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Coordination networks of Cu2+ ions with 1,3-bis[2-(4pyridyl)ethyl]benzene: strong structure-directing role of the counter ion (nitrate, acetate and sulphate), leading to clusters, sheets and chains M. John Plater, Ben M. De Silva, Mark R. St J. Foreman and William T. A. Harrison Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, Scotland (e-mail: [email protected]) Submitted to Polyhedron Abstract The crystal structures of three compounds formed from the crystallisation of different copper(II) salts (nitrate, acetate and sulphate) with the new ligand 1,3-bis[2-(4pyridyl)ethyl]-benzene, C 20H20N2 (L) are reported. The anion is incorporated into each strructure, but it plays a completely different role in each case: [Cu2L4(NO3)(H2O)2]·3(NO3) (1) contains discrete ‘paddlewheel’ bimetallic clusters incorporating a nitrate ion at their centres. [Cu 2L(Ac)3(OH)] n (2) contains unusual tetrametallic clusters in which the acetate ions display three different coordination modes: the L ligands link the clusters into (100) sheets. [CuL2(SO4)]n·2n(H2O) (3) contains looped [010] chains in which both the L ligands and sulphate ions bridge adjacent metal ions. Crystal data: 1, C80H84Cu2N12O14, Mr = 1564.67, tetragonal, space group I4/m, a = 15.2358 (4) Å, c = 16.5372 (6) Å, V = 3838.8 (2) Å 3, Z = 2, R(F) = 0.054, wR(F2) = 0.142. 2, C26H30Cu2N2O7, Mr = 609.60, monoclinic, space group P21/c, a = 15.9076 (6) Å, b = 11.8299 (4) Å, c = 14.4234 (4) Å, = 107.646 (1), V = 2586.56 (15) Å 3, Z = 4, R(F) = 0.043, wR(F2) = 0.109. 3, C40H44CuN4O6S, Mr = 772.39, triclinic, space group 𝑃1̅, a = 10.513 (2) Å, b = 12.925 (4) Å, c = 14.769 (4) Å, = 91.126 (11), = 109.749 (9), = 100.658 (9), V = 1848.9 (8) Å3, Z = 2, R(F) = 0.108, wR(F2) = 0.274. 1 Introduction Flexible dipyridyl bridging ligands, in which the pyridine rings are linked by an alkyl chain, are versatile ligands for constructing MOFs and coordination networks [1–20]. Such ligands can yield networks that can clathrate guest species because the flexible ligand may be able to ‘breathe’ to accommodate the guest, whilst maintaining its crystalline structure [21]. This might be termed an ‘induced-fit,’ resembling the binding of a substrate at the active site of an enzyme. Our previous crystallographic studies on a series of flexible dipyridyl alkanes showed that longer chains tend to be disordered although the gross structural features of the networks were easily discerned [22–24]. Flexible ligands, which can ‘fold back’ on themselves, may also offer an opportunity to form a discrete complex or a cluster [25]. An important consequence of using neutral dipyridyl ligands as metal-linkers rather than anions such as dicarboxylates is the requirement for charge-balancing anions, which may exert their own influence on the structure [26]. In this paper, we report three structures formed from the crystallisation of different copper(II) salts (nitrate, acetate and sulphate) with the new ligand 1,3-bis[2-(4pyridyl)ethyl]-benzene, C20H20N2 (see scheme below), in which the anion plays a key, but completely different, role in each case. So far as we are aware, this ligand is novel and no crystal structures containing it have been reported previously. 