyrene and by the two cobalticinium complexes in crystalline [CoIII(η5-C5H4COO)2](-)[(C16H9CH2NH3)](+) (4) [N(+)‚‚‚O(-) 2.676(5) and 2.849(5) Å] (a); the aromatic rings of the pyrene moieties establish π-stack interactions with an interplanar distance of 3.48 Å (b). [Hydrogen atoms belonging to the -NH3+ group are not shown for clarity].

Figure 3. The ferrocene dicarboxylic acid molecule and the diamido molecule [CoIII(η5-C5H4CONHC5H4NH)2] in [CoIII(η5C5H4CONHC5H4N)2][Fe(η5-C5H4COOH)2][PF6] 3 are linked via O-H‚‚‚N hydrogen bonds [N‚‚‚O 2.52(3) Å] (a) (compare to Figure 3). (b) Note how Cp rings of the ferrocene dicarboxylic acid are eclipsed, while those in the cobalt complex are staggered; furthermore, the pyridil groups are almost eclipsed and in cisoid conformation, while the amido oxygens point in opposite directions.

boxylic acid [CoIII(η5-C5H4COOH)(η5-C5H4COO)],13 which is formally neutral. In the crystal two cations [CoIII(η5-C5H4CONHC5H4NH)(η5-C5H4COO)]+ are linked together via a bifurcate N-H(+)‚‚‚O(-) hydrogen bond forming a dimer [N‚‚‚O 2.84(2) and 2.88(2) Å], as shown in Figure 2. The oxygen atoms of the carboxylate group and the N atom of the amido group are involved in lateral hydrogen bonds [O‚ ‚‚O 2.81(2) and 2.73(2) Å; N‚‚‚O 2.86(2) Å] with two water molecules, which are also interlinked [O‚‚‚O 2.71(2) and 2.86(2) Å]. The dimer is reminiscent of that formed by the dicarboxylic acid of [Fe(η5-C5H4COOH)2], in both its polymorphic forms, and also by compound 3 (see below). It is interesting to note, however, that while the dimer in crystalline [Fe(η5-C5H4COOH)2] is formed by neutral molecules, the dimer present in 2 is between organometallic cations, so that the whole system could be better described as a supramolecular dication, held together by hydrogen-bonding interactions between the protonated pyridine and the deprotonated -COO group. In crystalline 3 the ferrocene dicarboxylic acid molecule [Fe(η5-C5H4COOH)2] and the diamido molecule [CoIII(η5-C5H4CONHC5H4NH)2]‚2H2O (1) are linked by an O-H‚‚‚N hydrogen bond [N‚‚‚O 2.52(3) Å] forming a dimer, as shown in Figure 3a. The dimer ought to be compared to that formed by compound 2 (Figure 2). Despite the structural analogy between the two supramolecular dimers, there are substantial differences

in the hydrogen bonding lengths. The interdimer separation in 2 [Npyridine‚‚‚OCOOH/COO- 2.84(2) and 2.88(2) Å] is in fact much longer than in 3 [Npyridine‚‚‚OCOOH/COO2.52(3) Å]. Although the N‚‚‚O separation in 3 is shorter than most neutral O-H‚‚‚N bonds [the average OCOOHH‚‚‚Npyridine obtained from a search of the Cambridge Structural Database17 is 2.65(8) Å], the C-O bond length distribution within the carboxylic groups [C‚‚‚O 1.31(3) and 1.19(3) Å] indicates that proton transfer has not taken place. It is also worth noticing that, while the Cp rings of the ferrocene dicarboxylic acid are eclipsed, those in the diamido moiety are staggered; nonetheless, the pyridil groups are almost eclipsed and in cisoid conformation, while the amido oxygens point in opposite directions, as shown in Figure 3b. What is observed in 3 is thus a different isomeric arrangement of the ligands with respect to what is observed in compound 1, where the Cp rings are staggered and the amide oxygens point in the same direction. Two molecules of water are located on the sides of the dimers and form hydrogen bonds with the oxygen of the carboxylic groups and the hydrogen of the amido groups, joining the supramolecular adducts [O‚‚‚O 2.73(3) and 2.75(3) Å; N‚‚‚O 2.92(3) Å]. An example of an acid base adduct that bears some resemblance with compound 2 is provided by the salt [CoIII(η5-C5H4COO)2](-)[(C16H9CH2NH3)](+)‚4H2O (4). Compound 4 is obtained by reacting the cobalticinium diacid in its neutral zwitterionic form, namely, [CoIII(η5-C5H4COOH)(η5-C5H4COO)], with 2-methylamino pyrene. The acid-base reaction implies protonation of the base and removal of the second carboxylic proton from the zwitterion, with formation of the monoanionic form of this very versatile complex, namely, [CoIII(η5-C5H4COO)2](-). The monoanion accepts hydrogen bond formation from the protonated cation, as shown in Figure 4a. The -NH3(+) groups participate in two charge-assisted N-H(+)‚ ‚‚O(-) interactions [N‚‚‚O 2.676(5) and 2.849(5) Å] with two cobalticinium complexes, thus forming a sort of tetrameric unit, whereby the protonated 2-methylamino pyrene base bridges two cobalt complexes. In the crystal the acid-base adduct is accompanied by four water

