Metallohexacycles containing 4 ′ -aryl-4,2 ′ :6 ′ ,4 ′′ - terpyridines: conformational preferences and fullerene capture †

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Introduction
2][3][4] Two general approaches are typically adopted.The first uses the coordination geometry of the metal ion to define the internal angle of the metallomacrocycle in combination with rigid-rod ligands, for example, a molecular square predetermined by the 90°angle of square planar platinum(II). 5The second approach hinges upon the use of a ligand with a fixed internal angle.This strategy has been successfully exploited by the Newkome group to link {M(tpy)} 2 units (tpy = 2,2′:6′,2′′-terpyridine) into molecular hexagons 6 by using ditopic bis(4′-substituted tpy) ligands in which the metal ion is effectively a rigid linear metal node.These two design strategies tend to produce metallomacrocycles in which the metal ions are in a roughly planar array.In contrast, judicious choice of ligands can lead to the assembly of helical or grid-like 7 macrocycles. 4egular assemblies can also be obtained with flexible ligands, although a degree of control is lost.

General
Electrospray ionisation (ESI) and MALDI TOF mass spectra were measured using Bruker Esquire 3000plus and Bruker microflex instruments, respectively.Solution electronic absorption spectra were recorded using an Agilent 8453 spectrophotometer. 1 H NMR spectra were recorded at room temperature using a Bruker-Avance III-400 spectrometer.
Ligands 1, 23 224 and 325 were prepared according to literature methods.

[{ZnCl 2 (1)} 6 ]
A solution of 1 (19.1 mg, 0.050 mmol) in CHCl 3 (6.0mL) was placed in a long test tube.MeOH (3.0 mL) was layered on the top of the solution, followed by a solution of ZnCl 2 (6.76 mg, 0.050 mmol) in MeOH (5.0 mL).The test tube was sealed with parafilm and allowed to stand for 3 days at room temperature after which time, colourless crystals had formed.These were isolated by decantation (19 mg, 0.037 mmol, 73%

[{ZnCl 2 (2)} 6 ]
A solution of 2 (23.6 mg, 0.050 mmol) in CHCl 3 (6.0mL) was placed in a long test tube and MeOH (3.0 mL) was layered over the solution.A solution of ZnCl 2 (6.76 mg, 0.050 mmol) in MeOH (5.0 mL) was added carefully and the tube was sealed with parafilm and left for 3 days at room temperature.
Colourless crystals (blocks and spear-like blocks) formed and were isolated by decantation (15.6A solution of 2 (23.6 mg, 0.050 mmol) in CHCl 3 (6.0mL) was placed in a long tube.MeOH (3.0 mL) was layered on top of the solution, followed by a solution of ZnBr 2 (11.2 mg, 0.050 mmol) in MeOH (5.0 mL).The test tube was sealed with parafilm and allowed to stand for 3 days at room temperature.The colourless crystals that formed were isolated by decantation (12.7  A solution of 3 (21.8mg, 0.050 mmol) in CHCl 3 (6.0mL) was placed in a long tube and MeOH (3.0 mL) was layered on top, followed by a solution of ZnBr 2 (11.2 mg, 0.050 mmol) in MeOH (5.0 mL).The test tube was sealed with parafilm and left at room temperature for 3 days.The colourless crystals that formed were isolated by decantation (13.6 mg, 0.0206 mmol, 41.2%).Found C 57.38, H 3.81, N 6.21%; C 186 H 126 Br 12 N 18 Zn 6 requires C 56.35, H 3.20, N 6.36%.
A solution of 3 (21.8mg, 0.050 mmol) in a mixture of 1,2-Cl 2 C 6 H 4 (8.0 mL) and MeOH (2.0 mL) and a solution of C 60 (6 mg, 0.008 mmol) in 1,2-Cl 2 C 6 H 4 (2.0 mL) were placed in a long test tube.A mixture of MeOH (2.5 mL) and 1,2-Cl 2 C 6 H 4 (2.5 mL) was added as a new layer, followed by a solution of ZnCl 2 (6.76 mg, 0.050 mmol) in MeOH (8 mL).After sealing the tube with parafilm, it was left for 2 weeks at room temperature.During this time, purple-red blocks formed in addition to small crystals of C 60 .Satisfactory elemental analysis could not be obtained.

