A Planar Bibridged Cu10Br222- Oligomer: Dimensional Reduction and Recombination of the CuBr2 Lattice via the N-H‚‚‚Br- and the C-Br‚‚‚Br- Synthons Salim Haddad
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 501-505
Department of Chemistry, University of Jordan, Amman, Jordan
Firas Awwadi and Roger D. Willett* Department of Chemistry, Washington State University, Pullman, Washington 99164 USA Received February 13, 2003
ABSTRACT: A retro-crystal engineering analysis is given of the microporous stepped layer structure observed in the compound (35DBP)2Cu10Br22 (where 35DBP+ is the 3,5-dibromopyridinium cation). The assembly of the anionic layer structure can be envisioned as occurring in two steps. First, dimensional reduction of the parent CuBr2 structure into (Cu10Br222-)n ribbons occurs via the action of the 35DBP+ “molecular scissors”. These ribbons have the Geiser notation of n(t|, t⊥) ) 10(7/2, 1/2). Next, the ribbons are recombined into stepped layers with the concomitant reconstruction of the semi-coordinate Cu‚‚‚Br bonds. These layers aggregate through synthonic interactions between the 35DBP+ cations and the bromide ions in the layers. The combination of N-H‚‚‚Br- and the C-Br‚‚‚Br- synthons define bibridged [Br- - (35DBP+)2 - Br-] units that tie the layers together. These bibridged units are linked into cationic chains via C-H‚‚‚Br- interactions. It is observed that the nesting of these chains produces a cationic layer that is commensurate with the stepped anionic layer. This mutual commensuration gives rationale for the length of the Cu10Br222- oligomers. Introduction The use of crystal engineering concepts to design, influence, and analyze the structures of new crystalline materials is an area of growing scientific interest. The application of these techniques to hybrid organic/ inorganic systems will provide the basis for the design of new materials with novel and, hopefully, important properties. Dimensional reduction analysis allows the parentage of a lower dimensional structure to be traced back to a higher dimensional prototype structure.1 With hybrid systems based on organoammonium salts of metal halides, the organic cations can act as molecular “scissors” to reduce the dimensionality of the parent metal halide structure. In favorable cases, these lower dimensional metal halide units can be “fused” or recombined back into higher dimensional structures with different connectivities.2 The organic cations and metal halide frameworks then can be assembled into the final crystal structure via supramolecular synthons.3 These include hydrogen bonding interactions of various types, π-π stacking interactions, and other noncovalent interactions. We have recently had the opportunity to apply these techniques to a number of different hybrid organic/ inorganic metal halide systems. The dimensional reduction and recombination concepts have been utilized in the analysis of a series of Cd(II), Sn(II), and Pb(II) halide salts, where novel ribbons, stacks, and stepped layer structures were observed. In these studies, the parentages of the edge-shared metal halide octahedral frameworks were traced back to the layered hexagonal MX2 lattice.4 These studies extended our previous analyses of copper(II) halide salts, where an extensive series of structures based on the parent layered CuX2 lattice have been obtained.5 The parent CuX2 structure is a ferro-
distortive version of the layered hexagonal CdX2 structure induced by the Jahn-Teller elongation of the MX6 octahedra.6 It is most conveniently pictured as consisting of planar, bibridged (CuX2)n chains linked into layers by longer semi-coordinated Cu‚‚‚X bonds. A variety of ribbons and stacked structures exist that can be related to the parent CuX2 structure by dimensional reduction and recombination processes. These include simple (Cu2X2n+22-)n ribbons that are one-dimensional segments of the parent CuX2 structure,7 recombined (Cu2X2n+22-)n stacks obtained by reconstruction of the semi-coordinate Cu‚‚‚X bonds within the ribbons,8 and new stepped layer structures by reconstruction of the semi-coordinate Cu‚‚‚X bonds between the (Cu2X2n+22-)n ribbons.9 During this period of time, the results of a series of structures of copper(II) halide salts containing brominated pyridinium cations inspired our interest in the role of the C-Br‚‚‚Br- synthon in the control of the packing of CuBr42- anions in crystalline lattices.10 To further characterize this synthon, we have begun a systematic study of the structures of the monobromopyridinium and dibromopyridinium salts of copper(II) halides, as well as the copper(II) halide complexes with the corresponding neutral ligands. These studies are revealing that the C-Br‚‚‚X- synthon plays a major role in the determination of the three-dimensional structure of these compounds. This synthon is typically characterized by the existence of short, linear C-Br‚‚‚X- contacts in which the Br‚‚‚X- distance is substantially (0.3-0.4 Å) shorter than the sum of the van der Waals radii and the C-Br‚‚‚X- angle lies between 160 and 180°. This is consistent with theoretical calculations that indicate the presence of a positive electrostatic potential in the region along the extension of the C-Br bond, but a
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Figure 1. Anisotropic displacement illustration of hydrogen bonding in the molecular unit. Thermal ellipsoids shown at 50%.
