Cite this: DOI: 10.1039/c3ce26919a

Persistent C–I…p halogen-bonded layer motifsinvolving 4-iodobenzoate paddlewheel units,Cu2(4-Ibz)4(L)23

Received 25th November 2012,Accepted 11th January 2013

DOI: 10.1039/c3ce26919a

www.rsc.org/crystengcomm

Paul Smart, Angela Bejarano-Villafuerte, Rebecca M. Hendry and Lee Brammer*

The Cu2(4-Ibz)4 paddlewheel unit (4-Ibz = 4-iodobenzoate) has been synthesised with a series of

monodentate axial ligands, L = acetone, DMSO, EtOH, DMF, giving Cu2(4-Ibz)4(L)2, or with linear ditopic

ligands, L2 = dabco, 4,4-bipyridine, giving Cu2(4-Ibz)4(L2). The crystal structures of these compounds form

persistent layer structures which are propagated in the four orthogonal directions of the iodobenzoate

ligands around each paddlewheel unit via a combination of halogen bonds and offset p-stacking

arrangements. Halogen bonds are either C–I…p or C–I…I interactions. The axial ligands are coordinated

orthogonal to the layers and either fill spaces in the layers above and below (monodentate ligands, L) or

link layers together (ditopic ligands, L2). Only in the case of the longest ditopic ligand used,

4,49-bipyridylethane, does the paddlewheel fail to form.

Introduction

The identification of novel supramolecular synthons fromwhich new structures can be designed and reliably synthesisedis vital to advancing the field of crystal engineering.1 In orderto evaluate the propensity of a new synthon to direct the self-assembly process it is important to assess how the proposedsynthon competes with different types of intermolecularinteraction in the solid state.2,3 The use of inorganic tectonscan introduce interaction geometries which are unavailablewith purely organic species.4 However, as the metal-containingunits often form in situ during the self-assembly process, thisintroduces competition between formation of coordinationbonds and of the directional intermolecular interactions fromwhich the synthon is derived.5

The ‘‘paddlewheel’’ motif comprising two (divalent) metalions bridged by four carboxylate or other similar monoanionicligands has seen extensive use in the development of metal–metal multiple bond chemistry6 and more recently is afavoured secondary building unit (SBU)7 in the formation ofmetal–organic framework materials (MOFs).8 Such paddle-wheel units impose rigid geometrical constraints on thecarboxylate groups, maintaining a 90u angle between them,and can be further coordinated by axial ligands, which enablepaddlewheel units to be linked into linear assemblies via

ditopic ligands9 or assemblies of higher dimensionality viamultitopic ligands.10

Previously we examined the use of paddlewheels containingthe halogenated carboxylate ligand 3-Ibz (3-Ibz = 3-iodobenzo-ate). The Cu2(3-Ibz)4 units were combined with a set of neutralaxial ligands that were either capable of forming coordinationpolymers through the linking of paddlewheel units, or ofaccepting halogen bonds from the iodobenzoate ligands toform extended halogen-bond networks.9a A subsequent studyintroduced ligands with two different coordinating groups,and ligands capable of hydrogen bonding.9b It was found thatfor short linear linkers such as dioxane and diazabicyclcooc-tane (dabco), and ligands with a sterically hindered coordinat-ing group such as 2,6-dimethypyrazine, coordination polymersdid not form; instead networks were propagated via halogenbonds between the axial ligands and the iodobenzoate ligands.Axial ligands with hydrogen bonding capability such asnicotinamide and methanol were found to propagate networksformed through hydrogen bonds rather than halogen bonds.3-Cyanopyridine and 4-cyanopyridine, ligands with one strongand one weakly coordinating group, did not form coordinationpolymers but instead paddlewheels were linked throughhydrogen bonds or via p–p interactions between axial ligands.Longer linkers such as 4,49-bipyridine (bipy) and 1,2-bis(4-pyridyl)ethane (bpa) were found to form coordination poly-mers, although the halogen bonding of the 3-Ibz ligand wasstrong enough to disrupt the formation of the paddlewheelunits leading to linked mononuclear Cu(3-Ibz)2 units. Thebent linker pyrimidine was the only ligand which formed acoordination polymer comprised of linked Cu2(3-Ibz)4 paddle-wheel units, as the 120u angle subtended by the nitrogen

Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK.

E-mail: lee.brammer@sheffield.ac.uk

3 Electronic supplementary information (ESI) available: Tables providing detailof twin refinements of crystal structures of 2, 4?4DMF and 7?BnOH. CCDC912204–912210. For ESI and crystallographic data in CIF and other electronicformat see DOI: 10.1039/c3ce26919a

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atoms resulted in a zig-zag chain which allowed the paddle-wheel units to pack together efficiently.

The halogen-bonded networks in the structures presentedin our previous work form a number of different topologiesand use a number of different halogen bond acceptor sites,making structural predictions for related compounds difficult.The geometry of the 3-iodobenzoate ligand in the paddlewheelis such that rotation of the haloaryl ring relative to thecarboxylate group changes considerably the orientation of thehalogen relative to the coordination network direction. It isthis degree of freedom that allows the diverse halogen-bondnetwork topologies of the 3-iodobenzoate paddlewheel com-pounds to form. Paddlewheels employing 4-iodobenzoateligands do not possess this degree of freedom as the torsionalflexibility of the ligand does not change the orientation of theiodo group. Therefore, we reasoned that the type of halogen-bonded networks formed may be more consistent. Only threeexamples of coordination polymers comprising 4-iodobenzo-ate paddlewheels have been previously reported.11 All three arebridged by short linear linker molecules (pyrazine, 2-amino-pyrazine and dabco). 2-Aminopyrazine and dabco were foundto produce crystal structures with solvent filled channels,however they were found to collapse irreversibly upondesolvation. In the present study we present a series of Cu(II)coordination compounds incorporating 4-iodobenzoateligands, and including one of four monodentate coordinatingsolvents, or three ditopic linker ligands of differing length.

Experimental

General

All reagents and solvents were purchased from Sigma Aldrichor Alfa Aesar and used as received. Elemental analyses wereconducted by the Elemental Analysis Service, Department ofChemistry, University of Sheffield.

Synthesis of [Cu2(4-Ibz)4(acetone)2] (1) (4-Ibz = 4-iodobenzoate)

4-Iodobenzoic acid (149 mg, 0.6 mmol) was dissolved inacetone (10 mL) and carefully layered on top of a solution ofcopper(II) nitrate trihydrate (30 mg, 0.12 mmol) in ethyl acetate(10 mL). Small green block-shaped crystals of 1 started to formafter 7 days. At this point a single crystal was removed andused for crystal structure determination by X-ray diffraction.Yield 13.0 mg, 17.6% Calc.: C, 33.17; H, 2.29; I, 41.23%. Found:C, 34.20; H, 2.54; I, 41.52%.

Synthesis of [Cu2(4-Ibz)4(DMSO)2] (2) (DMSO =dimethylsulfoxide)

4-Iodobenzoic acid (149 mg, 0.6 mmol) was dissolved in DMSO(6 mL) and carefully layered on top of a solution of copper(II)nitrate trihydrate (30 mg, 0.12 mmol) in ethyl acetate (10 mL)Green block-shaped crystals of 2 started to form after 5 days. Atthis point a single crystal was removed and used for crystalstructure determination by X-ray diffraction. Yield 14.3 mg,19.9%. Calc.: C, 30.23; H, 2.22; I, 39.93%. Found: C, 29.88; H,2.09; I, 39.68%.

Synthesis of [Cu2(4-Ibz)4(EtOH)2] (3) (EtOH = ethanol)

4-Iodobenzoic acid (99.2 mg, 0.4 mmol) and ammoniumthiocyanate (15.2 mg, 0.2 mmol) were dissolved in benzylalcohol (6 mL). Benzyl alcohol and EtOH were mixed in a 1 : 1ratio and 1 mL of this mixture was layered on top of thissolution as a buffer layer. Copper(II) acetate monohydrate (39.9mg, 0.2 mmol) was dissolved in EtOH (2 mL) and this wascarefully layered on top of the buffer layer. Small blue blockcrystals of 3 started to form after 4 days along with a colourlessprecipitate which was collected separately. At this point asingle crystal was removed and used for crystal structuredetermination by X-ray diffraction. Yield 22 mg, 18.1%. Calc.:C, 31.84; H, 2.34; N, 0.00; I, 42.05%. Found: C, 32.15; H, 2.02;N, 0.00; I, 41.82%.