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Table 2. Crystal Data and Details of Measurements for Compounds 1-4 formula MW system space group a [Å] b [Å] c [Å] R [°] β [°] γ [°] V [Å3] Z F(000) µ(MoKR) [mm-1] measured reflns unique reflns parameters GOF on F2 R1 (on F [I >2σ(I)]) wR2 (on F2, all data)

1

2

3

4

C22H22CoF6N4O4P 610.34 monoclinic P21/c 10.11(1) 12.580(4) 19.493(5) 90 95.95(2) 90 2466(3) 4 1240 0.844 3484 3369 325 0.941 0.0949 0.3787

C16H14CoF6N2O4P 502.19 triclinic P1 h 8.256(3) 11.826(2) 11.854(5) 115.29(3) 101.24(4) 98.98(2) 988.3(6) 2 504 1.030 3463 3455 209 1.013 0.0818 0.2757

C34H32CoF6FeN4O8P 884.39 monoclinic C2/c 13.180(3) 22.060(4) 12.330(3) 90 105.70(3) 90 3451(1) 4 1800 1.702 1690 1592 98 1.001 0.0986 0.3566

C29H30CoNO8 579.47 monoclinic P21/n 8.235(5) 12.987(7) 24.566(6) 90 94.24(4) 90 2620(2) 4 1208 0.709 3680 3590 334 1.227 0.0507 0.1489

molecules in the asymmetric unit. The two carboxylate groups not involved in the interaction with pyrene interact with three water molecules, forming a complex network of hydrogen bonds. A further noticeable feature of this complex is shown in Figure 4b: the flat aromatic rings of the pyrene moieties are arranged in the solid to give π-stack interactions (distance between planes 3.48 Å). This interaction is probably responsible for the orange color of the crystals, while all other cobalticinium derivatives are bright yellow. Conclusions The main outcome of this study has been that of adding new molecules to the storehouse of building blocks that can be utilized in organometallic crystal engineering. More specifically, we have reported the synthesis of a novel molecular complex, namely, the carboxylic amido cobalticinium complex [CoIII(η5-C5H4CONHC5H4N)(η5-C5H4COOH)](+) derived from the cationic dicarboxylic acid [CoIII(η5-C5H4COOH)2](+). The complex is obtained in the solid state as its zwitterionic form, because of the transfer of the carboxylic proton from the -COOH to the pyridine group. Even though this transfer does not change the overall ionic charge, the complex dimerizes via N-H‚‚‚O interactions, with formation of an hydrogen bonded supramolecular dication. We have also determined the solid-state structure of the bis-amide 1 and exploited its hydrogen bonding acceptor capacity in the formation of the mixed metal supramolecular complex [CoIII(η5-C5H4CONHC5H4N)2][Fe(η5-C5H4COOH)2][PF6] (3). It is worth stressing that, in general, it is not easy to combine two different metal complexes in the solid state. The use of the hydrogen bonding acceptor and donor capacity of the ligands over the two components does not only provide a means to form a mixed metal supramolecular complex but also to place the two metal centers at a predefined M- - -M′ distance. The adduct 3 bears some resemblance with the dimer formed in the solid state by the cationic complex [CoIII(η5-C5H4CONHC5H4NH)(η5-C5H4COO)](+). However, the hydrogen bonding type between the N and O atoms of the pyridine and COOH group is reverted, i.e., while in compound 2 there is protonation of the N atom; hence, the hydrogen bond is of the charge-assisted