Figure 2. Illustration of the primary ribbon structure corresponding to a n(t|, t⊥) ) 10(7/2, 1/2) translational repeat structure.
negative potential in the π region. The presence of the C-Br‚‚‚X- synthon in addition to the N-H‚‚‚X- synthon transforms the pyridinium cation from a monosynthonic species into a polysynthonic species. Thus, these bromosubstituted pyridinium cations can be used to build various types of networks linking anionic species. In the (n-BrpyH)2CuX4 salts (n ) 2, 3, or 4), for example, various chain and layer networks are observed, built up from monobridged [CuX42- - (n-BrpyH+) - CuX42-] and bibridged[CuX42- - (n-BrpyH+)2 - CuX42-] linkage units.11 During these latter studies, a crystal was obtained as a byproduct in the attempt to prepare the CuBr42salt of the 3,5-dibromopyridinium cation, 35DBP+. The crystallographic analysis reveals this compound to have the stoichiometry (35DBP)2Cu10Br22 and that it contains the planar bibridged Cu10Br222- anion as well as the 35DBP+ cation. This anion (Figure 1) is the longest of a large series of pseudo-planar CunX2n+22- anions (X ) Cl, Br) that we have characterized over the past several decades.7-9 The majority of these oligomers are for n ) 2, 3, or 4. The previous longest oligomer found was the Cu7Br162- anion in the 1,2-dimethylpyridinium salt.9a In this paper, we analyze the structure of (35DBP)2Cu10Br22 and the role that the C-Br‚‚‚X- and N-H‚‚‚Xsynthons, as well as other synthons, play in the development of the structure. Structure Description. The structure will be analyzed in three steps. First, the local structure of the Cu ion will be described. Next, the dimensional reduction and recombination process that yields the observed perforated stepped layer structure will be detailed. This process involves the reconstruction of the semi-coordinated Cu‚‚‚Br- bonds within the structure. Finally, the role of the synthonic interactions involving the 35DBP+ cation will be described. The latter gives insight into the details of the reduction and recombination process as well as to the development of the three-dimensional structure. The Cu10Br222- oligomer is illustrated in Figure 1, along with the two 35DBP+ cations that hydrogen bond to a pair of the trans terminal bromide ions. These two 35DBP+ cations provide charge compensation for the
pair of bromide anions inserted into the chains during the chain breaking process. Within the oligomer, each copper ion has an approximate square planar coordination, with Cu1, Cu2, and Cu5 showing small tetrahedral distortions while Cu3 and Cu4 each have a pyramidal distortion. The Cu-Br distances range from 2.374(3) to 2.477(3) Å, while the trans Br-Cu-Br angles vary from 167.8(1) to 178.9(1)°. The bridging Cu-Br-Cu angles range from 90.9(1) to 94.8(1)°. The warping of the planar coordination geometry of the individual Cu atoms can be related to interactions between species in the crystalline lattice, as described later. In particular, there is asmall fold in the oligomer at Cu4 and Cu4a due to the stacking interactions between oligomers. The dihedral angle between the end and central segments of each oligomer is 3.6°. Each Cu(II) ion completes its elongated 4+2 JahnTeller distorted octahedral coordination by the formation of semi-coordinate bonds to neighboring oligomers. For each of the three Cu(II) ions with tetrahedral distortions, the two semi-coordinate bonds have similar distances (range 3.118-3.245 Å, average 3.19 Å). In contrast, the coordination for Cu4 and Cu5 could be more appropriately designated a 4+1+1 coordination since there is a 0.5-0.6 Å difference between the each pair of semi-coordinate bonds (range 2.907-3.561 Å). The process of the retro-crystal engineering analysis of the rearrangement of the semi-coordinate bonds between oligomers to yield the observed stepped layer structure can be broken down into two parts: (a) the dimensional reduction of the CuBr2 lattice into (Cu10Br22)n2n- ribbons via the supramolecular “scissors” action of the (35DBP)Br species and (b) the recombination of these ribbons into stepped, perforated layers via formation of new semi-coordinate bonds. A ribbon obtained via the dimensional reduction process is illustrated in Figure 2. Utilizing the Geiser notation previously developed,5 this stacking pattern of oligomers can be denoted by the symbol n(t|, t⊥) ) 10(7/2, 1/2) where n is the order of the oligomer, t| is the translation of the adjacent oligomer parallel to the oligomer axis (in units of the Cu‚‚‚Cu (or Br‚‚‚Br) repeat distance within the oligomer) and t⊥ is the corresponding
A Planar Bibridged Cu10Br222- Oligomer
Crystal Growth & Design, Vol. 3, No. 4, 2003 503
Figure 3. Illustration of the recombination of two ribbons to form the n(t|, t⊥)(t|′′, t⊥′′) ) 10[(7/2, 1/2)][(15/2, -1/2)] microporous layer structure.
Scheme 1
translation perpendicular to the oligomer axis. This stacking pattern is illustrated diagrammatically in Scheme 1. The large t| translation means that only the central four Cu(II) ions complete their 4+2 coordination geometry within the ribbon. This, then, provides a rationale for the fold at Cu4 and Cu4a. These ribbons are then recombined into the stepped, perforated layers by the formation of additional semicoordinate Cu‚‚‚Br bonds between ribbons. This process completes the 4+2 coordination geometry of the remaining six Cu atoms. In so doing, a second interlaced set of ribbons is developed. This is illustrated in Figure 3 and in Scheme 2. This layer has the extended Geiser notation5d of n(t|, t⊥)(t|′′, t⊥′′) ) 10[(7/2, 1/2)][(15/2, -1/ 2)], where the (t|′′, t⊥′′) notation in the second set of brackets denotes the translational process defining the interlaced ribbons. The net result of the dimensional reduction and recombination process is the introduction of a small gap between the ends of neighboring oligomers in adjacent stacks. This gap can be visualized as arising from the excision of a Cu(II) ion from the parent CuBr2 structure that occurs during the reconstruction of the layers, with a pair of 35DBP+ cations replacing the charge of the each excised Cu(II) ion. The above analysis provides a succinct description of the genesis of the stepped layer structure in terms of recombination of the semi-coordinate Cu‚‚‚Br between oligomers. However, it begs the question of why this particular structure arises and how the layers assemble into the final three-dimensional structure. To explore this question, we look at the synthonic interactions between the 35DBP+ cations and the Cu10Br222- oligomers. The 35DBP+ cation has the potential to be involved in several synthonic interactions. In addition to the primary N-H‚‚‚Br- and C-Br‚‚‚Br- interactions, several other potential interactions, particularly involving C-H‚‚‚Br- and C-H‚‚‚Br hydrogen bonding, play a significant role in the final structure. The role that the N-H‚‚‚Br- and C-Br‚‚‚Br- synthons play in developing the three-dimensional structure is illustrated in Figure 4. These synthonic interactions are denoted by the dashed lines in that figure.
Figure 4. Illustration of two sets of [Br- - (35DBP+)2 - Br-] synthonic linkages. The N-H‚‚‚Br- and C-Br‚‚‚Br- synthonic interactions are shown by dashed lines, while the C-H‚‚‚Brand C-H‚‚‚Br hydrogen bonding interactions are shown as dotted lines. Semi-coordinate Cu‚‚‚Br bonds omitted for clarity.
Figure 5. Nesting of the C-H‚‚‚Br hydrogen bonded cation chains.
It is to be noted that only one of the two aryl bromine atoms participates in this type of synthonic interaction. As illustrated in Scheme 3, two of the 35DBP+ cations act in concert to link pairs of oligomers from adjacent layers. The C-Br‚‚‚Br- synthon is characterized by a Br‚‚‚Br- distance of 3.430 Å and a C-Br‚‚‚Br- angle of 176.6°, while, in the N-H‚‚‚Br- synthon, the N‚‚‚Brdistance and N-H‚‚‚Br-angle are 3.281 Å and 146.2°, respectively. This bibridged [Br- - (35DBP+)2 - Br-] unit is very similar to that found in the (3BP)2CuX4 salts,11 with the caveat that the bromide ions are replaced by CuBr42- complex ions. The resultant orientation of the 35DBP+ cations is such that short, linear C-H‚‚‚Br- interactions are formed, both within and between the bibridged units (the H‚‚‚Br distances are
Scheme 2
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Crystal Growth & Design, Vol. 3, No. 4, 2003 Scheme 3
Table 1. Crystal Data and Structure Refinement for (35DBP)2Cu10Br22 empirical formula formula weight T (K) λ (Å) crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Fcalc (Mg/m3) µ (mm-1) F(000) indep. reflxs goodness-of-fit R1 a [I > 2σ(I)] wR2 b [I > 2σ(I)] a
C5H4Br13Cu5N 1434.62 295(2) 0.71073 triclinic P-1 9.8102(14) 10.0076(13) 12.8246(14) 103.151(8) 97.030(9) 98.851(10) 1195.1(3) 2 3.987 26.102 1282 3188 [R(int) ) 0.0442] 0.963 0.0577 0.1235
R1 ) ∑||Fo| - |Fc|/|∑|Fo|. b wR2 ) (∑w||Fo|2 - |Fc|2|/∑w|Fo|2)1/2.
3.07 and 3.10 Å, respectively). In this manner, hydrogen bonded chains of 35DBP+ cations are formed. In addition, two different weaker synthonic interactions are developed between the cations and the inorganic framework. A short C-H‚‚‚Br- hydrogen bond of 2.76 Å is formed to a bromide ion in the Cu10Br222- oligomer. Also two π-Br interactions are observed for each cation, with centroid-Br- distances of 3.573 and 3.630 Å, respectively, with the centroid-Br- vector making an angle of 16.9 and 10.7° to the normal. These further strengthen the interactions tying adjacent layers together. Not only do these synthonic interactions help hold the layers together, they are also intimately associated with the development of the observed ribbon and layer structure. The complementary nature of their role in the development of the n(t|, t⊥) ) 10(7/2, 1/2) ribbon structure described above is evident from Figure 4. The repeat spacing between 35DBP+ cations along the above hydrogen bonded chains just matches the displacement of adjacent oligomers within the ribbons. In addition, the chains of 35DBP+ cations nest together as shown in Figure 5. When this nesting arrangement is overlaid with the stepped layer structure shown in Figure 3, it
Haddad et al. Table 2. Bond Lengths [Å] and Angles [°] for Decamera atoms
distance
atoms
angles
Cu(1)-Br(11) Cu(1)-Br(1) Cu(1)-Br(10) Cu(1)-Br(2) Cu(1)-Br(3)#2 Cu(1)-Br(7)#3
2.374(3) 2.381(3) 2.436(3) 2.477(3) 3.228(3) 3.119(3)
Br(11)-Cu(1)-Br(1) Br(11)-Cu(1)-Br(10) Br(1)-Cu(1)-Br(10) Br(11)-Cu(1)-Br(2) Br(1)-Cu(1)-Br(2) Br(10)-Cu(1)-Br(2)
97.09(10) 89.19(11) 173.18(14) 172.86(12) 89.18(11) 84.71(9)
Cu(2)-Br(10) Cu(2)-Br(9) Cu(2)-Br(2) Cu(2)-Br(3) Cu(2)-Br(2)#2 Cu(2)-Br(6)#3
2.383(3) 2.393(3) 2.412(3) 2.431(3) 3.177(3) 3.245(3)
Br(10)-Cu(2)-Br(9) Br(10)-Cu(2)-Br(2) Br(9)-Cu(2)-Br(2) Br(10)-Cu(2)-Br(3) Br(9)-Cu(2)-Br(3) Br(2)-Cu(2)-Br(3)
93.61(12) 87.31(9) 176.49(14) 178.94(13) 87.40(9) 91.69(11)
Cu(3)-Br(9) Cu(3)-Br(8) Cu(3)-Br(4) Cu(3)-Br(3) Cu(3)-Br(1)#2 Cu(3)-Br(5)#3
2.398(3) 2.399(3) 2.417(3) 2.438(3) 2.947(3) 3.432(3)
Br(9)-Cu(3)-Br(8) Br(9)-Cu(3)-Br(4) Br(8)-Cu(3)-Br(4) Br(9)-Cu(3)-Br(3) Br(8)-Cu(3)-Br(3) Br(4)-Cu(3)-Br(3)
91.43(11) 168.70(14) 88.14(9) 87.12(9) 175.37(14) 92.42(11)
Cu(4)-Br(8) Cu(4)-Br(5) Cu(4)-Br(4) Cu(4)-Br(7) Cu(4)-Br(1)#4 Cu(4)-Br(4)#5
2.390(3) 2.417(3) 2.421(3) 2.439(3) 2.927(3) 3.540(3)
Br(8)-Cu(4)-Br(5) Br(8)-Cu(4)-Br(4) Br(5)-Cu(4)-Br(4) Br(8)-Cu(4)-Br(7) Br(5)-Cu(4)-Br(7) Br(4)-Cu(4)-Br(7)
171.74(15) 88.28(9) 91.36(11) 92.50(11) 86.12(9) 167.78(14)
Cu(5)-Br(7) Cu(5)-Br(5) Cu(5)-Br(6)#1 Cu(5)-Br(6) Cu(5)-Br(2)#4 Cu(5)-Br(3)#5
2.397(3) 2.397(3) 2.415(3) 2.424(3) 3.166(3) 3.189(3)
Br(7)-Cu(5)-Br(5) Br(7)-Cu(5)-Br(6)#1 Br(5)-Cu(5)-Br(6)#1 Br(7)-Cu(5)-Br(6) Br(5)-Cu(5)-Br(6) Br(6)#1-Cu(5)-Br(6) Cu(2)-Br(2)-Cu(1) Cu(2)-Br(3)-Cu(3) Cu(3)-Br(4)-Cu(4) Cu(5)-Br(5)-Cu(4) Cu(5)#1-Br(6)-Cu(5) Cu(5)-Br(7)-Cu(4) Cu(4)-Br(8)-Cu(3) Cu(2)-Br(9)-Cu(3) Cu(2)-Br(10)-Cu(1)
87.52(9) 178.78(14) 92.45(11) 92.42(11) 178.83(14) 87.62(9) 93.09(11) 91.16(11) 90.92(11) 93.21(11) 92.38(9) 92.64(11) 92.10(11) 93.07(11) 94.85(11)
N(1)-C(6) N(1)-C(2) C(2)-C(3) C(3)-C(4) C(3)-Br(13) C(4)-C(5) C(5)-C(6) C(5)-Br(15)
1.37(3) 1.39(2) 1.38(2) 1.34(2) 1.881(15) 1.37(2) 1.35(3) 1.886(19)
C(6)-N(1)-C(2) C(3)-C(2)-N(1) C(4)-C(3)-C(2) C(4)-C(3)-Br(13) C(2)-C(3)-Br(13) C(3)-C(4)-C(5) C(6)-C(5)-C(4) C(6)-C(5)-Br(15) C(4)-C(5)-Br(15) C(5)-C(6)-N(1)
122.3(17) 115.3(17) 124.0(16) 120.4(14) 115.4(14) 117.9(17) 122.1(19) 116.5(15) 121.3(14) 118.2(17)
a Symmetry transformations used to generate equivalent atoms: #1 -x, -y, -z.
is found that the vector AB shown in Figure 5 corresponds to the (15/2, -1/2) translations of the set of interlaced ribbons in the stepped layer structure. It is clear that the stoichiometry of the crystalline state is intimately associated with the stoichiometry and equilibria present in solution. Since the copper(II) halide species are very labile in solution, little correlation is expected between the structure in solution and in the crystalline state. The detailed understanding of these relationships is very complicated; nevertheless, certain generalizations can be made. With high counterion/ copper(II) or halide/copper(II) ratios, compounds containing monomeric halocuprate(II) species are obtained, typically CuX42- ions.12 As this ratio decreases, compounds containing various oligomers or polymeric species are now obtained. The predominate stoichiometry found is ACuX3, although many instances of A2CunX2n+2
A Planar Bibridged Cu10Br222- Oligomer
stoichiometries are known.7 It is clear that for stacked oligomer systems, like the one reported here, the principle driving force in their assembly is the semicoordinate bond formation. Unfortunately, thorough crystal engineering analyses of the secondary interactions between the cations and the inorganic framework have not been made.
Crystal Growth & Design, Vol. 3, No. 4, 2003 505
(3) (4) (5)
Experimental Section One millimole of 3,5-dibromopyridine hydrochloride and 1 mmol of copper(II) bromide were dissolved in 5 mL of absolute ethanol acidified with 1 mL of concentrated hydrobromic acid and warmed to 70 °C for 30 min. The volume of the solution was reduced on warm hotplate in a current of dry air until a solid black material formed. The solid material was redissolved in DMSO and the evaporation process repeated. The contents were transferred to a filtration flask to separate the solid material. A few small crystals later were obtained from the supernatant liquid left in the original flask. The X-ray data was collected on a Syntex P21 diffractometer upgraded to Bruker P4 specifications was utilized. Lattice dimensions were obtained from 25 accurately centered high angle reflections.13 Data were corrected for absorption utilizing psi scan data assuming an ellipsoidal shaped crystal. The structure solution was obtained via Patterson techniques and refinement were obtained using the SHELXTL package.14 A summary of data collection and refinement parameters is given in Table 1 and pertinent distances and angles are reported in Table 2.
(6) (7)
(8)
(9)
(10)
Acknowledgment. This research was supported by ACS-PRF 34779-AC5. Supporting Information Available: Crystal data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
(11) (12)
References (1) Tulsky, E. G.; Long, J. R. Chem. Mater. 2001, 13, 1149. (2) The Nomenclature Commission of the International Union of Crystallography in its report on Nomenclature of Inorganic Structure Types defines recombination structures as those “formed when topologically simple parent structures
(13) (14)
are periodically divided into blocks, rods or slabs which in turn are recombined into derivative structures by one or more structure building operations.” (a) Desiraju, G. R. Nature 2001, 412, 397. (b) Desiraju, G. R. Curr. Sci. 2001, 81, 1038. Willett, R. D.; Maxcy, K. R.; Twamley, B. Inorg. Chem., 2002, 41, 7024. Thorn, A.; Willett, R. D.; Twamley, B. Inorg. Chem., submitted for publication. (a) Weise, S.; Willett, R. D. Acta Crystallogr. 1993, B49, 283. (b) Geiser, U.; Willett, R. D.; Lindbeck, M.; Emerson, K. Inorg. Chem. 1986, 25, 1173. (c) Willett, R D.; Geiser, U. Acta Chem. Croat. 1984, 57, 751. (d) Bond, M. R.; Willett, R. D. Inorg. Chem. 1989, 28, 3267. Wells, A. F. J. Chem. Soc. 1947, 1670. (a) Halvorson, K. E.; Grigereit, T.; Willett, R. D. Inorg. Chem. 1987, 26, 1716. (b) Grigereit, T. E.; Ramakrishna, B. L.; Place, H.; Willett, R. D.; Pellacani, G. C.; Manfredini, T.; Menabue, L.; Bonamartini-Corradi, A.; Battaglia, L. P. Inorg. Chem. 1987, 26, 2235. (c) Murray, K.; Willett, R. D. Acta Crystallogr. 1991, C47, 2660. (a) Colombo, A.; Menabue, L.; Motori, A.; Pellacani, G. C.; Porzio, W.; Sandrolini, F.; Willett, R. D. Inorg. Chem. 1985, 24, 2900. (b) Fletcher, R., Hansen, J. J.; Livermore, J.; Willett, R. D. Inorg. Chem. 1983, 22, 330. (c) Bond, M. R.; Willett, R. D.; Rubins, R. S.; Zhou, P.; Zaspel, C. E.; Hutton, S. L.; Drumheller, J. E. Phys. Rev. B 1990, 42, 10280. (d) Willett, R. D.; Bond, M. R.; Pon, G. Inorg. Chem. 1990, 29, 4160. (a) Bond, M. R.; Place, H.; Wang, Z.; Willett, R. D.; Liu, Y.; Grigereit, T. E.; Drumheller, J. E.; Tuthill G. F. Inorg. Chem. 1995, 34, 3134. (b) Willett, R. D.; Rundle, R. E. J. Chem. Phys. 1964, 40, 835. (a) Place, H.; Willett, R. D. Acta Crystallogr. 1987, C43, 1497. (b) Place, H.; Willett, R. D., unpublished results. (c) Haddad, S.; Willett, R. D. Acta Cryst. 2000, C56, e437. (d) Turnbull, M. M.; Giantsidis, J.; Woodward, F. M.; Landee, C. P.; Richardson, C. Inorg. Chim. Acta 2001, 324, 324. (e) Luque, A.; Sertucha, J.; Lezama, L.; Rojo, T.; Roman, P. J. Chem. Soc., Dalton Trans. 1997, 847. Willett, R. D.; Awwadi, F.; Butcher, R.; Haddad, S.; Twamley, B. Cryst. Growth Des. 2003, 3, 301-311. Halvorson, K.; E., Patterson, K.; Willett, R. D. Acta Crystallogr. 1990, B46, 508. XSCANS, Siemen Analytical X-ray Instrument, Inc., Version 2.00, Madison, WI, USA, 1993. SHELXTL (XCIF, XL, XP, XPREP, XS). Version 6.10. Bruker AXS Inc., Madison, WI, USA, 2002.
CG030009N
