Crystal structure, thermal and fluorescence properties of 2,2′:6′,2′′-terpyridine-1,1′,1′′-triium tetrachloridonickelate(II) chloride

The synthesis, and structural determination of 2,2′:6′,2′′-terpyridine-1,1′,1′′-triium tetrachloridonickelate(II) chloride are reported. The crystal structure features N—H⋯Cl and C—H⋯Cl hydrogen bonds and Ni—Cl⋯π ring interactions.

The title compound, (C 15 H 14 N 3 ) [NiCl 4 ]Cl, comprises an Ni II cation tetrahedrally coordinated by four chloride anions, a non-coordinating chloride anion and an essentially planar terpyridinium trication (tpyH 3 3+ ), in which the central pyridinium ring forms dihedral angles of 5.7 (2) and 6.0 (2) with the peripheral pyridinium rings. Three inter-species N-HÁ Á ÁCl hydrogen bonds are formed with the Cl À anion, which also forms a link between the (tpyH 3 3+ ) cations through an aromatic C-HÁ Á ÁCl interaction, forming a zigzag chain extending along the 2 1 (b) screw axis. Two of the anionic Cl atoms of the [NiCl 4 ] 2À anions form Ni-ClÁ Á Á interactions with separate pyridinium rings [NiÁ Á ÁCg = 3.669 (3) and 3.916 (4) Å ]. In the crystal, successive undulating inorganic and organic layers are formed, extending across the (100) plane. Thermogravimetric and differential thermal analysis (TGA/DTA) indicate that the compound starts to decompose at 313 K and may be a candidate for use as a blue-light luminescent material.

Chemical context
The 2,2 0 :6 0 ,2 00 -terpyridine molecule (tpy) has been the object of numerous studies because of its excellent complexing properties on metal ions. The multitude of applications of this cation motivated a large development in the synthesis of terpyridines during the last decade. The compounds derived from the terpyridine molecule can be used in photochemistry for the realization of luminescent materials (Adeloye et al., 2012), the assembly of electrochemical sensors (Indelli et al., 1998), in photocatalysis (Mori et al., 2012) and as a sensitizing agent in photovoltaic conversion processes (Kohle et al., 1996). The literature reports some hybrid complexes of transition metal species incorporating tpy as a neutral ligand as well as complexes with its protonated forms [(tpyH + ), (tpyH 2 2+ ), (tpyH 3 3+ )] (Kochel, 2006). The title compound, which is a new hybrid complex, was characterized using IR spectroscopy and X-ray crystallography and its thermal and fluorescence properties have also been recorded.

Figure 2
The nickel tetrahedral environment.  decompose (ÁP/P = 23.14%, calculated = 22.51%). In addition, the corresponding endothermic peaks (at 394.16; 554.63 C and at 638 K) in the differential scanning ATD curve also record the processes of weight loss.

Luminescent properties
Photoluminescence spectra were measured using a Cary Eclipse (Agilent Technologies) fluorescence spectrophotometer.
The fluorescence properties of (C 15 H 14 N 3 )[NiCl 4 ]Cl and the free ligand tpy were investigated in the solid state at 298 K. As depicted in Fig. 6, the new compound (I) exhibits fluorescence emission at ca 481 nm (excited at 250 nm) compared to that of tpy (425 nm, excited at 250 nm), which can be attributed to -* electronic transitions. Thus, the title compound may be a candidate for use as a blue-light luminescent material and it is believed that more transition metal heterocyclic compounds with good luminescent properties may be developed (Wen et al., 2007;Zhang et al., 2010;Huang et al., 2013).

Synthesis and crystallization
All the chemicals and solvents were purchased commercially and used as received. The infrared spectra were recorded on a Perkin-Elmer spectrometer at room temperature in the range of 4000-500 cm À1 . tpy (1.67 g, 10 mmol) was dissolved in a 50/ 50 mixture of water and ethanol (20 ml) in a 50 ml roundbottom flask. Nickel(II) chloride (2.50 g, 10 mmol) was added to the flask to give a green-coloured solution that was stirred for 3 h under gentle heat, producing a green-coloured precipitate. The precipitate was filtered and washed twice with cold water/ethanol solvent then dried under vacuum for 20 min, producing a green powder (2.7g, 64% yield). Green prismatic crystals of the title complex (I) suitable for X-ray analysis were obtained from water/ethanol solvent. IR of (I) (cm   The thermogravimetric (TG) and differential thermal analysis (DTA) curves.

Figure 6
The solid-state fluorescence spectrum of tpy and the title compound (I) (excitation at 250 nm).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed at calculated positions and refined as riding atoms, with C-H = 0.93 Å , N-H = 0.86 Å and with U iso (H) = 1.2U eq (C,N).
Although not of relevance with this achiral molecule, the Flack parameter (Flack, 1983) was determined as 0.178 (16) for 4425 Friedel pairs. Minor non-merohedral twinning was identified and allowed for in the refinement, giving a BASF factor of 0.1783. Mercury (Macrae et al., 2008) and POVRay (Persistence of Vision, 2004).

(I)
Crystal data (C 15  Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles 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 >2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.