A Persistent Preference for Layer Motifs in Self-Assemblies of Squarates and Hydrogen Squarates by Hydrogen Bonding [X−H···O; X = N, O, or C]: A Crystallographic Study of Five Organic Salts

The synthesis and single-crystal X-ray structures of hydrogen
squarates of imidazole (1), histidine
(2),
and 2-aminopyridine (4) and squarates of histidine
(3) and 4-aminopyridine (5) are reported.
Both hydrogen squarate
[HSQ]1- and squarate
[SQ]2- ions participate in complementary
hydrogen bonding [X−H···O; X = N, O, or C]
networks. While the [HSQ]1- ion acts
both as a donor and as an acceptor of hydrogen bonds in salts
1, 2, and 4 and
creates head-to-tail infinite chains of anions in 1 and
head-to-head ten-membered cyclic dimers in 4, the
[SQ]2- ion
participates as a powerful acceptor of as many as six hydrogen bonds in
3 and 5. In all cases, extensive
hydrogen
bonding and ionic interactions between the components lead to layered
assemblies. In 1, the cross-linking of the
[HSQ]1- chains by protonated imidazolium
ions through N−H···O bonds creates a two-dimensional ionic
sheet
which is further stabilized by the presence of C−H···O bonds
resulting from the participation of the imidazolium
C-H groups. In 2 and 3, the layered
structures are formed by cross-linking of the cationic self-assemblies
(His-His
infinite helices in 2 and His-His zigzag ribbons in
3) by [HSQ]1- and
[SQ]2-, respectively. A novel feature
in 3 is
the presence of two C−H···O bonds between the more acidic CH
group and the O1 in [SQ]2-. In
4, the cyclic
dimers of [HSQ]1- are interwoven into a
two-dimensional sheet with a checker-board motif by the participation
of
2-aminopyridinium cation in horizontal and vertical N−H···O
bonds. The two-dimensional layered assembly in
5
arises by extensive participation of water molecules in cross-linking
of the alternating anion−cation conjugates both
in horizontal and vertical directions. Irrespective of the
presence of different types of hydrogen-bonding patterns
in
the structures of 1−5, the persistent formation
of a two-dimensional layered assembly is significant and may be
a
characteristic of squaric acid salts.