Synthesis of [Cu2(4-Ibz)4(DMF)2]?4DMF (4?4DMF) (DMF =dimethylformamide)

Copper(II) acetate monohydrate (78 mg, 0.39 mmol) and4-iodobenzoic acid (248 mg, 1.0 mmol) were dissolved inDMF (8 mL) and left to evaporate. Large blue blocks of 4?4DMFstarted to form after 4 days. At this point a single crystal wasremoved and used for crystal structure determination by X-raydiffraction. Yield 246.1 mg, 81.1%. Calc.: C, 35.56; H, 3.76; N,5.41; I, 32.67%. Found: C, 35.19; H, 3.48; N, 5.21; I, 31.79%.

Synthesis of [Cu2(4-Ibz)4(dabco)] (5) (dabco =diazabicyclo[2.2.2]octane)

4-Iodobenzoic acid (96 mg, 0.39 mmol) and dabco (24 mg, 0.21mmol) were dissolved in benzyl alcohol (1 mL). A buffer layerof benzyl alcohol (0.25 mL) was layered on top of this solution.Copper(II) acetate monohydrate (40 mg, 0.20 mmol) wasdissolved in methanol and this was carefully layered on topof the buffer layer. A blue gel formed after 2 days whichcontained small green crystals of 5. At this point a singlecrystal was removed and used for crystal structure determina-tion by X-ray diffraction. Yield 54.2 mg, 45.3%. Calc.: C, 33.27;H, 2.30; N, 2.28; I, 41.36%. Found: C, 33.27; H, 2.18; N, 2.44; I,41.30%.

Synthesis of [Cu2(4-Ibz)4(bipy)]?nMeOH (6?nMeOH) (bipy =4,49-bipyridine, MeOH = methanol)

4-Iodobenzoic acid (96 mg, 0.39 mmol) and bipy (18 mg, 0.11mmol) were dissolved in benzyl alcohol (1 mL). A buffer layerof benzyl alcohol (0.25 mL) was layered on top of this solution.Copper(II) acetate monohydrate (40 mg, 0.20 mmol) wasdissolved in methanol and this was carefully layered on topof the buffer layer. Small green needles of 6?nMeOH started toform after 3 days. At this point a single crystal was removedand used for crystal structure determination by X-ray diffrac-tion. Yield (for n = 2) 11.2 mg, 8.6%. Calc. (for n = 0): C, 35.90;H, 1.90; N, 2.20; I, 39.93%. Found: C, 34.44; H, 1.98; N, 2.05; I,39.66%.

[Cu(4-Ibz)2(bpa)]?BnOH (7?BnOH) (bpa = bis(4-pyridyl)ethane,BnOH = benzyl alcohol)

4-Iodobenzoic acid (96 mg, 0.39 mmol) and bpa (18 mg, 0.10mmol) were dissolved in benzyl alcohol (1 mL). A buffer layerof benzyl alcohol (0.25 mL) was layered on top of this solution.

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Copper(II) acetate monohydrate (40 mg, 0.20 mmol) wasdissolved in methanol and this was carefully layered on topof the buffer layer. Small purple needles of 7?BnOH started toform after 5 days. At this point a single crystal was removedand used for crystal structure determination by X-ray diffrac-tion. Yield 30.9 mg, 36.4%.

Crystallography

For compounds 1–7 a suitable crystal was mounted in a streamof cold N2 gas on a Bruker SMART or Kappa APEX-2 CCDdiffractometer, equipped with graphite-monochromated MoKa radiation from a sealed-tube source. Details of the crystal,data collection and refinement parameters for 1–7 aresummarized in Table 1. Data were corrected for absorptionusing empirical methods (SADABS) based upon symmetry-equivalent reflections combined with measurements at differ-ent azimuthal angles.12 The structures were solved by directmethods and refined by full-matrix least-squares based onweighted F2 values for all reflections using the SHELXTL suiteof programs13 or using the program olex2.14 Non-hydrogenatoms were refined anisotropically, except for 6?nMeOH whereonly iodine atoms were refined anisotropically, and in 7?BnOHwhere only a small subset of atoms were refined anisotropi-cally. Hydrogen atoms were placed in calculated positions,refined using idealized geometries (riding model), andassigned fixed isotropic displacement parameters. The crystalof compound 2 was a 2-component twin. In compound 3 theEtOH molecules are disordered. One was modelled in twoorientations with occupancies 0.685(3) and 0.315(3), whereasthe other was best modelled in a single orientation with largedisplacement ellipsoids. Compound 4?4DMF was a 3-compo-

