Polymorphism in (Tricyclohexylphosphine)gold(I) 2-MercaptobenzoatesA Tale of Two Structural Motifs Douglas R.
Smyth,†
Beverly R.
Vincent,‡
and Edward R. T.
Tiekink*,†
Department of Chemistry, The University of Adelaide, Adelaide, Australia 5005, and Molecular Structure Corporation, 9009 New Trails Drive, The Woodlands, Texas 77381-5290
CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 2 113-117
Received September 18, 2000
ABSTRACT: (Tricyclohexylphosphine)gold(I) 2-mercaptobenzoate is found in four distinct crystal forms. All forms show a linear P-Au-S geometry. In three of the polymorphs the molecule may be described as spherical, as a result of an intramolecular O-H‚‚‚S bond formed between the carboxylic acid group and the coordinated sulfur atom: i.e., a “ball” motif. These are in contrast to a “rod” motif (previously reported) that features association between centrosymmetric molecules via the familiar carboxylic acid dimer. Both motifs may be obtained under the same crystallizing conditions, a result which indicates that there is little energy difference between the different crystal forms. The process of crystallization may be described as a competition between the efficient crystal packing (ball motif) and the formation of carboxylic acid dimers (rod motif). Introduction A guiding principle of crystal engineering is the exploitation of hydrogen-bonding interactions, owing to their strength and directionality, in the design and construction of specific arrays in the solid state for both organic and transition metal systems. For gold(I) complexes, an area of recent focus has been the examination of the complementarity/competition between hydrogenbonding and aurophilic (Au‚‚‚Au) interactions.1 Individual Au‚‚‚Au interactions are known to contribute stability to a lattice in the range 6-11 kcal mol-1: i.e., on the same order of magnitude as hydrogen-bonding interactions.2 The use of bulky phosphine ligands (e.g. Cy3P) in (phosphine)gold(I) thiolates normally precludes association between molecules via Au‚‚‚Au interactions, and hydrogen bonding occurs when this is possible. Thus, it was no surprise when the crystal and molecular structure of one such example, i.e. [Cy3PAu(2-Hmba)] (2-Hmba is the anion derived from 2-mercaptobenzoic acid), showed that molecules aggregate via the familiar carboxylate dimer motif, as shown schematically in Figure 1.3 Interest in (phosphine)gold(I) mercaptobenzoate systems stems from their putative biological activity,4 and recently, prompted by some unexpected solution dependence in its NMR spectra,4b the crystallization of [Cy3PAu(2-Hmba)] from a variety of solvents was attempted. These studies revealed the presence of three additional crystal forms for [Cy3PAu(2-Hmba)], and remarkably, in each of these, the carboxylic acid dimer motif was absent. The results of this study are reported herein.
i.e. forms II-IV, were collected at -100 °C on a Rigaku/MSC Mercury CCD area detector (forms II (acetone, 0.08 × 0.20 × 0.24 mm; mp 173-174 °C) and III (DMSO, 0.15 × 0.20 × 0.25 mm; mp 173-176 °C)) or a Rigaku AFC7R diffractometer (form IV (MeOH, 0.13 × 0.19 × 0.32 mm; mp 175-176 °C)) employing Mo KR radiation; λ ) 0.710 73 Å. Crystallographic details are given in Table 1. The data were corrected for Lorentz and polarization effects5 and for absorption; see ref 6 for forms II and III and ref 7 for form IV. The structures were solved with DIRDIF PATTY8 and refined on F (teXsan5). Hydrogen atoms were included in their calculated positions. Oxygen-bound hydrogen atoms were refined to confirm their positions but were fixed in the final cycles of the respective refinements. Relatively large thermal motion was noted for some of the carbon atoms of the cyclohexyl groups of form IV. For the second molecule comprising the asymmetric unit, these could be resolved into distinct positions for several of the atoms, and these were assigned 50% site occupancies as determined from the refinement. For these groups the carbon atoms were refined with isotropic thermal parameters and hydrogen atoms were not included. Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre with deposition numbers 148396-148398.
Experimental Section
Results and Discussion
[Cy3PAu(2-Hmba)] was prepared according to the literature procedure.4b Colorless crystals were grown from a variety of solvents, as discussed later. Intensity data for three crystals,
[Cy3PAu(2-Hmba)] has been isolated in four distinct crystalline forms. The crystal and molecular structures of a triclinic form, hereafter called form I, isolated from the vapor diffusion of ether into an ethanol solution of the complex, have been reported previously.3 Molecules associate via the familiar carboxylic acid dimer motif as shown in Figure 1. The three remaining forms, i.e.
