The crystal structure of 4′-{4-[(2,2,5,5-tetramethyl-N-oxyl-3-pyrrolin-3-yl)ethynyl]phenyl}-2,2′:6′,2′′-terpyridine

The crystal structure of a nitroxide-substituted terpyridine molecule is presented and discussed.

The terpyridine group of the title compound, C 31 H 27 N 4 O, assumes an alltransoid conformation and is essentially planar with the dihedral angles between the mean planes of the central pyridine and the two outer rings amounting to 3.87 (5) and 1.98 (5) . The pyrroline-N-oxyl group commonly seen in such nitroxyls is found in the title structure and the mean plane of the pyrroline ring subtends a dihedral angle of 88.44 (7) to the mean plane of the central pyridine ring. The intramolecular separation between the nitrogen atom of the central pyridine unit of the terpyridine group and the nitroxyl group is 14.120 (2) Å . In the crystal, the molecules are arranged in layers stacked along [001]. Slipped face-to-faceinteractions between the pyridine rings are observed along this direction with the shortest centroid-centroid distances amounting to 3.700 (1) and 3.781 (1) Å . Furthermore, edge-on C-HÁ Á Á interactions between the phenylene rings of neighbouring molecules are observed along this direction. A two-dimensional C-HÁ Á ÁO hydrogen-bonded network is formed within the (010) plane. The shortest OÁ Á ÁO separation between neighbouring molecules is 5.412 (3) Å .

Chemical context
The title compound, (1), was synthesized as a ligand for 3d metal ions as part of a pulsed EPR study on metal-nitroxyl model systems. The molecule contains a paramagnetic nitroxyl group and a terpyridine group. Nitroxyls have been the subject of magnetic studies in which exchange interactions have been detected (see, for example, Rajca et al., 2006;Fritscher et al., 2002). Furthermore, nitroxyls are used as spin labels for structural investigations of biological macromolecules (Reginsson & Schiemann, 2011). The structures of terpyridines have been investigated by Fallahpour et al. (1999), Eryazici et al. (2006), Bessel et al. (1992) and Grave et al. (2003) to name a few examples. The terpyridine moiety is known to form complexes with various metals. Numerous studies on metal complexes of terpyridine have been conducted, examples include those by Hogg & Wilkins (1962), Constable et al. (1999), Narr et al. (2002) and Folgado et al. (1990). ISSN 2056-9890

Structural commentary
The structure of the title compound (1) is shown in Fig. 1. The terpyridine group of (1) assumes an all-transoid conformation and is essentially planar with angles between the mean planes of the central pyridine (N1, C1-C5, r.m.s deviation from the mean plane = 0.006 Å ) and the two outer rings amounting to 3.87 (5) (N4, C27-C31, r.m.s. deviation from the mean plane = 0.003 Å ) and 1.98 (5) (N2, C6-C10, r.m.s deviation from the mean plane = 0.006 Å ), respectively. The pyrroline-N-oxyl unit commonly found for such nitroxyls is seen in the structure and its mean plane (N3, C19-C22, r.m.s deviation from the mean plane = 0.006 Å ) subtends a dihedral angle of 88.44 (7) to the mean plane of the central pyridine ring (for similar structural motifs, see Margraf et al., 2009 andSchuetz et al., 2010). The subunits are linked by a 4-ethinylenephenylene group. The mean plane of the phenylene group (C11-C16, r.m.s deviation from the mean plane < 0.001 Å ) is tilted with respect to both the central pyridine ring [dihedral angle of 51.36 (5) ] and the pyrroline-N-oxyl [dihedral angle of 37.62 (7) ]. The angles C18-C17-C14 [177. 35 (19) ] and C17-C18- C19 [175.64 (18) ] are slightly lower than the 180 expected for a strictly linear shape of the molecular backbone. Two short intramolecular hydrogen-nitrogen distances are observed between the two meta-protons of the central pyridine subunit and the nitrogen atoms of the external pyridine rings (Table 1). Murguly et al. (1999) propose weak intramolecular hydrogen bonds for these atoms. The intramolecular separation between the terpyridine group and the nitroxyl amounts to 14.120 (2) Å (measured between O1 and N1). The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. H atoms have been omitted for clarity.

Figure 2
Crystal packing of the title compound viewed along the b axis. Weak C-HÁ Á ÁO hydrogen bonds are shown as dashed lines Table 1 Hydrogen-bond geometry (Å , ).
Cg is the centroid of the C11-C16 ring. . These terpyridine subunits are arranged in a slipped face-to-face alignment (Janiak, 2000) with the shortest intermolecular distances between the pyridine rings amounting to 3.700 (1) Å (measured from the centroid of N2, C6-C10 to the centroid of N4, C27-C31) and 3.781 (1) Å (centroid of N1, C1-C5 to the centroid of N4, C27-C31, see Fig. 5). Furthermore, the phenylene rings of neighbouring molecules show an edge-on C-HÁ Á Á interaction along the same axis (Table 1 and Fig. 5). The nitroxyl groups are arranged in an alternating manner pointing in opposite directions. The shortest oxygen-oxygen separation between neighbouring molecules amounts to 5.412 (3) Å . The oxygen-oxygen 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 been queried to find other terpyridine or 2,2,5,5-tetramethyl-N-oxyl-3-pyrroline derivatives. The terpyridine query revealed 3473 entries in the CSD Crystal packing of the title compound viewed along the c axis.

Figure 4
Crystal packing of the title compound viewed along the a axis.

Figure 5
Closest distances between pyridine rings and edge-on C-HÁ Á Á contact.
if metal complexes of terpyridine were included. For purely organic terpyridine compounds, the number of hits was reduced to 348. Only 33 results for 2,2,5,5-tetramethyl-N-oxyl-3-pyrroline derivatives were found in the CSD. A combined query for structures which include both terpyridine and 2,2,5,5-tetramethyl-N-oxyl-3-pyrroline derivatives did not result in any hit. However, the authors are aware of at least one published crystal structure of a compound which contains both structural motifs (Ackermann et al., 2015).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were fixed geometrically and allowed to ride on their parent C atoms, with 0.98 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for all other H atoms.

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