Crystallography
Single crystal data were collected on a Bruker APEX-II diffractometer with data reduction, solution and refinement using the programs APEX 26 and SHELXL97 or SHELX-13. 27The ORTEP-type diagram and structure analysis used Mercury v. 3.0. 22,28apid solvent loss and heavy disordering of solvent molecules influenced data quality in almost all structures, especially in [{ZnCl 2 (2)} 6 ] and [{ZnBr 2 (2)} 6 ].Therefore, SQUEEZE 29 was used to treat the data.
The packing of the hexacycles leads to the formation of a nanotube architecture with the tubes aligned parallel to the crystallographic c-axis (Fig. 2a).The tubes are filled with disordered solvent molecules and crystals are very sensitive to solvent loss, making structure determination difficult for the family of metallohexacycles reported in this work.The organization of molecules within each tube is best described by first considering the interdigitation of pendant phenyl rings of the biphenyl groups of every second metallohexacycle (Fig. 2b).Interlocking of two of the motifs shown in Fig. 2b results in the final nanotubular assembly shown in Fig. 2c.Between the hexacycles coloured red and blue in Fig. 2c, face-to-face π-stacking occurs between the pyridine ring containing atom N1 and the terminal phenyl ring with C22 iii (symmetry code iii = 1/3 + x, 2/3 + y, the angle between the planes = 6.7°and distance between ring centroids = 3.77 Å and analogous parameters are 7.5°and 3.77 Å in the chlorido derivative.As Fig. 3a shows, each hexacycle is involved in six such interactions which contribute significantly to the rigid architecture (see later).Adjacent tubes interact through π-stacking of 4,2′:6′,4′′-tpy domains involving the rings containing N1/N2 and N1 iv /N2 iv (symmetry code iv = 2/3 − x, 1/3 − y, −2/3 − z) (Fig. 3b).The angle between each pair of stacked pyridine rings is 4.4°, and the inter-centroid distance is 3.68 Å, making this an efficient interaction.Each hexamer is, by symmetry, involved in six such contacts.
The heavily disordered solvent molecules in the two structures have been modelled as partial occupancy H 2 O, CHCl 3 and MeOH, with best models fitting formulations for the compounds of [{ZnCl 2 (1) Despite the fact that the replacement of hydrogen by fluorine in an organic compound can significantly affect solid-state structures, 30,31 we have observed that coordination polymers formed between zinc(II) acetate and ligands 1 and 2 are isostructural. 24We were therefore intrigued to discover whether the same was true for the products of reactions between 1 and 2 with zinc(II) chloride or bromide.Ligand 2 was combined with ZnCl 2 or ZnBr 2 under the same room temperature conditions used for reactions with 1.In contrast to the growth of only one type of crystal in reactions with 1, treatment of 2 with either ZnCl 2 or ZnBr 2 resulted in the formation of colourless blocks and spear-like blocks, the latter being dominant in both reactions.
A combination of rapid solvent loss from the block-like crystals formed in the reaction of ZnBr 2 and 2, and heavily disordered solvent, meant that the program SQUEEZE 29 was used to treat the data.Structure determination confirmed the presence of [{ZnBr 2 (2)} 6 ] hexacycles.The trigonal space group and cell dimensions were consistent with those determined for all three structures described above, and Fig. 5 shows that the ring adopts conformer I mimicking that in [{ZnCl 2 (1)} 6 ], [{ZnBr 2 (1)} 6 ] (Fig. 1b) and [{ZnCl 2 (2)} 6 ].
