A Metallocyclic Calixarene Wheel and Axle Inclusion Compound

CRYSTAL GROWTH & DESIGN

A Metallocyclic Calixarene Wheel and Axle Inclusion Compound

2009 VOL. 9, NO. 3 1334–1338

Felicia Maharaj,† Roger Bishop,† Donald C. Craig,† Paul Jensen,‡ Marcia L. Scudder,† and Naresh Kumar*,† School of Chemistry, The UniVersity of New South Wales, UNSW Sydney NSW 2052, Australia, and School of Chemistry, UniVersity of Sydney, NSW 2006, Australia ReceiVed April 29, 2008; ReVised Manuscript ReceiVed NoVember 17, 2008

ABSTRACT: Reaction of the dipyridyl-substituted tetrapropoxycalix[4]arene 1 with palladium(II) chloride yields the unusual metallomacrocycle 2. The calixarene substituents of 1 and 2 show major conformational differences from X-ray determinations, and molecule 2 crystallizes as the inclusion compound (2).(CHCl3)x (x ∼ 7) in space group R3j. Three repeating layers stack to form parallel tubes that contain the disordered guests. Intruding host propoxy groups constrict these tubes at every third layer, and the formation of this tubulate structure is explained in terms of the host wheel and axle topology. Introduction Lattice inclusion (clathrate) compounds1,2 are produced frequently when host molecules, due to their awkward shape, can only pack together inefficiently in the solid state.3,4 This situation leads to increased likelihood of guest inclusion, with a concurrent decrease in the crystal packing energy.5 The wheel and axle compounds, first reported by Toda,6,7 are now recognized as being a significant class of such host molecules.8-10 Here, two bulky and relatively rigid end groups (wheels) are bonded to a linear rigid connecting link (axle). Typical end groups may be triptycyl, diaryl, or triaryl groups and common connecting links are azo, allenyl, alkynyl or transition metal complex functionalities. The latter variant can have either dumb-bell11 or wheel and axle12 topology depending on the length of the link. As an option, the end groups can carry additional hydrogen bonding groups, and these can then change the inclusion structure from a clathrate to the coordinatoclathrate type.13 Wheel and axle compounds, and structurally related types of host molecules, have been reviewed recently by Soldatov.14 Here we describe the crystal structures of the dipyridylsubstituted tetrapropoxycalix[4]arene ligand 1 and the palladiumcontaining macrocycle 2 prepared from it. Compound 2 acts as a tubulate host molecule due to the novel packing of its wheel and axle units. Results and Discussion Ligand 1 and Palladium Complex 2. The dipyridylsubstituted tetrapropoxycalix[4]arene derivative 115 was synthesized as part of a wider investigation into the chemistry of calixarenes carrying substitutuents on their upper rim.16,17 Reaction of 1 with palladium(II) chloride afforded the metallomacrocycle 2 by means of double pyridyl N-PdCl2-N pyridyl coordination. Hosseini and his colleagues have described a similar reaction involving a pyridyl-substituted cyclophane and zinc halide.18 The rigid nature of their ligand yielded a macrocycle containing a significant central cavity. In contrast, our more flexible ligand results in a macrocycle with wheel and axle topology and no significant internal void space. Crystal Structure of the Ligand 1. Crystals of 1 were obtained by slow concentration of a dichloromethane/methanol * Corresponding author. Fax: Int+61-2-9385-6141. E-mail: n.kumar@ unsw.edu.au. † The University of New South Wales. ‡ University of Sydney.

solution. The single crystal X-ray structure determination confirmed the expected molecular structure and showed the solid Table 1. Numerical Details of the Solution and Refinement of the Two Crystal Structures 1 formula formula mass space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 T/K Z Dcalc/g cm-3 radiation, λ/Å µ/mm-1 scan mode 2θmax/deg no. of intensity measurements criterion for obsd reflection no. of indep obsd reflections no. of reflections (m) and variables (n) in final refinement R ) ∑m|∆F|/∑m|Fo| Rw ) [∑mw|∆F|2/∑mw|Fo|2]1/2 s ) [∑mw|∆F|2/(m - n)]1/2 crystal decay R for multiple measurements max, min absorption correction largest peak in final diff map/e Å-3

2-CHCl3 compound

C100H108Cl4N4 O8Pd2 · (CHCl3)x (x ∼ 7) 747.0 2684.08 C2/c R3j 24.944(6) 37.3730(3) 9.171(2) 37.3730(3) 19.466(5) 23.8369(4) 90 90 112.66(1) 90 90 120 4109(2) 28833.4(6) 294(1) 150(1) 4 9 1.21 1.39 Mo KR, 0.7107 Mo KR, 0.7107 0.075 0.852 θ/2θ φ/ω 50 55 3596 14682 I/σ(I) > 2 I/σ(I) > 2 1889 11380 1889 11380 229 584 0.078 0.040 0.113 0.116 1.90 1.15 none none 0.016 0.042 0.91, 0.81 0.66 0.85 C50H54N2O4

