Crystal structure of dibenzylammonium hydrogen (4-aminophenyl)arsonate monohydrate

The title salt consists of three components, comprising one dibenzylammonium cation, [(C6H5CH2)2NH2]+, one hydrogen (4-aminophenyl)arsonate anion, [H2NC6H4As(OH)O2]−, and one molecule of water. In the crystal, these components are organized in infinite zigzag chains via intermolecular hydrogen bonds. Weak interactions between the chains lead to a three-dimensional network.


Chemical context
Organoarsenic compounds have been known for a long time and sparked great interest when they were discovered.Tetramethyldiarsine (Me 2 As-AsMe 2 ), commonly known as Cacodyl, was isolated in the middle of the 18 th century by Cadet de Glaussicourt (Garje & Jain, 1999).During the next century, in 1859, Antoine Be ´champ reported the synthesis of p-arsanilic acid sodium salt (named Atoxyl) by reacting aniline with arsenic acid.This compound was employed for pharmaceutical applications, in particular against trypanosomal infection.Subsequently, in the early 20th century, Paul Ehrlich was inspired by this work to develop a new organoarsenic derivative, called Arsphenamine or Salvarsan (Ehrlich & Bertheim, 1907).This molecule has proved particularly effective in the treatment of syphilis and sleeping sickness (African Trypanosomiasis) and is considered as being the first chemotherapeutic agent (Williams, 2009).The use of organoarsenicals as medicines was subsequently abandoned in favour of penicillin, as they were found to be highly toxic to humans, causing significant side effects (including blindness).However, they have continued to be used, until recently, as feed additives and veterinary drugs, particularly in the livestock and poultry breeding industry, but with serious negative effects on the environment.Soil and groundwater contamination resulting from the excessive use of aromatic organoarsenic compounds is now a major environmental concern (Fei et al., 2018).Current investigations involving academics focus on improving analytical detection (Depalma et al., 2008;Yang et al., 2018) and remediation methods (Jun et al., 2015;Chen et al., 2022).

Structural commentary
The asymmetric unit of the title salt, which is depicted in Fig. 1 10) A ˚and As-O3 =1.6699 (10) A ˚, which can be considered to be identical.The As-O1 distance is consistent with the presence of a hydroxyl group (Yang et al., 2002), while the As-O2 and As-O3 distances, which are shorter, reflect rather a double-bond character.In the literature, based on a comparison of structural examples, the average length of the As-O bond is defined as 1.77A ˚and that of the As O bond as 1.67 A ˚ (Nuttall & Hunter, 1996).The nature of the As O2 and As O3 double bonds implies that the negative charge is delocalized on the arsonate.The three oxygen atoms of the arsonate function are engaged in hydrogen bonding, the O1 and O2 atoms being linked head-to-tail [O1-H� � �O2 iv , D� � �A = 2.5444 (15) A ˚; symmetry code: (iv) À x, À y + 1, À z + 1, Table 1].The length of the As-C1 bond [1.8955 (13) A ˚] is within the range of values measured for related compounds such as ammonium 4-nitrophenylarsonate (Yang et al., 2002) and guanidinium phenylarsonate (Smith & Wermuth, 2010).An amino group is positioned on the phenyl ring in the para position to the arsonate function.Both functional groups are contained in the plane of the phenyl ring.The negative charge of [H 2 NC 6 H 4 As(OH)O 2 ] À is compensated by the presence of one dibenzylammonium cation, [(C 6 H 5 CH 2 ) 2 NH 2 ] + , whose NH 2 + group is hydrogen bonded to the oxygen atom O3 of the arsonate function [N1-H1A� � �O3, D� � �A = 2.6842 (16) A ˚, N1-H1B� � �O3 iii , D� � �A = 2.7260 (15) A ˚; symmetry code: (iii) À x + 1, À y + 1, À z + 1].Moreover, the dibenzylammonium cation shows a syn-anti conformation, displaying C-C-N-C torsion angles of 57.65 (16) � and À 178.14 (11) � , which are in the range of previous examples of X-ray structures involving [(C 6 H 5 CH 2 ) 2 NH 2 ] + (Trivedi & Dastidar, 2006).A water molecule (co-solvent of the reaction) participates in a hydrogen-bond interaction with the oxygen atom O2 of -As(OH)O 2 À [O4-H4A� � �O2 V , D� � �A = 2.8074 (18) A ˚; symmetry code: (v) 1 + x, y, z] completes the composition of salt I. From a spectroscopic point of view, the infrared spectrum of I (ATR mode) highlights �(As-C) and �(As-O) absorption bands, which are characteristic of the arsonate function (Cowen et al., 2008), at 1096 cm À 1 and between 925-690 cm À 1 , respectively.The percentages of C, H, N and O determined by elemental analysis support the chemical composition of I, but show that the salt is partially dehydrated (see the Synthesis and crystallization section).

Supramolecular features
At the supramolecular stage, two levels of organization can be observed in the crystal structure of I: (i) The propagation of one-dimensional zigzag chains along the a-axis direction resulting from the hydrogen-bonding The molecular structure of I with displacement ellipsoids at the 30% probability level.The water molecule was found to be disordered over two positions, the minor part was omitted and the major part is represented with the following symmetry code: (i): À 1 + x, y, z.Dotted lines indicate hydrogen bonds.
The hydrogen atoms for the major component of the water molecule were refined geometrically as a rigid group (O-H = 0.87 A ˚) with U iso (H) = 1.5U eq (O).C-bound hydrogen atoms were placed at calculated positions [C-H = 0.95 A ˚(aromatic) or 0.99 A ˚(methylene group)] and H atoms of the NH 2 and OH terminal groups were placed geometrically (N-H = 0.83-0.84A ˚, O-H = 0.83 A ˚) and refined as riding with U iso (H) = 1.2U eq (N, C).

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Figure 2
Figure 2 Mercury representation (Macrae et al., 2020; colour code: C = grey, N = blue, O = red, As = pink, H = white] of the infinite chain structure of I propagating along the a-axis direction via hydrogen bonds (dotted cyan lines).

Figure 3
Figure 3 Mercury representation (Macrae et al., 2020; colour code: C = grey, N = blue, O = red, As = pink, H = white) highlighting the hydrogen-bonding network (cyan dotted lines) involving the components of I (the benzyl H atoms have been omitted for clarity).

Figure 4
Figure 4 Arrangement of the chains in the crystal of I and along the b-axis, leading to a three-dimensional network (Mercury representation; Macrae et al., 2020; colour code: C = grey, N = blue, O = red, As = pink, H = white).H atoms of phenyl and benzyl groups are omitted for clarity.The hydrogen bonds propagating the infinite chains are represented by dotted cyan lines.

Figure 5
Figure 5 View of the �-� stacking interactions between phenyl rings of the dibenzylammonium cations of I [along the a-axis, Mercury representation (Macrae et al., 2020); colour code: C = grey, N = blue, H = white).H atoms of phenyl rings, anions and water molecules have been omitted for clarity.

Table 2
Experimental details.