Correspondence between Molecular Functionality and Crystal Structures. Supramolecular Chemistry of a Family of Homologated Aminophenols

The crystal structures and packing features of a family of 13 aminophenols, or supraminols, are analyzed and correlated. These compounds are divided into three groups:  (a) compounds <b>1</b>−<b>5 </b>with methylene spacers of the general type HO−C<sub>6</sub>H<sub>4</sub>−(CH<sub>2</sub>)<i><sub>n</sub></i>−C<sub>6</sub>H<sub>4</sub>−NH<sub>2</sub> (<i>n</i> = 1 to 5) and both OH and NH<sub>2</sub> in a <i>para</i> position; (b) compounds <b>1a</b>, <b>2a</b>, <b>2b</b>, <b>2c</b>, and <b>3a</b> in which one or more of the methylene linkers in <b>1</b> to <b>3</b> are exchanged with an S-atom; and (c) compounds <b>2d</b>, <b>1b</b>, and <b>6a </b>prepared with considerations of crystal engineering and where the crystal structures may be anticipated on the basis of structures <b>1</b>−<b>5</b>,<b> 1a</b>, <b>2a</b>, <b>2b</b>, <b>2c</b>, and <b>3a</b>. These 13 aminols can be described in terms of three major supramolecular synthons based on hydrogen bonding between OH and NH<sub>2</sub> groups:  the tetrameric loop or square motif, the infinite N(H)O chain, and the β-As sheet. These three synthons are not completely independent of each other but interrelate, with the structures tending toward the more stable β-As sheet in some cases. Compounds <b>1</b>−<b>5 </b>show an alternation in melting points, and compounds with <i>n</i> = even exhibit systematically higher melting points compared to those with <i>n</i> = odd. The alternating melting points are reflected in, and explained by, the alternation in the crystal structures. The <i>n</i> = odd structures tend toward the β-As sheet as <i>n</i> increases and can be considered as a <i>variable series</i> whereas for <i>n</i> = even, the β-As sheet is invariably formed constituting a <i>fixed series</i>. Substitution of a methylene group by an isosteric S-atom may causes a change in the crystal structure. These observations are rationalized in terms of geometrical and chemical effects of the functional groups. This study shows that even for compounds with complex crystal structures the packing may be reasonably anticipated provided a sufficient number of examples are available.