10.1021/cg800440d CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

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Molecules of 1 exist in a flattened cone conformation with the unsubstituted phenyl rings almost parallel to each other. The pyridyl rings are splayed outward like a pair of wings and are twisted slightly out of plane relative to their phenyl substituents. The propyl groups on the pyridyl edges are also close to parallel, while those on the phenyl edges are splayed outward as illustrated in Figure 1. Molecules of 1 align in rows in the crystal with no significant strong interactions between neighbors (Figure 2). Along c, the molecules of 1 have their bowl-shaped cavities alternately up and down, while those along a are all oriented in the same direction. Figure 1. The conformation adopted by the dipyridyl-substituted tetrapropoxycalix[4]arene 1 in its crystal structure. Color code: C green, H light blue, N dark blue, O red.

to be solvent-free material in the monoclinic space group C2/c. Numerical details of the solution and refinement of this crystal structure are presented in Table 1.

Crystal Structure of (2) · (CHCl3)x (x ∼ 7). The yellow palladium complex 2 was recrystallized to yield crystals of (2) · (CHCl3)x in the trigonal space group R3j (see Table 1). These crystals slowly turn opaque on standing due to the gradual loss of guest molecules, and the H:G ratio is approximately 1:7 (see Experimental Section).

Figure 2. The crystal packing of the dipyridyl ligand 1 projected onto the ac plane.

Figure 3. The molecular structure and conformation of 2 in its crystalline inclusion compound with chloroform. Color code: Pd purple, Cl pink.

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Maharaj et al.

Figure 4. Projection in the ab plane of part of one host layer in (2) · (CHCl3)x, and showing one partial void space (A) at the center where the propoxy groups of six molecules of 2 abut. This void is surrounded by six further partial void spaces (B) that do not involve propoxy group intrusions from molecules of 2 belonging to this particular layer.

Figure 5. Partial crystal structure of the compound (2) · (CHCl3)x, showing the addition of the second (purple) layer of host molecules. Hydrogen atoms are omitted for clarity. The central (green) partial void in Figure 4 is now surrounded by three purple partial voids also containing protruding propoxy groups (purple), plus a further three that are unaffected.

The structure and conformation of 2 in this solid is illustrated in Figure 3. Formation of the complex produces a dimeric adduct with reversal of the conformational orientations seen in the solid ligand 1. It is now the unsubstituted phenyl groups, and the propyl groups on the pyridyl edges, that are splayed outward. There is also a change in symmetry: 1 has its two halves related by a 2-fold axis, while 2 has its two halves related by a center of inversion. The N-Pd-Cl angles are all close to the expected 90° (range 89.4-90.3°), and the Pd-Cl distances of 2.31 Å are close to anticipated literature values.19,20 The cross-ring nonbonded Pd · · · Pd separation is 3.62 Å. The bulky calixarene groups and rigid linear pyridyl N-PdCl2-N pyridyl links provide 2 with a wheel and axle topology. A considerable volume of literature14 indicates that molecules of this shape cannot fit together efficiently by

themselves. They do not have self-complementarity, cannot adopt classical Kitaigorodskii close-packing, and invariably crystallize using guest inclusion to increase their crystal density.5 The molecules of 2 choose to do so in a most interesting manner, by unusually creating a 6-fold arrangement unrelated to their own individual symmetry.21 These molecules associate as layers in the ab plane of the crystal structure. Where the propoxy groups of six calixarene groups (wheels) abut in one such layer, partial void spaces (A) result (Figure 4). Each of these voids is surrounded by six further partial void spaces (B) that do not involve propoxy group intrusions from the molecules of 2 present in this particular layer. (All the void sites are identical when the other layers are added to complete the structure. In the various layer diagrams only one disorder component is shown for each propoxy group). There are three repeating layers (carbon atoms colored green, purple and blue in the following diagrams) which are relatively rotated by 120°, and then inverted through the center of the cavity. As a consequence, parallel tubes are formed along the c direction and these contain the disordered chloroform guest molecules. The protruding propoxy groups create constrictions in these tubes, but only one-third of the partial voids in a given layer is affected by these intrusions. Figure 5 illustrates the effect of adding the second host layer. The host crystal structure is complete when the third (blue) host layer is added, as shown in Figure 6. The nature of the nonequivalent contributions of the three layers to the construction of each tube can be seen more clearly using an alternative representation. Figure 7 shows a projection of one tube built up (as described earlier) from the central partial void containing protruding green propoxy groups shown in Figure 4. It illustrates how the propoxy group intrusions into the tubes only occur in every third layer of the repeat. The ease by which the calixarene derivative 1 was converted into the wheel and axle metallocyclic structure 2 is noteworthy, since most work in this area has concentrated on calixarenes

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Figure 6. The three layers of host molecules (C atoms colored green, blue, and purple; H atoms omitted) seen edge-on, and as a projection in the ab plane showing five tubes running along c. Note that each identical tube is constricted by only green, or blue, or purple, propoxy groups.

