A strongly fluorescent NiII complex with 2-(2-hydroxyethyl)pyridine ligands: synthesis, characterization and theoretical analysis and comparison with a related polymeric CuII complex

A novel mononuclear nickel complex coordinated by 2-(2-hydroxyethyl)pyridine has been synthesized and structurally characterized by X-ray diffraction techniques and photoluminescence spectroscopy. TDDFT calculations have been performed to rationalize the structure explored for this and a related polymeric Cu complex.

In the two complexes, the 2-(2-hydroxyethyl)pyridine ligands coordinate the metal ions through the N atom of the pyridine ring and the O atom of the hydroxy group, creating a chelate ring. The Ni II or Cu II ion lies on an inversion centre and exhibits a slightly distorted MO 4 N 2 octahedral coordination geometry, build up by O and N atoms from two 2-(2-hydroxyethyl)pyridine ligands and two water molecules or two O atoms belonging to sulfate anions. The sulfate anion bridges the Cu II ions, forming a polymeric chain. The photoluminescence properties of these complexes have been studied on assynthesized samples and reveal that both compounds display a strong blue-light emission with maxima around 497 nm. From DFT/TDDFT studies, the blue emission appears to be derived from the ligand-to-metal charge-transfer (LMCT) excited state. In addition, the IR spectroscopic properties and thermogravimetric behaviours of both complexes have been investigated.

Chemical context
A wide variety of nitrogen-containing heterocyclic ligands has been used to construct coordination complexes (Lin et al., 2015;Kim et al., 2015;Huang et al., 2015). In particular, pyridine alcohol derivatives and their metal complexes have been studied extensively in recent years, focusing on the rational design and synthesis of coordination monomers and polymers because of their intriguing structural features as well as potential applications in catalysis and fluorescence and as chemical sensors (Ley et al., 2010). Moreover, luminescent compounds have also attracted attention because of their applications, particularly in modern electronics, as materials for producing organic light-emitting diodes (OLEDs) (Kelley et al., 2004). The 2-(2-hydroxyethyl)pyridine (hep-H) ligand may adopt many coordinating variants because of its donating capabilities: N-monodentate (N) (Martínez et al., 2007), N,Ochelating ( 2 N,O) (Antonioli et al., 2007); deprotonated ISSN 2056-9890 chelating ( 2 N,O) (Antonioli et al., 2007) and bridging (N:O) (Antonioli et al., 2007) (Stamatatos, Boudalis et al., 2007).
We are in particular interested in the hep-H ligand, which has attracted much attention in biology and chemistry because it is a useful model and for its practical applications (Kong et al., 2009;Mobin et al., 2010). The hep-H ligand could be a good candidate to construct simultaneously nitrogen heteroaromatic alcohol coordination monomers and polymers with interesting magnetic behaviour. On the other hand, with Ni II and Cu II metals, the hep-H ligand could also be a desirable candidate for fluorescent materials. The flexible coordination sphere around the Ni II and Cu II ions, in combination with steric and packing forces, is one of the effects that gives rise to a wide structural diversity in Ni II /Cu II coordination chemistry (Comba & Remenyi, 2003

Structural commentary
In the mononuclear title Ni II complex 1 as well as in the polymeric Cu II complex 2 (Zeghouan et al., 2016;Zienkiewicz-Machnik et al., 2016), the metal ions are located on inversion centers with the neutral hep-H molecule acting as a bidentate ligand in a 2 N,O fashion and forming the equatorial plane of an octahedron, the apex of which is occupied by the water molecules in the case of the Ni II complex or an O atom of an SO 4 2À anion in the Cu II complex ( Figs. 1 and 2). The main difference between the two structures is the occurrence of the SO 4 2À anion in 2, which links complex molecules, forming a polymeric chain. Moreover, in this structure the asymmetric unit contains two half molecules of the complex. In the Ni II complex, two nitrate anions balance the charges. The coordination environment around the nickel ions can be described as a nearly perfect octahedron. The O1-Ni1-N1 [88.68 (3) ], N1-Ni1-O1W [90.00 (4) ] and O1-Ni1-O1W [89.26 (4) ] angles are all very close to 90 (Table 1). The two hep-H ligands are trans with respect to each other. The hydroxyl O atom and the pyridine N atoms define the equatorial plane while the water molecules occupy the apices. In the case of the Cu II complex, the octahedron is slightly distorted with the angles around the metal ranging from 83.39 (5) to 96.62 (5) . This distortion might result from the influence of the SO 4 2À linking the Cu complex to form a polymeric chain.
In both complexes, the chelate ring displays a twist-boat conformation with puckering parameters = 81.9 and ' = 162 for 1 and = 79.2 and ' = 159.9 and = 87.75 and '= 176.08 for the two molecules of 2. research communications Figure 1 View of the Ni complex, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as circles of arbitrary radii. Hydrogen bonds are shown as dashed lines.

