Structural Diversity of Methyl-Substituted Inclusion Complexes of Calix

Sep 13, 2007 - An investigation was undertaken into the structural diversity of ... xylenes with p-H-calix[5]arene according to the inequality m- > o-...
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Structural Diversity of Methyl-Substituted Inclusion Complexes of Calix[5]arene Clark,†

Makha,†

Thomas E. Mohamed Alexandre N. Jerry L. Atwood,‡ and Colin L. Raston*,†

Sobolev,†

Scott J.

Dalgarno,‡

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 10 2059-2065

Centre for Strategic Nano-Fabrication, School of Biomedical, Biomolecular and Chemical Sciences, UniVersity of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia, and Department of Chemistry, UniVersity of MissourisColumbia, Columbia, Missouri 65211 ReceiVed June 21, 2007; ReVised Manuscript ReceiVed July 23, 2007

ABSTRACT: The three isomers of xylene form inclusion complexes with p-H-calix[5]arene with different spatial interplay of the cavitands, which is relevant to using crystallization as a means of separating mixtures of o-, m-, and p-xylenes. Competition experiments involving mixtures of o-/m-xylene, o-/p-xylene, and m-/p-xylene and a mixture of all three isomers showed some selectivity in the inclusion of the xylenes with p-H-calix[5]arene according to the inequality m- > o- > p-xylene. Also reported are the toluene and mesitylene solvates, which are isostructural with the o-xylene and m-xylene inclusion complexes, respectively. Introduction Calix[4,5]arenes are versatile 3D molecules which preferentially take on the polar cone or bowl shaped conformations,1 the larger calixarenes being conformationally more flexible with a tendency to form more flattened arrangements of the phenolic moieties. The cavity of the cone-shaped calix[4,5]arenes can bind a variety of molecules. In the case of the ubiquitous calix[4]arenes, the size of the cavity restricts the binding to small molecules, most notably solvent molecules.2 In the case of calix[5]arenes, solvent molecules can also be included along with larger molecules such as ferrocene and globular-like fullerenes and carborane molecules.3 In addition, calix[4,5]arenes can associate whereby the cavity of one calixarene is occupied by part of the rim of another calixarene. Noteworthy examples here include trimeric calix[4]arene,4 p-phenylcalix[4,5]arenes,5 and p-But-calix[5]arene,6 where the self-association depends on the solvent of crystallization or lack thereof. The larger size of the cavity for calix[5]arene compared to calix[4]arene makes it an attractive host molecule; however, it is less well investigated mainly due to poor synthetic yields.7 In this paper we report the crystal structures of five new inclusion complexes of p-H-calix[5]arene involving methylsubstituted benzenes. The addition and placement of extra methyl groups onto the phenyl ring of toluene has a profound effect of the supramolecular interplay with the calix[5]arene host molecule, with the three isomers of xylene, o-, m-, and p- having different crystal packing arrangements. Also reported are the toluene and mesitylene solvates, which are isostructural with the o-xylene and m-xylene inclusion complexes, respectively. The separation of xylenes is a challenging process mainly due to the close proximity of the boiling points of m- and p-xylene. Extractive distillation and adsorption onto Y zeolites or metalorganic frameworks are among some of the ways to separate the different isomers.8 The ability of p-H-calix[5]arene to differentiate between the three isomers of xylene is without precedent and sets the scene for being able to separate a mixture of xylenes via inclusion processes using calixarenes. We note that p-H-calix[5]arene forms a number of inclusion complexes with dichloromethane,3a acetonitrile,3b and acetone.9 * To whom correspondence should be addressed. E-mail: clraston@ chem.uwa.edu.au. Tel.: +618 6488 3045. Fax: +618 6488 1005. † University of Western Australia. ‡ University of MissourisColumbia.

Scheme 1.

