Cocrystallization with Acetylene: Molecular Complex with Methanol Michael T. Kirchner, Dinabandhu Das,† and Roland Boese* Institut für Anorganische Chemie der UniVersität Duisburg-Essen, 45117 Essen, Germany ReceiVed February 23, 2007; ReVised Manuscript ReceiVed NoVember 17, 2007
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 763–765
ABSTRACT: Methanol forms a cocrystal with acetylene, of which single crystals were grown in situ. The methanol chains of the β-methanol are retained, and acetylene is intercalated between the layers. The weight ratio of host to guest is 1.23, the lowest and most efficient value for all known acetylene cocrystals. Further, the first single crystal structure for the low-temperature R-form of methanol is reported confirming previous neutron powder data. For the β-form of methanol, the space group Cmc21 is favored in contrast to former data in Cmcm. Chemical co-condensation or cocrystallization of two chemical entities is an alternative to physical condensation for the storage and transport of gaseous fuel such as hydrogen or small hydrocarbons.1 Instead of pressurizing or lowering of temperature, the use of a highly self-binding coagent leads to host networks with cavities where guest molecules are incorporated. Clathrates with methane are a prominent example.2 Channels can also be filled with guests, and intercalation is possible with layered structures. Such host–guest structures usually have nonspecific and nondirectional interactions. This is in contrast to the binding situation in molecular complexes, as it was found in the complex of acetone with acetylene, where additional ≡CH · · · O hydrogen bridges add specificity.3 For storage and transportation purposes, the weight ratio of host to guest is of some concern. For acetylene, a key compound in organic synthesis, this is of major importance, because acetylene cannot be compressed in pure form without risk of a highly exothermal reaction.4 In the gas hydrate of acetylene and in the molecular complex with acetone the weight ratio is unfavorably high. The maximum mole (and weight) ratio, respectively, for water-acetylene is 46:8 (3.98), for acetone-acetylene is 2:1 (4.46) or 1:1 (2.23), and for dimethyl sulfoxide (DMSO)-acetylene is 1:2 (1.50). Among the potential partners for the formation of cocrystals with acetylene, methanol is the smallest and most lightweight molecule. It has hydrogen bond acceptor and networking abilities with an OH group similar to water and is expected to form a stable entity. Neat methanol is polymorphic. The β-phase exists between the melting point at 175.37 K and a phase transition at 156–159 K;5 below, the R-phase is formed.6 An intermediate phase has been proposed by some authors based on calorimetric and NMR experiments, while others suggest that anomalies are due to formation of an eutectic with water impurities.7 Beside these phases, a high pressure phase above 4.0 Gpa and an amorphous phase formed by fast cooling are known.8 A hexagonal cell without structural data has been reported.9 The low temperature R-phase (P212121) has been so far characterized by powder diffraction with a deuterated sample only.5 We could grow a single crystal and redetermine the structure of the same phase, 1.10 Crystallization was performed by in situ zone melting of the nondeuterated methanol when trying to cocrystallize with allene.11 The cell constants and packing of deuterated and nondeuterated methanol are comparable. The differences in volume of 3% reflect the respective temperatures of measurement (15 K/122 K). Methanol molecules form chains of OH · · · O hydrogen bridges down [100] (d ) 1.714 Å, D ) 2.693 Å, θ ) 174°) (Figure 1a). * To whom correspondence should be addressed. E-mail: roland.boese@ uni-due.de. Web: http://www.structchem.uni-due.de/. Tel.: +49201183-2416. Fax: +49201183-2535. † On leave from School of Chemistry, University of Hyderabad, India.
Figure 1. Chain of methanol molecules (a) along [100] in 1, (b) along [001] in 2 (disorder not shown), and (c) along [100] in 3.
