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, C44H35N4O3·2.5C6H6 (1), consists of a terpyridine and a N-oxylpyrroline-3-formate group separated by an aromatic spacer, viz. 4-(phenyl-ethyn-yl)-1,1'-biphenyl. It crystallized in the triclinic space group P-1 with two and a half benzene solvate mol-ecules (one benzene mol-ecule is located about an inversion center), while the di-chloro-methane solvate (2) of the same mol-ecule [Ackermann et al. (2015 ▸). Chem. Commun. 51, 5257-5260] crystallized in the tetra-gonal space group P42/n, with considerable disorder in the mol-ecule. In (1), the terpyridine (terpy) group assumes an all-trans conformation typical for terpyridines. It is essentially planar with the two outer pyridine rings (B and C) inclined to the central pyridine ring (A) by 8.70 (15) and 14.55 (14)°, respectively. The planes of the aromatic spacer (D, E and F) are nearly coplanar with dihedral angles D/E, D/F and E/F being 3.42 (15), 5.80 (15) and 4.00 (16)°, respectively. It is twisted with respect to the terpy group with, for example, dihedral angle A/D being 24.48 (14)°. The mean plane of the N-oxylpyrroline is almost normal to the biphenyl ring F, making a dihedral angle of 86.57 (16)°, and it is inclined to pyridine ring A by 72.61 (15)°. The intra-molecular separation between the O atom of the nitroxyl group and the N atom of the central pyridine ring of the terpyridine group is 25.044 (3) Å. In the crystal, mol-ecules are linked by pairs of C-H⋯O hydrogen bonds, forming inversion dimers. The dimers stack along the c axis forming columns. Within and between the columns, the spaces are occupied by benzene mol-ecules. The shortest oxygen-oxygen separation between nitroxyl groups is 4.004 (4) Å. The details of the title compound are compared with those of the di-chloro-methane solvate (2) and with the structure of a related mol-ecule, 4'-{4-[(2,2,5,5-tetra-methyl-N-oxyl-3-pyrrolin-3-yl)ethyn-yl]phen-yl}-2,2':6',2''-terpyridine (3), which has an ethynylphenyl spacer [Meyer et al. (2015). Acta Cryst. E71, 870-874].