CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 3 163-169
Articles On the Reliability of C-H‚‚‚O Interactions in Crystal Engineering: Synthesis and Structure of Two Hydrogen Bonded Phosphonium Bis(aryloxide) Salts Charlotte K. Broder,† Matthew G. Davidson,*,‡ V. Trevor Forsyth,§ Judith A. K. Howard,† Sarah Lamb,† and Sax A. Mason§ Department of Chemistry, University of Bath, Bath, BA2 7AY, United Kingdom, the Department of Chemistry, University of Durham, Durham, DH1 3LE, United Kingdom, and the Institut Laue Langevin, BP 156, 38042 Grenoble, Cedex 9, France Received January 24, 2002
W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: Two related phosphonium aryloxides, [Ph3PCH3]+2[CH2(C6H2-3,5-tert-butyl-4-O)2]2-, 1a, and [Ph3PC2H5]+2[CH2(C6H2-3,5-tert-butyl-4-O)2]2-, 1b have been synthesized by protonation of nonstabilized phosphorus ylides with 4,4′-methylenebis(2,6-di-tert-butylphenol) LH2. The crystal structures of 1a and 1b have been determined by single-crystal X-ray diffraction, and that of 1b has also been determined by low-temperature (20 K) neutron diffraction. Both crystallize as CH3CN solvates, and both exhibit polymeric supramolecular structures via extensive C-H‚‚‚O hydrogen bonding. The predominant structural motif is chelation of the aryloxide oxygen atom of the anion by phosphonium alkyl and aryl C-H groups, such as has been previously observed for phosphonium monoaryloxide salts. The use of this motif as a supramolecular synthon in crystal engineering is explored. In the case of 1a, in addition to the expected intermolecular C-H‚‚‚O hydrogen bonding pattern, C-H‚‚‚π interactions between the anion and this solvent are observed. In the structure of 1b, C-H‚‚‚π interactions between the cation and anion lead to an unpredicted supramolecular structure. The possible significance of this feature is discussed the context of the general use of weak hydrogen bonds in crystal engineering. Introduction Conventional “strong” hydrogen bonds (e.g., O-H‚‚‚O, N-H‚‚‚O, etc.) have long been recognized as being of fundamental importance in determining the supramolecular structure of organic solids.1,2 It has been shown that these interactions may be utilized in the deliberate design of organic solids, possessing controlled supramolecular structures with the intention that such materials may possess specific and useful chemical, physical, or optical propertiessso-called crystal engineering.3,9 In an attempt to rationalize and systemize the relationship between molecular and supramolecular structures, Desiraju has developed the concept of the supramolecular synthon.5,7 A supramolecular synthon may be a combi* Corresponding author: Professor M. G. Davidson, Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K. Tel: +44 1225 826443, Fax: +44 1225 826231, e-mail: m.g.davidson@ bath.ac.uk. ‡ University of Bath. † University of Durham. § The Institut Laue Langevin.
nation of complementary donor and acceptor groups that are recognizable at the molecular level and are able to generate reproducible well-defined noncovalent supramolecular interactions in the solid state. Whereas stronger hydrogen bonds are ideal tools in this context, weaker C-H‚‚‚X (where X ) N, O, π, etc.) interactions are more problematic.13 Although specific examples have been noted,10-12 the general application of C-H‚‚‚X based synthons in crystal engineering is open to question. The relative weakness of these interactions, their electrostatic and therefore nondirectional nature, and the availability of many potential C-H donor groups in most organic solids, are all factors that reduce the consistency and reliability of a particular C-H‚‚‚X interaction and mediate against their general use for crystal engineering. For example, Dance et al. have recently highlighted the “multiple phenyl embrace” as a persistent structural motif involving Ph3P (and related) groups and based on C-H‚‚‚π interactions.14-20 Despite involving up to six mutual interactions, being geometrically robust and relatively strong, a detailed
10.1021/cg025503q CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002
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Figure 2. Schematic representation of the anticipated monomeric (a) or polymeric (b) supramolecular structures for phosphonium salts derived from LH2. Cations are represented by light circles, anions by dark squares, and each arrow represents one chelating alkyl/aryl (C-H)2‚‚‚O interaction. Figure 1. Schematic representation of monomeric (a) and dimeric (b) structures exhibited by a range of phosphonium aryloxides (OAr ) OC6H2-2,4,6-Me3, OC6H3-2,6-Ph2, OC6H22,6-tBu2-4-Me; R ) H or Me).22 Cations are represented by light circles, anions by dark squares, and each arrow represents one chelating alkyl/aryl (C-H)2‚‚‚O interaction.