* To whom correspondence should
[email protected]. † The University of Adelaide. ‡ Molecular Structure Corporation.
be
addressed.
E-mail:
Figure 1. Schematic representation of carboxylic acid dimer formation in the crystal structure of [Cy3PAu(2-Hmba)].
10.1021/cg0000114 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/23/2000
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Table 1. Crystallographic Data and Refinement Details for [Cy3PAu(2-Hmba)]a cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dexptl, g cm-3 µ, cm-1 no. of rflns measd θmax, deg no. of unique rflns no. of refined params no. of rflns with I g 3.0σ(I) R Rw a
form I3
form II
form III
form IV
triclinic P1 h 11.048(2) 13.752(1) 9.858(1) 107.72(1) 97.99(1) 111.46(1) 1273.7(4) 2 1.644
monoclinic P21/n 11.0165(6) 10.819(1) 21.5237(4) 90 102.4420(4) 90 2505.2(2) 4 1.672 60.59 28 086
monoclinic P21/n 10.0680(5) 21.042(1) 13.4015(2) 90 118.9620(3) 90 2484.1(2) 4 1.686 61.10 27 711
triclinic P1 h 11.533(3) 12.269(3) 18.119(5) 83.28(2) 87.06(2) 84.57(2) 2533(1) 4 1.654 59.93 12 142
26.4 5388
26.4 5213
27.5 11 601
271
271
497
4125
4271
6024
0.024 0.029
0.031 0.040
0.045 0.043
Formula C25H38AuO2PS, Mr ) 630.6.
Figure 3. Projections down the P-Au axis for Cy3PAu in forms I-IV of [Cy3PAu(2-Hmba)]. Note that (i) the thiolate groups have been omitted for clarity and (ii) the second molecule comprising the asymmetric unit of form IV has disorder in two of the cyclohexyl groups.
Figure 2. Molecular structure and crystallographic numbering scheme for [Cy3PAu(2-Hmba)], form II (50% displacement ellipsoids). This numbering scheme is the same as that adopted for the other structures of [Cy3PAu(2-Hmba)].
forms II-IV, adopt a distinct motif in the solid state and feature an intramolecular S‚‚‚H hydrogen bond. Forms II and III are monoclinic (P21/n), and each crystallizes with a single molecule in the asymmetric unit. Form IV is triclinic (P1 h ) and crystallizes with two independent molecules in the asymmetric unit. Thus, [Cy3PAu(2-Hmba)] is found in four different crystalline environments and is characterized in five independent molecules. Form II of [Cy3PAu(2-Hmba)] is illustrated in Figure 2,9 and selected interatomic parameters for all crystallographically independent molecules are listed in Table 2. The gold atom in form II exists in a linear geometry defined by the phosphorus and sulfur atoms such that the P-Au-S angle is 172.84(4)°. The Au-S distance of 2.325(1) Å is longer than the Au-P(1) distance of 2.2726(9) Å, and the C-S distance of 1.803(5) Å confirms that the 2-Hmba anion functions as a thiolate ligand.
The disparity of the C-O distances confirms localization of the hydroxyl hydrogen atom on O(2). To a first approximation, the molecular geometry for each molecule in forms III and IV is the same as described above; however, a number of conformational differences associated with both the cyclohexyl groups and thiolate ligands exist. Projections down the P-Au axis for each molecule (the thiolate ligands have been omitted for clarity) are shown in Figure 3. From this diagram, quantified in Table 2, it is clear that the relative orientations of the methine hydrogens differ in the molecules. Other conformational