Two 8-Hydroxyquinolinate Based Supramolecular Coordination Compounds: Synthesis, Structures and Spectral Properties

Two new Cr(III) complexes based on 2-substituted 8-hydroxyquinoline ligands, namely [Cr(L1)3] (1), (HL1=(E)-2-[2-(4-nitro-phenyl)-vinyl]-8-hydroxy-quinoline) and [Cr(L2)3] (2), (HL2=(E)-2-[2-(4-chloro-phenyl)vinyl]-8-hydroxy-quinoline), were prepared by a facile hydrothermal method and characterized thoroughly by single crystal X-ray diffraction, powder X-ray diffraction, FTIR, TGA, ESI-MS, UV-Visible absorption spectra and fluorescence emission spectra. Single crystal X-ray diffraction analyses showed that the two compounds featured 3D supramolecular architectures constructed from noncovalent interactions, such as π···π stacking, C-H···π, C-H···O, C-Cl···π, C-H···Cl interactions. The thermogravimetric analysis and ESI-MS study of compounds 1 and 2 suggested that the Cr(III) complexes possessed good stability both in solid and solution. In addition, the ultraviolet and fluorescence response of the HL1 and HL2 shown marked changes upon their complexation with Cr(III) ion, which indicated that the two 8-hydroxyquinolinate based ligand are promising heavy metal chelating agent for Cr3+.


Introduction
Complexes of various metals with 8-hydroxyquinoline (HQ) or its derivatives have attracted considerable attention owning to their biological activities and promising application in organic light-emitting diodes (OLEDs), optical sensing, and so on [1][2][3][4][5][6]. In recent years, particular interest has been given to the supramolecular coordination compounds based on 8-hydroxyquinoline derivatives [7][8][9][10][11]. Because of the ease of preparation and chemical modification of 8-HQ, various 8-hydroxyquinoline derivatives with different substituents are employed to construct metal complexes with desired structure and properties relying on the non-covalent intra-and intermolecular forces [12][13][14]. For example, Yuan et al. recently reported that the luminescent properties of trimeric Zn(II) 8-hydroxyquinolinates tuned by functional substituents [8]. In addition, a wide range of metal ions, including main group, transition, and rare earth metal ions, have been used in preparing 8-hydroxyquinolinates-based supramolecular coordination compounds with the aim to understand the effect of the central metals on supramolecular architectures as well as the properties of the final products [15][16][17][18][19][20][21].
It is known that Cr(III) and Al(III) have very similar ionic radii, coordination numbers (typically six) as well as comparable thermodynamic stabilities. In addition, 8-hydroxyquinolinate complexes

Synthesis of 2
Using the same procedure as that used for 1, except using HL 2 (8.43 mg, 0.03 mmol) instead of HL 1 , brown block crystals of 2 were obtained with a yield of~62.3% based on Cr.

X-ray Crystallography
Single-crystal XRD data for compounds 1 and 2 were collected on a Bruker SMART Apex II CCD-based X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with Mo-Kα radiation (λ = 0.7103 Å) at 296 K, respectively. The structures of compounds 1 and 2 were solved using direct method, and refined by full-matrix least-squares on F 2 , Technical details of the crystal structure solutions and refinements are listed in Table 1. Powder X-ray diffraction (PXRD) data were collected on a DMAX2500 diffractometer (Rigaku, Tokyo, Japan) using Cu Kα radiation. The calculated PXRD patterns were produced using the SHELXTL-XPOW program (Version 5.102, Bruker AXS, Germany) and single crystal reflection data.