statistical analysis using the Cambridge Structural Database (CSD) recently suggested that “phenyl embraces” may be insufficiently reliable for general use in crystal engineering.21 We have also been interested in the supramolecular structure of phosphonium salts and have synthesized and characterized structurally a series of simple alkyltriphenylphosphonium salts containing organic anions (e.g., aryloxide, diarylamide, diarylphosphide). We have found them to be extensively aggregated via very short C-H‚‚‚X (where X ) O, N, or P) hydrogen bonds.22-24 Furthermore, all the phosphonium aryloxides we have characterized so far contain the same structural motif of chelation of the anionic hydrogen bond acceptor, by alkyl C-H and ortho-aryl C-H groups of the phosphonium cation [subsequently abbreviated to (C-H)2‚‚‚O in this paper]. We have found that the degree of aggregation can be controlled by varying the steric bulk of the cation and/or anion to give either monomeric or dimeric structures (Figure 1). To investigate the potential of using this seemingly persistent and strong (C-H)2‚‚‚O synthon in crystal engineering, we report in this paper the structural characterization of two phosphonium bis(aryloxide) salts synthesized by double deprotonation of the bis(phenol) 4,4′-methylenebis(2,6-di-tert-butylphenol), LH2 (Scheme 1). In light of our previous results, and the rich supramolecular chemistry of neutral bisphenols which have been shown to polymerize via strong hydrogen bonds,25-27 we reasoned that the structures of these phosphonium bisaryloxides would shed further light on the current debate concerning the suitability of C-H‚‚‚O interactions in crystal engineering. Specifically, we expected to generate either monomeric [Figure 2a] or polymeric [Figure 2b] supramolecular structures, depending on the steric requirements of the cation and anion.
Scheme 1. Synthesis of Bis(aryloxide) Phosphonium Salts
Experimental Section General Synthetic Procedures. All reactions and manipulations were carried out under an atmosphere of dry argon or dinitrogen gas using standard Schlenk and glovebox techniques. Solvents were freshly distilled over suitable drying agents and degassed prior to use. Triphenylphosphonium methylide and triphenylphosphonium ethylide were prepared as described in the literature.28 4,4′-methylenebis(2,6-di-tertbutylphenol), LH2, was purchased from Aldrich and used as received. 1H and 31P{1H} NMR spectra were recorded on either a Varian 200 or a Bruker AM 250 spectrometer. Synthesis of 1a. Ph3PCH2 (0.55 g, 2 mmol) and LH2 (0.42 g, 1 mmol) were stirred vigorously under vacuum in a Schlenk tube for 1 h. Dry acetonitrile (10 mL) was then added, resulting in the rapid precipitation of a pale red solid. On warming of the sample, the precipitate dissolved into in a clear dark red solution. Standing at room temperature yielded a crop of red crystals suitable for X-ray diffraction. Yield, 0.80 g (75%), m.p., 139 °C. Anal. Calc. % (found %) for C69H82P2O2: C, 82.5 (81.2); H 8.2 (8.8). 1H NMR (C6D6, 200 MHz, 25 °C, TMS) δ 1.3 (36H, s, tBu), 1.6 (3H d of t, Ph3PCH2Me), 3.8 (2H, s, CH2 of anion), 6.9 (4H, s ArH of anion), 7.7-7.9 ppm (30H, m, ArH of cation) (nB Ph3PCH2Me not observed). 31P{1H} NMR (C6D6, 101.2 MHz, 25 °C, 85% H3PO4) δ 26.6 ppm. Synthesis of 1b. Synthetic procedure as for 1a using 0.58 g (2.0 mmol) of Ph3PC(H)Me and 3 mL of acetonitrile. Crystals suitable for single-crystal neutron diffraction were grown by slow cooling of a more dilute acetonitrile solution. Yield, 0.60 g (62%), m.p., 162 °C. Anal. Calc. % (found %) for C65H78P2O2: C, 81.9 (79.9); H 8.3 (8.1). 1H NMR (CDCl3, 200 MHz, 25 °C, TMS) δ 1.4 (36H, s, tBu), 3.8 (2H, s, CH2 of anion), 7.0 (4H, s
Reliability of C-H‚‚‚O Interactions in Crystal Engineering Table 1. Summary of Crystallographic Data for Compounds 1a and 1b
emp form form wt crystal color crystal size, mm cryst system space group a, Å b, Å c, Å β, deg V, Å3 Z T, K Dcalc, g cm-3 µ, mm-1 λ, Å
(1a‚1.5CH3CN)2
1b‚CH3CN (X-ray)
1b‚CH3CN (neutron)
C140H165N3O4P4 2077.62 red 0.5 × 0.4 × 0.4
C73H88N2O2P2 1087.39 red 0.7 × 0.6 × 0.2
C73H88N2O2P2 1087.39 red 3.9 × 2.7 × 2.6
orthorhombic P212121 18.0475(2) 18.7762(2) 19.1060(2) 90 6474.3(1) 4 100(2) 1.116 0.112 0.71073 (Mo KR) 55.2 1.115 0.0346 0.0852
orthorhombic P212121 17.8957(4) 18.6462(4) 18.9466(4) 90 6322.2(1) 4 20(2) 1.142 2.56 1.3099(1) [Ge(115)] 121 1.067 0.0605 0.1518
monoclinic P21/c 37.233(7) 19.987(4) 17.011(3) 97.82(3) 12 541(4) 4 150(2) 1.100 0.113 0.71073 (Mo KR) 2θmax, deg 55 GOF (F2) 1.041 R1 [I > 2σ(I)] 0.0651 wR2 (all data, 0.1771 F2)
ArH of anion), 7.7-7.8 ppm (30H, m, ArH of cation) (nB Ph3PCMe not observed). 31P{1H} NMR (C6D6, 80.9 MHz, 25 °C, 85% H3PO4) δ 22.9 ppm. X-ray Structures of 1a and 1b. Crystals suitable for an X-ray structural determination were obtained as described above. Crystals were removed directly from solution under a stream of dinitrogen and coated in an inert perfluorinated oil prior to mounting on a glass fiber and immediate transfer to the diffractometer. Details of the data collection and refinement are shown in Table 1. The data were measured on a Siemens SMART 3-circle diffractometer with a CCD area detector using graphite-monochromated Mo KR radiation and equipped with an Oxford Cryosystems cryostream. Intensities were measured using ω scans. The structures were solved by direct method, and refinement, based on F2 of all data, was by full matrix least-squares techniques, using SHELX-97.29 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were either located using a difference map or placed in idealized positions and allowed to ride on heavy atoms. Neutron Structure of 1b. A single crystal of dimensions 3.85 × 2.70 × 2.60 mm was mounted in an argon-filled glovebox onto an aluminum pin using a low-temperature epoxy resin, and sealed under a thin-walled quartz dome to maintain
Crystal Growth & Design, Vol. 2, No. 3, 2002 165 the inert atmosphere around the crystal during subsequent measurement. This sample was later mounted on a Displex cryorefrigerator30 on the thermal-beam instrument D19 at ILL equipped with a 4° × 64° position-sensitive detector.31 The crystal was cooled slowly (2 degrees/min) to 20 K while monitoring a strong reflection, and no change in mosaic was observed. The unique reflections to 2θ ) 40° were measured in equatorial geometry with omega scans, with 2 to 3 s per step. The space group P212121 was confirmed at 20 K. Higher angle data were recorded with either normal-beam or equatorial geometry (ILL programs HKLGEN and MAD) up to 2θ ) 121°. Three standard reflections were monitored regularly and showed no significant variation. A total of 17 603 reflections were measured, 6777 of which were unique [R(int) ) 0.0622]. The unit cell dimensions were calculated from 6497 strong reflections in the range 5.65° < 2θ < 103° (ILL program RAFD19). Bragg intensities were integrated in 3-d using the ILL program RETREAT.32 The intensities were corrected for attenuation by the cylindrical heat shields (minimum and maximum transmission coefficients 0.8170 and 0.8961, respectively), and by the crystal itself (minimum and maximum transmission coefficients 0.4842 and 0.6182, respectively) with the program D19ABS, based on the ILL version of the CCSL system.33 Refinement, using the X-ray structure as a starting point, and based on F2 of all data, was by full matrix leastsquares techniques, using SHELX-97.29 The coherent scattering amplitudes used in the refinement were those tabulated by Sears.34 All atoms were refined anisotropically. Further details of the data collection and refinement are shown in Table 1.