2 Experimental Synthesis of 1,3-Bis[2-(4-pyridyl)ethyl]benzene (L) Pyridine-4-carboxaldehyde (2.72 g, 25.4 mmol) was added to a solution of 1,3bis(methyltriphenylphosphonium)benzene di-bromide (10 g, 12.7 mmol) in dry ethanol (50 ml). A fresh solution of sodium ethoxide was slowly added to the stirred mixture over 30 min. After a further 3.5 h the ethanol was removed and water (80 ml) followed by CH2Cl2 (8 ml) was added. The triphenylphosphine oxide that remained as a precipitate was filtered off and washed with water. The aqueous washings were combined and neutralized with sodium hydroxide (2 M) and the mixture was extracted with CH2Cl2 (2 50 ml). The CH2Cl2 was removed in vacuo to give a brown solid, which was dissolved in ethanol (15 ml) to which 10% palladium on carbon (200 mg) was added. This vigorously stirred mixture was hydrogenated at 1 atm pressure and at 24 °C until no more hydrogen was absorbed. The ethanol was removed and the crude product was purified by column chromatography (silica gel; eluent 90% ethanol:10% ethyl acetate) to give L (2.2 g, 62 %) as a light brown solid, m.p. 86–89 C. (KBr)/cm–1 3010s, 2928s, 2866w, 1556s, 1454s, 1413s, 1217w, 990w and 704s; H(250 MHz; CDCl3) 2.86(8H, s), 6.87–7.20(8H, m) and 4.00(4H, d, J 1.9); C(62.9 MHz; CDCl3) 36.5, 37.1, 124.0, 126.3, 128.6, 140.9, 150.0, 150.4 (one resonance is missing); m/z 289.1705 (C20H20N2 requires 289.1704). Synthesis of [Cu2L4(H2O)2(NO3)]·3(NO3) (1) L (0.10 g, 0.35 mmol) was dissolved in ethanol (5 ml) and carefully layered onto a solution of Cu(NO3)2·2.5H2O (0.081 g, 0.35 mmol) in water (5 ml) in a sample vial, which was then sealed. The mixture was left to stand for two weeks and during this time royal blue crystals of 1 grew at the layer interface. The crystals were harvested by filtration and air dried (0.106 g, 59%); (KBr)/cm–1 3382s, 3050m, 3016m, 2922s, 2856s, 1672s, 1430s, 1228s, 1184w, 1066m, 1030m, 936w, 822s, 773s and 698s. Synthesis of [Cu2L(OAc)3(OH)]n (2) L (0.10 g, 0.35 mmol) was dissolved in EtOH (5 ml) and layered onto a solution of Cu(OAc)2·H2O (0.070 g, 0.35 mmol) in water (5 ml). The vial was sealed and the solution was left to stand for two weeks: during this time turquoise crystals of 2 grew at the layer interface, which were collected and air dried (0.138 g, 74%); (KBr)/cm–1 3430s, 3060m, 2922s, 2858s, 1617s, 1436s, 1229s, 1050s, 1032s, 967s, 825s, 795s, 704s and 613s. Synthesis of [CuL2(SO4)]n·2n(H2O) (3) L (0.10 g, 0.35 mmol) was dissolved in ethanol (5 ml) and layered onto a solution of CuSO4·5H2O (0.086 g, 0.35 mmol) in water (5 ml). The solution was left to stand for two weeks in a sealed vial and during this time blue crystals of 3 grew at the layer interface. The crystals were collected and air dried (0.095 g, 59%). (Found: C, 51.7; H, 5.0; N, 4.55. C26H30Cu2N2O7 requires C, 51.2; H, 5.0; N, 4.6%) (KBr)/cm–1 3445m, 1603s, 1506w, 1445s, 1332s, 1224s, 1070s, 1037s, 910w, 819w, 705w and 671s. X-Ray Crystallography Intensity data for 1–3 were collected at 120 K using graphite-monochromated MoK radiation ( = 0.71073 Å) on a Bruker-Nonius KappaCCD diffractometer. The structures were solved by direct methods and completed and optimised by least-squares refinement against F2 with SHELXL-97 [27]. The C-bound H atoms were geometrically placed (C– H = 0.95–0.99 Å) and refined as riding atoms. The O-bound H atoms were located in difference maps and refined as riding atoms in their as-found relative positions. The constraint Uiso(H) = 1.2Ueq(carrier) was applied in all cases. The nitrate ion at the centre of the ‘paddlewheel’ cluster in 1 (vide infra) is disordered by symmetry over four orientations rotated by 90. The three charge-balancing (uncoordinated) nitrate ions in 1 were found to be highly disordered and after unsuccessful attempts to plausibly model them, their contribution to the scattering was remved with the SQUEEZE option incorporated in PLATON [28]. The H atoms of the each of the –CH3 groups of the acetate anions in 2 were modelled as being disordered over two sets of sites related by a 60 rotation about the H3C–C bond axis. One of the Cu 2+ ions in 3 is statistically disordered over two sites adjacent to an inversion centre. One of the uncoordinated water molecules in 3 is disordered over three adjacent positions; its H atoms could not be located. The key crystallographic and data collection parameters for 1–3 are summarized in Table 1 and full details are available in the archived cifs. 3 Results Structure of [Cu2L4(NO3)(H2O)2]·3NO3 (1) The crystal structure of 1 contains novel cationic ‘paddlewheel’ [Cu 2L4(NO3)(H2O)2]3+ clusters, which are templated by nitrate ions at their centres. The copper(II) ion, which lies on a fourfold rotation axis, is bonded to a square-plane of four symmetry equivalent ligand N1 atoms [Cu–N = 2.025 (3) Å] and its Jahn–Teller, asymmetrically axially-distorted octahedral CuN4O2 coordination sphere is completed by a water molecule and the O atom of a nitrate group [Cu–O = 2.289 (5) and 2.620 (5) Å, respectively]. The asymmetric unit (Fig. 1) of 1 contains half an L ligand, which is completed by mirror symmetry. The dihedral angle between the planes of any two adjacent py rings of the ligands bonded to Cu is 77.92 (9), and the metal ion is displaced from the mean plane of each py ring by 0.386 (4) Å. The conformation of the ethyl chain linking the aromatic rings of the ligand is gauche [C3– C6–C7–C8 torsion angle = –63.9 (4)] and the dihedral angle between the central benzene ring and the terminal pyridyl ring is 44.5 (2). Each ligand is twisted back to bond to a second, symmetry equivalent metal ion; the nitrate group at the centre of the cluster, which is disorderd about the [001] 4-fold axis also bonds to the second copper ion, to result in a ‘paddleweel’ (Fig. 2) arrangement for the cluster: its O–CuCu–O axis lies in the [001] direction and the complete cluster has 4/m point symmetry. The axial water molecules probably form O–HO hydrogen bonds to the disordered charge-balancing nitrate groups. The unit-cell packing shows (Fig. 3) shows that the clusters stack in the [001] direction; the disordered (and unmodelled) nitrate ions separate adjacent clusters in the c-dirction. Structure of [Cu2L(OAc)3(OH)]n (2) The crystal structure of 2 consists of unusual tetra-metallic copper–acetate–hydroxo clusters connected into (100) sheets by the L ligands. Cu1 (Fig. 4) has four near-neighbours arranged in a square plane: one ligand N atom [Cu–N = 1.977 (2) Å], two cis hydroxide groups [Cu–O = 1.948 (2) and 1.973 (2) Å] and an acetate O atom [1.925 (2) Å]. Its axially distorted octahedral coordination geometry is completed by two further acetate O atoms [2.543 (2) and 2.678 (2) Å]: the bond angle for the axial O–Cu–O grouping is 163.33 (7). Cu2 has five near-neighbours in a square pyramidal arrangement: one N atom [2.018 (3) Å], a hydroxide group trans to N [1.989 (2) Å] and two acetate O atoms [1.973 (2) and 2.002 (2) Å] form the base of the pyramid and another acetate (O2) atom [2.198 (2) Å] the apex. A highly distorted octahedron is completed by the long Cu2–O5 bond [2.647 (2) Å]: the O2–Cu2–O5 bond angle is 149.94 (7). The hydroxide ion plays a key role is constructing the cluster: it bridges the two Cu1 atoms (as part of a centrosymmetric Cu2O2 square) and also bonds to Cu2. It is notable that the three acetate groups have very different coordination environments: the C21-containing ion is 1 + 1 bridging between Cu1 and Cu2, whereas the C23 ion is chelating to Cu2 [O–Cu–O = 54.88 (8)] and also bridges to Cu1 from