The spear-like blocks from the reactions of ZnCl 2 or ZnBr 2 and 2 proved to be a second conformer (conformer II) of the metallohexacycle.The X-ray crystal structure of the chlorido complex [{ZnCl 2 (2)} 6 ] confirmed the presence of a chair conformer that replicates that observed in [{ZnCl 2 (4)} 6 ] (4 = 4′-(4-ethynylphenyl)-4,2′:6′,4′′-terpyridine). 19Excessive solvent disorder in the large void space was handled using the program SQUEEZE. 29[{ZnCl 2 (2)} 6 ] crystallizes in the monoclinic space group P2 1 /n, with half of the metallomacrocycle in the asymmetric unit; the second half is generated through an inversion centre.Each Zn atom is tetrahedrally coordinated with Zn-N bond distances in the range 2.026(2) to 2.070(2) Å and Zn-Cl bond lengths ranging from 2.2157(8) to 2.2519(10) Å. Fig. 6 shows two views of the structure of [{ZnCl 2 (2)} 6 ], and a comparison with Fig. 1 highlights the differences between conformations I and II.One pentafluorobiphenyl unit in conformer II of [{ZnCl 2 (2)} 6 ] is disordered and has been modelled over two positions of fractional occupancies 0.79 and 0.21; only one site is shown in Fig. 6.The chair-conformers pack into columns which run parallel to the a-axis (Fig. 7a), with protruding pentafluorophenyl units of one column interdigitated with those of an adjacent column.However, interdigitation involves pyridine-phenyl π H ⋯π H contacts and does not involve the pentafluorophenyl domains.Intermolecular π F ⋯π H (pyridine) interactions operate between adjacent [{ZnCl 2 (2)} 6 ] molecules within a column (Fig. 7a), but only involve one of the three independent 4,2′:6′,4′′-tpy ligands (that containing N8 and F13, see Fig. S5 † for labelling).The angle between the planes through the rings containing N8 and F13 i (symmetry code i = −1 + x, y, z) is 5.0°and the distance between ring centroids = 3.96 Å.Each hexacycle participates in four such stacking interactions.
Preliminary data only were obtained for the spear-like blocks obtained from reaction of ZnBr 2 and 2. Crystals of this habit were repeatedly obtained as the major product in a number of crystallization attempts, but were always of poor quality.Solvent loss was a persistent problem.The preliminary structure determination established the presence of the chair-like conformer of [{ZnBr 2 (2)} 6 ], thus confirming that [{ZnBr 2 (2)} 6 ], like [{ZnCl 2 (2)} 6 ], crystallizes with conformers I and II.Although both conformers pack into tube-like assemblies, the intermolecular interactions between molecules of conformer I, both within a tube and between adjacent tubes, generate a more rigid architecture than those of conformer II.The void spaces (calculated using PLATON 29 6)°.The naphthalen-1-ylphenyl unit is disordered and has been modelled over two sites (related by a wagging motion) of occupancies 0.41 and 0.59.The slight bowing of the tpy backbone (angles between the planes of adjacent pyridine rings = 9.7 and 3.3°) and the twisting of the phenyl ring with respect to pyridine and naphthyl units to which it is bonded (interplane angles = 40.8and 44.3°) are consistent with the related structures described above.[{ZnBr 2 (3)} 6 ] hexamers stack into tubes along the c-axis in an analogous manner to that detailed in Fig. 2 and the accompanying discussion.Interdigitation of naphthalen-1-ylphenyl units occurs between every second [{ZnBr 2 (3)} 6 ] molecule, and adjacent metallohexacycles engage in face-to-face π-stacking of naphthyl and pyridine rings (Fig. 9).The angle between the least squares planes through the pyridine ring containing N3 and the naphthyl unit is 7.3°, and the distances from the centroid of the pyridine ring to those of the rings comprising the naphthyl unit are 3.67 and 4.00 Å.The closest separation of any pair of naphthyl units on one rim of the [{ZnBr 2 (3)} 6 ] hexacycle is ≈11 Å (C29⋯C29 ii = 11.1 Å, symmetry code ii = −x + y, 1 − x, z, see Fig. S6 † for atom labels) and this compares to the diameter of a C 60 molecule of ≈7 Å (10 Å van der Waals diameter).Thus, the cavity is suited to acting as a host for the fullerene.
Preliminary data from structural analysis of crystals grown from reactions of ZnCl 2 and ligand 3 confirmed the assembly of the anticipated hexamer [{ZnCl 2 (3)} 6 ] with conformation I as described for the analogous bromido complex.However, the naphthyl units were heavily disordered, and this was a persistent problem despite modifying the crystallization conditions.
