Synthon robustness and solid-state architecture in substituted gem-alkynols

Abstract

Analysis of crystal packing of related compounds is a necessary ingredient of crystal engineering. We report here the crystal structures of 14 cyclic and fused ring gem-alkynols, compounds that are known for their supramolecular inconsistency because of the juxtaposition of two hydrogen-bond donors (O-H, C≡C-H) and two hydrogen-bond acceptors (O-H, C≡C-H) in the molecules. Because of the competition between these hydrogen-bond donors, or hydrogen-bond acceptors, the packing pattern in any particular crystal structure is highly sensitive to substitution by functional groups. The compounds studied here are methyl and chloro derivatives of the parent compounds 1a (benzo series, 1b-1f), 2a (naphtha series, 2b-2f), 3a and 4a (anthra series, 3b, 4b-4d). Structures 2a and 4a are prototypes because the O-H···O, C-H···O, O-H···∏ , and C-H···∏ hydrogen bonds form sheets in these cases with the interdigitating hydrocarbon residues perpendicular to the sheets. The hydrogen bonds are arranged to give centrosymmetric synthons I and II, which are noteworthy for their robustness. Orthogonality of the hydrogen-bonded and hydrocarbon regions in these crystals leads to structural insulation so that the addition of an extra fused ring in going from, say, 2a to 4a, leaves the structure unaltered. Many types of methyl substitution on these rings also preserve the packing so that it may be inferred that methyl substitution is supramolecularly akin to benzo annelation in these compounds. A new synthon [(I)_0.5(II)_0.5], which is a hybrid of synthons I and II, is also observed. This hybrid, H, is thermodynamically comparable to synthons I and II because it contains the same cooperative network of strong and weak hydrogen bonds, O-H···O-H···C≡C-H···C≡C-H···O-H···, which is the key structural element in this family. Chloro substitution is in general more aggressive than methyl substitution, and Cl/Me exchange does not operate in the benzene series. In contrast, Cl/Me exchange perturbs only some elements of the crystal packing in the naphthalene series. Also observed is a homology between crystal structures related by a naphtha/anthra exchange. Identifying robust supramolecular synthons and proving their repetition is a challenging task in a system as fragile as the gem-alkynols, and we note that prior to this work there was no repetition of a major hydrogen-bonded supramolecular synthon in the 144 gem-alkynols with published crystal structures. Also noteworthy is that synthons I, II, and H are fairly largethe real challenge in crystal engineering is to find a big enough synthon that occurs often enough. Large synthons that occur frequently constitute the most useful structural domains in crystal families, and we believe that we have identified such a domain in the gem-alkynol group of compounds.