CRYSTAL GROWTH & DESIGN
C-H‚‚‚Br-M Interactions at Work: Tetrabromometalates of the Bolaamphiphilic N,N′-Dodecamethylenedipyridinium Cation
2001 VOL. 1, NO. 5 387-393
Francesco Neve* and Alessandra Crispini Dipartimento di Chimica, Universita` della Calabria, 87030 Arcavacata di Rende (CS), Italy Received June 18, 2001
ABSTRACT: The organic-inorganic hybrid salts [Py-C12-Py][MX4] (M ) Pd, Cu; X ) Cl, Br) were prepared by reaction of the N,N′-dodecamethylenedipyridinium cation [Py-C12-Py]2+ and the appropriate halide metal salt. The crystal structures of the bromide derivatives [Py-C12-Py][PdBr4] (3) and [Py-C12-Py][CuBr4] (4) were determined by X-ray crystallography. The presence of several C-H‚‚‚Br-M contacts between organic cations and inorganic anions leads to segregation of the polymethylene chains. These are arranged in columns which are surrounded by bound pyridinium cation headgroups and isolated tetrabromometalates. The role of C-H‚‚‚Br-M interactions in crystalline species has been investigated through a CSD search and subsequent analysis. Introduction One of the most important ways to govern the bulk properties of individual chemical species is through the control of their supramolecular solid-state organization. Crystal engineering (in its organic,1 inorganic,2 coordination,3 and organometallic4 variants) has taught us how to master noncovalent interactions to build up molecular crystals, host-guest compounds, or infinite 1-D, 2-D, and 3-D coordination networks with a wide range of topologies. As recently recalled by Braga and Grepioni,5 the range of noncovalent bonding tools encompasses both “organic” (electrostatic, H-bonding, π-π stacking, van der Waals) and “inorganic” (closed-shell, metallophilic, pseudoagostic, etc.) types of interactions, with the latter rapidly growing in number. Nevertheless, only the strongest (individually or collectively) intermolecular interactions may become true topological directors, thus making crystal programming a challenging, and therefore interesting, synthetic problem. In an attempt to relate the field of molecular crystal engineering and that of liquid crystals (one of the most paradigmatic examples of self-organized supramolecular systems), we have recently prepared and studied the condensed state of organic-inorganic hybrids [Cn-Py]2[MX4] based on N-substituted pyridinium cations with
types of organizations (crystalline and liquid crystalline), the solid-state architecture of such solids is also governed by the presence of extensive C-H‚‚‚X hydrogen bonding. C-H bonds of heteroaromatic cations (e.g., pyridinium, imidazolium, and benzimidazolium) are indeed effective donors in the formation of hydrogen bonds to electronegative atoms.9-11 This observation is complemented by the recent recognition of the special ability of M-X moieties (M is a transition metal, X is usually chlorine) to act as hydrogen bond acceptors (metal-assisted H-bonding).12-14 Both the above tendencies are also expected to become stronger in the electrostatic field of ionic solids (charge-assisted H-bonding).5,15 We report here a variation of our previous studies presenting the new hybrids [Py-C12-Py][MX4] (X ) Cl, Br; 1-4) containing planar ([PdX4]2-) or tetrahedral ([CuX4]2-) complex metal ions. The cation of choice, the bolaamphiphilic N,N′-dodecamethylenedipyridinium (PyC12-Py2+), was designed in order to preserve the H-bonding ability of N-substituted pyridinium groups while restricting the conformational freedom of the paraffinic component. The hybrid salts [Py-C12-Py][MBr4] (M ) Pd (3), Cu (4)) were structurally characterized, and their solid-state features are reported. Experimental Section
alkyl substituents of various chain lengths (Cn-Py+) and tetrahalometalate [MX4]2- anions.6-8 While electrostatic and segregation effects are common to both * To whom correspondence should be addressed. Phone: +39-0984/ 492060. Fax: +39-0984/492044. E-mail:
[email protected].