Porphyrin and calixarene 32,33 (in particular calix [5]arene) 34 derivatives are popular choices as hosts for C 60 , with the fullerene typically occupying the bowl-shaped cavity of the host.An example of a dumb-bell shaped metallo-bis(calixarene) (in which two tetrathiacalix [4]arenes are connected through coordination to manganese(II) centres) breaks this pattern with the C 60 guests interacting with the concave outer surface of the dumb-bell in preference to occupying the calixarene cavities. 35In contrast, both the inner and outer faces of a porphyrin barrel (a tetrameric metalloporphyrin complex) interact with C 60 , giving rise to a 1 : 3 host : guest compound in the solid state. 36Examples of metallomacrocycles hosting C 60 appear to be limited.Maverick and coworkers have described C 60 encapsulation by a molecular square comprising ditopic β-ketonate ligands bound to square planar copper(II) ions. 379][40][41][42] Of particular relevance to the results reported below is the use of pyridyldecorated bis(nickellaporphyrin) domains which assemble into one-dimensional tubes in the solid state by virtue of  pyridine⋯pyridine face-to-face π-interactions augmented by CH⋯N pyrrole 41,42 We considered two approaches for the formation host-guest complexes using [{ZnX 2 (L)} 6 ] metallomacrocycles as hosts.The first strategy of trapping guest molecules within pre-formed hosts relies upon the retention of the metallohexacycles in solution.Unfortunately, we have no unambiguous evidence that the [{ZnX 2 (L)} 6 ] complexes remain intact in solution.Attempts to obtain ESI mass spectra were unsuccessful; the MALDI TOF mass spectrum of [{ZnCl 2 (1)} 6 ] showed a base (and dominant) peak at m/z 869.5 which was assigned to the fragment [Zn(1) 2 Cl] + (calc.m/z 869.2).The electronic absorption spectrum of a solution made by dissolving crystalline [{ZnCl 2 (1)} 6 ] in MeOH (1 × 10 −5 mol dm −3 ) was identical to that of the free ligand, 23 suggesting dissociation of the complex.The aromatic region of the 1 H NMR spectrum of a CD 3 OD solution of [{ZnCl 2 (3)} 6 ] matched that of the free ligand 3 in the same solvent.A similar result was obtained for a CDCl 3 solution.
The second strategy for the formation of the host-guest complex involves the assembly of the [{ZnX 2 (L)} 6 ] host from ZnX 2 and L in the presence of the guest species.Single crystals suitable for X-ray diffraction were obtained within 2 weeks by carefully layering 1,2-Cl 2 C 6 H 4 -MeOH solutions of 3, C 60 and ZnCl 2 at room temperature.A ratio of ZnCl 2 : 3 : C 60 = 6 : 6 : 1 was chosen in anticipation of encapsulation of one C 60 molecule per metallohexacycle.Subsequent experiments with different amounts of C 60 resulted in crystals with the same structure as that described below and of crystals of excess C 60 .The product was confirmed to be [2{ZnCl 2 (3)} 6 •C 60 ]•6MeOH•16H 2 O and crystallized in the trigonal space group R3 ¯, with a unit cell having a c-axis approximately double the length of those found for the hexamers with conformer I described above, and with two independent {ZnCl 2 (3)} units and one-sixth of a fullerene molecule in the asymmetric unit.Two mutually stacked hexamers and one C 60 molecule (Fig. 10a) are generated using 3-fold rotoinversion.The general architectures of the two independent [{ZnCl 2 (3) ], the {ZnCl 2 (3)} 6 hexamer that associates most closely with C 60 contains an ordered naphthyl group, while in the second, the naphthyl unit is disordered and has been modelled over two positions with site occupancies 0.67 and 0.33.In the discussion below, we consider only the major occupancy site.The twist angles between the bonded phenylpyridine rings are 37.8 and 33.2°in the molecules coloured green and yellow in Fig. 10a  The angles between the planes through the phenyl and naphthyl units in the molecules coloured green and yellow in Fig. 10a   (3)} 6 ] molecule, coupled with an