nent twin; one of the DMF molecules is disordered over twoorientations with occupancies 0.76(3) and 0.24(3). In com-pound 5 the dabco molecule sits on a 4-fold rotation axis andis therefore disordered over 4 orientations. In compound6?nMeOH one of the iodo substituents of each independentpaddlewheel is disordered over two positions with occupancies0.524(3) : 0.476(3) and 0.703(3) : 0.297(3), respectively. Thecrystals of compounds 5, 6?nMeOH and 7?BnOH were verysmall and weakly diffracting, leading to lower quality intensitydata; the latter is also a 2-component twin. The diffuse solventlocated in the pores of 6?nMeOH were accounted for using thesolvent masking function implemented in the refinementprogram olex2.14 The pores account for 20.4% of the unit cellvolume; the radius of the largest pore is 2.8 Å. The totalelectron count per unit cell is 352.8, corresponding to 2.32MeOH molecules per formula unit (i.e. n = 2.32). Furtherdetails of twin refinements for compounds 2, 4?4DMF and7?BnOH are provided in the ESI.3

Results

Reaction in suitable solvents of copper(II) acetate or copper(II)nitrate with 4-iodobenzoic acid and a number of potentialaxial ligands has been used to prepare a series of crystallinecoordination complexes and networks comprising copper(II)ions coordinated by the 4-iodobenzoate ligands (Scheme 1).The crystal structures of these compounds highlight thepropensity of the iodobenzoate ligands to form extendedhalogen-bonded networks. The reliability and flexibility of the

Table 1 Crystal data, data collection and structure refinement parameters for 1–7

1 2 3 4?4DMF 5 6?nMeOH 7?BnOH

Crystal colour Blue Blue Blue Blue Green Green PurpleCrystal size/mm 0.18 6 0.13

6 0.050.23 6 0.156 0.06

0.34 6 0.216 0.15

0.20 6 0.206 0.16

0.02 6 0.0156 0.01

0.02 6 0.016 0.01

0.03 6 0.026 0.02

Crystal system Triclinic Triclinic Triclinic Monoclinic Tetragonal Monoclinic TriclinicSpace group, Z P1, 1 P1, 2 P1, 2 P21/n, 2 I4/m, 2 P21/c, 8 P1, 4a/Å 6.6678(5) 6.5847(5) 12.196(2) 12.997(3) 13.4825(4) 15.230(3) 8.2079(19)b/Å 11.9530(9) 15.8533(13) 13.011(2) 12.621(3) 13.4825(4) 22.664(5) 16.029(4)c/Å 12.7015(10) 18.2578(15) 13.015(2) 16.880(3) 9.8363(3) 27.692(5) 24.699(6)a/u 93.451(4) 92.436(6) 74.517(7) 90 90 90 102.764(17)b/u 103.761(4) 90.283(6) 68.073(7) 96.15(3) 90 102.821(10) 98.004(16)c/u 102.981(4) 93.469(6) 87.380(8) 90 90 90 101.353(14)V/Å3 951.20(13) 1900.7(3) 1842.9(5) 2753(1) 1788.02(9) 9320(3) 3049.9(12)Density/Mg m23 2.149 2.221 2.176 1.874 2.332 1.812 1.851Temperature/K 100 100 100 100 100 100 100m/mm21 4.417 4.530 4.557 3.082 4.699 3.608 2.7852h range/u 3.32 to 51.98 2.24 to 55.18 3.26 to 55.02 3.76 to 54.80 4.28 to 55.00 2.34 to 42.10 1.72 to 49.38Reflns collecteda 17 854 16 142 37 498 10 752 7097 25 328 16 794Independent reflns, n, (Rint)

