Comparison crystal structure conformations of two structurally related biphenyl analogues: 4,4′-bis[3-(pyrrolidin-1-yl)prop-1-yn-1-yl]-1,1′-biphenyl and 4,4′-bis{3-[(S)-2-methylpyrrolidin-1-yl]prop-1-yn-1-yl}-1,1′-biphenyl

The crystal structures of the two title compounds each display the chair conformation of their piperidine rings. In 4,4′-bis[3-(pyrrolidin-1-yl)prop-1-yn-1-yl]-1,1′-biphenyl, the biphenyl rings are coplanar because the molecules sit on crystallographic centres of inversion. In 4,4′-bis{3-[(S)-2-methylpyrrolidin-1-yl]prop-1-yn-1-yl}-1,1′-biphenyl, the biphenyl ring system has a twisted conformation with a dihedral angle of 28.76 (11)°.


Structural commentary
The title compounds, (I) and (II) are shown in Figs. 1 and 2, respectively. X-ray crystallographic studies were carried out in order to determine the geometry of the biphenyl ring systems, ISSN 2056-9890 as well as to obtain more detailed information about the conformation of the pyrrolidino headgroups. Structure (I) is triclinic, space group P1, while crystal (II) is monoclinic, space group P2 1 .
In each compound, individual bond lengths and angles are unremarkable. For compound (I), the asymmetric unit contains two half molecules (denoted A and B in Fig. 1) such that the biphenyl rings straddle crystallographic inversion centres. As a result, the biphenyl groups are coplanar. In compound (II), however, the biphenyl rings (C9-C14) and (C15-C20) are non-coplanar, with a dihedral angle of 28.76 (11) . In crystals of (I), the two independent molecules differ in the orientation of the pyrrolidine ring. In molecule A, the nitrogen lone pair points inward towards the biphenyl rings, but in molecule B the nitrogen lone pair is directed away from the rings). The torsion angles about the ethynyl groups between the planes of the phenyl rings and the pyrrolidine ring N atoms are 84.15 (10) and À152.89 (10) (defined by atoms N1A-C5A-C8A-C9A and N1B-C5B-C8B-C9B, respectively). In compound (II), the corresponding torsion angles are 122.0 (3) and 167.0 (3) (defined by atoms N1-C6-C9-C14 and N2-C23-C18-C17, respectively), with the nitrogen lone pair directed away from the biphenyl rings at both ends of the molecule.

Supramolecular features
Aside from weak van der Waals interactions, there are no noteworthy intermolecular contacts in either (I) or (II).

Database survey
A search of the November 2014 release of the Cambridge Structure Database (Groom & Allen, 2014), with updates through May 2015, using the program Mogul (Bruno et al., 2004) for 4,4 0 substituted biphenyl fragments was conducted. The search was restricted to non-organometallic, solvent-free structures with R < 5% and Cl as the heaviest element. There were over 1000 matches, which gave a bimodal distribution of biphenyl torsion angles with a tight peak at 0 and a broader peak centred at 30 . The biphenyl torsion angles in (I) and (II) are thus not unusual.

Figure 2
The molecular structure of (II), with ellipsoids drawn at the 50% probability level.
Crystallization: Yellow crystals of compounds (I) and (II) suitable for X-ray analysis were grown from a mixture of dichloromethane/methanol (2:1) by slow evaporation of the solution at room temperature over 24 h.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 1. In both structures, H atoms were found in difference Fourier maps, but subsequently included in the refinement using riding models. Constrained distances were set to 0.95 Å (C sp2 H), 0.98 Å [RCH 3 , (II) only], 0.99 Å (R 2 CH 2 ) and 1.00 Å (R 3 CH). U iso (H) parameters were set to values of either 1.2U eq or 1.5U eq [RCH 3 in (II) only] of the attached atom.
In (II), the Flack parameter, x = À0.3 (10) is indeterminate, which is to be expected for a light-atom structure refined against Mo K data. However, the synthesis used pure (S)-2methylpyrrolidine, so the absolute configuration for the model of (II) was dictated by the synthesis.