Formation of Inclusion Complexes 1-5 from Their Respective Solvents

In all three cases the calix[5]arene adopts a cone-shaped conformation due to the hydrogen-bonded network of hydroxyl groups on the lower rim. This is in contrast to the sublimed form of R-calix[5]arene, which adopts an inverted cone conformation and forms helical stacks in the extended structure.10 These helices are held together by hydrogen-bonded chains running though the hydroxyl groups. The unsolvated β-polymorph of calix[5]arene adopts the usual cone conformation and was shown to be active for CO2 sorption. Results and Discussion The calix[5]arene inclusion complexes 1-5 were prepared by slow evaporation of the respective solvent from a solution of pure calix[5]arene, with crystals suitable for single-crystal diffraction studies forming over a period of 1-7 days, Scheme 1. From o-xylene, calix[5]arene crystallizes as a 1:1 inclusion complex,1,withthreehost-guestsupermolecules,[o-xylene∩calix[5]arene], in the asymmetric unit, in the space group P31c. Crystallization from toluene affords a 1:1 toluene inclusion complex, 2, which is isostructural with complex 1 with three host-guest supermolecules, [toluene∩calix[5]arene], in the asymmetric unit, in the same space group. From m-xylene, calix[5]arene crystallizes in the space group Pbcn with one calixarene and one m-xylene molecule in the asymmetric unit, [calix[5]arene]‚m-xylene, 3, with the xylene exo to the calixarene cavity. Crystallization from mesitylene affords inclusion complex 4, [calix[5]arene]‚mesitylene, which is isostructural with the mxylene inclusion complex 3, with a molecule of the calixarene and an exo cavity mesitylene molecule in the asymmetric unit,

10.1021/cg0705658 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/13/2007

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Figure 1. Projections and packing diagrams for complex 1: (i) Asymmetric unit showing supermolecules 1-3; (ii) view down the c-axis of the extended packing showing the hexagonal close packing arrangement (pink) and the star shape motif (yellow); (iii) view down the c-axis of the extended packing with space filling showing the differing columnar arrangement (supermolecule 1, gold; supermolecule 2, purple; supermolecule 3, blue); (iv) tris(calix[5]arene) segment, which forms the columnar arrays. For all pictures, the following coloring scheme is adopted except where otherwise stated: purple and gold for calix[5]arene carbons; green for solvent carbons; gray for hydrogens.

in the same space group. From p-xylene, calix[5]arene crystallizes in the space group P1h with two and a half p-xylene molecules and five calixarenes in the asymmetric unit, [calix[5]arene]5‚2.5p-xylene, 5, with all the xylene molecules exo relative to the calixarene cavity. In complex 1, each of the three crystallographically independent host-guest supermolecules, [o-xylene∩calix[5]arene], have a similar interplay of the two components in the form of a perched structure. The calixarenes have a slightly distorted cone conformation as evident by analysis of the angles between the least-squares planes of the “O” centers and phenyl rings varying from 125.8 to 136.6°. Within each calixarene the five O-H groups form the expected disordered H-bonded array, with O‚‚‚O distances in the range 2.73-2.92 Å. The supermolecules are associated through aromatic and methyl C-H‚‚‚π interactions ranging from 2.74 to 3.03 and 3.11-3.25 Å, respectively, Figure 1i. o-Xylene resides in the cavity of calix[5]arene with the aromatic ring directed toward the bottom of the cavity, with the dihedral angle between the principal axis of the calixarene and the principal axis of o-xylene varying from 32.4 to 35.0°. The extent of inclusion of the o-xylene residing in the cavity is measured by the distance between the centroid of the aromatic ring of the o-xylene and the centroid of the five O-centers, which varies from 4.01 to

Clark et al.

Figure 2. Projections and packing diagram for complex 2: (i) asymmetric unit showing the extra aromatic C-H‚‚‚π interaction compared to complex 1; (ii) tris(calix[5]arene) segment, which forms the columnar arrays.

4.02 Å. Moreover one methyl group of o-xylene is directed toward the cleft associated with two adjacent phenyl rings with the other methyl group partially straddling one of the phenyl rings. This is expected on steric considerations and the longer range methyl CH‚‚‚π interactions at 3.40-3.50 Å. The overall structure in 1 is intricate and can be regarded as an assembly of three supermolecules. Spatial interplay of the supermolecules 1 and 2 involves a methylene C-H‚‚‚π interaction at a distance of 3.16 Å between supermolecules, Figure 1i. There are two longer range aromatic C-H‚‚‚π contacts at distances of 3.35 and 3.43 Å. Supermolecules 2 and 3 have back to back contacts of their lower rims offset by about half the radius of the lower rim itself, the closet (O‚‚‚O) contact being 3.00 Å. The extended structure can be considered as columnar arrays of supermolecules located around 3-fold symmetry axes with the peripheral of each column lined with the lower rim of the calixarenes, Figure 1ii. The guest molecules are embedded in the columnar arrays, and the arrangement is reminiscent of the arrangement of the host-guest molecules in the 1:1 complex of fullerene C60 and calix[5]arene.3c The principal axes of the calixarenes are not orthogonal to the principal axis of the columns but are offset by 132.1-134.6°. The columns form a hexagonal closed-packed arrangement with supermolecule 3 in columns facing one direction (downward) and supermolecules 1 and 2 in columns facing in the opposite direction (upward), as viewed down the c-axis, Figure 1iii. Each downward facing column is surrounded by six upward facing columns with aromatic and methylene C-H‚‚‚π interactions between columns ranging from 3.00 to 3.33 Å. The reversal in column relates to back-to-back stacking between calixarenes

Inclusion Complexes of Calix[5]arene

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Figure 3. Projections and packing diagram for complex 3: (i) side view of one columnar array showing the hydrogen-bonding interactions; (ii) top view looking down a columnar array showing the hydrogenbonding interactions; (iii) view down the b-axis with space filling for calix[5]arene and m-xylene molecules showing the up and down packing of neighboring columns; (iv) view down the b-axis showing the hydrogen-bonding interactions between calixarenes and m-xylene. For (iii) and (iv), upward facing columns are colored purple and downward facing columns are colored gold.

Figure 4. Projections and packing diagram for complex 4: (i) view down the b-axis showing the hydrogen-bonding interactions between calixarenes and mesitylene molecules; (ii) view down the b-axis showing the orientation of mesitylene molecules in the extended packing. The disorder of the methyl groups has been removed for clarity.

in neighboring columns. Each hexagon contains one full column and 1/3 of 6 columns giving 3 columns in total. Within each hexagon, the circumferences around segments of the six neighboring columns that surround a complete central column create the outline of a star shape motif, Figure 1ii. The columns are composed of a repeating unit of two segments which are offset by 60° relative to each other, and the segments in the repeating unit interplay through aromatic and methylene C-H‚‚‚π interactions at a range 2.93-3.35 Å. Each segment is comprised of 3 calixarenes, and there are no obvious hydrogen-bonding interactions between the supermolecules, Figure 1iv. The toluene inclusion complex 2 is isostructural with the o-xylene complex, showing all the same extended packing features. The interplay of the two components in the supermolecule involves aromatic C-H‚‚‚π interactions between the aromatic hydrogens of toluene and the π aromatic rings of the calixarene and range from 2.57 to 3.13 Å for the m-hydrogens and 2.98-3.09 Å for the p-hydrogens relative to the methyl group, Figure 2i. There are no methyl CH‚‚‚π interactions, and the methyl group of the toluene molecule resides in a cleft associated with two adjacent phenyl rings. This is similar to the positioning of one methyl group of o-xylene in complex 1. The only other major difference is an aromatic C-H‚‚‚π

interaction at 3.21 Å between supermolecules 1 and 2. The tris(calix[5]arene) segment is analogous to that of complex 1 leading to an identical packing arrangement, Figure 2ii. In complex 3 the calixarene molecules adopt a slightly distorted cone conformation as evident by analysis of the angles of the phenyl rings relate to the plane of the oxygen atoms varying from 122.54 to 148.62°. Within each calixarene, the five O-H groups form the expected H-bonded array for the cone conformation, albeit with the associated hydrogen atoms disordered in both directions around the rim, with O‚‚‚O distances in the range 2.75-2.79 Å. The calixarenes pack in a “self-included” manner with a phenyl ring of one calixarene protruding into the cavity of another, Figure 3i. The included calixarene points vertically out of the cavity so the planes of the “O” centers of these two calixarenes are almost perpendicular to each other. This is repeated to form a columnar array where the principal axes of the calixarenes are set at about 45° relative to the principal axes of the columns, Figure 3i. The columnar arrays are held together by aromatic C-H‚‚‚π interactions ranging from 2.91 to 2.99 Å and methylene C-H‚‚‚π interactions ranging from 2.79 to 3.07 Å, Figure 3i,ii. The included calixarene has one phenyl ring protruding into the cavity of the neighboring calixarene, and this association has a dihedral angle of 34.9°. This is similar to

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Clark et al.

Figure 5. Projections of the asymmetric unit for complex 5: (i) front view of the asymmetric unit showing the dumbbell feature between calixarenes 1, 3, and 4; (ii) side view of the asymmetric unit showing the offset alignment of calixarene 5; (iii) view of calixarenes 1-3 showing the hydrogenbonding interactions; (iv) view of calixarenes 2-4 showing the hydrogen-bonding interactions; (v) view of calixarenes 3-5 showing the hydrogenbonding interactions. The following coloring scheme is adopted: calixarene 1 and 4, purple; calixarene 2, blue; calixarene 3, gold; calixarene 5, violet. Solvent molecules have removed for clarity.

Figure 6. Projections and packing diagram for complex 5: (i) view down the a-axis of the extended packing showing the alternating orientation of calixarene 3 molecules within a row and the same orientation within a column; (ii) expanded view down the a-axis of the back-to-back stacking of calixarene 5 molecules with one hydrogen-bonded solvent molecule.

the dihedral angle seen for o-xylene in complex 1, which has a dihedral angle ranging from 32.4 to 35.0° indicating that the calix[5]arene is mimicking the interactions of o-xylene due to the fact that m-xylene is too large to reside in the cavity.

The columns pack together into sheets with alternating rows of upward and downward facing columns, Figure 3iii. The columns associate through offset π‚‚‚π stacking with a centroid‚‚‚centroid distance of 3.82 Å between columns in the

Inclusion Complexes of Calix[5]arene

same sheets. The columns in the same sheet are also linked via m-xylene molecules involving a network of aromatic C-H‚‚‚π interactions, Figure 3iv. The aromatic C-H‚‚‚π interactions range from 2.75 to 2.88 Å and involve two meta protons on the same m-xylene molecule simultaneously. Columns in adjacent sheets are linked via aromatic C-H‚‚‚π interactions with a m-xylene molecule at a range 2.79-3.06 Å. There are no obvious methyl C-H‚‚‚π interactions, and the methyl groups presumably just occupy the available interstitial space. The mesitylene inclusion complex, 4, is isostructural with the m-xylene complex, 3, and shows all the same extended packing features. Mesitylene partakes in the same hydrogen bonding as m-xylene with the extra methyl group occupying the interstitial space between neighboring sheets, Figure 4i. The aromatic C-H‚‚‚π interactions between sheets ranges from 2.90 to 3.00 Å and within the same sheet range 2.97-3.04 Å. All three methyl groups are disordered over two positions, and there are no obvious methyl C-H‚‚‚π interactions indicating that the methyl groups are just occupying the available interstitial space, Figure 4ii. The calixarenes in complex 5, [calix[5]arene]5‚2.5p-xylene, adopt a slightly distorted cone conformation with the angles of the phenyl rings relative to the plane of the oxygen atoms varying from 120.1 to 147.6°. The five O-H groups within each calixarene form the expected disordered H-bonded array for the cone conformation, with O‚‚‚O distances in the range 2.73-2.88 Å. The calixarenes pack together in a “self-included” manner with a striking feature of the asymmetric unit being a calixarene encapsulated sideways by two other calixarenes, calixarenes 1, 3, and 4, Figure 5i,ii. This takes on a dumbbell shape appearance, and the encapsulated calixarene 3 forms aromatic and methylene C-H‚‚‚π interactions at a range 2.62-2.88 Å with calixarene 1, Figure 5iii, and 2.83-3.06 Å with calixarene 4, Figure 5iv. Calixarene 2 is associated by having one of its phenyl rings penetrating the cavity of calixarene 3 forming two methylene C-H‚‚‚π interactions at a range 2.99-3.16 Å, Figure 5iii,iv. There is an aromatic C-H‚‚‚π interaction with calixarene 1 at distance of 2.96 Å, and calixarene 2 partakes in no hydrogen-bonding interactions with calixarene 4. Calixarene 5 is associated with its cavity occupied by one phenyl ring of calixarene 4 with aromatic and methylene C-H‚‚‚π interactions at a distance of 2.84-2.97 and 2.84 Å, respectively, Figure 5v. Longer range aromatic C-H‚‚‚π contacts with calixarene 3 are in the range 3.27-3.28 Å. The included calixarenes have one phenyl ring that protrudes into the cavity of the neighboring calixarene, and this association has a dihedral angle ranging from 40.9 to 44.0°. It is intriguing that the calix[5]arenes pack together in a self-included rather than endo-cavity binding of the xylene. This presumably relates to the p-methyl substituents on the xylene geometrically circumventing interplay with the calixarene cavity through C-H‚‚‚π interactions, which are more energetically favored than analogous interactions involving the methyl H-atoms.11 The extended packing consists of sheets with alternating upward and downward facing dumbbells within a row, with respect to the orientation of calixarene 3, Figure 6i. Two dumbbells are connected together through back to back stacking between two calixarene 1 molecules with the closet (O‚‚‚O) contact being 3.01 Å. The outer calixarene 5 molecule to a dumbbell shows no obvious hydrogen-bonding interactions with an adjacent calixarene 5 molecule within the same row. However it is associated with a calixarene 5 molecule from an adjacent row through back-to-back stacking with the closet (O‚‚‚O)

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Figure 7. Projections and packing diagram for complex 5: (i) expanded view down the a-axis showing the role of the solvent molecules in linking the columns and rows together; (ii) view of the unit cell contents showing the hydrogen-bonding interactions involved.

contact being 2.91 Å, Figure 6ii. A molecule of p-xylene is hydrogen bonded to this back-to-back stacked dimer via methylene, methyl, and aromatic C-H‚‚‚π interactions at distances of 3.13, 2.67, and 2.59 Å, respectively. The solvent molecules in the middle of 4 dumbbells do not connect dumbbells across a row or a column. Instead they link two dumbbells that are in adjacent columns with one of these dumbbells in an adjacent row that is one row deeper than the front row shown in Figure 7i. This is more clearly depicted in Figure 7ii, which shows the unit cell contents. This association involves aromatic and methyl C-H‚‚‚π interactions at a distance of 2.86-3.20 Å. To determine the selectivity of p-H-calix[5]arene toward the three xylene isomers, crystallization competition experiments were performed. Using a commercial mixture of all three xylene isomers (composition: 23.0% o-xylene; 42.5% m-xylene; 19.6% p-xylene; 14.9% ethylbenzene), a white precipitate was formed, which was shown to have a composition comprised of 10.0% o-xylene, 86.1% m-xylene, 3.9% p-xylene, and 95% selective over p-xylene. Conclusions We have structurally authenticated five new inclusion complexes of p-H-calix[5]arene, including three complexes containing the isomers of xylene which have different crystal packing arrangements. The preference of p-H-calix[5]arene for the three isomers has been determined by competition experiments and was shown to have modest selectivity, m- > o- > p-xylene. This has implications in purifying mixtures of xylene using the inclusion phenomena of calixarenes. The toluene and mesitylene solvates of p-H-calix[5]arene have been shown to be isostructural with the o-xylene and m-xylene solvates, respectively. This shows a remarkable versatility of the calixarene in forming specific arrays, even with structurally similar solvents differing slightly in the degree and pattern of methyl substitution, as well as a rich structural diversity of the packing of the calixarene in the presence of small molecules. Experimental Section All solvents were purchased from commercial suppliers and used without further purification. p-H-calix[5]arene12 was prepared from p-But-calix[5]arene7a as described previously in the literature. Solvates 1-5 were prepared by slow evaporation of a pure solution of p-Hcalix[5]arene (15 mg/mL) in the respective solvent, with crystals suitable for single-crystal diffraction studies forming over 1-7 days. Crystal uniformity of each sample was checked by determining unit cell dimensions on crystals from the same preparation and from different preparations. Competition experiments were performed by slow evaporation of a pure solution of p-H-calix[5]arene (15 mg/mL) in the respective solvent mixture with a white solid forming over 3-4 days. The percentage composition of xylene present was determined by 1H NMR on the white solids formed using the integration of the methyl signals of the xylene isomers. 1H NMR (500 or 600 MHz) spectra were recorded in CDCl3 on Bruker spectrometers and internally referenced to the solvent signal. X-ray Crystallography. The X-ray diffracted intensities were measured from single crystals at about 100 K on an Oxford Diffraction Xcalibur or Gemini-R Ultra CCD diffractometer or at about 153 K on a Bruker ASX CCD diffractometer using monochromatized Mo KR (λ ) 0.710 73 Å) or Cu KR (λ ) 1.541 78 Å). Data were corrected for Lorentz and polarization effects and an absorption correction applied using multiple symmetry-equivalent reflections. The structures were solved by direct methods and refined on F2 using the SHELX-97 crystallographic package13 and the X-Seed interface.14 A full matrix least-squares refinement procedure was used, minimizing w(Fo2 - Fc2), with w ) [σ2(Fo2) + (AP)2 + BP]-1, where P ) (Fo2 + 2Fc2)/3. Agreement factors R ) Σ|Fo| - |Fc|/Σ|Fo|, wR2 ) {Σ[w(Fo2 - Fc2)2]/ Σ[w(Fo2)2]}1/2, and GOF ) {Σ[w(Fo2 - Fc2)2]/(n - p)}1/2 are cited,

where n is the number of reflections and p the total number of parameters refined. Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated, and their atomic parameters were constrained to the bonded atoms during the refinement. The CCDC reference numbers are 645492-645496. The distances of several C-H···π interactions are greater than the van der Waals’ radii, and such interactions have been well reviewed.15 Crystal/refinement details for solvate 1: C43H40O5, M ) 636.75; F(000) ) 6084 e; trigonal, P31c; Z ) 18; T ) 100(2) K; a ) 34.3092(16), c ) 14.4613(4) Å; V ) 14742.1(8) Å3; Dc ) 1.291 g/cm3; sin θ/λmax ) 0.5946; N(unique) ) 8620 (merged from 251 404; Rint ) 0.1900, Rsig ) 0.0888); No(I > 2σ(I)) ) 6011, R ) 0.0707, wR2 ) 0.1370 (A, B ) 0.06, 12.00); GOF ) 1.005; |∆rmax| ) 0.27(5) e Å-3. Crystal/refinement details for solvate 2: C42H38O5, M ) 622.72; F(000) ) 5940 e; trigonal, P31c (No. 159); Z ) 18; T ) 153(2) K; a ) 33.821(4), c ) 14.570(2) Å; V ) 14433(3) Å3; Dc ) 1.290 g/cm3; sin θ/λmax ) 0.5946; N(unique) ) 8378 (merged from 90 029; Rint ) 0.0701, Rsig ) 0.0408); No(I > 2σ(I)) ) 6411, R ) 0.0563, wR2 ) 0.1290 (A, B ) 0.06, 23.00); GOF ) 1.016; |∆rmax| ) 0.34(5) e Å-3. Crystal/refinement details for solvate 3: C43H40O5, M ) 636.75; F(000) ) 2704 e; orthorhombic, Pbcn; Z ) 8; T ) 100(2) K; a ) 26.780(1), b ) 8.5173(4), c ) 29.317(1) Å; V ) 6687.0(5) Å3; Dc ) 1.365 g/cm3, sin θ/λmax ) 0.5946; N(unique) ) 5841 (merged from 38 481, Rint ) 0.0992, Rsig ) 0.1088); No(I > 2σ(I)) ) 3006, R ) 0.0788, wR2 ) 0.1776 (A, B ) 0.10, 0.00); GOF ) 1.004, |∆rmax| ) 0.50(7) e Å-3. Crystal/refinement details for solvate 4: C44H42O5, M ) 650.78; F(000) ) 2768 e; orthorhombic, Pbcn; Z ) 8; T ) 100(2) K; a ) 27.6777(5), b ) 8.3899(2), c ) 29.4830(7) Å; V ) 6846.3(3) Å3; Dc ) 1.263 g/cm3; sin θ/λmax ) 0.5946; N(unique) ) 6013 (merged from 60 104, Rint ) 0.0992, Rsig ) 0.1082); No(I > 2σ(I)) ) 2603, R ) 0.0400, wR2 ) 0.0646 (A, B ) 0.021, 0.0); GOF ) 1.004, |∆rmax| ) 0.30(5) e Å-3. Crystal/refinement details for solvate 5: C78H70O10, M ) 1167.34; F(000) ) 3090 e; triclinic, P1h (No. 2); Z ) 5; T ) 100(2) K; a ) 11.9659(6), b ) 14.989(6), c ) 43.029(4) Å; R ) 84.36(1), β ) 87.57(1), γ ) 78.03(1)°; V ) 7511(3) Å3; Dc ) 1.290 g/cm3, sin θ/λmax ) 0.5878; N(unique) ) 25 282 (merged from 84 583, Rint ) 0.0849, Rsig ) 0.1377); No(I > 2σ(I)) ) 10 849, R ) 0.0424, wR2 ) 0.0657 (A, B ) 0.015, 0.0); GOF ) 0.916, |∆rmax| ) 0.34(4) e Å-3.

Acknowledgment. We thank the ARC and NSF for financial support of this work and the University of Western Australia for a SIRF award to T.E.C. Supporting Information Available: Crystallographic information file (CIF). This material is available free of charge via the Internet at http://pubs.acs.org

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