Supramolecular features
Although not coordinated to the Ni atom, the nitrate anion in 1 participates in the packing motif. The hydroxyl group and water molecules are involved in strong O-HÁ Á ÁO hydrogen bonds (Table 2) with the O atoms of the nitrate anions, resulting in the formation of R 4 4 (12) and R 4 4 (16) graph-set motifs, as shown in Fig. 3, building up a three-dimensional network. C-HÁ Á ÁO hydrogen bonds also occur.  Table 3. There are no notable differences between the Ni-O(H) distances, which range from 2.057 (2) to 2.114 (1) Å , and the Ni-O(ligand) bonds, ranging from 2.052 (1) to 2.112 (2) Å . Clearly the organic substituent attached to the O atom in the axial position has no real influence on the Ni-O(R) bond length. The dihedral angles between the pryridine ring and the NiN 2 O 2 basal square plane range from 28.3 to 37.6 . The largest angle is observed for two polymeric structures in which the succinato or adipato organic ligand bridge the Ni atoms, forming a chain; this is possibly related to steric effects. A similar search on the Cu(Hep-H) 2 O 2 fragment gave seven hits. The major differ- Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 2; Ày þ 1; Àz þ 2; (ii) Àx þ 1; Ày; Àz þ 2; (iii) x À 1; y; z; (iv) Àx þ 1; Ày þ 1; Àz þ 2; (v) Àx þ 1; Ày; Àz þ 1.

Thermogravimetric and differential thermal analysis
Thermal analyses were performed on a SETARM 92-16.18 PC/PG 1 instrument from 303 to 1273 K under a dynamic air atmosphere and under nitrogen at 200.0 ml min À1 with a heating rate of 283 K min À1 . The stability of the two complexes was measured by TGA and the experimental results are in agreement with the calculated data.
The TG curve for 1 (Fig. 4a) shows that the monomer is stable up to 424 K with the first weight loss of 33.55% (calculated 34.21%) at 303 À438 K corresponding to the loss of two coordinated water molecules and the organic hep-H ligand. The second loss of 47.51% (calculated 40.02%) at 438-488 K corresponds to the loss of the second hep-H ligand and the nitrate anion, and then the second nitrate anion decomposes (DP/P = 12.14%, calculated =13.34%). In addition, the corresponding endothermic and exothermic peaks (at 424.26, 475.06 and 631.85 K) in the differential scanning ATD curve also record the processes of weight loss. As illustrated in Fig. 4b, the TG curve for 2 shows that the polymer is stable up to 470 K with the first weight loss of 24.16% (calculated 23.66%) at 470-483 K corresponding to the loss of the sulfate anion and the second loss of 29.42% (calculated 30.30%) at 483-573 K to the loss of the first hep-H ligand, and then the second hep-H ligand decomposes (DP/P = 28.55%, calculated =30.30%). In addition, the corresponding endothermic and exothermic peaks (at 473, 558 and 773 K) in the differential scanning ATD curve also record the processes of weight loss.

Luminescence properties
Photoluminescence spectra were measured using a Cary Eclipse (Agilent Technologies) fluorescence spectrophotometer with quartz cell (1 Â 1 cm 2 cross-section) equipped with a xenon lamp and a dual monochromator. The measurements were carried out at ambient temperature (298 K) with the slit ex/em = 10 nm/10 nm. The photoluminescence properties of 1, 2 and free hep-H in an ethanolwater (v/v = 1:1) solution were investigated in the visible region. As shown in Fig. 5  The thermogravimetric (TG) and differential thermal analysis (DTA) curves for (a) the monomer and (b) the polymer. Table 3 Comparison of selected geometrical parameters (%, Å , ) for Ni II and Cu II complexes bearing the hep-H ligand.
Á is the dihedral angle between the basal MO 2 N 2 square plane and the pyridine ring.

Ref.
R  (2)  is combined with Ni II or Cu II in 1 or 2, an intense blue emission band is seen at em / ex = 498.03 nm/250.93 nm or 496.96 nm/ 250.00 nm respectively. This should probably be assigned to the -* charge-transfer interaction of the hep-H ligands. The observed blue shift of the emission maximum between 1, 2 and free hep-H is considered to originate mainly from the influence of the coordination of the metal atoms to the hep-H ligand (Leitl et al., 2016). Thus, these compounds may be candidates for blue-light luminescent materials which suggests that more transition metal, pyridine alcohol compounds with good luminescent properties can be developed.

TDDFT calculations
In an effort to better understand the nature of the electronic transitions exhibited by compounds 1 and 2, DFT calculations using the Amsterdam density function (ADF) software (Baerends et al., 1973) along with generalized gradient approximations, exchange and correlation functional GGA (PBE) (Perdew et al., 1997), employing the TZP (triple zeta polarized) basis set. The singlet excited state was optimized using time-dependent density functional theory calculations (TDDFT) (Bauernschmitt & Ahlrichs, 1996;Gross & Kohn, 1990;Gross et al., 1996). The ground-state geometry of 1 and 2 was adapted from the X-ray data. The calculated structural parameters show a good agreement with the original X-ray diffraction data (Table 1); the root-mean-square deviation f between the X-ray and the DFT structure for non-hydrogen atoms is 0.603 and 0.620 Å for 1 and 2, respectively. The computed absorption bands, dominant transitions, characters, and oscillator strengths (f) are given in Table 4. As shown in this table, two absorption features are predicted for the monomer; these mainly consist of absorption peaks located at = 286 and 280 nm, resulting from the HOMO-2 to LUMO transition and the HOMO-3 to LUMO transition, which is attributed to a ligand-metal charge transfer (LMTC) (Fig. 6a). Three absorption features are predicted in the polymer, consisting mainly of absorption peaks that are located at = 507, 443 and 244 nm, resulting from HOMO-4 to LUMO, HOMO-5 to LUMO and HOMO-2 to LUMO transitions, which are attributed to a ligand-metal charge transfer (LMTC) (Fig. 6b) Figure 6 Plots of the molecular orbitals dominating the contribution of the low-energy transitions for (a) the monomer and (b) the polymer.

Figure 5
The fluorescence spectrum of the hep-H ligand and the title compounds (excitation at 250 and 269.70 nm for the complexes and hep-H, respectively)

Synthesis and crystallization
All chemicals and solvents were commercially purchased 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 . The hep-H ligand was obtained from commercial sources. The synthesis of the two compounds followed the same procedures as previously described for the Co II analog (Zeghouan et al., 2013). (2-Hydroxyethyl)pyridine (10.0 mmol, 1.67 g) was reacted in a mixture of ethanol-water (v/v = 1:1) with Ni(NO 3 ) 2 Á6H 2 O (10.0 mmol, 2.50 g) for the Ni II analogue and with Cu(SO 4 ) 2 Á6H 2 O (10.0 mmol, 2.3 g) for the Cu II analogue. The solutions were maintained under agitation for 24 h at room temperature. Green prisms of the monomer and green prisms of the polymer were obtained by slow evaporation of the solvents within three weeks. The crystals formed were filtered and washed with 15 ml of water.