variations are also noted for these residues (Table 2). Perhaps more dramatic is the variation in the relative dispositions of the thiolate ligands. Projections down the P-Au-S axis of the five molecules are shown in Figure 4. Relevant torsion angles are listed in Table 2, and of these, the Au-S(2)-C(2)-C torsion angles show variations of up to 30°. However, the major difference between form I and forms II-IV is seen in the relative disposition of the carboxylic acid groups. In form I, the carbonyl oxygen is syn to the sulfur atom, while in the other structures the carbonyl is anti. Systematic variations are found in the angles about the carboxyl carbon atom, C(1)′. Whereas in forms II-IV the distribution of angles is relatively narrow, in form I the C(1)-C(1′)-O(2) angle has contracted somewhat to 113.7(4)°. This variation is traced to the formation of the carboxylic acid dimer in form I. To maximize the hydrogen-bonding interactions, the O(1)-
(Tricyclohexylphosphine)gold(I) 2-Mercaptobenzoate
Crystal Growth & Design, Vol. 1, No. 2, 2001 115
Table 2. Selected Interatomic Parameters (Distances in Å and Angles in deg) for [Cy3PAu(2-Hmba)]a form IV Au-S(2) Au-P(1) S(2)-C(2) P(1)-C(11) P(1)-C(21) P(1)-C(31) C(1′)-O(1) C(1′)-O(2) S(2)-Au-P(1) Au-S(2)-C(2) Au-P(1)-C(11) Au-P(1)-C(21) Au-P(1)-C(31) S(2)-C(2)-C(1) C(1)-C(1′)-O(1) C(1)-C(1′)-O(2) O(1)-C(1′)-O(2) Au-S(2)-C(2)-C(1) Au-S(2)-C(2)-C(3) Au-P(1)-C(11)-C(12) Au-P(1)-C(11)-C(16) Au-P(1)-C(21)-C(22) Au-P(1)-C(21)-C(26) Au-P(1)-C(31)-C(32) Au-P(1)-C(31)-C(36) S(2)-Au-P(1)-C(11) S(2)-Au-P(1)-C(21) S(2)-Au-P(1)-C(31) S(2)-C(2)-C(1)-C(1′) P(1)-Au-S(2)-C(2) O(1)-C(1′)-C(1)-C(2) O(2)-C(1′)-C(1)-C(2) a
form I
form II
form III
molecule a
molecule b
2.313(1) 2.271(1) 1.769(4) 1.855(4) 1.844(4) 1.834(4) 1.209(5) 1.303(5) 176.8(1) 102.8(1) 112.0(1) 111.4(1) 110.8(1) 122.8(3) 123.6(4) 113.7(4) 122.7(4) -147.5(2) 34.3(2) 58.5(3) -64.8(3) -155.6(3) -28.8(3) -53.3(3) 179.8(3) -101.8(6) 19.6(6) 139.7(5) 0.7(4) 83.6(5) -19.8(4) 160.3(3)
2.325(1) 2.2726(9) 1.803(5) 1.835(4) 1.838(4) 1.849(4) 1.204(6) 1.329(7) 172.82(4) 96.2(1) 109.7(1) 112.7(1) 110.9(1) 123.8(3) 121.9(5) 119.6(4) 118.4(5) -119.4(3) 60.7(3) -54.7(3) 67.7(3) 176.7(3) 49.2(3) 24.7(3) 151.5(2) 45.9(3) 165.6(3) -74.8(3) -2.8(6) -41.8(4) -174.3(5) 5.1(7)
2.311(1) 2.267(1) 1.784(4) 1.839(4) 1.843(4) 1.840(4) 1.204(6) 1.325(7) 177.18(4) 103.3(1) 110.9(1) 111.0(1) 109.3(1) 123.7(3) 121.6(6) 119.2(4) 119.2(5) -133.6(3) 49.8(3) -62.7(3) 59.8(3) 166.0(3) 38.1(3) -66.8(3) 163.8(3) 85.2(7) -155.9(7) -32.0(8) 3.6(6) -113.8(7) 179.3(5) -0.1(7)
2.306(2) 2.267(2) 1.772(7) 1.841(7) 1.825(9) 1.848(7) 1.191(9) 1.30(1) 177.16(7) 106.1(2) 111.1(2) 110.9(2) 109.7(2) 122.6(5) 122.0(8) 119.4(6) 118.6(8) -130.7(6) 54.3(7) 62.4(6) -62.6(5) 35.6(8) -172.5(5) 169.3(5) -60.4(6) 114(1) -126(1) -3(2) 9(1) -153(1) -178.1(8) 1(1)
2.311(3) 2.264(2) 1.763(9) 1.837(7) 1.88(1) 1.84(1) 1.21(1) 1.32(2) 176.09(9) 101.2(3) 112.1(2) 111.4(4) 108.8(4) 125.7(7) 123(1) 119(1) 118(1) -113.8(9) 72.2(8) 64.9(5) -55.7(5) -14(1)/34(1) -172.5(6) 9(1)/-38(1) 159.6(6) 102(1) -137(1) -17(1) 9(2) -132(1) -178(1) 3(2)
The inclusion of two entries for some parameters of form IVa reflects the disorder in the cyclohexyl groups.
