Crystal structures of the 2:2 complex of 1,1′-(1,2-phenylene)bis(3-m-tolylurea) and tetrabutylammonium chloride or bromide

The title compounds both comprise a tetrabutylammonium cation, a halide anion and an ortho-phenylene bis-urea molecule. Each halide ion shows four N—H⋯X (X = Cl or Br) interactions with two urea receptor sites of different bis-urea moieties. A crystallographic inversion centre leads to the formation of a 2:2 arrangement of two halide anions and two bis-urea molecules.


Chemical context
Hydrogen bonding,interactions, anion-interactions, halogen bonds, and anion-macrodipole interactions are some of the crucial principal forces that determine structure, selfassembly and recognition in chemical and biological systems (Lehn, 1990;Jentzsch et al., 2013). Various urea-based anion receptors of varying complexity and sophistication have been designed and prepared (Amendola et al., 2010;Wei et al., 2011;Bregovic et al., 2015). It has been shown that the efficiency of urea to act as a receptor subunit depends on the presence of two parallel polarized N-H fragments, capable of (i) chelating a spherical anion or (ii) donating two parallel hydrogen bonds to the oxygen atoms of a carboxylate or of an inorganic oxoanion (Custelcean, 2013). In our ongoing research on N-rich organic ligand design and synthesis (Wang et al., 2015), we report herein the synthesis of the title orthophenylenediamine based methyl substituted neutral organic bis-urea receptor L and crystal structures of the 2:2 adducts of L with tetrabutylammonium chloride (TBACl) or bromide (TBABr) (I) and (II). ISSN 2056-9890

Structural commentary
The molecular structures of the title compounds are illustrated in Figs. 1 and 2. The receptor L displays a trans orientation of two urea groups showing non-cooperativity to each other. In the presence of 1.5 equivalents of tetrabutylammonium chloride or bromide in acetone and Et 2 O the 2:2 host-guest complexes (I) and (II) crystallize in the monoclinic space groups P2 1 /n and P2 1 /c, respectively. The 2:2 adducts are formed via N-HÁ Á ÁX hydrogen bonds between the halide anions and the urea subunits of two bis-urea receptors. Both NH functions of each urea group are trans to the C O double bond across the respective C-N bond, thereby the aromatic substituents are cis, with small C Ar -N-C O torsion angles [C1-N1-C13-O2 = 2.7 (4) and C15-N2-C12-O1 = 11.4 (3) in complex (I), C12-N1-C1-O1 = À0.7 (5) and C14-N3-C13-O2 = 8.5 (4) in complex (II)]. Moreover, it is also evident that the distance between the two terminal aromatic functions varies considerably due to the torsion angles between the two urea groups and between the two phenylene groups. The angles between the planes through the two urea planes are 55.67 (4) and 54.51 (5) in (I) and (II), respectively, with the receptors arranging themselves in a way that in the anion complex the urea groups on the two receptors are oriented in opposite directions therefore establishing interactions with two symmetry related anions. This results in the formation of a 2:2 non-capsular assembly via non-cooperative equally shared hydrogen-bonding interactions between the urea groups and respective anions. This is possibly additionally ascribed for the both syn geometrical orientation of the meta-substituent (-CH 3 ) with respect to the adjacent N-H part of the urea moiety of a particular receptor.

Figure 1
The molecular structure of (I), with atom labels and 50% probability displacement ellipsoids for non-H atoms.
stabilized by another two C-HÁ Á ÁO interactions and four weak C-HÁ Á Á supportive interactions between the two peripheral TBA units and respective receptor molecules. Additional interactions between TBA cations, halide anions and receptor molecules L in terms of several short C-HÁ Á ÁX contacts and C-HÁ Á ÁO contacts connect the 2:2 adducts into infinite layers (Tables 1 and 2

Database survey
The crystal structure of L with a meta-substitution of methyl group present in complex (I) and (II) appears not to have been reported previously. However, a search for orthophenylenediamine bis-urea with no methyl or any other substitutions on the phenyl ring resulted in some hits. For example, a 1:1 adduct between the bis-urea ligand and benzoate bound in the bis-urea cleft via four hydrogen bonds has been reported (Brooks et al., 2005a). Similarly, a single terephthalate anion is encapsulated by two bis-urea receptors in another case (Brooks et al., 2005b). Furthermore, an orthophenylenediamine bis-urea with para-nitro substitution receptor has also been reported, three of which enclose one PO 4 3anion by 12 hydrogen bonds (Li et al., 2013), whilst the bis-urea isomer with meta-nitro substitution displayed good selectivity for carboxylate anions forming a 2:1 complex between receptor and anion (Moore et al., 2013). Very recently, an ortho-phenylenediamine based 3-chloro-4-methyl disubstituted bis-urea receptor and its isomeric 4-bromo-3methyl disubstituted bis-urea receptor have been reported and their affinity with the common anions such as Cl À , AcO À , CO 3 2-, SO 4 2and SiF 6 2has also been studied (Manna et al., 2016). Especially, the 4-bromo-3-methyl disubstituted bis-urea forms non-capsular 2:2 host-guest assemblies with chloride ions via non-cooperative hydrogen-bonding interactions of the urea moieties. This phenomenon is consistent with that of L in the present study. Similarly to our work, structural elucidation reveals that two symmetry-identical chloride anions accept four strong N-HÁ Á ÁCl bonds [N1Á Á ÁCl 3.226 (6); N2Á Á ÁCl 3.312 (5); N3Á Á ÁCl 3.305 (6); N4Á Á ÁCl 3.270 (6) Å ; average 3.278 (8) Å ].

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms bonded to N were located from a difference map and refined with distance restraints of N-H = 0.86 (0) Å , and with U iso (H) = 1.2U eq (N). Other H atoms were positioned geometrically and refined using a riding model, with C-H = 0.96-0.97 Å and with U iso (H) = 1.2 (1.5 for methyl groups) times U eq (C).  used to refine structure: olex2.refine (Bourhis et al., 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).     (4)