N-H(+)‚‚‚O(-) type. In compound 3 proton transfer is not observed, and the bond between the two complexes is of the O-H‚‚‚N type. This might be due to the different acidity of the -COOH group in the neutral complex Fe(η5-C5H4COOH)2, with respect to the cationic complex [CoIII(η5-C5H4CONHC5H4N)(η5-C5H4COOH)](+). Finally, we have reported synthesis and structural characterization of the supramolecular adduct [CoIII(η5C5H4COO)2](-)[(C16H9CH2NH3)](+) (4) derived from the zwitterionic form of the dicarboxylic cobalticinium acid, namely, [CoIII(η5-C5H4COOH)(η5-C5H4COO)]. The adduct shows in the solid state an interesting combination of supramolecular interactions, hydrogen bonds, ionic charges, and π-stack interactions between the pyrene moieties. Further studies are in progress to utilize the monoand the bis-amido complexes as ligands in the formation of complexes of complexes, for applications in coordination chemistry and crystal engineering. Experimental Section General Information. All the starting materials were purchased from Aldrich and used without further purification. 1,1′-Cobalticinium dicarboxylic acid (zwitterionic form) was prepared according to literature procedure.13 Synthesis of [CoIII(η5-C5H4CONHC5H4N)2][PF6]. Compound 1 was synthesized following the procedure reported by Beer et al.14 Yields 70%, elemental analysis calc. for C22H22N4O4CoPF6: C, 43.3; H, 3.6; N, 9.2%. Found: C, 43.0; H, 3.8; N, 9.0%. Single crystals of 1 suitable for single-crystal X-ray diffraction were obtained by slow cooling of a solution obtained dissolving 50 mg of 1 in 4 mL of boiling water. Synthesis of [CoIII(η5-C5H4CONHC5H4NH)(η5-C5H4COO)][PF6] (2). To a solution of 4-aminopyridine (0.250 g, 2.6 mmol) and triethylamine (0.270 g, 2.8 mmol) in dry CH3CN (40 mL) a solution of 1,1′-bis(chlorocarbonyl)cobalticinium hexafluorophosphate (1.201 g, 2.6 mmol) in CH3CN was added dropwise and under nitrogen. The mixture was allowed to stir for 12 h and the yellowish product was then filtered. The crude product was taken up in hot water and an excess of [NH4][PF6] was added. On cooling, the pure product was obtained as a fine yellow powder. Yields 66%, elemental analysis calc. for C17H14N2O4CoPF6: C, 39.7; H, 2.7; N, 5.4%. Found: C, 40.1; H, 3.1; N, 5.3%. Single crystals for X-ray diffraction were grown by slow evaporation of a water solution of 1 obtained dissolving 10 mg of the compound in 1 mL of water.

Novel Organometallic Building Blocks Synthesis of [CoIII(η5-C5H4CONHC5H4N)2][Fe(η5-C5H4COOH)2][PF6] (3). Single crystals of 3 were obtained by slow evaporation at room temperature of a solution obtained dissolving 1 (20 mg, 0.033 mmol) and the 1,1′-ferrocenedicarboxylic acid (9 mg, 0.033 mmol) in 3 mL of methanol. Yields 87%, elemental analysis calc. for C34H32CoF6FeN4O8P: C, 46.2; H, 3.6; N, 6.3%. Found: C, 45.9; H, 3.7; N, 6.0%. Synthesis of [CoIII(η5-C5H4COO)2](-)[(C16H9CH2NH3)](+) (4). Single crystals of 4 were obtained by slow evaporation at -4 °C of a solution obtained dissolving the zwitterionic form of the dicarboxylic acid [CoIII(η5-C5H4COOH)(η5-C5H4COO)] (15 mg, 0.054 mmol) and the 1-pyrene methylamine (13 mg, 0.054 mmol) in a 1:1 water/ethanol solution. Yields 84%, elemental analysis calc. for C29H30CoNO8: C, 60.1; H, 5.2; N, 2.4%. Found: C, 59.7; H, 5.0; N, 2.4% Crystallography. Crystal data and details of measurements for all compounds are summarized in Table 2. The following details of the structure determination are common to all compounds: MoKR radiation, λ ) 0.71073 Å, graphite monochromator, ψ-scan absorption correction. Data for compound 3 were collected up to θmax ) 20 deg because of heavy crystal decay. All non-hydrogen atoms in 1, 2, and 4 (except for the disordered F atoms in 2) and the metal atoms in 3 were refined anisotropically. H atoms bound to C atoms were added in calculated positions. The [PF6]- anion in 2 is disordered over two positions (occupancy ratio 50:50). The computer program SHELX9718a was used for structure solution and refinement. The computer program SCHAKAL9918b was used for all graphical representations. Hydrogen-bonding interactions were evaluated by the program PLATON.18c Correspondence between the structures determined by single-crystal X-ray diffraction and those of the bulk materials precipitated from solution was confirmed by comparing the experimental powder diffractograms obtained from the bulk materials with those calculated on the basis of the single-crystal structures. Powder data were collected on a Philips PW-1710 automated diffractometer with CuKR radiation and a graphite monochromator. The program PowderCell 2.218d was used for calculation of X-ray powder patterns.

Acknowledgment. We thank MIUR (COFIN and FIRB), the Universities of Bologna (D.B.) and Sassari (F.G.) for financial support. Supporting Information Available: Details about the X-ray crystal structures, including tables of crystal data and structure refinement, atomic coordinates, bond lengths and angles, anisotropic displacement parameters, and ORTEP figures for all compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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Braga et al. 1990, A46, C31. (d) PowderCell programmed by W. Kraus and G. Nolze (BAM Berlin), subgroups derived by Ulrich Muller (Gh Kassel).

CG049942W