Results and Discussion Synthesis and Structure of 1a‚1.5CH3CN. The reaction of two equivalents of triphenylphosphonium methylide with LH2 in acetonitrile solution led to the immediate precipitation of a red solid, which on warming redissolved into a red solution. From this solution, a high yield of X-ray quality crystals was recovered after standing at room temperature overnight. Preliminary characterization of this solid revealed the expected double deprotonation of the phenol (Scheme 1). A singlecrystal X-ray structure confirmed that the diphosphonium bis(aryloxide) [Ph3PCH3]+2[CH2(C6H2-3,5-tBu-4O)2]2-‚1.5CH3CN, 1a‚1.5CH3CN had been synthesized and isolated as an acetonitrile solvate. The crystallographic asymmetric unit of 1a‚1.5CH3CN contains four cations and two anions and is shown in Figure 3. All four cations are of similar and unremarkable geom-
Figure 3. The asymmetric unit of 1a‚1.5CH3CN; aryl H atoms not involved in hydrogen bonding, and tert-butyl methyl groups omitted for clarity. Selected bond lengths and nonbonded distances (Å): P1-C100, 1.791(2); P2-C200, 1.789(2); P3-C300, 1.780(2); P4-C400, 1.787(2); O501-C511, 1.299(3); O502-C521, 1.307(3); O601-C611, 1.307(3); O602-C621, 1.301(3); C100‚‚‚O501, 3.428(3); C200‚‚‚O601, 3.281(3); C200‚‚‚O601′, 3.439(3); C212‚‚‚O601′, 3.623(2); C222‚‚‚O601, 3.458(2); C300‚‚‚O501, 3.151(3); C300‚‚‚O602, 3.103(3); C336‚‚‚O501, 3.117(2); C400‚‚‚O502, 3.316(3); C400‚‚‚O502′, 3.251(3); C426‚‚‚O502′, 3.517(2); C436‚‚‚O502, 3.726(2). W A 3D rotatable image in XYZ format is available.
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Figure 5. Anion-CH3CN host-guest interactions in the structure of 1a‚1.5CH3CN. Nonbonded C‚‚‚X distances (Å), where X is the centroid of the relevant aryl ring: C12X‚‚‚X, 3.405(4); C12X‚‚‚X, 3.362(4); C22X‚‚‚X, 3.334(6); C32X‚‚‚X, 3.395(7). W 3D rotatable images of W (a) and W (b) in XYZ format are available.
Figure 4. Schematic representation of the polymeric supramolecular structure of 1a. Cations are represented by light circles, anions by dark squares. Arrows and broken lines represent short and longer-range C-H‚‚‚O interactions, respectively.