C(1)-O(2) angle is splayed by approximately 3°, which has the effect of placing in closer proximity lone pairs on O(1) and S(2). To alleviate this repulsion, the C(1)C(1′)-O(1) angle widens and the C(1)-C(1′)-O(2) angle contracts. The other major difference also relates to the relative disposition of the carboxylic acid residues. In form I this group is out of the plane of the aromatic ring, but in the other forms this group is effectively coplanar. These differences allow for the formation of intramolecular O-H‚‚‚S interactions in forms II-IV, as detailed in Table 3. In addition to the intramolecular S‚‚‚H contacts, there are significant C-H‚‚‚O contacts10 evident in forms IIIV. In each of the molecules there is an intramolecular O(1)‚‚‚H(6) contact present (Table 3). There are also intermolecular O‚‚‚H contacts involving each of the oxygen atoms and symmetry-related cyclohexyl-bound hydrogen atoms with the shortest contacts detailed in Table 4. Whereas in form I molecules are aligned so as to place carboxylic acid groups in close proximity, enabling the formation of a carboxylic acid dimer, in forms II-IV the carboxylic acid groups are “capped” by symmetry-related cyclohexyl groups leading to C-H‚‚‚O contacts. A recent survey of hydrogen-bonded dimers for organic molecules has indicated that the carboxylic acid dimer motif is not pervasive, particularly in the presence of competing hydrogen-bonding functionalities.11 The results of the present study suggest that this conclusion may also be appropriate for metal complexes. Thus, while an extended structure is found in form I of [Cy3PAu(2-Hmba)] where two molecules associate via the carboxylic acid dimer motif, a more spherical motif
Figure 4. Projections down the P-Au-S axis for Cy3PAu in forms I-IV of [Cy3PAu(2-Hmba)]; only the phosphorus-bound carbon atoms of the cyclohexyl rings are shown.
is found in forms II-IV, facilitated by an intramolecular S‚‚‚H interaction. From a packing perspective form I
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Table 3. Intra- and Intermolecular Hydrogen-Bonding D-H‚‚‚A Contacts (Distances in Å and Angles in deg) in [Cy3PAu(2-Hmba)]a form II
III
IVa
IVbb
D
H
A
H‚‚‚A
D‚‚‚A
D-H‚‚‚A
O(2) C(6) C(15) C(13) O(2) C(6) C(11) C(26) O(2) C(6) C(11) C(34) O(2) C(6) C(11)
H(1) H(6) H(151) H(132) H(1) H(6) H(111) H(261) H(1) H(6) H(111) H(341) H(1) H(6) H(11)
S(2) O(1) O(1) O(2) S(2) O(1) O(1) O(2) S(2) O(1) O(1) O(2) S(2) O(1) O(1)
1.99 2.39 2.77 2.81 2.06 2.41 2.49 2.57 2.08 2.42 2.39 2.87 2.07 2.36 2.50
2.917(4) 2.731(7) 3.656(7) 3.676(6) 2.887(5) 2.745(8) 3.402(8) 3.374(6) 2.895(6) 2.752(11) 3.328(8) 3.74(1) 2.879(9) 2.718(11) 3.45(1)
149 101 155 152 145 101 158 143 143 101 168 154 143 102 175
a
sym
-1 + x, y, z 0.5 - x, -0.5 + y, 0.5 - z 1 + x, y, 1 + z 0.5 + x, 0.5 - y, 0.5 + z x, 1 + y, z -x, -y, -z 1 + x, y, z
b
D-H is 1.03 Å for form II and is fixed at 0.95 Å for the remaining molecules. No entry for a C-H‚‚‚O interaction is given for IVb, owing to disorder in the cyclohexyl group. Table 4. Summary of Crystallization Conditions Employed for [Cy3PAu(2-Hmba)] solvent
form I
DMSO DMF CH3CN MeOH EtOH EtOH/ether acetone iPrOH/CH2Cl2 THF EtOAc CHCl3
X
form II
form III
form IV
X X X X
X X X
X X X
X
X X
X X
may be thought of as being comprised of “rods” as opposed to “balls” for forms II-IV. The appearance of the rod or ball motif can be monitored conveniently by infrared spectroscopy measured in KBr disks. The rod motif showed characteristics consistent with carboxylic acid dimer formation: ν(CdO) at 1719 and 1684 cm-1, a weak shoulder at 1659 cm-1, and a broad ν(O-H) band in the region 25003300 cm-1. In contrast, the ball motif shows a strong ν(CdO) stretch at 1717 cm-1 and three sharp but weak bands around 2600, 2645, and 2700 cm-1 that have been assigned to the OH stretch. Using a combination of crystallography (structure determination and unit cell determination) and infrared spectroscopy, the influence of recrystallization solvent on the appearance of the rod and ball motifs was investigated. The polymorphic forms of [Cy3PAu(2-Hmba)] characterized in this study have been isolated using a common crystallization procedure. In a typical experiment, [Cy3PAu(2-Hmba)] was dissolved in the chosen solvent with heating (40-60 °C). The resulting solution was left to stand under ambient conditions until crystals were formed. A range of solvents and solvent combinations deposited single crystals suitable for analysis. These had dielectric constants12 from 2.3 (benzene) to 47 (DMSO) and ET values13 from 34.8 (benzene) to 55.5 (MeOH). No correlation between either of these indicators and the adoption of one motif over another was found. Indeed, both ball and rod motifs were deposited simultaneously from the slow evaporation of ethyl acetate, EtOH, MeOH, and DMSO solutions of the complex. This result clearly indicates that the respective energies of stabilization in the crystal lattices are similar. Consis-