a 3718 (0.0419) 5059 (0.0640) 8303 (0.0679) 2502 (0.0368) 1094 (0.0745) 10 006 (0.1888) 5627 (0.3200)Reflns used in refinementa 3718 5059 8303 2502 1094 10 006 5627L.S. parameters, p 228 455 446 342 65 337 448Restraints, r 0 12 4 0 0 25 0R1(F)b, I . 2s(I) 0.0302 0.0512 0.0411 0.0429 0.0859 0.1425 0.1080wR2(F2)b, all data 0.0660 0.1300 0.1091 0.1149 0.2756 0.4112 0.3417S(F2)b, all data 1.029 1.060 1.057 1.031 1.057 0.906 1.030

a The no. of reflections and Rint values for compounds 2, 4?4DMF and 7?BnOH correspond to the largest twin component. Further details ofthe twin refinements can be found in Tables S1–S3, ESI.3 b R1(F) = S (|Fo| 2 |Fc|)/S|Fo|; wR2(F2) = [Sw(Fo

2 2 Fc2)2/SwFo

4]1/2; S(F2) = [Sw(Fo2 2

Fc2)2/(n + r 2 p)]1/2.

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halogen-bonded networks has been investigated through theuse of three coordinating solvents with differing stericdemands. The possibility of using the halogen bond as asynthon for the formation of porous materials has beeninvestigated through the use of three linear ditopic linkerligands of increasing size. All of the compounds synthesizedform extended halogen-bonded networks using one or more ofthe three motifs shown in Scheme 2.

[Cu2(4-Ibz)4(acetone)2], 1, [Cu2(4-Ibz)4(DMSO)2], 2, [Cu2(4-Ibz)4(EtOH)2], 3 and [Cu2(4-Ibz)4(DMF)2]?4DMF, 4?4DMF

The crystal structure of 1 comprises Cu(II) paddlewheels (SBU-I, Scheme 1) with acetone solvent molecules coordinated at theaxial sites of the paddlewheel. The paddlewheel sits on aninversion centre, and the aryl rings are slightly twisted withrespect to the carboxylate groups with C–C–C–O torsion angles12.9(4) and 10.0(4)u. One of the independent iodobenzoategroups forms a C–I…p halogen bond to the p system of aneighbouring paddlewheel (I…C 3.432(4) Å, C–I…C176.64(13)u, cf. motif 1 in Scheme 2). This iodo group alsoaccepts a halogen bond from the second halobenzoate groupof a third paddlewheel unit (I…I 3.7142(5) Å, C–I…I170.35(11)u; I…I–C 80.04(11)u, cf. motif 2 in Scheme 2). Thesehalogen bonds are supported by anti-parallel p-stacking of thehalobenzene units. The resulting halogen-bonded layer shownin Fig. 1a possesses rectangular voids, and the layers stacksuch that the coordinated solvent molecules of one layer arelocated in the rectangular voids of the adjacent layers resultingin a close packed structure.

The crystal structure of 2 is very similar to that of 1,comprising Cu(II) paddlewheels (SBU-I) with DMSO solvent

molecules at the axial sites of the paddlewheel, however thepaddlewheel is not located on a symmetry element. Thepaddlewheels pack in an identical manner to 1, forming thesame extended halogen-bonded layer involving motifs 1 and 2(Fig. 1b). The C–I…p halogen bonds are slightly longer (I…C3.476(10) Å and 3.443(10) Å) and the angle at the donordeviates slightly more from linearity for one of the interactions(C–I…C 172.7(4) and 177.8(4)u). Similarly, one of the C–I…Ihalogen bonds formed is longer than in 1 with a correspond-ingly smaller angle (C–I…I 3.7825(10) Å, C–I…I 164.7(3)u, I…I–C81.4(3)u) and the second of these interactions is much shorter(I…I 3.6448(11) Å, C–I…I 172.5(3)u, I…I–C 84.1(3)u). Therectangular voids within the halogen-bonded layers are slightlydistorted to accommodate the larger DMSO molecules requir-ing changes in the halogen-bond geometries relative to thosein 1 (vide supra). In addition to this, the halobenzoate groupsare bent slightly away from their ideal planar geometry, andthe range of ring-carboxylate torsion angles is greater (between2 and 18u).

