A dihydrogen arsenate-mediated supramolecular network: crystal structure and magnetic properties of {[(bipy)Cu(m-H2AsO4)(H2AsO4)]2}n
Paper
Robert P. Doyle,a Paul E. Kruger,*a Miguel Julve,b Francesc Lloretb and Mark Nieuwenhuyzenc a
Department of Chemistry, Trinity College, Dublin 2, Ireland. E-mail:
[email protected] Department de Quı´mica Inorga`nica, Facultat de Quı´mica, Universitat de Vale`ncia, Dr. Moliner 50, E-46100 Burjassot, Vale`ncia, Spain c Chemistry Department, Queen’s University, Belfast, UK BT9 5AG b
Received 13th December 2001, Accepted 4th January 2002 Published on the Web 16th January 2002
Treatment of an aqueous suspension of Cu(OH)2 and 2,2’-bipyridine (bipy) with either Na2HAsO4?7H2O (1 : 1 : 2) or As2O5 (1 : 1 : 1) yields single crystals of {[(bipy)Cu(m-H2AsO4)(H2AsO4)]2}n, 1, on standing. The solid-state structure of 1 consists of a three dimensional supramolecular network, supported by a combination of coordination covalent, hydrogen bonding and face-to-face p–p interactions. Variable temperature magnetic susceptibility measurements reveal very weak antiferromagnetic coupling between Cu(II) centres across the dihydrogen arsenate bridges (J ~ 20.58 cm21).
Introduction The efficacy of employing hydrogen bonding in the development of supramolecular architectures via crystal engineering is well established.1 Robust organic and inorganic networks, or composites of them, may be readily generated by taking advantage of these hydrogen bond donor (D) and acceptor (A) interactions. The nature of the association is primarily driven by an electrostatic interaction and thus the charge distribution on the D/A system dictates the strength of the connection. Multiple D/A interactions similarly enhance the stability of the resultant framework. We are currently employing a combination of supramolecular and coordination chemistry in the development of topical architectures and novel networks.2 In this way we aim to incorporate into the superstructures both the versatility, strength and directionality, and hence predictability, of hydrogen bonding with the chemical and physical properties of transition metals. In our current studies we have been drawn to the polyprotic inorganic oxy-acids of some Group 15 elements (H2PO42, PO432, P2O742, H2AsO42, AsO432, etc.) as they are potentially powerful hydrogen bond D/A moieties. These moieties are prevalent throughout nature and have been extensively studied in solid-state and materials chemistry and more recently in the hydrothermal synthesis of porous materials.3 Apart from a few notable exceptions4 they have been somewhat neglected in the development of metallo-supramolecular coordination chemistry. It has been recognised that some of these species, in their various deprotonated states in combination with simple cations, have been shown to form extended network structures in which hydrogen bonding is the main ordering mechanism.5 This has given rise to crystalline solids possessing such phenomena as non-linear optical and ferroelectric properties. Furthermore, it has been proposed that tetrahedrally disposed oxo-anions of this group should propagate ferromagnetic coupling between copper(II) centres bridged by them.6 Following on from our recent success of employing phosphate and pyrophosphate to assemble Cu(II) and Zn(II) ions into extended networks,7 we also desired a ligand that would
itself promote intermolecular association. We therefore chose 2,2’-bipyridine, as it has been shown to engage in p–p interactions throughout ensuing crystal lattices and within the metal complexes it forms.8 We describe here our latest results with this strategy and present the synthesis, structural characterisation and magnetic behaviour of {[(bipy)Cu(m-H2AsO4)(H2AsO4)]2}n, 1, which features a three dimensional supramolecular network, supported by a combination of coordination covalent, hydrogen bonding and face-to-face p–p interactions.
