A new crystal phase of N,N,N′,N′-tetraphenyl-1,1′-biphenyl-4,4′-diamine

The complete molecule of the title compound, C36H28N2, is generated by a crystallographic centre of inversion. The biphenyl unit is forced by symmetry to be essentially flat (r.m.s. deviation = 0.008 Å); the dihedral angles between it and the two terminal phenyl rings are 69.39 (5) and 59.53 (5)°.

The complete molecule of the title compound, C 36 H 28 N 2 , is generated by a crystallographic centre of inversion. The biphenyl unit is forced by symmetry to be essentially flat (r.m.s. deviation = 0.008 Å ); the dihedral angles between it and the two terminal phenyl rings are 69.39 (5) and 59.53 (5) .

Comment
Triarylamine-based organic semiconductors have been intensively investigated as hole transport materials for electro-optic devices (Kennedy et al., 2002;Shirota, 2000;Shirota, 2005). Recently, the organic field-effect transistor (OFET) based on a cyclic triphenylamine has been reported, which shows the relatively good mobility and high on/off ratio (Song et al., 2006). The title compound,N,N,N',, has been synthesized by coupling of two triphenylamine molecules using a copper catalyst (Ullman coupling) and also using methanesulfonic acid as catalyst in the yield of 10% . We have synthesized compound (I) from the coupling of 4,4'-dibromobiphenyl and diphenylamine using a palladium catalyst in the yield of 71% (Hartwig, 1999). The crystal structure of compound (I) has been reported , where the biphenyl part of the molecule is highly twisted with a dihedral angle of 32.9 (4)°. Herein, we report another crystal phase of compound (I).
The molecule (I) possesses a inversion center, thus the biphenyl moiety of the molecule shows a completely flat conformation with an average deviation of 0.0079Å (Fig. 1). The other fused rings make 69.39 (5)° and 59.53 (5)° dihedral angles with this flat biphenyl moiety. The N atom displays an almost trigonal geometry with slight distortion of the N-C bond lengths and C-N-C bond angles. The angle sums around the N atom is ca 360°. The molecular geometry of (I) is quite different from the previously reported one , and also different from those of the related biphenyl-diamine compounds (Kennedy et al., 2002, Low et al., 2004Zhang et al., 2004). The molecular packing pattern in the present crystal is also different from the reported one , and a herringbone-like packing motif governed by the van der Waals interactions is observed (Fig. 2).

Experimental
A suspension of 4,4'-dibromobiphenyl (3.1 g, 10 mmol), diphenylamine (3.9 g, 21 mmol) and sodium t-butoxide (2.3 g, 24 mmol) in xylene (20 ml) was refluxed in the presence of palladium(II) acetate (4.4 mg, 0.020 mmol) and triphenylphosphine (42 mg, 0.016 mmol) for 5 h under an argon atmosphere. After removing the solvent in vacuum, the residue was separated by column chromatography on silica gel using benzene as eluent to afford the title compound in the yield of 71%. The single crystals of (I) suitable for the X-ray crystallographic analysis were obtained by recrystallization from benzene/hexane as colorless blocks.

Refinement
All of the H atoms were positioned geometrically with C-H of 0.95Å and were constrained in a riding motion on their parent carbon atoms with U iso (H) = 1.2 U eq (C).  N,N,N',N'-tetraphenyl-1,1'-biphenyl-4,4

Special details
Geometry. ENTER SPECIAL DETAILS OF THE MOLECULAR GEOMETRY Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . Rfactor (gt) are based on F. The threshold expression of F 2 > 2.0 σ(F 2 ) is used only for calculating R-factor (gt).