Crystal structure of 4′-{[4-(2,2′:6′,2′′-terpyridyl-4′-yl)phenyl]ethynyl}biphenyl-4-yl (2,2,5,5-tetramethyl-1-oxyl-3-pyrrolin-3-yl)formate benzene 2.5-solvate

The title compound, consisting of a terpyridine group linked to a N-oxylpyrroline-3-formate group by a phenylethynylbiphenyl spacer, crystallized as a benzene two and a half solvate. Its structure is compared to that of the same molecule that crystallized as a dichloromethane solvate and to a similar molecule with a shorter spacer unit viz. ethynylphenyl.


Structural commentary
The molecular structure of the title compound, (1), is shown in Fig. 1. The crystal structure of the dichloromethane solvate (2) of the title compound has been reported (Ackermann et al., 2015). However, these authors used a different protocol for the crystallization of (1) and the conformation of (2) differs markedly from the one presented herein, as shown in the structural overlay of the two compounds (Fig. 2). The structural overlay of compounds (1) and (3) also illustrate the differences in their conformations (Fig. 3).

Supramolecular features
In the crystal of (1), Fig. 4, molecules form layers which are nearly coplanar with the (011) plane. Neighbouring layers differ in the orientation of the molecules and each layer is separated by layers of solvent molecules. This arrangement possibly leads to favorable dispersive interactions although only one short C-HÁ Á Á contact is observed between the solvent molecules and molecules of (1) ( Table 1). Shortcontacts are observed between the C rings of neighbouring molecules and between the B and C rings (Fig. 5). The centroid-to-centroid distances are 3.678 (2) and 3.8915 (18) Å , respectively, and can be classified as slipped face-to-faceinteractions (Janiak, 2000).
Within the planes, there are weak C-HÁ Á ÁO hydrogen bonds between the nitroxyl-O atom and the para-hydrogen atom of pyridine ring B (Table 1). Furthermore, two weak hydrogen bonds per molecule are formed between pairs of layers ( Table 1). One of these hydrogen bonds involves the nitroxyl O atom and a hydrogen atom of a methyl group of a molecule from a neighboring layer. The other hydrogen bond is formed between the carbonylic O atom of the carboxylate group and a meta-hydrogen atom of one of the outer pyridine rings of a molecule from a neighboring layer. As the layers are Crystal packing of the title compound viewed along the a axis. Weak C-HÁ Á ÁO hydrogen bonds are shown as dashed lines (see Table 1). H atoms not involved in C-HÁ Á ÁO bonds have been omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ).
It is noteworthy that the arrangement of the molecules of the title compound strongly depends upon the solvents of crystallization. In compound (1), the molecules are arranged in layers and the benzene molecules fill out the channels between the layers formed by the aromatic spacers of the molecule. Close intermolecule contacts exist only between the functional groups. In the structure of (2) (Ackermann et al., 2015), the solvent of crystallization is dichloromethane instead of benzene and molecules are arranged having fourfold rotational site symmetry. The solvent molecules fill out channels between the molecules of (2), as in (1). However, the CH 2 Cl 2 solvent molecules in (2) are in close proximity to the terpyridine groups instead of to the aromatic spacer. Weak hydrogen bonds are formed predominantly involving the O atoms as acceptors and the pyrroline and the pyridine rings as donors, as observed in (2) and (3). The shortest oxygen-oxygen separation between neighboring nitroxyl groups is 4.004 (4) Å . This OÁ Á ÁO distance is an important factor determining the strength of through space exchange interactions of nitroxyls (Rajca et al. 2006).

Database survey
The Cambridge Structural Database (CSD, Version 5.36; Groom & Allen, 2014) has not been updated since our presentation of the structure of (2). The CSD query revealed, that non-coordinated terpyridines are arranged in an all-trans conformation, unless they are either protonated, lithiated or cannot assume an all-trans conformation for reasons of steric hindrance.

Refinement
Crystal data, data collection and structure refinement details are summarized in

Figure 6
The synthesis of the title compound (1).
0.98 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms. 16 reflections with bad agreement were omitted from the final refinement cycles.