General Methods. All reactions and manipulations were performed in air with reagent-grade solvents unless otherwise noted. Pyridine was dried by distillation from KOH. 1H NMR (300.13 MHz) spectra were recorded at room temperature on a Bruker AC 300 spectrometer. Chemical shifts were referenced to internal SiMe4. Melting temperatures were measured by differential scanning calorimetry (DSC) with a PerkinElmer DSC-7 instrument operating at a scanning rate of 5 °C/ min. Elemental analyses were performed with a Perkin-Elmer 2400 microanalyzer by the Microanalytical Laboratory at the Universita` della Calabria. Reagents were purchased from Aldrich and used as received. [Py-C12-Py][X]2 (X ) Br, Cl). Freshly dried pyridine (10 mL) was added to 1,12-dibromododecane (1.58 g, 4.82 mmol) and the resulting colorless solution heated to reflux under a nitrogen atmosphere for 24 h. Cooling of the reaction mixture
10.1021/cg0155346 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/17/2001
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Crystal Growth & Design, Vol. 1, No. 5, 2001
to room temperature led to the formation of an off-white precipitate. The solid was collected by filtration, washed several times with diethyl ether, and vacuum-dried to afford a white crystalline solid in 98% yield (2.3 g). Mp: 140 °C. Anal. Calcd for C22H34Br2N2: C, 54.33; H, 7.05; N, 5.76. Found: C, 54.29; H, 7.28; N, 5.65. 1H NMR (CDCl3-CD3OD): δ 9.21 (d, 2 H, J ) 6.1 Hz), 8.54 (t, 1 H, J ) 7.3 Hz), 8.15 (m, 2 H), 4.79 (t, 2 H, J ) 7.3 Hz), 2.05 (m, 2 H), 1.36-1.26 (m, 8 H). [PyC12-Py][Cl]2 was prepared by metathesis from the corresponding bromide using silver chloride16 and recrystallization from ethanol-diethyl ether. The product was obtained in quantitative yield as a hygroscopic yellowish white crystalline solid. Mp: 110 °C. Anal. Calcd for C22H34Cl2N2‚H2O: C, 63.60; H, 8.73; N, 6.74. Found: C, 63.62; H, 8.96; N, 6.60. 1H NMR data were similar to those reported above for the bromide salt. Synthesis of [Py-C12-Py][MCl4] (M ) Pd (1), Cu (2)) and [Py-C12-Py][MBr4] (M ) Pd (3), Cu (4)). [Py-C12Py][PdCl4] (1). Addition of [Py-C12-Py][Cl]2 (0.125 g, 0.32 mmol) to a warm solution of PdCl2 (0.056 g, 0.32 mmol) in CH3CN (30 mL) led to the immediate formation of a pinkish brown microcrystalline precipitate. The reaction mixture was heated at 70 °C for 2 h, and then the solid was collected by filtration, washed with diethyl ether and n-hexane, and vacuum-dried. Yield: 0.175 g (98%). Mp: 239 °C dec. Anal. Calcd for C22H34Cl4N2Pd: C, 45.98; H, 5.96; N, 4.87. Found: C, 46.69; H, 6.21; N, 5.03. [Py-C12-Py][CuCl4] (2). A mixture of equimolar amounts of CuCl2‚2H2O (0.1 g) and [Py-C12-Py][Cl]2 (0.24 g) in CH3CN (15 mL) was heated under reflux. A microcrystalline yellow precipitate separated almost immediately from the orange solution. After 2 h the solid was filtered, washed with diethyl ether, and vacuum-dried. Yield: 0.26 g (80%). Mp: 159 °C. Anal. Calcd for C22H34Cl4N2Cu: C, 49.68; H, 6.44; N, 5.27. Found: C, 49.76; H, 6.42; N, 5.35. [Py-C12-Py][PdBr4] (3). Addition of a water solution (2 mL) of a 10-fold excess of KBr to a yellow-brown warm solution of PdCl2 (0.11 g, 0.62 mmol) in CH3CN (24 mL) resulted in the immediate formation of a dark red solution and a white precipitate. After the solid was removed by filtration, the filtrate was treated with [Py-C12-Py][Br]2 (0.3 g, 0.62 mmol). Immediate formation of a dark red microcrystalline precipitate was