tent with this conclusion is that each of the four crystal structures feature close packing and there are simple relationships between the respective unit cells. Thus, to a first approximation, between forms I and IV, a doubling of c occurs, and between forms II and III, an interchange between b and c occurs. Furthermore, the values of the calculated densities do not show any systematic variations. Table 4 summarizes crystallization conditions for isolation of forms I-IV. In keeping with the notion that there are apparently only small differences in the lattice energies for the ball and rod motifs, the crystals generally had sharp melting points within the range 171-176 °C. Although a detailed morphological study of the different crystal forms was not undertaken, a clear difference between the habits of the ball and rod motifs was evident. Thus, crystals of the rod motif (form I) tended to be rectangular blocks. In contrast, crystals of the ball motif were manifested as multifaceted blocks. The results of the study reported herein allow the following qualitative argument to be postulated. The process of crystallization of [Cy3PAu(2-Hmba)] appears to be a competition between the formation of intermolecular forces between the carboxylic acid groups on one hand and the adoption of a spherical molecule on the other. Each phenomenon would be expected to contribute to the stability of the resulting crystal lattice, i.e. the formation of directional hydrogen bonds and, presumably, more efficient crystal packing. Expressed in another way, it is possible to state that the competition is such as to avoid the formation of the less favorable rod form (from a global packing perspective) and the formation of less favorable intramolecular S‚‚‚H interactions. Clearly, in [Cy3PAu(2-Hmba)] the balance between these competing factors is subtle. Similar arguments have been invoked previously to rationalize the presence of intramolecular Au‚‚‚O or Au‚‚‚S interactions in a series of R3PAu(S2COR′) complexes.14 In these systems favorable Au‚‚‚S interactions led to annular molecules, whereas unfavorable Au‚‚‚O interactions gave rise to approximately spherical molecules. The [Cy3PAu(2-Hmba)] system may be described as a tetramorphic system in that, thus far, four distinct crystalline forms have been detected for the one molecule. However, given that significant conformational
(Tricyclohexylphosphine)gold(I) 2-Mercaptobenzoate
differences exist between form I and forms II-IV, a better description differentiating between the two motifs would be “conformational polymorphism”.15 Nevertheless, the isolation of both ball and rod forms concurrently (see above) suggests that “concomitant polymorphism” may be appropriate.15 The availability of three forms of the ball motif of [Cy3PAu(2-Hmba)] allows an examination of the influence of crystal packing on the molecular geometry. The respective ranges of Au-S and Au-P and S-C bond distances for forms II-IV are 2.306(2)-2.325(1), 2.264(2)-2.2726(9), and 1.763(9)-1.803(5) Å. It is noteworthy that the comparable parameters for form I lie within these ranges. The differences between the longest and shortest distances are respectively 0.02, 0.009, and 0.04 Å, suggesting a certain robustness in the Au-P interaction but flexibility in the other bonds. More remarkable are the variations in the S-Au-P (range 172.82(4)-177.18(4)°) and Au-S-C (range 96.2(1)106.1(2)°) angles of 4 and 10°, respectively; again, the comparable parameters for form I fall within these ranges. Some of the differences in these generally welldefined parameters are far greater than those indicated by the conventional crystallographic errors and point to an influence exerted by the crystalline environment. Comparable variations in geometric parameters have been noted in other transition-metal systems16 and, indeed, in main-group-element systems.17 It would seem that adjustments are made to molecular geometry in order to accommodate the requirements of efficient crystal packing; i.e., the molecular structure is syntactic with the crystalline manifold in which it exists.
Crystal Growth & Design, Vol. 1, No. 2, 2001 117
(2)
(3) (4)
(5) (6)
(7) (8)
(9) (10)
(11) (12)
Acknowledgment. We gratefully acknowledge The University of Adelaide for a Postgraduate Research Award (D.R.S.) and the Australian Research Council for support of the X-ray facility. Supporting Information Available: Further details of the structure determination in CIF format, including atomic coordinates, bond distances and angles, and thermal parameters. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG0000114