Experimental 2,2’-Bipyridine, Na2HAsO4?7H2O and As2O5 were purchased from Aldrich and Alfa Chemicals. Caution! Arsenate salts are potentially extremely toxic and are known carcinogens and should be handled with due care and attention. Copper hydroxide was freshly prepared as detailed previously.9 Triply distilled deionised water was employed throughout. The syntheses were performed under an inert dinitrogen atmosphere. IR spectra were recorded as KBr pellets on a Perkin Elmer Paragon 1000 FT-IR spectrometer in the 4000–400 cm21 region. Synthesis of {[(bipy)Cu(m-H2AsO4)(H2AsO4)]2}n, 1 An aqueous solution of Na2HAsO4?7H2O (1.59 g, 5 mmol), or As2O5 (0.57 g, 2.5 mmol), was added dropwise to a stirring aqueous suspension of Cu(OH)2 (0.25 g, 2.5 mmol) and 2,2’bipyridine (0.4 g, 2.5 mmol). An intense blue solution resulted after 10 min, and the solution was allowed to stir for a further 6 h. After filtration the solution was left to evaporate at ambient temperature in an uncovered beaker. Stable, deep blue, diamond-shaped crystals suitable for a diffraction study became evident after standing for 7–10 days and were isolated in 90% yield. Calc. for C20H24N4Cu2O16As4: C, 23.95; H, 2.41; N, 5.58. Found: C, 23.84; H, 2.41; N, 5.55%. n/cm21: 1600 vs, 1550 m, CrystEngComm, 2002, 4(3), 13–16
DOI: 10.1039/b111350j This journal is # The Royal Society of Chemistry 2002
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1500 m, 1473 s, 1446 vs, 1315 m, 1251 w, 1150 sh, 1114 vs, 1050 w, 1031 m, 830 br, 771 vs, 730 s, 617 m, 474 w. X-Ray crystallographic analysis Diffraction data were collected using a Bruker SMART diffractometer with Mo-Ka radiation, with data collection in the range 4.4 w 2h w 52.8, corrected for Lorentz, polarization and absorption effects. The structure was solved by direct methods and refined by full-matrix, least-squares on F2. Nonhydrogen atoms were refined anisotropically and hydrogen atoms were located from a difference Fourier map and refined using a riding model with Uij ~ 1.2 Ueq. Details of data collection and refinement are given in Table 1. Magnetic susceptibility measurements The magnetic susceptibility measurements of a polycrystalline sample of 1 were performed in the temperature range 1.9–290 K with a Quantum Design SQUID susceptometer and using an applied magnetic field of 0.1 T as previously described.10 The complex (NH4)2Mn(SO4)2?6H2O was used as a susceptibility standard. Diamagnetic corrections of the constituent atoms were estimated from Pascal’s constants. A value of 60 6 1026 cm3 mol21 was used for the temperature-independent paramagnetism of the copper(II) ion.
Results and discussion Synthesis of 1 It is imperative to conduct the synthesis under an inert atmosphere, as basic solutions of Cu(II) may ‘fix’ carbon dioxide (as carbonate), at the expense of oxo-anion ligand coordination.11 This was indeed found to be the case here, since when syntheses were conducted in air, other crystalline products were produced that did not analyse well for the presence of arsenate in any form. However, it was not necessary to exclude air (CO2) during crystallisation. It would appear therefore that carbon dioxide fixation occurs during synthesis, as crystallisation occurred without the production of these side-products under ambient conditions. There is also a subtle interplay between the pH, the nature of the ‘arsenate’ anion in solution and the fixation of carbon dioxide. Nevertheless, irrespective of which arsenate salt was used, under the conditions employed here, the generation of 1 always resulted.
Parameter
a
C20H24Cu2N4O16As4 1003.19 Triclinic P1¯ 8.5303(7) 9.4703(8) 10.1087(8) 88.9515(13) 65.3746(12) 71.8427(12) 669.48(10) 1 153(2) 2.382 6.302 52.8 0.359, 1.000 2855 2673 0.0618 0.0461, 0.1194 0.0477, 0.1209
Click here for full crystallographic data (CCDC 176064).
