(E)-6,6′-(Diazene-1,2-diyl)bis(1,10-phenanthrolin-5-ol) trichloromethane disolvate: a superconjugated ligand

The preparation and structural characterization of the diazo-diphenanthroline compound, (E)-6,6′-(diazene-1,2-diyl)bis(1,10-phenanthrolin-5-ol) are described. The fully conjugated bis-phenanthroline molecule is expected to offer exciting new physical and chemical properties, and should form the basis of novel metal coordination complexes as a consequence of the dual N,N′-1,10-phenanthroline chelating moieties situated on the opposite ends of the molecule.

Phenanthroline ligands are important metal-binding molecules which have been extensively researched for applications in both material science and medicinal chemistry. Azobenzene and its derivatives have received significant attention because of their ability to be reversibly switched between the E and Z forms and so could have applications in optical memory and logic devices or as molecular machines. Herein we report the formation and crystal structure of a highly unusual novel diazo-diphenanthroline compound, C 24 H 14 N 6 O 2 Á2CHCl 3 .
The bond lengths indicate some delocalization through the central part of the molecule. The C6-O1and C5-N2 bonds are short [1.318 (3) and 1.376 (3) Å , respectively] and the N N bond, at 1.316 (4) Å , is significantly longer than in most free diazo molecules [mean of 1.24 (4) Å for 2730 CSD entries]. Fig. 2 shows the unit-cell packing, the molecules lie parallel to the (5 7 15) or (5 7 14) planes with a mean interplanar distance of 3.228 (2) Å (under 1 À x, 1 À y, 1 À z) and the axis of the stack runs parallel to the a axis. The shortest ring centroidcentroid distance is 3.5154 (15) Å between the C 6 rings; however, there is a more direct overlap between the diazo group and an imine group in the next layer (ca 3.246 Å between the mid-point of the N N bond and the mid-point of the C11 N3 bond under symmetry operation 1 À x, 1 À y, 1 À z) (Fig. 3). The most notable interactions between stacks are type 1 R-ClÁ Á ÁÁCl-R packing interactions (Mukherjee et al., 2014;Cavallo et al., 2016), the shortest ClÁ Á ÁCl distance being 3.5353 (11) Å for Cl1Á Á ÁÁCl3 under symmetry operation À1 + x, y, z.

Spectroscopy studies of 1 in solution
Compound 1 has a very low solubility in all organic solvents investigated (CH 3 Cl, CH 2 Cl 2 , DMSO, CH 3 CN and alcohols).  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Unit-cell packing diagram viewed parallel to the plane of 1Á2CHCl 3 . Hydrogen atoms not involved in hydrogen bonding have been omitted for clarity.

Figure 3
Principal intermolecular interactions in 1Á2CHCl 3 . Purple spheres represent ring centroids and orange spheres show bond mid-points; distances in Å .

Figure 1
Perspective view of 1Á2CHCl 3 showing the labelling scheme for the asymmetric unit with displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms are shown as spheres of arbitrary radius and C-HÁ Á ÁN hydrogen bonds are indicated by dashed red lines.
UV/vis spectra of 1Á2CHCl 3 recorded in ethanol, methanol, CHCl 3 and CH 2 Cl 2 are given in Fig. 4. Compound 1 exhibits significant solvatochromism showing a broad band in CHCl 3 and CH 2 Cl 2 with max at 543 nm. This band undergoes a bathochromic shift and separates into two bands in the alcohols with max values at 643 nm and 600 nm in ethanol and at 636 nm and 591 nm in methanol. No accurate measurement of the extinction coefficient could be made as 1 was not fully soluble in the solvents and precipitation from the solvent occurred upon standing. The solubility of 1 was so low that only a very poorly resolved 1 H NMR of the compound was obtained in d 4 -methanol showing a series of peaks in the region expected for the phenanthroline H-atom signals and no definitive assignments of the peaks could be made. Interestingly, 1 was more soluble in strongly acidic solutions due to the protonation of one or more of the N atoms. Compound 1 dissolved in CF 3 COOD to form a bright-red solution. Six signals are observed in the 1 H NMR spectrum in the region associated with the phenanthroline peaks. This finding is consistent with a compound which has a centre of symmetry, as found for the crystal structure, and suggests that at room temperature 1 remains in the E form in this solvent.

Synthesis and crystallization
The mechanism to form 1 from the reaction mixture is unclear; however, the formation of isonicotinic acid N 0 -(pyridine-4carbonyl)-hydrazide (2) from the mixture is an indication that radical chemistry is occurring. Isoniazid is well known to react to form radical species and these radicals are important in its role as an anti-tuberculosis drug (Timmins et al. 2006). Significantly one LC-MS study has shown that isoniazid will photo-degrade to form 2 (Fig. 5) and radical intermediates are proposed in its formation (Bhutani, 2007). Attempts were made to try to favour the formation of 1 using UV irradiation and the radical initiators azobisisobutyronitrile and 2,2 0azobis(2-amidinopropane) dihydrochloride but these were unsuccessful. However, although the reaction to form 1 was low yielding, attempts to make 1 by reaction of 6-amino-1,10phenanthrolin-5-ol and 6-nitroso-1,10-phenananthrolin-5-ol using known conditions to form diazo compounds (Zhao et al. 2011) did not form the desired product. Studies are ongoing to improve the yield of the reaction to form 1. Phendione (0.210 g, 1.000 mmol) was added to solution of isoniazid (0.137 g, 1.000 mmol) in EtOH (25 cm 3 ). p-Tolouenesulfonic acid (10%, 0.02 g) was added and the solution refluxed for 6 h. The resulting suspension was filtered whilst hot and the filtrate allowed to stand in the dark overnight. Precipitated yellow (Z)-N 0 -(6-oxo-1,10-phenanthrolin-5(6H) ylidene)isonicotinohydrazide (0.263 g, 0.799 mmol, 80%) was filtered off and the bright-orange filtrate was concentrated to ca 10 cm 3 using rotary evaporation and then allowed to stand in the dark. Over a period of four weeks, the bright-orange filtrate changed to a dark-green suspension. This mixture was heated to reflux and filtered whilst hot to give a green filtrate (see below) and a dark-purple powder. The powder was dissolved in CHCl 3 and allowed to crystallize over several days to produce dark-purple crystals of (E)-6,6 0 -(diazene-1,2-diyl)bis(1,10-phenanthrolin-5-ol) trichloromethane disolvate (1Á2CHCl 3 ) (0.026 g, 0.039 mmol, 6.2% based on isoniazid starting material). Upon leaving the above green filtrate to evaporate further white isonicotinic acid N 0 -(pyridine-4-carbonyl)-hydrazide (compound 2) precipitated. The supernatent was decanted off and the solid dissolved in hot acetone. This colourless solution was evaporated to dryness on a rotary evaporator to give 2 (0.015 g, 0.062 mmol, 12.4% based on isoniazid starting material).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-bound H atoms were included in calculated positions and treated as riding, with C-H = 0.95-1.00 Å and U iso (H) = 1.5U eq (C) for methyl H atoms or 1.2U eq (C) otherwise. The H atom (H1A) bonded to oxygen was located in a difference-Fourier map and its coordinates were refined with U iso (H) = 1.5U eq (O).  program(s) used to solve structure: SHELXT (Sheldrick 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.