both its’ oxygen atoms (i.e. 2 + 2). Finally, the C25 acetate ion is simple monodentate (1) to Cu2; its uncoordinated O atom accepts a hydrogen bond [HO = 1.85 (4) Å; O–HO = 165 (4)] from the hydroxide group at the centre of the cluster. Overall, a centrosymmetric [Cu4(OH)2(OAc)6N4] entity arises (Fig. 5), where N is part of the ligand. The Cu1Cu1i (i = 1–x, 2–y, –z) and Cu1Cu2 separations are 2.9851 (5) Å and 3.0920 (5) Å, respectively. In terms of the L ligand in 2, the dihedral angles between the central ring and the N1 and N2 pyridine rings are 8.72 (15) and 45.26 (9), respectively. The conformation of the C3–C6–C7– C8 inter-ring link is anti [torsion angle = 179.9 (3)] whereas the C18–C15–C14–C10 link is gauche [–67.0 (4)]. The extended structure of 2 results in (100) infinite sheets (Fig. 6), in which the clusters are linked by the contorted ligands in the [010] and [001] directions. Inter-sheet bonding occurs via van der Waals’ contacts between the ligand atoms. There are no aromatic – stacking interactions in 2 (shortest centroid–centroid separation > 4.3 Å). Structure of [CuL2(SO4)]n·2n(H2O) (3) The crystal structure of 3 (Fig. 7) consists of infinite [010] chains with adjacent copper ions bridged by both L ligands and sulphate ions. Cu1 (site symmetry ̅1) is coordinated by four L nitrogen atoms in a square planar arrangement (mean Cu–N = 2.030 Å) and its Jahn–Teller axially-distorted octahedral geometry is completed by two sulphate O atoms [Cu–O = 2.414 (6) Å]. Cu2, which is disordered over adjacent sites [CuCu = 0.957 (4) Å] related by inversion, has essentially the same coordination mode. The N1 ligand has a gauche conformation for both the C3–C6–C7–C8 [50.5 (12)°] and C10–C14–C15–C18 [63.0 (12)°] linking groups. The dihedral angles between the central ring and the N1 and N2 pyridine rings are 61.2 (4) and 38.5 (4)°, respectively. For the N3 ligand, both its linking ethyl groups are gauche [C23–C26–C27–C28 = 54.0 (12)° and C30–C34–C35–C38 = 61.9 (12)°] and the central ring subtends dihedral angles of 43.2 (3) and 68.4 (3)° with the N3- and N4-pyridine rings, respectively. Because of the coordination of the N2- and N3-rings to Cu2 and Cu1, respectively (Fig. 7), the ring planes are well aligned for aromatic – stacking with a short centroid–centroid separation of 3.486 (6) Å; the angle between the ring planes is 4.9 (5). Each Cu1 atom in the [010] chain is linked to two Cu2 atoms via a pair of L ligands and a bridging sulphate ion and, mutatis mutandis, Cu2 has the same situation with respect to Cu1 (Fig. 8). The separation of adjacent copper ions in the chain is either 6.073 (3) or 6.863 (3) Å, depending on which disorder component of Cu2 is considered. The crystal of 3 is consolidated by water-to-sulphate O5–HO hydrogen bonds, which link adjacent chains in the [xxx] direction. 4 Conclusion The room-temperature interface syntheses of the new flexible ligand 1,3-bis[2-(4pyridyl)ethyl]benzene with different Cu2+ salts has generated single crystals of a novel cluster, a sheet and a chain structure. The flexibility of the ligand has been demonstrated, with both gauche and anti conformations for the ethyl linkers and a variety of dihedral angles between the aromatic rings. The meta-attachment of the side chains to the central benzene ring of the ligand leads to a “bent back” (rather than extended) conformation in each case: in 1 and 3 the ligand bonds to two metal ions bridged by an anion [in 1, CuCu = 7.4229 (12) Å; in 3, CuCu = 6.073 (3) or 6.863 (3) Å] whereas in 2, even though the CuCu separation of 7.3061 (5) Å is slightly shorter than in 1, the