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CrystEngComm, 2002, 4(3), 13–16
Crystal structure of 1 The atomic numbering scheme and atom connectivity for 1 are shown in Fig. 1. The structure of 1 consists of a centrosymmetric Cu(II) dimer with bridging between copper centres furnished by dihydrogen arsenate [Cu(1)–O(21)–As(1)–O(23)– Cu(1a)]. Further coordination about copper is provided by bidentate 2,2’-bipyridine [N(1) and N(12)] and an additional monodentate dihydrogen arsenate [O(11)], resulting in a slightly distorted square pyramidal metal coordination chromophore [CuN2O3]. Slight distortion toward trigonal bipyramidal is evident as O(21) lies some 7.1u out of the plane containing N(1)–Cu(1)–N(12)–O(23a). Analysis of the shapedetermining angles using the approach of Reedijk and coworkers12 yields a t value of 0.25 (t ~ 0 for ideal tetragonal and 1 for trigonal-bipyramidal geometries, respectively). The ˚, copper centres [Cu(1)…Cu(1a)] are separated by 5.287 A … … whereas the copper to arsenic [Cu(1) As(1)] and [Cu(1) ˚ , respectively. The As(2)] separations are 3.312 and 3.447 A bonding about each arsenate group deviates slightly from that of a perfect tetrahedron. Selected bond lengths and angles are collected within Table 2. The dinuclear unit displays extensive hydrogen bonding both intramolecularly and throughout the crystal lattice, Table 3. Firstly, intramolecular hydrogen bonding through ˚ ) and angles (u) for 1 Table 2 Selected bond lengths (A
Table 1 Crystal data for 1a
Chemical formula M Crystal system Space group ˚ a/A ˚ b/A ˚ c/A a/u b/u c/u ˚3 V/A Z T/K Dc/g cm23 m(Mo-Ka)/mm21 2hmax./u Min., max. transmission factor Reflections collected Reflections observed Rint R, wR2 [I w 2s(I)] R, wR2 (all data)
Fig. 1 Molecular structure of 1 showing the intramolecular hydrogen bonding between H(24) and O(11) and the atomic numbering scheme used. Click image or here to access a 3D representation.
Cu(1)–O(11) Cu(1)–O(21) Cu(1)–O(23a)
2.325(3) 1.944(3) 1.917(3)
O(23a)–Cu(1)–O(21) O(23a)–Cu(1)–N(12) O(21)–Cu(1)–N(12) O(21)–Cu(1)–O(11) N(12)–Cu(1)–N(1) As(1)–O(21)–Cu(1) As(2)–O(11)–Cu(1)
92.35(12) 169.58(13) 95.17(13) 97.53(11) 80.96(14) 133.92(17) 118.16(14)
Cu(1)–N(1) Cu(1)–N(12) Cu(1)–Cu(1a) O(23a)–Cu(1)–N(1) O(23a)–Cu(1)–O(11) O(21)–Cu(1)–N(1) N(1)–Cu(1)–O(11) N(12)–Cu(1)–O(11) As(1)–O(23)–Cu(1a)
2.012(3) 2.001(3) 5.287(3) 89.17(13) 93.11(12) 154.91(13) 107.40(12) 93.04(12) 126.93(17)
Symmetry code: a ~ 2x, 2y, 2z.
Table 3 Parameters of hydrogen bonding interactions within the structure 1 D2H…A
˚ d(H…O)/A ˚ d(O…O)/A ˚ /(OH…A)/u d(D–H)/A
O13–H13…O11i O12–H12…O14ii O22–H22…O12iii O24–H24…O11
0.84 0.84 0.84 0.84
1.90 1.95 1.84 1.88
2.733(5) 2.639(5) 2.669(4) 2.715(4)
172 139 170 174
Symmetry codes: i ~ 2x, 2 2 y, 2z; ii ~ 1 2 x, 2 2 y, 2z; iii ~ x, 21 z y, z.
O(24)–H(24)…O(11), and intermolecular hydrogen bonding via O(22)–H(22)…O(12), link the bridging dihydrogen arsenate, centred around As(1), to apically bound ones, centred around As(2). Secondly, these apically bound dihydrogen arsenates interact with themselves intermolecularly through self-complementary ‘head-to-tail’ hydrogen bonding, Fig. 2. This generates an R22 (8) chain that continues along the crystallographic a-axis which, through linking with As(1), propagates a two dimensional dihydrogen arsenate sheet in the ab-plane, Fig. 2. Cu(1) and Cu(1a), etc. are situated slightly above and below this sheet such that the bipy ligands project into the space between sheets. In this way the hydrophobic ligands minimise their contact with the ‘hydrophilic’ dihydrogen arsenate sheets. The bipy ligands themselves form an ‘offset’ p-stack (separation between adjacent bipy ligands ca. ˚ ) down the crystallographic b-axis which serves not only to 3.4 A separate sheets but also to link them into three dimensions, Fig. 3. Additional interactions are also evident between the bipy hydrogens H(5), H(8) and H(9) and the oxygen atoms O(12), O(14) and O(21), and O(24), respectively, from an adjacent dihydrogen arsenate sheet. It is interesting to note that the efficacy of packing within 1 is at the total exclusion of water from which it crystallised, as we have found that the extremely hydrophilic nature of the Group 15 oxo-anions means that highly hydrated crystals are usually formed.7 Furthermore, the crystals themselves are practically insoluble in water and other polar solvents (DMF, DMSO, EtOH, etc.) attesting to the robust nature of the network. Whilst the structure of 1 is novel, the hydrogen bonding nature of the H2AsO4 moiety has been observed in simple organic salts of it and also within its phosphate and phosphonate analogues, H2PO4 and H2PO3R, respectively.4,13 A survey of the CSD14 does show, however, that hydrogen bonding between H2XO42 (X ~ P, As) is less commonly observed within coordination complexes containing them, although there is only a small sample size of such compounds. A pertinent example to the study here is [Mn(bipy)(HPO4)(H2PO4)], which shows similar R22 (8) chain formation and bipy p–p interactions, although in this instance each phosphate species is bridging between Mn(III) ions within the coordination polymer.15 It therefore appears that species of this type will form hydrogen bonded networks within the
Fig. 2 Packing diagram of 1 looking down the crystallographic c-axis showing the 2D sheet formed through hydrogen bonding. The sheet lies in the ab-plane with the ‘head-to-tail’ R22 (8) chain formed by As(2) running along the a-axis. Copper, and bipy atoms omitted for clarity. Click image or here to access a 3D representation.
