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The title compounds, 2-chloroanilinium dihydrogen phosphate (2CADHP) and 4-chloroanilinium dihydrogen phosphate (4CADHP), both C6H7NCl+·H2PO4 −, form two-dimensional supramolecular organic–inorganic hybrid frameworks. In 2CADHP, the dihydrogen phosphate anions form a double-stranded anionic chain generated parallel to the  direction through O—HO hydrogen bonds, whereas in 4CADHP they form a two-dimensional supramolecular net extending parallel to the crystallographic (001) plane into which the cations are linked through strong N—HO hydrogen bonds.
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., 2002 ; 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 (H3PO4), an inorganic oxy-acid, forms dihydrogen phosphate salts with organic amines, resulting in organic–inorganic hybrid systems with potentially powerful hydrogen-bonded D/A moieties. The dihydrogen phosphate anions (H2PO4 −) form substructures in these compounds, generating anionic networks via O—HO hydrogen bonds which act as a template for the assembly of cations (Shylaja et al., 2008 ). 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., 1998 ), the H2PO4 − anions form a one-dimensional chain network, while in 3-amino-2-chloropyridinium dihydrogen phosphate (Hamed et al., 2007 ) they form chains of fused (8) ring motifs [for graph-set analysis, see Bernstein et al. (1995 )]. Two-dimensional nets of anionic substructures were also observed in dimethylammonium dihydrogen phosphate (Pietraszko et al., 1999 ) and 2-methylpiperazinediium dihydrogen phosphate (Choudhury et al., 2000 ). Interestingly, in the structure of imidazolinium dihydrogen phosphate (Blessing, 1986 ), the H2PO4 − 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 P21/n, whereas 4CADHP crystallizes in Pbca. The asymmetric units of both 2CADHP and 4CADHP contain a dihydrogen phosphate anion and a singly protonated 2- or 4-chloroanilinium cation, respectively. In the tetrahedral dihydrogen phosphate group of both 2CADHP and 4CADHP, 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., 2006 ). The geometries of the 2- and 4-chloroanilinium cations show characteristic values compared with other reported structures (Muthamizhchelvan et al., 2005 ; Glidewell et al., 2005 ). 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.
In 2CADHP, the inversion-related H2PO4 − anions are linked through an O1—H1DO3iii hydrogen bond [symmetry code: (iii) −x + 1, −y + 1, −z + 1], forming an O—HO hydrogen-bonded dimer with a ring motif of (8), with its centroid occupying the inversion centre. These dimers are interlinked through an O2—H2DO3ii hydrogen bond [symmetry code: (ii) x, y + 1, z] to form a ring motif of type (12). The alternately fused (8) and (12) supramolecular motifs in turn generate a double-stranded inorganic H2PO4 − chain made of P—OHO=P hydrogen bonds extending infinitely along the  direction (Fig. 3 ). The 2-chloroanilinium cations are linked to the anionic substructure through three N—HO hydrogen bonds and a ClO short contact [ClO = 3.1705 (14) Å]. The N1—H1AO4 and N1—H1CO4ii hydrogen bonds [symmetry code: (ii) x, y + 1, z], along with O2—H2DO3ii, 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 ClO1 interaction, which acts as a pseudo-hydrogen bond (Bryant et al., 1998 ; Kubicki & Wagner, 2007 ), with the Cl1 atom at (x, y, z) as donor and atom O1 at (−x + , −y + , −z + ) as acceptor, along with the N—HO hydrogen bonds, forms a chain of fused (10) motifs extending along the  direction. The 21 screw-related chains of (10) and (10) motifs along (, y, ) (Fig. 4 ) link the anionic substructure, resulting in the formation of an organic–inorganic sheet framework parallel to (10) (Fig. 5 ).
In 4CADHP, the H2PO4 − anions form dimers through an O2—H2DO3iv hydrogen bond [symmetry code: (iv) −x + 1, −y, −z + 1], with the characteristic ring motif of (8), in which the centroid of the dimer occupies the crystallographic inversion centre. The O1—H1DO4iii hydrogen bond [symmetry code: (iii) −x + , y − , z] generates a C4 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 H2PO4 − anions is built from (8) and (24) ring motifs (Fig. 6 ). The 4-chloroanilinium cations are anchored to the H2PO4 − anionic net through N1—H1AO4i [symmetry code: (i) −x + 1, −y + 1, −z + 1], N1—H1BO3 and N1—H1CO3ii [symmetry code: (ii) −x + , y + , z] hydrogen bonds, forming fused-ring motifs of (14) and (12) types with O—HO 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., 2001 ), 2-(methoxycarbonyl)anilinium dihydrogen phosphate (Shafiq et al., 2009 ) and 3,5-dimethoxyanilinium dihydrogen phosphate (Kaabi et al., 2004 ) (Z′ = 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., 2001 ), 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., 2004 ) and 4-ethylanilinium dihydrogen phosphate (Kaabi et al., 2003 ), but it has markedly different cell dimensions from 4-bromoanilinium dihydrogen phosphate, even though they belong to the same Pbca space group.
Ethanol solutions containing equimolar quantities of 2-chloroaniline and orthophosphoric acid were mixed to produce a white precipitate, which was filtered off, dried for a few hours, dissolved in ethanol and allowed to recrystallize to afford colourless single crystals of 2CADHP after a period of about two weeks.
Colourless crystals of 4CADHP were obtained from a solution of 4-chloroaniline and orthophosphoric acid mixed at a 1:1 stoichiometric ratio in a mixed solvent of ethanol and water in equal proportions (50:50 v/v) upon gentle heating. The solution thus prepared was allowed to crystallize and crystals were obtained by slow evaporation of the solvent.
The positions of the H atoms bound to N and O atoms were identified from difference electron-density maps, but were subsequently geometrically optimized (O—H = 0.82 Å and N—H = 0.89 Å) and allowed to ride at the best staggered positions, with U iso(H) = 1.5U eq(O,N), except for O2 of 2CADHP whose O—H vector was allowed to rotate around the P—O bond. H atoms bound to C atoms were treated as riding atoms, with C—H = 0.93 Å and U iso(H) = 1.2U eq(C).
For both compounds, data collection: APEX2 (Bruker, 2004 ); cell refinement: APEX2 and SAINT (Bruker, 2004 ); data reduction: SAINT and XPREP (Bruker, 2004 ); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: ORTEP-3 (Farrugia, 1997 ) and Mercury (Macrae et al., 2006 ); software used to prepare material for publication: PLATON (Spek, 2009 ).
Crystal structure: contains datablocks 2CADHP, 4CADHP, global. DOI: 10.1107/S0108270110001940/gd3320sup1.cif
Structure factors: contains datablocks 2CADHP. DOI: 10.1107/S0108270110001940/gd33202CADHPsup2.hkl
The authors thank Dr Babu Varghese, Senior Scientific Officer, and SAIF at IITM for providing the data-collection facility.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: GD3320). Services for accessing these data are described at the back of the journal.