metal ions are in separate clusters. The structural role of the anion in these structures is notable, with each species generating a completely different structure. Acknowledgements We thank the EPSRC National Mass Spectrometry Facility (University of Swansea) for the high-resolution mass spectrometry data and the EPSRC National Crystallography Service (University of Southampton) for the X-ray data collections and preliminary structural analyses. References 1 J. C. Dai, X. T. Wu, S. M. Hu, Z. Y. Fu, J. J. Zhang, W. X. Du, H. H. Zhang and R. Q. Sun, Eur. J. Inorg. Chem. (2004) 2096. 2 X. J. Li, R. Cao, W. H. Bi, Y. Q. Wang, Y. L. Wang and X. Li, Polyhedron 24 (2005) 2955. 3 G. A. Farnum, A. L. Pochodylo and R. L. LaDuca, Cryst. Growth Des. 11 (2011) 678. 4 P. Lama, R. K. Das, V. J. Smith and L. J. Barbour, Chem. Commun. 50 (2014) 6464. 5 B. Li, G. Li, D. Liu, Y. Peng, X. Zhou, J. Hua, Z. Shi and S. Feng, CrystEngComm 13 (2011) 1291. 6 P. F. Wang, G. Z. Wu, X. Wang and X. H. Yang, Jiegou Huaxue (Chin. J. Struct. Chem.) 30 (2011) 1775. 7 A. M. Atria, G. Corsini, M. T. Garland and R. Baggio,. Acta Cryst. C67 (2011) m367. 8 L. Carlucci, G. Ciani, J. M. Garcia-Ruiz, M. Moret, D. M. Proserpio and S. Rizzato, Cryst. Growth Des. 9 (2009) 5024. 9 W. Yang, X. Lin, A. J. Blake, C. Wilson, C. Hubberstey, N. R. Champness and M. Schroeder, Inorg. Chem. 48 (2009) 11067. 10 J. Zhang, E. Chew, S. Chen, J. H. T. Pham and X. Bu, Inorg. Chem. 47 (2008) 3495. 11 F. J. Meng, H. Q. Jia, N. H. Hu and J. W. Xu, Inorg. Chem. Comm. 28 (2013) 41. 12 J. Ji, Y. Zhang, Y. F. Yang, H. Xu and Y. H. Wen, Eur. J. Inorg. 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Table 1: key crystallographic parameters for 1–3 1 Chemical formula Formula Mass Crystal system a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) T (K) Space group Z Reflections measured Independent reflections RInt R(F) [I > 2σ(I)] wR(F2) (all data) CCDC deposition number 2 3 C80H84Cu2N12O14 1564.67 Tetragonal 15.2358 (4) 15.2358 (4) 16.5372 (6) 90.00 90.00 90.00 3838.8 (2) 120 (2) I4/m (No. 87) 2 6489 C26H30Cu2N2O7 609.60 Monoclinic 15.9076 (6) 11.8299 (4) 14.4234 (4) 90.00 107.646 (1) 90.00 2586.56 (15) 120 (2) P21/c (No. 14) 4 24917 C40H44CuN4O6S 772.39 Triclinic 10.513 (2) 12.925 (4) 14.769 (4) 91.126 (11) 109.749 (9) 100.658 (9) 1848.9 (8) 120 (2) 𝑃1̅ (No. 2) 2 10508 1765 5070 6232 0.053 0.054 0.142 1032648 0.094 0.043 0.109 1032649 0.136 0.108 0.274 1032650 Fig. 1: The asymmetric unit of 1 extended to show the complete L ligand showing 50% displacement ellipsoids. Only one orientation of the disordered nitrate ion is shown. The complete ligand is generated by the symmetry operation (x, y, –z). Fig. 2: A [Cu2L4(H2O)2(NO3)]3+ paddlewheel cluster in 1. Only one orientation of the disordered nitrate ion at the centre of the cluster is shown. The C-bound H atoms are omitted for clarity. Fig. 3: The unit-cell packing in 1 viewed down the fourfold c-axis. The Cu–O and Cu–N bonds are highlighted in yellow. Fig. 4: the asymmetric unit of 2 showing 50% displacement ellipsoids. The O–HO hydrogen bond is indicated by a double-dashed line. Fig. 5: the tetra-metallic cluster in 2 (atom colours: Cu orange, C dary grey, O red, N blue, H white). Fig. 6: the packing in 2 showing part of a (100) sheet of clusters connected by the L ligands. Fig. 7: the asymmetric unit of 3 showing 50% displacement ellipsoids. The H atoms of the disordered water molecule O6a/O6b/O6c could not be located. Fig. 8: fragment of a [010] chain in 3 showing that adjacent copper ions are each bridged by a sulphate ion and a pair of contorted L ligands.