etries. Those of the previously uncharacterized anion, two of which are present in the asymmetric unit, differ from each other and will be discussed in more detail below. Of particular interest in the context of this paper is the supramolecular structure of 1a‚1.5CH3CN. Cations and anions associate via the (C-H)2‚‚‚O interactions leading to the anticipated formation of polymeric chains of cations and anions. However, the actual polymeric structure is more complex than expected. Of the four phosphonium cations in the asymmetric unit, two (those containing P2 and P4) participate in the anticipated centrosymmetric “dimeric” structural motif mediated by short (C-H)2‚‚‚O interactions. The other two cations (those containing P1 and P3) are both involved in the same asymmetric link. However, only one cation (that containing P3) is able to participate in the expected short (C-H)2‚‚‚O interactions. The close approach of the cation containing P1 is sterically hindered by two cisoid
anions, resulting in fewer, longer-range C-H‚‚‚O interactions involving only its methyl group. A schematic representation of this complex polymeric supramolecular arrangement is shown in Figure 4. The conformations of the two anions in 1a‚1.5CH3CN differ, and both are solvated by either one or two CH3CN molecules via short C-H‚‚‚π interactions between the C-H acidic methyl group of the solvent and the electron rich aryl rings of the anion (Figures 3 and 5). One anion (that containing C520) is in a “butterfly” conformation (approximate C2v symmetry) and the other (that containing C620) is in a “propeller” conformation (approximate C2 symmetry). The cleft created by the two aryl rings of the butterfly anion acts as a host for only one guest solvent molecule [Figure 5a], while the more open propeller anion interacts with two solvent molecules [Figure 5b]. It is not clear whether the degree of CH3CN solvation is the significant factor in determining the geometry of the dianions or whether it is simply a consequence of the two conformations which are determined by other factors. Thus, 1a‚1.5CH3CN conforms, at least in general terms, to the polymeric supramolecular structure anticipated in Figure 2b, although a deviation from the expected structure involving disruption of the (CH)2‚‚‚O supramolecular synthon casts some doubt on its integrity.
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Figure 6. Neutron structure of 1b‚CH3CN, thermal ellipsoids at the 60% probability level, aryl H atoms not involved in hydrogen bonding, tert-butyl methyl groups and lattice CH3CN omitted for clarity. Selected bond lengths (Å) and angles (°): P1-C101, 1.802(6); P2-C201, 1.815(6); O1-C11, 1.295(6); O2-C21, 1.302(5); C20-C14, 1.509(5); C20-C24, 1.522(5); P1-C101-C102, 111.6(3); P2-C201-C202, 113.6(3); C14-C20-C24, 115.5(3). W A 3D rotatable image in XYZ format is available.
Synthesis and Structure of 1b‚CH3CN. By increasing the steric demand of the cation, it was envisaged that the bis(aryloxide) structural analogue, containing two (C-H)2‚‚‚O synthons, of the known monomeric aryloxide could be isolated [Figure 2a]. Reaction of triphenylphosphnium ethylide with LH2, again in acetonitrile solution, resulted in the precipitation of a red crystalline solid. Preliminary characterization and an X-ray structure confirmed this to be [Ph3PC2H5]+2[CH2(C6H2-3,5-tBu-4-O)2]2-‚CH3CN, 1b‚CH3CN (Scheme 1). Large crystals were subsequently grown by slow cooling of a dilute acetonitrile solution, and a low temperature (20 K) single-crystal neutron diffraction study was carried out (Figure 6). Structural parameters used in the subsequent discussion are taken from these neutron data. The molecular parameters within both the cation and anion are as expected and the bisaryloxide moiety adopts a butterfly conformation similar to that found in one anion of 1a. In contrast to the C-H‚‚‚π bonded solvent in 1a, the CH3CN of 1b resides in the lattice and has no short contacts to the cation or the anion. The reason for this lies in the mode of supramolecular aggregation of the cation. One phosphonium cation (containing P2) interacts with O2 of the bisaryloxide anion atom via the now normal (C-H)2‚‚‚O motif, but, unexpectedly, the mode of aggregation of the second cation (containing P1) deviates from that predicted on the basis of previous results. Rather than chelating the second aryloxide group, this cation instead sits in the cleft formed by the two aryl rings of the anion and interacts via two short C-H‚‚‚π interactions. This mode of interaction is similar to that involving the CH3CN solvent in 1a, and is also topologically similar to the Ar‚‚‚H-X-H‚‚‚Ar interactions found in water (X ) O)35 and dichloromethane (X ) C)36 inclusion complexes of calixarenes and in ammonium tetraphenylborate (X ) N).37 Despite being “charge-assisted”, the distances from the cation to the aryl rings of the anion are slightly greater than those of the CH3CN-anion interactions found in 1a, presumably reflecting the greater steric
bulk of the Ph3PEt+ cation. The C-H‚‚‚π bonded cation interacts further, with O1 of a neighboring anion via one aryl para C-H group, completing the polymeric supramolecular structure shown in Figures 7 and 8. Although until now the (C-H)2‚‚‚O supramolecular synthon had proved reliable, its use to predict the structure of 1b fails. This highlights difficulty in applying the principles of predictive crystal engineering to relatively weak intermolecular interactions. A number of factors lead to the possibility of many supramolecular arrangements with similar energies being possible in our system, making the “correct” structure difficult to predict. First, the alkyl(triaryl)phosphonium group is very flexible C-H donor. Almost any of its C-H groups can participate in short intermolecular interactions.38 Second, the predicted C-H‚‚‚O interactions are weak, providing only a small differential in energy between the predicted and the other potential modes of aggregation, including that observed. Third, the bis(aryloxide) anion is a stereochemically flexible host. The former two of these observations are relevant to C-H‚‚‚X interactions in general. The Hydrogen Bond Parameters in 1b‚CH3CN. The determination of the structure of 1b‚CH3CN by single-crystal neutron diffraction allows accurate location of all hydrogen atoms and full characterization of their interactions in the solid state. The geometries of all H-bond parameters are given in Table 2. All C-H‚‚‚O interactions are unusually short with the C‚‚‚O and H‚‚‚O lengths being at the lower end of the normal range,39 while the C-H‚‚‚O angles are within the expected range. The shortness of the hydrogen bonds reflects a combination of good hydrogen bond donors and highly basic aryloxide anion acceptors present in 1b as has been noted for other phosphonium aryloxides.22-24 The C-H bond lengths of the donor groups are of significance, particularly for such short C-H‚‚‚O interactions. Elongation of C-H (as has been observed in stronger hydrogen bonds, e.g., O-H‚‚‚O40) would imply some degree of covalent three-center-four-electron bonding. Previously, consideration of the geometry of C-H
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Figure 7. The supramolecular structure of 1b‚CH3CN; aryl H atoms not involved in hydrogen bonding, and tert-butyl methyl groups omitted for clarity. W A 3D rotatable image in XYZ format is availble.
Figure 8. Schematic representation of the polymeric supramolecular structure of 1b. Cations are represented by light circles, anions by dark squares. Arrows and broken lines represent C-H‚‚‚O and C-H‚‚‚π interactions, respectively. Table 2. Geometrical Parameters for Selected C-H and Hydrogen Bonds Obtained from Neutron Data for 1b‚CH3CNa interaction
C-H distance
C‚‚‚X distanceb
H‚‚‚X distanceb
C-H‚‚‚X angleb
C201-H21a‚‚‚O2 C201-H21b C226-H226‚‚‚O2 C115-H115‚‚‚O1′c C101-H11a‚‚‚ π C101-H11b‚‚‚ π
1.105(8) 1.099(8) 1.084(9) 1.089(9) 1.084(9) 1.084(8)
3.200(5)
2.117(8)
165.7(6)
3.321(5) 2.911(6) 3.491 3.466
2.245(9) 1.981(9) 2.464d 2.488e
157.6(6) 141.2(6) 157.6 149.1
a Distances in Å, angles in degrees. b Where X ) O or the centroid of the relevant aryl ring. c Symmetry transformation used to generate O1′: -x, 1/2 + y, 1/2 - z. d Range of H‚‚‚C distances: 2.606(9) - 3.090(9) Å. e Range of H‚‚‚C distances: 2.741(9) 2.992(8) Å.