Compound 3 also comprises copper(II) paddlewheels (SBU-I)that do not possess inversion symmetry. Two of the iodo-benzoate ligands are almost planar (ring-carboxylate torsionangle 5u) whist the other two ligands have a larger ring-carboxylate torsion angle (13–16u). The axial sites of thepaddlewheels are occupied by ethanol molecules, one of whichis disordered over two orientations. 3 forms the same halogen-bonded layer as 1 and 2 (Fig. 1). The C–I…p halogen bonds arethe shortest of the three compounds (I…C 3.400(5) and3.424(5) Å, C–I…C 176.35(15) and 176.33(16)u), whereas theC–I…I halogen bonds are longer than those in 1 and 2 (I…I3.7578(8) and 3.7719(7) Å, C–I…I 169.36(14) and 169.62(15)u,I…I–C 71.97(13) and 71.64(13)u). The halogen-bonded layerspack in a similar manner to those in 1 and 2, allowing thehydroxyl groups of the coordinated ethanol molecules to formO–H…O forms hydrogen bonds to the carboxylate oxygen of apaddlewheel from a neighbouring layer (O…O 2.836(5) and3.189(5) Å for the ordered and the disordered ethanol ligands,respectively).

As in compounds 1, 2 and 3, the axial sites of the Cu(II)paddlewheels in 4?4DMF are occupied by coordinated solventmolecules, in this case DMF. 4?4DMF also forms an extendedhalogen-bonded layer, although in this case it is propagatedonly through motif 1 type interactions (I…C 3.589(3) Å and3.387(3) Å, C–I…C 173.9(1) and 176.5(1)u, respectively; seeFig. 2). These halogen bonds are shorter than in 1 and 2, as theresulting voids within the layer are square rather thanrectangular in cross-section and are not occupied by theaxially coordinated solvent molecules of adjacent layers.Instead the layers stack such that the adjacent layers arerotated by y45u relative to each other, thereby situating thecoordinated DMF solvent molecules above and below thesquare voids in the halogen-bonded layer.

As a consequence of this less efficient layer packing, twomolecules of DMF per paddlewheel are required in order to fillthe additional space between the layers. The DMF moleculesform a number of weak C–H…O hydrogen bonds.

Scheme 2 The halogen bonding motifs present in compounds 1–6. (a) Motif 1.Pairs of C–I…p halogen bonds between iodobenzoate ligands supported byantiparallel p-stacking. (b) Motif 2. Involves pairs of C–I…I and C–I…p halogenbonds. (c) Motif 3. Similar to motif 1, but involving SBU-II, and the halogen bondacceptor is the p-system of the diimine linker ligand rather than thehalobenzoate.

Scheme 1 The secondary building units (SBUs) which occur in compounds 1–7.a) SBU-I: paddlewheel b) SBU-II: mononuclear. Arrows indicate the direction ofthe coordination polymer chains.

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[Cu2(4-Ibz)4(dabco)], 5, [Cu2(4-Ibz)4(bipy)]?nMeOH, 6?nMeOHand [Cu(4-Ibz)2(bpa)]?BnOH, 7?BnOH

The crystal structure of 5 comprises Cu(II) paddlewheels (SBU-I) linked through their axial sites by a dabco molecule to forma 1D coordination polymer. The iodobenzene rings are verynearly coplanar with the carboxylate groups (torsion angles0.7(18) and 2.4(17)u). The iodobenzoate groups form anextended 2D layer perpendicular to the coordination polymerchains. This 2D layer closely resembles that found in 4?4DMF,propagated by motif 1 type interactions (Fig. 3). The structureis interpenetrated, with the square voids of the halogen-bonded halobenzoate layer being filled by a dabco molecule ofthe interpenetrated structure. This results in a large distortionof the halogen-bonded layer, forcing the halobenzoate groupsto bend significantly (Fig. 3b), and also lengthening thehalogen bonds (I…C 3.749(17) and 3.746(16) Å, C–I…C 159.7(7)and 160.1(7)u). The structure of 5 differs considerably from theZn analogue [Zn2(4-Ibz)4(dabco)]?3.25DMF reported byBurrows et al.11 In the Zn compound the iodobenzoate ligandsform halogen bonds to the DMF solvent molecules rather thanforming any of the motifs found in Scheme 2. This leads tooffset arrangement of the chains and leads to the formation ofsolvent-filled channels within the structure. The differencebetween the two compounds can be attributed to the differentsynthesis methods used. [Zn2(4-Ibz)4(dabco)]?3.25DMF wassynthesized by heating a sealed tube containing the metalsalt and ligands dissolved in DMF, with crystals forming uponcooling of a homogenous solution of the reactants. 5 wasformed through the ambient temperature diffusion of themetal salt into a solution that contains the ligand molecules,with the crystals forming upon mixing of the BnOH and MeOH

