Crystal structure and photoluminescent properties of bis(4′-chloro-2,2′:6′,2′′-terpyridyl)cobalt(II) dichloride tetrahydrate

In the title hydrated complex, [Co(C15H10ClN3)2]Cl2·4H2O, the complete dication is generated by symmetry. In the crystal, O—H⋯Cl and C—H⋯O hydrogen bonds link the components into (001) sheets. The complex exhibits blue-light emission.


Chemical context
Since the pioneering work of Tang et al. (1987), there has been increasing interest in chelating organic compounds being employed as charge-transporting materials in electronic devices such as OLEDs. Transition-metal complexes are promising candidates for use as hole-transporting materials as the metal ions can assume variable oxidation states and are found to exhibit low kinetic barriers for self-exchange reactions (Marcus, 1965).
Single-layer device structures that make use of Ru II complexes involving bipyridine and its derivatives not only show the potential to transport both holes and electrons but also exhibit luminescent properties (Rudmann & Rubner, 2001;Gao & Bard, 2000). Reports of the application of cyclometalated Ir III complexes in vapour-deposited OLEDs both as efficient emissive and charge-transporting materials (Adamovich et al., 2003;Grushin et al., 2001) and the luminescent properties of a distorted octahedral Ni II complex with 5,5 0 -dimethyl-2,2 0 -bipyridine have been published (Abedi et al., 2015). The synthesis and a study of the thermal and luminescent properties of d 8 transition-metal complexes with the incorporation of substituted 2,2 0 ;6 0 ,2 00 -terpyridine ligands were described by Momeni et al. (2017). ISSN 2056-9890 As an extension of such studies, we now report the synthesis, structure, spectroscopic characterization and thermal behaviour of the title complex, (I).

Structural commentary
The [Co(C 15 H 10 N 3 ) 2 Cl 2 ] 2+ cation in (I) is symmetric (the metal atom lies on a special position with 4 site symmetry; atoms N2, C8 and Cl1 lie on a crystallographic twofold axis), thus the asymmetric unit contains half of the ligand coordinated to the cobalt ion, one water molecule of crystallization (O atom site symmetry 1) and half of a chloride counter-ion (site symmetry 2) (Fig. 1). The complex shows distortion from an ideal octahedral geometry for the metal ion with two N1-Co1-N1 bond angles being 160.62 (9) . However, the N2-Co1-N2 bond angle is 180 , as it lies on the rotoinversion axis. The coordinated ligand is almost planar with the r.m.s. deviation of all the non-hydrogen atoms being 0.025 Å . Moreover, the dihedral angle between the ligands is 90.0 , as constrained by the presence of the rotoinversion axis.

Supramolecular features
The unit cell of (I) contains four cations, which are electrically balanced by eight chloride ions along with sixteen water molecules of crystallization (Fig. 2). In the crystal structure, two pairs of O-HÁ Á ÁCl hydrogen bonds between water molecules and chloride ions [O2-H2O1Á Á ÁCl2 and O2-H1O1Á Á ÁCl2] link the components into infinite (001) sheets (Table 1).

Thermal and photoluminescence studies
Thermogravimetry (TG) and differential thermal analysis (DTA) on (I) show progressive decomposition in several steps.   The molecular structure of (I) with displacement ellipsoids drawn at the 50% probability level. The complete cation of the complex is generated by applying the symmetry operations (a) Ày + 5 4 , x + 1 4 , Àz + 5 4 , (b) Àx + 1, Ày + 3 2 , z and (c) y À 1 4 , Àx + 5 4 , Àz + 5 4 . Table 1 Hydrogen-bond geometry (Å , ). The first mass loss (obs. 10.0%, calc. 9.8% over the temperature range 60-140 C) is attributed to the loss of the water molecules of crystallization, accompanied by endotherms at 78 and 134 C. The second mass loss over the temperature range 200-310 C accompanied by a DTA peak at 306 C is probably due to the decomposition of one ligand with an estimated mass loss of 36.1% (calcd. mass loss 36.2%). Powder XRD of the final residue after heating to 800 C indicated the presence of cobalt oxy hydroxide, CoO(OH) and Co 3 O 4 (Sulikowska et al. , 2000). The diffuse reflectance (DR) spectrum of (I) was scanned in the wavelength range 200-1100 nm and an absorption band appeared in the visible region as shown in Fig. 3a. A prominent peak with a diffuse reflectance percentage of 5.4 is observed at 640 nm. The Kubelka-Munk function (Harry, 1976) ( Fig. 3b) was used in order to determine the HOMO-LUMO gap for (I): the band gap energy obtained from the plot was found to be 2.23 eV (Morales et al., 2007).
The excitation and emission spectra of (I) recorded at room temperature are shown in Fig. 4a and b. The excitation spectrum shows features at 318, 339, 382 and 395 nm. From the emission spectrum, three well-defined peaks at 436, 541 and 653 nm are apparent for (I). The determination of chromaticity co-ordinates [Publication CIE No 15.2 (1986) and17.4 (1987)] was carried out at an excitation wavelength of 395 nm.
The estimated CIE values for the probable excitation are incorporated in the left corner of Fig. 4c. The colour of emission for the highlighted phosphor is indicated in the chromaticity diagram by the solid circle sign (star), which indicates that the emission colour is blue.

Database survey
A search of the Cambridge Structural Database gave 90 matches for crystal structures containing the 4 0 -chloro-2,2 0 ;6 0 ,2 00 -terpyridine (L) ligand. Closely related complexes to (I) with a pair of chelating L ligands generating an MN 6 coordination sphere include the nickel and iron complexes

Synthesis and crystallization
A solution of 4 0 -chloro-2,2 0 ;6 0 ,2 00 -terpyridine (2) (0.535 g, 2.00 mmol) in 3 ml of ethanol was stirred at 333 K for about 30 min and an aqueous solution of cobalt(II) chloride hexahydrate (1) (0.2379 g, 1.00 mmol) dissolved in 2 ml of water was added slowly and the resulting solution was refluxed for one h. The brown solution obtained was subjected to slow evaporation at room temperature and was finally triturated with toluene to recover the powdered form of the title complex. The solid product was then kept in a desiccator in order to achieve constant weight (yield 0.584 g; 87.8%).
Simultaneous TG/DTA measurements were carried out using a Perkin-Elmer Diamond TG/DTA analyser. A Perkin-Elmer Lambda-35 UV-visible spectrophotometer and Moriba spectrofluorimeter equipped with a 450 W xenon lamp as an excitation source were used to obtain the diffuse reflectance and photoluminescence spectra, respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in   Photoluminescence spectra of (I); (a) excitation spectrum (b) emission spectrum (c) CIE graph O-H = 0.82 (2) Å . The carbon-bound H atoms were placed in calculated positions (C-H = 0.93 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C).  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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )