The construction of organic–inorganic hybrid compounds has been of considerable interest and importance in recent years, not only because they are a powerful means of generating interesting supramolecular frameworks but also due to their potential for providing new materials with magnetic, semiconducting, optical and electrolytic properties (Doyle et al.
; Zaccaro & Ibanez, 2000
; Chisholm & Haile, 2000
). The supramolecular frameworks of these organic–inorganic compounds are generated by hydrogen-bond interactions between donor (D
) and acceptor (A
) moieties. Orthophosphoric acid (H3
), an inorganic oxy-acid, forms dihydrogen phosphate salts with organic amines, resulting in organic–inorganic hybrid systems with potentially powerful hydrogen-bonded D
moieties. The dihydrogen phosphate anions (H2
) form substructures in these compounds, generating anionic networks via
O hydrogen bonds which act as a template for the assembly of cations (Shylaja et al.
). A considerable number of dihydrogen phosphate salts are recorded in the Cambridge Structural Database (CSD, Version 5.28; Allen, 2002
). In the crystal structure of benzylammonium dihydrogen phosphate monohydrate (Elaoud et al.
), the H2
anions form a one-dimensional chain network, while in 3-amino-2-chloropyridinium dihydrogen phosphate (Hamed et al.
) they form chains of fused
(8) ring motifs [for graph-set analysis, see Bernstein et al.
)]. Two-dimensional nets of anionic substructures were also observed in dimethylammonium dihydrogen phosphate (Pietraszko et al.
) and 2-methylpiperazinediium dihydrogen phosphate (Choudhury et al.
). Interestingly, in the structure of imidazolinium dihydrogen phosphate (Blessing, 1986
), the H2
anions form a three-dimensional cage-type framework inside which the imidazolinium cations are trapped. We have prepared the dihydrogen phosphate salts 2-chloroanilinium dihydrogen phosphate (2CADHP
) and 4-chloroanilinium dihydrogen phosphate (4CADHP
), and have determined their structures and studied the supramolecular networks in these salts.
The salt 2CADHP
crystallizes in the space group P
, whereas 4CADHP
crystallizes in Pbca
. The asymmetric units of both 2CADHP
contain a dihydrogen phosphate anion and a singly protonated 2- or 4-chloroanilinium cation, respectively. In the tetrahedral dihydrogen phosphate group of both 2CADHP
, the protonated P—O bond distances are P1—O1 = 1.5696 (11) Å and P1—O2 = 1.5529 (12) Å for 2CADHP
, and P1—O1 = 1.541 (2) Å and P1—O2 = 1.557 (2) Å for 4CADHP
. These values are as expected and are longer than the other two P—O bonds, viz.
P1—O3 = 1.5036 (12) Å and P1—O4 = 1.5031 (12) Å for 2CADHP
, and P1—O3 = 1.5168 (17) Å and P1—O4 = 1.4957 (19) Å for 4CADHP
. The identical P1—O3 and P1—O4 bond distances observed in 2CADHP
indicate delocalization of negative charge between them (Demir et al.
). The geometries of the 2- and 4-chloroanilinium cations show characteristic values compared with other reported structures (Muthamizhchelvan et al.
; Glidewell et al.
). The C—N distances of the 2- and 4-chloroanilinium cations [C1—N1 = 1.4545 (18) and 1.467 (3) Å, respectively] are longer than the neutral C—NH2
value [1.386 (4) Å; Ploug-Sørenson & Andersen, 1985
] and this lengthening is due to the transfer of an H atom to the N atom from the orthophosphoric acid.
The hydrogen-bonded organic–inorganic supramolecular frameworks of 2CADHP
are determined primarily by a combination of O—H
O and N—H
O hydrogen bonds (Tables 1 and 2).
Table 1 Hydrogen-bond geometry (Å, °) for 2CADHP
Table 2 Hydrogen-bond geometry (Å, °) for 4CADHP
, the inversion-related H2
anions are linked through an O1—H1D
hydrogen bond [symmetry code: (iii) −x
+ 1, −y
+ 1, −z
+ 1], forming an O—H
O hydrogen-bonded dimer with a ring motif of
(8), with its centroid occupying the inversion centre. These dimers are interlinked through an O2—H2D
hydrogen bond [symmetry code: (ii) x
+ 1, z
] to form a ring motif of type
(12). The alternately fused
(12) supramolecular motifs in turn generate a double-stranded inorganic H2
chain made of P—OH
O=P hydrogen bonds extending infinitely along the  direction (Fig. 3). The 2-chloroanilinium cations are linked to the anionic substructure through three N—H
O hydrogen bonds and a Cl
O short contact [Cl
O = 3.1705 (14) Å]. The N1—H1A
O4 and N1—H1C
hydrogen bonds [symmetry code: (ii) x
+ 1, z
], along with O2—H2D
, form a chain of edge-fused
(10) ring motifs extending along the  direction, as observed in the structure of 3-acetylanilinium dihydrogen phosphate (Cinčić & Kaitner, 2008
). The Cl
O1 interaction, which acts as a pseudo-hydrogen bond (Bryant et al.
; Kubicki & Wagner, 2007
), with the Cl1 atom at (x
) as donor and atom O1 at (−x
) as acceptor, along with the N—H
O hydrogen bonds, forms a chain of fused
(10) motifs extending along the  direction. The 21
screw-related chains of
(10) motifs along (
) (Fig. 4) link the anionic substructure, resulting in the formation of an organic–inorganic sheet framework parallel to (10
) (Fig. 5).
, the H2
anions form dimers through an O2—H2D
hydrogen bond [symmetry code: (iv) −x
+ 1, −y
+ 1], with the characteristic ring motif of
(8), in which the centroid of the dimer occupies the crystallographic inversion centre. The O1—H1D
hydrogen bond [symmetry code: (iii) −x
] generates a C
4 chain which connects the glide-related anionic dimers with the glide plane perpendicular to the  direction, the glide component of which is [0,
, 0]. This forms an infinite two-dimensional layer in the form of a net extending parallel to the (001) plane. This inorganic supramolecular net of H2
anions is built from
(24) ring motifs (Fig. 6). The 4-chloroanilinium cations are anchored to the H2
anionic net through N1—H1A
[symmetry code: (i) −x
+ 1, −y
+ 1, −z
+ 1], N1—H1B
O3 and N1—H1C
[symmetry code: (ii) −x
] hydrogen bonds, forming fused-ring motifs of
(12) types with O—H
O hydrogen bonds (Fig. 7). The 4-chloroanilinium cations are pendant on both faces of the anionic net, thus resulting in the formation of two-dimensional sheet of an organic–inorganic supramolecular framework (Fig. 8) extending infinitely parallel to the crystallographic (001) plane.
It is of interest to note that in both the title compounds, although they form different types of anionic substructures, the overall anionic–cationic supramolecular framework results in the formation of infinite two-dimensional sheets. In 2CADHP
, the formation of an anionic double-stranded substructure and the linking of the cations to it is analogous with other reported structures. In the crystal structures of 2,4-dimethylanilinium dihydrogen phosphate (Fábry et al.
), 2-(methoxycarbonyl)anilinium dihydrogen phosphate (Shafiq et al.
) and 3,5-dimethoxyanilinium dihydrogen phosphate (Kaabi et al.
′ = 2Z
), the respective cations bound to the anionic substructures form two-dimensional sheets. In the last compound, the substructure was formed with different ring motifs than the other two structures. A three-dimensional hydrogen-bonded framework was observed for 1,3-propanediammonium bis(dihydrogen phosphate) (Marsh, 2004
), in which the cation contains an additional three N—H bonds involved in hydrogen bonding. The crystal structure of 4CADHP
is isomorphous with 4-bromoanilinium dihydrogen phosphate (CSD refcode UGISEI; Zhang et al.
), but no H atoms are reported in CSD. In 4CADHP
, the hydrogen-bonded anionic substructure formation and the linking of cations pendant from the supramolecular net are analogous to the structures of 4-methylanilinium dihydrogen phosphate (Smirani et al.
) and 4-ethylanilinium dihydrogen phosphate (Kaabi et al.
), but it has markedly different cell dimensions from 4-bromoanilinium dihydrogen phosphate, even though they belong to the same Pbca