solutions. The structure of 5 is most similar to that of [Cu2(4-Ibz)4(pyrazine)],9a which forms the same halogen-bonded layerand is interpenetrated in the same manner as 5; it wassynthesized via a similar ambient temperature diffusionmethod using the same solvent system as for 5.

The crystal structure of compound 6?nMeOH comprisesCu(II) paddlewheels linked through their axial sites by bipyligands to form a 1D coordination polymer which runs parallelto the c-axis. Two crystallographically independent paddle-wheels exist in the unit cell, here designated type A and type B.Each type of paddlewheel forms a coordination polymer chaincomposed exclusively of that type of paddlewheel (Fig. 4a). Thehalogen bonding motif in 6 is similar to that of 1 and 2,however the network extends infinitely in only one direction,forming halogen-bonded tapes which are four paddlewheelswide. The edges of the tapes are made up of type Bpaddlewheels whilst the inside rows are formed by type Apaddlewheels. Similarly to 1 and 2, the halogen-bonding motifresults in rectangular voids in the tapes which in this case arefilled by bipy molecules from the other polymer chainresulting in pseudo-interpenetration (Fig. 4b). Running alongthe coordination polymer chain, paddlewheel units are tiltedin an alternating manner (Fig. 4a), which allows the halogen-bonded tapes to stack in a herringbone pattern (Fig. 4c). Inthis way efficient space filling is achieved with a minimumnumber of solvent molecules being included in the structure.Although coordination polymers comprising paddlewheelSBUs linked by bipy have been previously synthesized incrystalline form, e.g. [Cu2(O2CPh)4(bipy)],15 it proved verydifficult to prepare such material with 4-Ibz; many failedsynthesis attempts yielded uncharacterised amorphous pow-

Fig. 1 The crystal structures of (a) 1 (b) 2 and (c) 3. The paddlewheel unit is shown at the top and the corresponding halogen-bonded layers (comprising motif 1 andmotif 2 halogen bonds) are shown at the bottom.

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ders. In practice only a few very small crystals of 6?nMeOHwere synthesized. Attempts to produce this type of compoundby Burrows et al. also failed, instead yielding coordinationpolymers comprising mononuclear units,11 which reflects ourexperience with the 3-IBz analogue9a and in the present workwhen using the still longer ligand 4,49-bipyridylethane in7?BnOH.

The crystal structure of compound 7?BnOH (Fig. 5) com-prises mononuclear copper centres chelated by two iodo-benzoate ligands linked into a 1D coordination polymerthrough the final coordination sites by bpa molecules (SBU-II). The halobenzoate groups form halogen bonds to the p

system of the bpa ligand (motif 3; I…C 3.608(17) Å, C–I…C169.0(6)u). This motif is similar to motif 1 and is alsosupported by antiparallel p-stacking between iodobenzoategroups. The halogen bonds link the coordination polymerchains into a sheet (Fig. 5b), and these sheets stack throughweak C–H…O hydrogen bonds between the CH2 groups of thebpa linkers and the carboxylate oxygen atoms of the adjacent

layers (C…O 3.49(3) Å), creating voids which are filled withBnOH solvent molecules. The alcohol oxygen of the solventaccepts a weak C–H…O hydrogen bond from the second CH2

group of the bpa ligand (C…O 3.23(2) Å), and also donates ahydrogen bond to the carboxylate oxygen of the adjacent layer(O…O 2.793(19) Å).

In all of the structures presented herein a commonstructural arrangement is present, the antiparallel p-stackingof iodobenzoate groups combined with C–I…p halogen bonds,which is present in the three motifs represented in Scheme 2.

Fig. 2 (a) The paddlewheel unit of 4?4DMF. (b) The corresponding halogen-bonded layer, comprising type 1 motifs only.

