4,4 -Diamino-2,2 ,6,6 -tetramethylbiphenyl

Each of the three independent mol­ecules of the title compound, C16H20N2, has near-perpendicular benzene rings and pyramidal N atoms. Hydrogen bonding in the structure is rather inefficient.


Comment
Benzidine derivatives are now widely used in the solid state as hole-injecting and transport materials in applications ranging from the Xerox process to organic light-emitting diodes (OLEDs) (Thelakkat, 2002). As noted by Shankland et al. (2005), the solid-state structures of these materials are of more than simply crystallographic interest, with the physical structure playing a critical role in determining the mechanism and energetics of the charge transport process. In the course of a wider study of tetra(aryl)benzidines (Littleford et al., 2004;Low, Paterson, Puschmann et al., 2004;Low et al., 2005) we have determined the structure of 4,4 0 -diamino-2,2 0 ,6,6 0 -tetramethylbiphenyl, (I), reported here.
All N atoms in (I) have substantially pyramidal geometry with a mean bond angle of 113 (1) , and are nearly coplanar with the attached rings, except atoms N1 and N4, which are displaced from these planes by 0.102 (2) and 0.129 (2) Å , respectively. The degree of pyramidalization is similar to that observed in solid aniline (Fukuyo et al., 1982); the mean N-C bond distance of 1.410 (3) Å in (I) is marginally longer than in aniline [1.392 (6) Å ].

Experimental
Compound (I) was prepared according to Carlin (1945) and recrystallized from benzene and hexane (1:1).  Amino H atoms were located in a difference map and were refined isotropically [N-H = 0.86 (2)-0.95 (3) Å ]. Methyl groups were refined as rigid bodies [C-H = 0.98 Å ] rotating around the C-C bonds, with a common refined U iso value for the three H atoms; the benzene H atoms were treated as riding on the C atoms [C-H = 0.95 Å and U iso (H) = 1.2U eq (C)].

Data collection
Bruker SMART 6K CCD area-detector diffractometer Radiation source: fine-focus sealed tube Graphite monochromator Detector resolution: 8 pixels mm -1 ω scans 76630 measured reflections 12148 independent reflections 9388 reflections with I > 2σ(I) R int = 0.032 θ max = 30.0°, θ min = 1.5°h Special details Experimental. The data collection nominally covered full sphere of reciprocal space, by a combination of 5 sets of ω scans; each set at different φ and/or 2θ angles and each scan (5 sec exposure) covering 0.3° in ω. Crystal to detector distance 4.84 cm. 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.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement.