Crystal structures of 1′-aminocobaltocenium-1-carboxylic acid chloride monohydrate and of its azo dye 1′-[2-(1-amino-2,6-dimethylphenyl)diazen-1-yl]cobaltocenium-1-carboxylic acid hexafluoridophosphate monohydrate

Two asymmetrically substituted cobaltocenium carboxylic acid compounds were synthesized and their crystal structures determined. Both crystallize as hydrates and exhibit an extended hydrogen-bonding network.

The molecular and crystal structures of compounds 3 and 5 are reported in this communication.

Structural commentary
Compounds 3 and 5 both crystallize as their monohydrates. Compound 3 forms crystals with one formula unit per asymmetric unit (Fig. 2). The cobalt atom is coordinated in a nearly eclipsed manner by the planar cyclopentadienide rings with a torsion angle of 15 between the substituents, but the bond lengths between Co and C are not equal. In the carboxylsubstituted ring, the shortest distance [2.028 (3) Å ] is found between Co1 and C10, the atom bearing the carboxyl group, as is to be expected from the electron-poorest carbon atom. Bond lengths involving the other four carbon atoms in this ring are considerably longer [Co-C averaged = 2.052 Å ]. On the other hand, in the amino-substituted ring, the N-bonded carbon atom C1 shows a significantly longer bond length [2.153 (3) Å ] to Co1 than the other four carbon atoms in this ring [Co-C averaged = 2.031]. In addition, the formal C-N single bond [C1-N1 = 1.343 (4) Å ] of the amino substituent is considerably shortened, as has also been observed in aminocobaltocenium tetraphenylborate [C-N = 1.340 (3) , 2014]. This is caused by the contribution of a mesomeric structure featuring an 4 -bound cyclopentadiene with an iminium group, a general effect observed in donorsubstituted cobaltocenium salts (Sheats, 1979). The bond lengths and angles of the carboxyl substituent are unexceptional and in line with expectations.
In the cobaltocenium cation of 5, the cyclopentadienide rings are almost staggered with the substituents oriented in roughly the same direction and a torsion angle of 29 (s.u.?) (Fig. 3). The Co-C ring distances show no great variation, with the exception being the bond to C6, i.e. the carbon atom connected to the azo group [2.064 (2) Å ]. This bond is slightly elongated but not as much as the corresponding bond to the amino group in the structure of 3. The azo group features a trans-configuration with distances typical for asymmetric azo compounds.

Supramolecular features
The water molecule of crystallization, carboxyl group, amino group and chloride anion of 3 are part of an extended hydrogen-bonding network in the crystal (Fig. 4 The molecular entities in the structure of 3 with displacement ellipsoids for non-H atoms drawn at the 50% probability level.

Figure 3
The molecular entities in the structure of 5 with displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms were omitted for clarity.
Zigzag chains are aligned parallel to the c axis (Fig. 5), in which every other molecule shows the same orientation. These chains are formed by an infinite hydrogen-bonding network, comprised of water molecules connecting the carboxyl groups of two neighboring cations and also forming a bond to the chloride anion. The chloride anions are also hydrogen-bonded to the NH 2 groups of two more cations, therefore forming a ladder-type network in which the ladders are connected to each other by the cobaltocenium moieties (Fig. 6). Overall, this arrangement results in an undulating layer structure extending parallel to (100) (Fig. 7 Hydrogen-bonding interactions between the amino group, the carboxyl group, the water molecule of crystallization and the counter-anion in the crystal structure of 3. Displacement ellipsoids as in Fig. 2. [Symmetry codes: (i) Àx þ 1 2 ; y; z þ 1 2 ; (ii) x; y þ 1; z; (iii) Àx þ 1 2 ; y; z À 1 2 .]

Figure 5
A view along the b axis of the crystal structure of 3 showing the formation of zigzag chains parallel to the c axis. Displacement ellipsoids as in Fig. 2.

Figure 6
Ladder-type hydrogen-bonded network in the crystal structure of 3. Displacement ellipsoids as in Fig. 2.

Figure 7
Formation of undulating layers parallel to (100) in the crystal structure of 3. Displacement ellipsoids as in Fig. 2.
In the crystal structure of 5, the azo, carboxyl, amino groups and the water molecule of crystallization are part of a hydrogen-bonded network ( Table 2). Dimers result from hydrogen bonds between the amino function (N3-H) of one molecule and the carboxylic acid group (O1) of a neighbouring molecule. Additionally, these dimers are connected to one another by water molecules (O3), forming hydrogen bonds involving the carboxylic acid group (O1) and the azo group (N1). In addition, the disordered hexafluoridophosphate ions interact with the otherwise unbound second hydrogen atom of the water molecule and the second hydrogen atom of the amino functionality ( Fig. 8), thereby forming layers parallel the bc plane that separate layers of cations ( Fig. 9).