Figure 7. Projection in the ab plane of one host tube constructed from all three layers (green, blue and purple). Left: ball and stick representation. Right: diagrammatic representation. The green lines represent the propoxy chains protruding into the tube, located at the center of the diagram; the triangles connect the Pd atoms surrounding the partial voids in the purple and blue layers. (The other, identical, tubes simply exchange the layer colors shown here.) determined using the DEPT procedure. FT-IR spectra were recorded bearing four ligands in order to synthesize cavitand cage using a Perkin-Elmer 298 instrument and UV data with a Varian Cary compounds.22-25 Preparation of rodlike axles employing calix100 UV-visible spectrophotometer. arenes with just two ligands is largely unexplored, and the Palladium Complex (2). To a solution of 5,17-bis(4′-pyridinyl)behavior of 2 extends the wheel and axle host concept in new 25,26,27,28-tetrapropoxycalix[4]arene (1)15 (50 mg, 6.70 × 10-5 mol) directions. Our use of calixarene wheels and metal-ligand axles in chloroform (5 mL) was added a concentrated solution of palladium(II) confirms the importance of topological shape and symmetry, chloride (60 mg) in methanol (5 mL), and the mixture was refluxed rather than molecular functionality, in the design and function for 24 h prior to being reduced to dryness. The yellow solid was of host molecules of this type.26 recrystallized from chloroform/methanol to yield the palladium complex 2 (85 mg, 69%) as dark yellow needles: mp >300 °C (decomp); UV Experimental Section λmax (CHCl3) 303 (86440), 242 (44201) nm; 1H NMR (CDCl3) δ 8.46 (bs, 4H, pyr-H), 7.19 (d, 4H, J 7.5 Hz, Ar-H), 7.05 (t, 2H, J 7.2 Hz, NMR data were recorded using a Bruker DPX300F instrument (300 p-Ar-H), 6.44 (d, 4H, J 5.3 Hz, pyr-H), 6.31 (s, 4H, Ar-H), 4.48 and MHZ for 1H, 75.5 MHz for 13C) at 25 °C, and carbon substitution was

1338 Crystal Growth & Design, Vol. 9, No. 3, 2009 3.18 (d, 8H, JAB 13.4 Hz, Ar-CH2-Ar), 4.06 (t, 4H, J 7.9 Hz, O-CH2), 3.69 (t, 4H, J 6.8 Hz, O-CH2), 2.07-1.95 (m, 4H, OCH2-CH2-CH3), 1.95-1.83 (m, 4H, O-CH2-CH2-CH3), 1.11 (t, 6H, J 7.2 Hz, CH3), 0.91 (t, 6H, J 7.1 Hz, CH3); 13C NMR (CDCl3) δ 157.5, 157.2, 152.5, 136.5, 134.4, 129.0, 125.9, 122.7, 120.6, 77.2, 77.1, 30.8, 23.4, 22.9, 10.7, 9.7; IR (KBr disk) 2960m, 2930m, 2873m, 1616s, 1595s, 1462s, 1386m, 1290w, 1221w, 1210w, 1183s, 1160w, 1086m, 1037w, 1004s, 964m, 886w, 828s, 766s cm-1. A 1H NMR spectrum of the inclusion crystals in d6-DMSO showed an additional signal to those of 2 at 8.32 δ (hydrogen-bonded CHCl3) but no further signal due to included methanol. Integration indicated a H:G value close to (2) · (CHCl3)7, which is in good agreement with the electron density located within the molecular tubes. This stoichiometry is the closest estimate currently available for this inclusion compound. Solution and Refinement of the Crystal Structures. Reflection data were measured with an Enraf-Nonius CAD-4 diffractometer (1) or a Bruker-Nonius FR591 Kappa Apex II diffractometer (2-CHCl3 compound). Only data for 2 were corrected for absorption, using a multiscan technique. For both structures, the positions of all atoms in the asymmetric unit were determined by direct phasing (SIR92)27 with hydrogen atoms included in calculated positions. In 1 the molecule is situated around a 2-fold axis. The thermal motion of the pyridyl group was treated as a TLX rigid group (where T is the translation tensor, L is the libration tensor and X is the origin of libration28); all other atoms were refined as individual isotropic atoms. In the 2-CHCl3 compound, the propyl groups were almost invariably disordered over two or three sites. The total occupancy of each was refined to equal 1. The guest chloroform was also highly disordered and its effect on the structure factors was eliminated using the SQUEEZE29,30 routine in SHELX. The total number of electrons accounted for in this way led to the proposed stoichiometry. Crystallographic data (CIF) for the crystal structures are available from the Cambridge Crystallographic Data Centre (CCDC 682115-682116) by e-mailing [email protected].

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CG800440D