Crystal structure of 4,4′-diethynylbiphenyl

4,4′-Diethynylbiphenyl crystallizes with four unique molecules in the asymmetric unit. The crystal structure is stabilized by weak C—H⋯π(ring) and C C—H⋯π(alkyne) contacts.


Chemical context
Donor-acceptor (D-A) dyads with the innate ability to generate long-lived charge separation in their excited states have elicited a great deal of current interest. Their applications cover fields ranging from artificial photosynthesis to solar cell technology (Rogozina et al., 2013;Fukuzumi et al., 2014). We have produced a variety of such dyads based on ferrocene as the donor and with a variety of acceptors (see for example : Flood et al., 2007;Cuffe et al., 2005;McAdam et al., 2003). More recently, we have been interested in expanding the range of donor-acceptor dyads by interpolating a potentially conductive spacer between the donor and the acceptor to yield donor-spacer-acceptor (D-S-A) dyads. Biphenyl is a conductive spacer that we have used with some recent success, joined to a ferrocene donor through an alkene unit and to an acceptor via an alkyne link (McAdam et al., 2010;Tagg et al., 2015). We are interested in further developing the chemistry of biphenyl as a potential spacer, with alkyne links to both the donor and the acceptor. Surprisingly, the molecular and crystal structure of the precursor molecule, 4,4 0 -diethynylbiphenyl , has not been previously studied and we report its structure here.

Structural commentary
The title compound, (I), crystallizes with four unique molecules in the asymmetric unit, identified by the leading digits 1-4 in the numbering schemes, Fig. 1. Each molecule comprises a central biphenyl ring system symmetrically substituted at the 4 and 4 0 positions by terminal alkyne units. None of the molecules is planar, with the two benzene rings of each molecule inclined to one another at angles of 42.41 (4), 24.07 (6), 42.59 (4) and 46.88 (4) for molecules 1-4, respectively. Bond distances and angles in the biphenyl ring systems are not unusual and compare well, both internally, over the four unique molecules, and with those observed in related systems (see for example: O'Brien et al., 2010, Butler et al., 2008Muller, et al., 2006, Nitsche et al., 2003. The Cn4-Cn7 and Cn4 0 -Cn7 0 distances (n = 1-4) [mean 1.445 (2) Å ] are generally somewhat long, enough indeed to raise alerts in the checkCIF procedure. However analysis in Vista (Groom & Allen, 2014) of comparable values for eight other biphenyl systems, with terminal alkyne functions in the 4-position, provides a mean value of 1.442 (16) Å , not at all dissimilar to the values observed here (see for example : Langley et al., 1998;Mague et al., 1997;McAdam et al., 2010;Laliberté et al., 2006).
The C C distances are also generally reasonable, with the exception of C27 0 -C28 0 , 1.130 (2) Å , which is unusually short compared to more typical C C distances of 1.181 (14) Å (Allen et al. 1987). There is no obvious explanation for this, except to note that the adjacent C27 0 -C24 0 distance 1.4507 (19) Å is the longest of those reported here.

Supramolecular features
The absence of donor and acceptor components, to provide classical hydrogen bonding or even C-HÁ Á ÁE (E = O, N, halogen) contacts, challenge the packing in this system. There has been considerable speculation on the factors influencing the formation of structures with Z 0 > 1 (Desiraju, 2007;Steed & Steed, 2015;Anderson & Steed 2007, Nichol & Clegg, 2007, and the nature, extent and degree of the intermolecular contacts are clearly contributory factors. In this instance, the packing in the structure is profoundly influenced by an extensive series of weak edge-to-face C-HÁ Á Á(ring) interactions (Table 1)  The asymmetric unit of (I), showing the numbering schemes for the four unique molecules designated as types 1-4 with the types discriminated by the leading characters in the atom labels. Table 1 C-HÁ Á Á interactions (Å , ).

Figure 2
Complementary chains of 1, 3 and 2, 4 molecules extending along the caxis direction. In this and subsequent figures, C-HÁ Á Á(ring) contacts are drawn as dotted lines with ring centroids shown as coloured spheres.
of these contacts may influence the adoption of a Z 0 > 1 structure.
A complementary set of C-HÁ Á Á contacts, involving in one case molecules 1 and 3 and in the second molecules 2 and 4, sandwiches a molecule of 1 between two molecules of 3 and a molecule of 2 between two molecules of 4. These contacts generate infinite chains approximately along the c-axis direction. The two chains lie approximately orthogonal to one another, Fig. 2. Weak C16 0 -H16 0 Á Á ÁCg1 contacts form inversion dimers between two adjacent 1 molecules, Fig. 3, and dimers also result from C-HÁ Á Á contacts involving both rings of adjacent 2 and 3 molecules, Fig. 4; both these sets of contacts contribute to the overall packing. In addition to these C-HÁ Á Á(ring) interactions, one further set of somewhat unusual contacts is formed, again involving all four molecules in the structure. These are weak C C-HÁ Á Á(alkyne) contacts (Desiraju & Steiner, 1999) involving the relatively acidic C-H donors of the alkyne substituents. These again involve pairs of molecules with C18-H18Á Á ÁC37 C38 and C38 0 -H38 0 Á Á ÁC17 0 C18 0 contacts generating one set of zigzag chains along b with an adjacent and complementary zigzag produced by C28-H28Á Á ÁC47 C48 and C48 0 -H48 0 Á Á ÁC27 0 C28 0 interactions, These chains generate layers of molecules in the ac plane, Fig. 5. The contacts display the classic T shape, found also in the neutron structure of acetylene (McMullan et al., 1992), but not perfectly so. The Hn8Á Á ÁCn7 distances are consistently slightly shorter [mean of the four distances = 2.77 (3) Å ] than the Hn8Á Á ÁCn8 equivalents [mean 2.97 (4) Å ]. The mean Hn8Á Á ÁC C centroid distance is 2.82 (4) Å and these values all fall well within projected ranges for such contacts (Desiraju & Steiner, 1999).
The overall effect of this plethora of weak interactions is to stack molecules into 'multiple-decker sandwich' columns, linked together along the c-axis direction, Fig. 6.

Database survey
Structures of 4-4 0 -disubstituted biphenyls abound with 2891 hits on the CSD (Groom & Allen, 2014). However, those with 4,4 0 -alkyne substituents are far less plentiful with only 29 entries. These fall into two distinct categories. First compounds with one or both of the alkyne substituents on the biphenyls bound to carbon or silicon atoms, 14 entries (see Inversion dimers formed through C-HÁ Á Á(ring) contacts between molecules of type 1.

Figure 5
Zigzag chains of molecules generated by C-HÁ Á ÁC C contacts between molecules of types 1 and 3 and molecules of types 2 and 4. The centroids of the C C bonds are drawn as coloured spheres and the C-HÁ Á ÁC C contacts are shown as dotted lines. ides, also referred to as ethynyl compounds. These have either the terminal hydrogen atoms of the alkyne groups both replaced by a transition metal complex moiety (see for example: Shanmugaraju et al., 2011;Gao et al., 2007;Ibn Ghazala et al., 2006;Liu, Poon et al., 2005) or, much less frequently, only a single terminal hydrogen atom is replaced to afford ethynyl complexes with terminal C C-H substituents (Zeng et al., 2013;Saha et al., 2005).

Synthesis and crystallization
The title compound (I) was prepared by a literature procedure  and recrystallized from dichloromethane/hexane (1:1) to give pale-yellow plates suitable for X-ray analysis.