Biphenyl-4,4′-diyl bis(2,2,5,5-tetramethyl-1-oxyl-3-pyrroline-3-carboxylate)

In the title compound, C30H34N2O6, the complete molecule is generated by a crystallographic 2/m symmetry operation. The 1-oxyl-3-pyrroline-3-carboxylate group lies on a mirror plane. The dihedral angle between the ring planes of the biphenyl fragment is constrained by symmetry to be zero, resulting in rather short intramolecular H⋯H contact distances of 2.02 Å. In the crystal, molecules are connected along the a-axis direction by very weak intermolecular methyl–phenyl C—H⋯π interactions. The C—H bond is not directed to the center of the benzene ring, but mainly to one C atom [C—H⋯C(x − 1, y, z): H⋯C = 2.91 Å and C—H⋯C = 143°].

In the title compound, C 30 H 34 N 2 O 6 , the complete molecule is generated by a crystallographic 2/m symmetry operation. The 1-oxyl-3-pyrroline-3-carboxylate group lies on a mirror plane. The dihedral angle between the ring planes of the biphenyl fragment is constrained by symmetry to be zero, resulting in rather short intramolecular HÁ Á ÁH contact distances of 2.02 Å . In the crystal, molecules are connected along the a-axis direction by very weak intermolecular methyl-phenyl C-HÁ Á Á interactions. The C-H bond is not directed to the center of the benzene ring, but mainly to one C atom [C-HÁ Á ÁC(x À 1, y, z): HÁ Á ÁC = 2.91 Å and C-HÁ Á ÁC = 143 ].
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: LH2852).

Comment
The title compound was prepared as a reference compound for pulsed electron-electron double resonance measurements (Weber et al., 2002).
The molecular structure is shown in Fig. 1. The molecule has 2/m symmetry: atoms C1, C4, O1, C5, O2, C6, C7, C8, C9, N1 and O3 lie on a mirror plane. There is a twofold axis perpendicular to this mirror plane and passing through the center of the central C-C single bond. There also is an inversion center at the midpoint of the central C-C single bond.
The two six-membered rings of the biphenyl group are coplanar by symmetry, resulting in rather short intramolecular H···H contact distances of 2.02 A. The 1-oxyl-3-pyrroline-3-carboxylate group is planar. Approximate planarity of this group also has been observed in a number of related crystal structures (Papoutsakis et al., 1999;Boeyens & Kruger, 1970;Bolte, 2006;Duskova et al., 2001;Godt et al., 2000;Wiley et al., 1989 andWiley et al., 1991) The crystal packing is shown in Fig 2. The molecules are connected along the a-direction by four symmetry-equivalent very weak intermolecular C methyl -H···π(phenyl) interactions (Table 1). The C methyl -H bond is not directed to the center of the phenyl ring, but mainly to one C atom. There are no other short intermolecular contacts.

Experimental
The title compound was prepared similar to the procedure described by Weber et al. (2002). Single crystals were obtained by recrystallization of the compound from a mixture of toluene and n-hexane (3:1).

Refinement
The H atoms were positioned geometrically and treated as riding: C methyl -H=0.98 Å, C planar -H=0.95 Å, U iso (H)=1.2U eq (C non-methyl ) and U iso (H)=1.5U eq (C methyl ). The torsion angles about the C-C methyl bonds were refined for the methyl groups. Fig. 1. The structure of the title compound shown with 50% probability displacement ellipsoids. The H atoms are drawn as small spheres of arbitrary radius. Symmetry equivalent atoms are related by i: x, -y, z, ii: 2 -x, -y, 1 -z and iii: 2 -x, y, 1 -z.   (2)