Synthesis and crystallization
Compound 3: 1 0 -Aminocobaltocenium-1-carboxylic acid chloride hydrate, 3, was obtained in varying yields starting from cobaltocenium-1,1 0 -bis carboxylic acid hexafluoridophosphate by converting it first to its mono carboxylic azide followed by Curtius rearrangement, in a variant analogous to monosubstituted cobaltocenium carboxylic acid hexafluoridophosphate (Vanicek et al., 2016). Column chromatography on alumina using methanol/water as eluent, separated it from 1,1 0 -diaminocobaltocenium, which was eluted before with acetonitrile. After addition of hydrochloric acid to hydrolyze the methoxyaluminum species, the volatiles were evaporated, the residue extracted with ethanol, filtered and dried first on a rotary evaporator and then in vacuo. Single crystals were obtained via slow concentration of a solution in methanol. 1 H NMR (CD 3 OD), ppm: = 5.16 (pseudo-t, J = 2.1 Hz), 5.48 (pseudo-t, J = 2.1 Hz), 5.51 (pseudo-t, J = 2.1 Hz), 5.97 (pseudo-t, J = 2.1 Hz). ESI-MS showed a signal at 248.0139 m/z in accordance to the molecular cation.
Compound 5: 1 0 -Aminocobaltocenium-1-carboxylic acid chloride hydrate (3) (100.9 mg, 0.3345 mmol, 1 equivalent) was dissolved in 5 ml of concentrated HCl and the mixture was cooled to 273 K. Then NaNO 2 (26.6 mg, 0.3850 mmol, 1.15 equivalent) was added and the yellow solution was stirred for 15 min. After addition of 2,6-dimethylaniline (63.5 ml, 0.5134 mmol, 1.5 equivalents), the solution immediately turned red and was stirred for a further 30 min. When neutralized with saturated Na 2 CO 3 solution, the reaction mixture again changed color to a darker red. The mixture was concentrated on a rotary evaporator and the salts were precipitated with ethanol. The solution was filtered, evapo-research communications Table 1 Hydrogen-bond geometry (Å , ) for 3.

Figure 8
Formation of hydrogen-bonded dimers in the crystal structure of 5. Displacement ellipsoids as in Fig. 3; hydrogen atoms were omitted for clarity.

Figure 9
Molecular packing of the crystal structure of 5 in a view along the c axis, showing the alternating anionic and cationic layers parallel to the bc plane. Displacement ellipsoids as in Fig. 3. rated to dryness, the residue taken up in acetonitrile and after filtering and evaporating to dryness the product was dissolved in small amounts of water, and a few drops of aqueous HPF 6 (60%) were added. The solution was extracted three times with dichloromethane, the combined dark-violet-colored organic phases were evaporated to dryness and the product (5) was dried in vacuo. Yield: 92.1 mg (52.2%) as a dark orange-red powder. Slow concentration of a solution in ethanol yielded single crystals suitable for X-ray analysis. 1 H NMR (CD 3 OD), ppm: = 2.3 (2,6-Me, t, J = 0.6 Hz), 5.80 (pseudo-t, J = 2.1 Hz), 5.89 (pseudo-t, J = 2.1 Hz), 6.15 (pseudo-t, J = 2.1 Hz), 6.29 (pseudo-t, J = 2.1 Hz), 7.52 (3,5-CH, t, J = 0.6 Hz). ESI-MS showed a signal at 380.0836 m/z in accordance with the molecular cation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. In both compounds, C-bound H atoms were positioned geometrically (C-H = 0.95-0.98) and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (Cmethyl). For the refinement of 3, H atoms bound to N1, O2 and O3 were found in difference-Fourier maps and were treated with restraints on bond lengths (d = 0.89 Å for N and d = 0.83 Å for O) and refined with isotropic displacement parameters. The crystal studied was refined as an inversion twin. For 5, H atoms bound to N3 and O2 were treated in the same way as for 3 while the H atoms of the water molecule (also found from a difference-Fourier map and treated with restraints on the bond length) were refined with U iso (H) = 1.2U eq (O3). The hexafluoridophosphate ion shows positional disorder. Each of the six F atoms was refined with two sets of sites in a 1:1 ratio.  For both structures, data collection: APEX3 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction:

1′-Aminocobaltocenium-1-carboxylic acid chloride monohydrate (3)
Crystal data Absolute structure: Refined as an inversion twin Absolute structure parameter: 0.067 (17) 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. Refinement. Refined as a two-component inversion twin. Hydrogens at N1, O2 and O3 were found and refined isotropically with bond restraints (d = 89 pm for N and d = 83 pm for O).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Co1 0.52686 (2) Hydrogen-bond geometry (Å, º)  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. Refinement. Hydrogen atoms at N3 and O2 were found and refined isotropically with bond restraints (d=89pm for N and d=83pm for). Also the hydrogens at water molecule were found, refined with bond restraints but with isotropic displacement parameter of 1.2 higher than U(iso) of O3. The flourine of the anion PF6-show a nearly 1:1 positional disorder F1-F1: F1A-F6A.