followed by heating under reflux conditions for 2 h. Cooling of the reaction mixture to room temperature allowed the complete formation of the solid product. This was washed with water, ethanol, and diethyl ether and vacuum-dried. Yield: 0.38 g (82%). Mp: 225 °C. Anal. Calcd for C22H34Br4N2Pd: C, 35.11; H, 4.55; N, 3.72. Found: C, 35.51; H, 4.61; N, 3.74. [Py-C12-Py][CuBr4] (4). CuBr2 (0.09 g, 0.4 mmol) was added to a colorless solution of [Py-C12-Py][Br]2 (0.2 g, 0.4 mmol) in CH3CN (12 mL). Immediately the reaction mixture turned dark violet. After 1.5 h of reflux, the mixture was cooled to room temperature, and the product was recovered as violetblack needles and plates. A further crop of solid product was obtained upon cooling the mother liquor to -20 °C. Yield: 0.19 g (65%). Mp: 139 °C. Anal. Calcd for C22H34Br4N2Cu: C, 37.23; H, 4.83; N, 3.95. Found: C, 37.43; H, 4.83; N, 3.93. Crystallographic Studies. Single-crystals of [Py-C12Py][PdBr4] (3) and [Py-C12-Py][CuBr4] (4) suitable for X-ray analysis were obtained from acetonitrile solutions upon overnight cooling at -18 °C. Crystals of 3 appeared as dark red regular prisms, while 4 crystallized as large, almost black needles. Crystal data and data collection parameters are summarized in Table 1. Intensity data for both crystal structure determinations were collected on a Siemens R3m/v diffractometer using graphite-monochromated Mo KR radiation with θ- and ω-scan methods for 3 and 4, respectively. Lorentz and polarization corrections were applied to the data. An empirical absorption correction based on ψ scans was applied in both cases. The intensities of 2135 (3) and 4318 (4) unique reflections were measured in the 2θ range 3.0-48.0°. The structure solutions (direct methods) and full-matrix leastsquares refinements based on F2 were perfomed using SHELXS/L programs of the SHELXTL-NT-5.1 program package. In both cases, all non-hydrogen atoms were refined
Neve and Crispini Table 1. Crystallographic Data for [Py-C12-Py][MBr4] (M ) Pd (3), Cu (4))
chem formula fw cryst syst space group (No.) a, Å b, Å c, Å β, deg V, Å3 Z Fcalcd, g cm-3 temp, K λ, Å µ(Mo KR), mm-1 collected data obsd data (I g 2σ(I)) parameters R1/wR2a (I g 2σ(I)) R1/wR2a (all data) GOF a
[Py-C12-Py][PdBr4] (3)
[Py-C12-Py][CuBr4] (4)
C22H34Br4N2Pd 752.55 monoclinic P21/c (No. 14) 8.939(2) 11.225(2) 14.077(3) 104.12(3) 1369.8(5) 2 1.825 293(2) 0.710 73 6.523 2280 1654 134 0.0481/0.1164 0.0662/0.1286 1.05
C22H34Br4CuN2 707.68 monoclinic P21/c (No. 14) 8.215(2) 18.448(4) 18.196(4) 96.74(3) 2738.6(9) 4 1.716 293(2) 0.710 73 6.645 4651 2190 279 0.0830/0.1884 0.1670/0.2340 0.99
R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.
anisotropically. Hydrogen atoms were included as idealized atoms riding on the respective carbon atoms with C-H bond lengths appropriate to the carbon atom hybridization. The bromine atoms of the [CuBr4]2- anion in 4 are disordered over two positions with calculated site-occupation factors of 0.843 and 0.157. Only the bromine atoms with the highest siteoccupation factor were refined anisotropically. Database Analysis. The search of the Cambridge Structural Database (April 2000 version) was conducted on the generic C-H fragment with C-H‚‚‚Br intermolecular contacts to any M-Br bond (where M is a transition metal). C-H distances were normalized to the standard neutron diffraction value of 1.083 Å (CSD default). The search was restricted to ordered and error-free structures (R factor