donor groups within hydrogen bonds has been limited, although a correlation between C-H bond length and H‚‚‚O distance was noted for a series of neutron structures of amino acids,41 and a short acetylenic C-H‚‚‚O interaction was reported to exhibit a lengthened C-H bond.11 In 1b, of the four crystallographically unique C-H bonds associated with R-C atoms of the phosphonium groups, only one is involved in a short C-H‚‚‚O interaction (C201-H21a), and this is indeed slightly longer than the other three, which are either
not hydrogen bonded or are involved in longer-range C-H‚‚‚π interactions (see Table 2). However, the degree of lengthening is at the limit of experimental precision and is also in close agreement with a “corrected” value of 1.103 Å recently suggested for alkyl C-H bonds on the basis of variable-temperature neutron experiments.42 Thus, it seems that any elongation in the C-H bonds is very slight in these short charge-assisted hydrogen bonds, highlighting their overwhelmingly electrostatic nature. The interactions between the alkyl C-H groups of the second cation and the aryl rings of the anion are to our knowledge the first neutron determination of a C-H‚‚‚π interaction involving an acidic alkyl group. As expected, they are longer-range than the C-H‚‚‚O hydrogen bonds but again are toward the lower limit of other C-H‚‚‚Ar contacts found in the Cambridge Structural Database.43 The H atoms are not located centrally over the aryl rings but are closer to the methylene bridge of the dianion than they are to the aryloxide fragment. This probably reflects a mismatch in the geometries of the phosphonium and aryloxide groups rather than any particular preference for an interaction with this part of the host ion. Conclusions In this work, we have shown that the synthetic strategy of deprotonation of phenols with phosphonium ylides can be extended beyond monofunctional examples and bis(phenols) can be doubly deprotonated to give phosphonium bis(aryloxide) salts yielding polymeric supramolecular structures. In the two crystal structures we have determined, which include accurate low-temperature neutron data, the structural motif of multiple C-H‚‚‚O interactions between cations and anions involving alkyl and ortho-aryl CH groups of the cation is reproduced persistently as we had envisaged. However, the difficulty in achieving precise control over structures using this motif, and C-H‚‚‚O interactions in general, is emphasized by the unpredictable and competing feature of C-H‚‚‚π interactions between the anions and either acetonitrile solvent or cations. Our results reinforce the view that, while C-H‚‚‚X interactions often
Reliability of C-H‚‚‚O Interactions in Crystal Engineering
have a significant influence on the supramolecular structure of organic solids, their flexibility and inherent weakness make their general use in predictive crystal engineering uncertain. Acknowledgment. We thank the EPSRC for a studentship (C.K.B.) and a Senior Research Fellowship (J.A.K.H.), the Royal Society for an Industry Fellowship (M.G.D.), and the ILL for beam time and for the support of V.T.F. as a Senior Visiting Scientist on secondment from the Physics Department, Keele University. Supporting Information Available: X-ray crystallographic information files (CIF) are available for compounds 1a (X-ray data) and 1b (X-ray and neutron data). This material is available free of charge via the Internet at http:// pubs.acs.org.
References (1) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press Inc: New York, 1997. (2) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1999. (3) For recent reviews on the use of hydrogen bonds in crystal engineering see refs 4-8. (4) Row, T. N. G. Coord. Chem. Rev. 1999, 183, 81. (5) Desiraju, G. R. Curr. Opin. Solid State Mater. Sci. 1997, 2, 451. (6) Aakeroy, C. B. Acta Crystallogr., Sect. B 1997, 53, 569. (7) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311. (8) Subramanian, S.; Zaworotko, M. J. Coord. Chem. Rev. 1994, 137, 357. (9) For specific application of C-H‚‚‚X interactions to crystal engineering, see refs 10-12. (10) Thalladi, V. R.; Goud, B. S.; Hoy, V. J.; Allen, F. H.; Howard, J. A. K.; Desiraju, G. R. Chem. Commun. 1996, 401. (11) Allen, F. H.; Howard, J. A. K.; Hoy, V. J.; Desiraju, G. R.; Reddy, D. S.; Wilson, C. C. J. Am. Chem. Soc. 1996, 118, 4081. (12) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1999, chapter 4. (13) Indeed, even the existence of some weaker hydrogen bonds as distinct from van der Waals interactions is still questioned in the recent literature. See for example Steiner, T.; Desiraju, G. R. Chem. Commun. 1998, 891; Mascal, M.; Marjo, C. E.; Blake, A. J. Chem. Commun. 2000, 1591. (14) Lewis, G. R.; Dance, I. J. Chem. Soc., Dalton Trans. 2000, 299. (15) Lewis, G. R.; Dance, I. J. Chem. Soc., Dalton Trans. 2000, 3176. (16) Scudder, M.; Dance, I. J. Chem. Soc., Dalton Trans. 1998, 329.
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