interlocking of two sets of these assemblies (compare Fig. 10 with Fig. 2).Fig. 10b shows two non-adjacent [{ZnCl 2 (3)} 6 ] hexamers and highlights the interdigitated naphthalen-1ylphenyl units.Adjacent metallohexacycles (yellow and green in Fig. 10) participate in face-to-face π-stacking of naphthyl and pyridine rings.Each C 60 molecule is captured between six naphthyl units, three from one [{ZnCl 2 (3)} 6 ] hexamer, and three from its interdigitated partner (centre of Fig. 10b).Further, the fullerene-six-naphthyl (green in Fig. 10) assembly lies at the heart of the second (yellow in Fig. 10 to centroids of the two rings making the naphthyl group are 3.78 and 4.08 Å.It is noteworthy that only every set of interdigitated naphthyl units (green in Fig. 10) hosts a fullerene.The cavity between the naphthyl units of the [{ZnCl 2 (3)} 6 ] hexamers coloured yellow in Fig. 10 is dimensionally similar to that between the hexamers coloured green, but is filled with disordered solvent.The latter have been modelled as partial occupancy H 2 O and MeOH molecules.
Our attempts to introduce further fullerene into the host (see above) were not successful, begging the question as to why only every other cavity is occupied.The spatial properties of each centrosymmetric cavity are essentially the same, and the distance between the middles of empty (yellow in Fig. 10c) and occupied (green) cavities is 11.27 Å.The corresponding separation in crystalline C 60 or co-crystallized C 60 •Z where Z is a small organic molecule, is close to 10 Å, [43][44][45][46] indicating that steric crowding is not the origin of the half-filling of cavities by ordered C 60 in [2{ZnCl 2 (3)} 6 •C 60 ]•6MeOH•16H 2 O.We propose that the observed structure and periodic occupancies of cavities by the fullerene are a consequence of the assembly process, and that capture of C 60 by a three-naphthyl domain of one hexacycle is probably an early recognition event.A search of the CSD 21 (v.5.34 with November 2012 updates using Conquest v. 1.15 22 ) indicates that the structure of hexamers along a tube appears to be a critical feature that prevents the C 60 molecules from occupying every six-naphthyl host.A relevant example for comparison is metallocycle 5 (Scheme 3).In the solid state, these molecules form one-dimensional tubes, supported by intermolecular pyridine⋯pyridine π-stacking interactions and CH⋯N pyrrole contacts.The tube-like assembly is more open than that formed by the [{ZnCl 2 (3)} 6 ] hexamers and 5 forms a 1 : 1 host-guest complex with C 60 , i.e. every macrocyclic cavity hosts a C 60 molecule. 41,42Other metallomacrocyclic hosts crystallize with C 60 in 1 : 1 assemblies, but there is no interlocking of the metallomacrocycles to form tubes. [37][38][39][40]
The naphthalen-1-ylphenyl-containing ligand 3 reacts with ZnCl  (2)} 6 ] in conformer I.Each crystallographically-ordered C 60 is trapped between six ordered naphthyl units, three from one hexamer and three from its interdigitated partner, and the C 60 -six-naphthyl unit sits at the centre of a second [{ZnCl 2 (3)} 6 ] macrocycle.The structure is highly unusual in having an ordered array of C 60 guests occupying every other available cavity in a tube.All spatial properties of all the six-naphthyl cavities in the lattice are essentially the same, and the distance between them is greater than the separation of C 60 molecules in crystalline C 60 and related structures.Thus, on steric grounds, the 'empty' cavity could, in principle, host a fullerene.Thus, we suggest that the observed structure and periodic occupancies of cavities by the fullerene are intimately associated with the assembly process.

Fig. 2
Fig. 2 (a) Hexacycles of [{ZnBr 2 (1)} 6 ] pack into tubes which follow the c-axis.(b) Within each tube, biphenyl domains of every second hexamer are interdigitated.(c) Interlocking of two of the motifs shown in (b).Solvent molecules are omitted.