Crystal structure of the complex of 2,4,6-triethyl-1,3,5-tris[(4-methyl-1H-indazol-1-yl)methyl]benzene with NH4PF6

The title complex crystallizes in the monoclinic space group P21 with two molecules of the receptor, and two NH4 + and two PF6 − ions in the asymmetric unit. In each of the complexes, the ammonium ion resides in the cavity of the receptor molecule and is fixed in its position by three N—H⋯N bonds, while the remaining hydrogen atom of the cation acts as a bifurcated binding site for N—H⋯F bonding to the counter-anion.

The complex of 2,4,6-triethyl-1,3,5-tris[(4-methyl-1H-indazol-1-yl)methyl]benzene with ammonium hexafluorophosphate, C 39 H 42 N 6 ÁNH 4 + ÁPF 6 À , crystallizes in the monoclinic space group P2 1 with two molecules of the receptor, two NH 4 + and two PF 6 À ions in the asymmetric unit. In each of the complexes the ammonium ion resides in the cavity of the receptor molecule and is fixed in its position by three N-HÁ Á ÁN bonds, while the remaining hydrogen atom of the cation acts as a bifurcated binding site for N-HÁ Á ÁF bonding to the counteranion. The crystal is composed of one-dimensional supramolecular aggregates extending along the a-axis direction.

Chemical context
The development of efficient artificial receptors that exhibit high selectivity for ammonium versus potassium ions is of great interest (Bü hlmann et al., 1998;Chin et al., 1999;Spä th & Kö nig, 2010;Pazik & Skwierawska, 2014;Jonah et al., 2017;Schulze et al., 2018). Both acyclic and macrocyclic receptors have been designed to achieve this goal. Tripodal and hexapodal benzene derivatives bearing pyrazolyl or indazolyl groups have proven to be promising as receptors for NH 4 + (Chin et al., 1999(Chin et al., , 2002Koch et al., 2015;Jonah et al., 2017;Schulze et al., 2018). The ability of these compounds to act as ammonium receptors has been examined both in solution and in the crystalline state. Structural variations are to be used to develop receptor molecules that exhibit a more pronounced selectivity. As part of our studies on structure-binding affinity relationships, we have synthesized various acyclic molecules and investigated their binding properties. In this work we describe the crystal structure of a complex between NH 4 PF 6 and a tripodal benzene derivative bearing 4-methyl-indazol-1yl groups.

Structural commentary
Crystallization from a mixture of the title compound and NH 4 PF 6 in ethanol yields colourless prisms of the monoclinic space group P2 1 with two molecules of the receptor, two NH 4 + ions and two PF 6 À ions in the asymmetric unit. These components are connected to form complexes of the structures shown in Fig. 1. In each of them, the NH 4 + ion resides in a cavity created by the indazolyl groups of the receptor and is held in its position by three N-HÁ Á ÁN bonds [d(HÁ Á ÁN) 2.01 (2)-2.10 (4) Å ], involving the nitrogen atoms designated N2, N4, N6 and N2A, N4A, N6A. The remaining H atom of the ammonium acts as a bifurcated binding site for N-HÁ Á ÁF bonding with the counter-ion. The conformational difference between the receptor molecules is seen in the orientation of the three ethyl groups, leading to an ab 0 ab 0 ab 0 (complex I) and an ab 0 aa 0 ab 0 arrangement (complex II) (a = above, b = below, a 0 /b 0 = ethyl; see Koch et al., 2017;Schulze et al., 2017) of substituents with respect to the plane of the central arene ring (Fig. 2). The dihedral angles between the planes of the indazole units are 67.8 (1), 8.1 (2), 72.6 (1) for complex I and 62.0 (1), 6.9 (2), 65.4 (1) for complex II.

Supramolecular features
The crystal is composed of one-dimensional supramolecular aggregates of C-HÁ Á ÁF-bonded ammonium complexes [d(HÁ Á ÁF) 2.33-2.52 Å ; Table 1] extending in the a-axis direction. The packing is shown in Fig. 3. Multiplearene contacts connect these aggregates into a three-dimensional network. For the analysis of this type of interactions, the PLATON program (Spek, 2020) was used. The centroidcentroid distances between the interacting indazole units range from 3.776 (4) to 4.257 (4) Å with shifts of 1.154-1.830 Å .

Figure 2
Ball-and-stick representation (side view) of the complexes in the crystal structure of the title complex.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms were positioned geometrically and refined isotropically using the riding model with C-H = 0.95-0.99 Å and U iso (H) = 1.2 or 1.5U eq (C). The hydrogen atoms of the ammonium ions were located in a difference-Fourier map and the N-H bond lengths refined to a target value of 0.90 Å . The crystal studied was refined as a two-component twin. SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012).