Each chain is surrounded by six others and is tilted against neighboring chains in [001], leading to weak CH · · · O interactions (d ) 2.616 Å, D ) 3.622 Å, θ ) 154°) (Figure 2a). The high-temperature β-phase was determined as early as 1952 by Tauer and Lipscomb applying the space group Cmcm.5 Since molecular mm symmetry requires a complicated disorder in two directions, we redetermined the structure and here present a solution in Cmc21, 2, with consistent systematic extinctions and disorder only of the O-atom out of the plane.12,13 This could not be modeled by the methods available in 1952. Again, the methanol molecules are arranged in chains with OH · · · O hydrogen bridge (d ) 1.75 Å, D ) 2.71 Å, θ ) 165°; large errors by disorder), (Figure 1b). One chain is surrounded by six further chains, but in contrast to the low temperature R-phase here the chains are not tilted against each other (Figure 2b). As a result chains in [010] interdigitate with their
10.1021/cg0701877 CCC: $40.75 2008 American Chemical Society Published on Web 02/09/2008
764 Crystal Growth & Design, Vol. 8, No. 3, 2008
Communications contact of the other H-atom of acetylene is to the π-system of another acetylene molecule, but the distance is above the sum of van der Waals radii (d ) 2.823 Å, D ) 3.857 Å, θ ) 160°).14 This is in contrast to the 1:2 molecular complex of methanol with dibromine which binds to two molecules of methanol.15 The relationship of the β-phase of methanol and the complex with acetylene is emphasized by a comparison of the cell parameters. The cell axes representing the methanol layer are comparable in length 4.647 Å/4.565 Å and 7.220 Å/7.3135 Å. Perpendicular to the layer the distance is increased from 6.401 to 13.165 Å to accommodate the acetylene molecules (cf. Figure 2b,c). The weight ratio of host to guest is 1.23 in 3, which is more efficient than in any other cocrystal with acetylene. The strong hydrogen bridge containing layers of methanol are preserved, and the acetylene is intercalated between them with weaker hydrogen bonds. Surprisingly, only one side of the acetylene is active. The cocrystal has features of both, the host network of strong hydrogen bridges enclathrating guests and the molecular complex where each guest is strongly attached to another molecule. A careful crystal engineering considering weak and strong interactions also might prove useful for other systems relevant for gas storage.
Acknowledgment. R.B. and M.T.K. thank the DFG FOR 618 for financial support. We thank the DFG/DST PPP for financial support and G. R. Desiraju for discussion. Supporting Information Available: Crystallographic data of 1, 2, and 3 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 2. View down the chains, (a) in 1 with chains tilted against each other, (b) in 2 (disorder not shown), (c) in 3 with intercalation of acetylene between methanol layers.
methyl groups, while neighboring chains in [100] are translated by half a unit cell in [010]. CH · · · O distances are longer than in the R-phase (d ) 2.7 Å, D ) 3.8 Å, θ ) 166°). Methanol and acetylene could be readily co-condensed at 98 K in a capillary, attached to a high-vacuum line. Subsequent transfer to a diffractometer and application of the in situ crystallization technique at 161 K with an IR-laser as described earlier3 gave single crystals of 3 in the space group P212121. The asymmetric unit contains one molecule of methanol and acetylene each. As in the β-phase, methanol molecules form chains in [100] with OH · · · O hydrogen bridges (d ) 1.690 Å, D ) 2.666 Å, θ ) 171°), and the chains in [010] interdigitate. As a result, methanol maintains the layer packing when cocrystallized. The acetylene molecules are intercalated in the otherwise intact methanol β-structure (Figure 2c), and hydrogen bridged to the O-atoms (d ) 2.399 Å, D ) 3.358 Å, θ ) 147°), perpendicular to the chain. When compared to the ≡CH · · · O distances in the molecular complex with acetone (1:2 D(C · · · O) ) 3.172 Å; 1:1 D(C · · · O) ) 3.246 Å/3.430 Å) the hydrogen bridge is longer and therefore weaker. Interestingly, only one acetylene H-atom is used for a hydrogen bridge with methanol in the 1:1 molecular complex. The closest
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