Fig. 3 The crystal structure of 5. (a) The paddlewheels of 5 interact throughmotif 1 interactions. (b) The halogen-bonded layer is very similar to that formedin 3. Only one copy of the interpenetrated network is shown for clarity. (c) Thehalogen-bonded coordination polymer chains interpenetrate to form a close-packed network (second copy of the network shown in grey).

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This arrangement can therefore be regarded as a reliablesynthon for cross-linking of coordination polymers containing4-iodobenzoate ligands. It is also a flexible arrangement, asdemonstrated by the range of values for the halogen bonddistances and angles. For these reasons it is reasonable tohypothesise that a porous structure propagated in part by suchinteractions may show some dynamic guest-responsive beha-viour. Even though the synthon itself forms reliably, there area number of ways that the ligands can orient themselves whichsatisfy the geometric demands of synthon formation. For thisreason attempts to build porous structures from the 4-iodo-benzoate paddlewheel through the pillaring of the resulting

Fig. 5 The crystal structure of 7?BnOH. (a) The benzyl alcohol molecules formhydrogen bonds to the carboxylate oxygen of the adjacent layers. (b) Thecoordination polymer chains of 7 form motif 3 interactions resulting in poresthat are filled by benzyl alcohol solvent molecules (not shown).

Fig. 4 The crystal structure of 6?nMeOH. (a) View of the independentcoordination polymer chains, each comprising only one type of paddlewheel.Type A paddlewheels are shown in green, type B paddlewheels are shown inyellow. (b) View of assembly of coordination polymer chains into halogen-bonded tapes, which are four paddlewheel units wide. The halogen-bondedtape is assembled via motifs 1 and 2. The rectangular voids are filled by bipymolecules from other coordination polymers that thread through. Suchpolymers are represented only by the (isolated) bipy molecules. (c) Schemeshowing the herringbone arrangement of stacked tapes viewed directly downthe direction of propagation of the tapes. Bipy molecules are represented byblue lines. Haloaryl rings are represented by green or yellow sticks. The positionof one of the tapes is highlighted by a red box. The blue box highlights theparallel coordination polymer chains shown in (a).

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halogen-bonded layers were unsuccessful, resulting in eithernetwork interpenetration (compound 5), the breakdown of thelayer (6?nMeOH), or the breakdown of the paddlewheel SBUcompletely (7?BnOH).

Conclusions

4-Iodobenzoate paddlewheels, here Cu2(4-Ibz)4, do not possessthe degree of conformational freedom available to their 3-Ibzanalogues9 and as such the halogen-bonded networks whichform in these compounds are more consistent. In thecompounds reported herein a common structural arrange-ment defines the intermolecular interaction between iodo-benzoate groups, whereby antiparallel p-stacking ofiodobenzoates is combined with C–I…p halogen bonding toan aromatic ring that is perpendicular to the p-stacked ligands(motifs 1–3, Scheme 2).

Two different halogen-bonded layers form in this set ofcompounds. One is built entirely from the aforementionedinteractions involving iodobenzoate groups (motifs 1 and 3),whereas the other type of layer includes a similar motifwhereby the iodine atom involved in one motif 1 interactionalso acts as a halogen bond acceptor. The type of layer thatforms is determined by how efficiently the resulting layerspack once the axial ligand (i.e. perpendicular to the layer) iscoordinated. Motif 2 interactions leave a larger void within thelayer, allowing interdigitation of the layers with the void filledby the axial ligands of a neighbouring layer, leading to veryefficient stacking.

Attempts to prepare porous frameworks by use of axiallycoordinated pillar ligands to covalently link the Cu2(4-Ibz)4

layers was only partially successful. Although a pillared layeredstructure was obtained when using dabco or 4,49-bipy as thepillar, these structures were doubly interpenetrated andexhibited no porosity. Use of the still longer bipyridylethaneligand to extend the separation between layers in a manneranalogous to many MOFs built from paddlewheel layerscomprising metal dicarboxylates instead resulted in break-down of the halogen-bonded layer and paddlewheel SBU.

Acknowledgements

We thank the CCDC, STFC and EPSRC for support for PS. ABVwas supported by the Erasmus-Socrates exchange programmeand RMH was supported by the Sheffield UndergraduateResearch Experience (SURE) programme.

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