Fig. 3 Packing diagram of 1 looking down the crystallographic a-axis (in the bc-plane) showing the hydrogen bonding and p–p interactions along b. Click image or here to access a 3D representation.
coordination complexes they form, but that the nature of the interaction is highly dependent upon the metal they are bound to. Magnetic properties Variable temperature magnetic susceptibility studies were carried out on powdered samples of 1 over the temperature range 1.9–290 K. Plots of xmT vs. T [xm being the magnetic susceptibility per two copper(II) ions] displayed typical Curie behaviour in the range 290–80 K, remaining practically invariant with temperature at a value of 0.81 cm3 mol21, as would be expected for two magnetically isolated spin doublets. A slight decrease occurred below these temperatures resulting in a value of 0.71 cm3 mol21 at 1.9 K. This behaviour is typical of a very weak antiferromagnetically coupled system. The data could be adequately described through a simple Bleaney– Bowers16 expression for a copper(II) dimer, derived through the isotropic Hamiltonian H ~ 2 JS1S2 where J is the exchange coupling parameter and S1 ~ S2 ~ 1/2 (interacting local spins). Least-squares fitting leads to the following parameters: J ~ 20.58(1) cm21, g ~ 2.07(1) and R ~ 4.3 6 1026 {R is the agreement factor defined as Si[(xmT)obs(i) 2 (xmT)calc(i)]2/ Si [(xmT)obs(i)]2}. The magnitude and nature of this very weak interaction probably results from a number of factors including: (a) the slight distortion of the copper chromophore toward a trigonalbipyramidal disposition may be expected to diminish any interaction across the bridge when the magnetic orbital is of the dx2 2 y2 type; and (b) the deviation of the bridgehead (O–As– O) angles centred around As(1) away from that of a perfect tetrahedron (109.5u) might be expected to negate effective coupling through it, leading to very weak net antiferromagnetism. Close inspection of 1 shows that those angles about As(1) are quite removed from the ideal tetrahedron, and that the O(21)–As(1)–O(23) angle of 117.8u is considerably so. Clearly, more examples of arsenate-bridged copper(II) dimers will have to be magnetically and structurally characterised. In conclusion we have shown that treatment of an aqueous suspension of Cu(OH)2 and 2,2’-bipyridine with either Na2HAsO4 or As2O5 yields single crystals of {[(bipy)Cu(mH2AsO4)(H2AsO4)]2}n, 1. The solid-state structure of 1 consists CrystEngComm, 2002, 4(3), 13–16
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of a 3D supramolecular network, supported by a combination of coordination covalent, hydrogen bonding and face-to-face p–p interactions. Variable temperature magnetic susceptibility measurements reveal very weak antiferromagnetic coupling between Cu(II) centres. We have recently extended this work to include phosphate salts and have found that instead of isolating isostructural analogues of dinuclear 1, tetranuclear phosphato complexes of the type [Cu4L4(m-PO4)2(m-X)(H2O)2]nz (where L ~ 1,10-phenanthroline or 2,2’-bipyridine; X ~ HO2, CO322; n ~ 0, 1) are formed. We will shortly be reporting on these and related systems.7b–d
Acknowledgements We are grateful to the EC (contract ERBMRXCT98-0226), the Higher Education Authority and Trinity College Academic Development Fund for financial support.
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