Single Crystal Structures
As shown in Scheme 1, two 2-substituted 8-hydroxyquinoline ligands were readily synthesized according the reported procedure [14,28]. A solvothermal reaction of Cr(OAc)3 with HL1 or HL2 (1:3 molar ratio) in DMSO/H2O/EtOH afforded brown crystals of Cr(L1)3 (1) or Cr(L2)3 (2) in good yields. A single-crystal X-ray diffraction study reveals that 1 crystallizes in the triclinic space group P-1, with two crystalgraphic independent Cr(L1)3 molecules in the asymmetric unit. Three 8-hydroxyquinolinate ligands L1 chelate to one Cr centers in the mer-geometry to form a propeller-like Cr(L1)3, thus, the Cr centers adopt an octahedral coordination environment, which was confirmed by the paramagnetic S = 3/2 system (d 3 electron configuration) of Cr center in complex 1 (see Figure 1) [29,30]. In the octahedral geometry, the Cr-O, Cr-N distances are fall in the range of 1.9176(2)-1.949(2) Å and 2.102(2)-2.186(2) Å, respectively (See Table 2), which are comparable with those reported [26]. The three 8-hydroxyquinolinolates are almost perpendicular to each other, with dihedral angles of 85.7, 86.90, 87.31° for Cr1(L1)3 unit and 87.55, 89.10, 86.35° for Cr2(L1)3 unit, respectively. As shown in Figure  2a, multiple intramolecular C-H···O and C-H···N hydrogen bonds could be found between the C-H group of ethenyl and quinoline rings (for Cr1L1, C···O        (Figure 3). The potential solvent-accessible volume for 1 is calculated using Platon, and it suggests void space only constitutes 2.4% of the total crystal volume, i.e., 1 is nearly a non-porous solid (Figure 4a) [31]. To compare the influence of different substituents at 2-position on the supramolecular self-assembly, ligand HL2 was employed to prepare supramolecular coordination compound 2 [12,32]. Compound 2 crystallizes in space group P-1 and is isostructural with compound 1, but the arrangement of building units (Cr(L2)3 molecules) in the supramolecular network is different due to the non-covalent interactions originated from Cl. As depicted in Figure 2b, each of Cr(III) ions in 2 adopts a distorted octahedral geometry constructed from three oxygen and three nitrogen atoms of three L2 ligands. The bond lengths around Cr centers are similar with those in 1 (see Table 1). The two crystallographically unique Cr(L2)3 molecules in the asymmetric unit with a Cr1-Cr2 separation of 8.853(1) Å are linked into a Cr2(L2)6 dimer via the combination of weak π···π stacking (face-to-face distance: 3.828 (18)  . This is primarily due to the replacement of -NO2 group by the chlorine atom on the bidentate chelate ligand, leading to different intermolecular interactions in the solid state [8,9]. In addition, compound 2 is almost non-porous supramolecular coordination polymers with only 2.1% of void space as calculated by PLATON (see Figure 4b) [31].  To compare the influence of different substituents at 2-position on the supramolecular self-assembly, ligand HL2 was employed to prepare supramolecular coordination compound 2 [12,32]. Compound 2 crystallizes in space group P-1 and is isostructural with compound 1, but the arrangement of building units (Cr(L2)3 molecules) in the supramolecular network is different due to the non-covalent interactions originated from Cl. As depicted in Figure 2b, each of Cr(III) ions in 2 adopts a distorted octahedral geometry constructed from three oxygen and three nitrogen atoms of three L2 ligands. The bond lengths around Cr centers are similar with those in 1 (see Table 1). The two crystallographically unique Cr(L2)3 molecules in the asymmetric unit with a Cr1-Cr2 separation of 8.853 (1)   To compare the influence of different substituents at 2-position on the supramolecular self-assembly, ligand HL 2 was employed to prepare supramolecular coordination compound 2 [12,32]. Compound 2 crystallizes in space group P-1 and is isostructural with compound 1, but the arrangement of building units (Cr(L 2 ) 3 molecules) in the supramolecular network is different due to the non-covalent interactions originated from Cl. As depicted in Figure 2b, each of Cr(III) ions in 2 adopts a distorted octahedral geometry constructed from three oxygen and three nitrogen atoms of three L 2 ligands. The bond lengths around Cr centers are similar with those in 1 (see Table 1). The two crystallographically unique Cr(L 2 ) 3 molecules in the asymmetric unit with a Cr1-Cr2 separation of 8.853(1) Å are linked into a Cr 2 (L 2 ) 6 dimer via the combination of weak π···π stacking (face-to-face distance: 3

The stability of 1 and 2
In order to confirm the crystal structure of 1 and 2 are truly representative of their bulk samples, the PXRD experiments have been carried out on the as-prepared sample. As shown in Figure 5, the experimental diffraction patterns fit perfectly with the patterns simulated from their single-crystal structure, indicating the phase purity of bulk samples of 1 and 2. The thermal stabilities of the two crystalline solids were determined by thermal analysis technique. Thermogravimetric analyses (TGA) of 1 and 2 shown no appreciable weight loss until the temperature reaches around 370 • C for 1 and 395 • C for 2, respectively, this thermal behavior is similar to that of other metal quinolates [9,33] (see Figure 6). The TGA results suggest that the two supramolecular architectures are thermally robust, which are attributed to highly polarized Cr-N and Cr-O bonds. In comparison with compound 1, the higher thermal stability may derive from C-Cl···π interactions and C-H···Cl interactions.
The stability of chromium(III) tris-(8-hydroxyquinolinates) in solution is confirmed from the electrospray ionization mass spectra of 1 and 2, where the molecular ion peaks located at m/z = 925.2 and 892.1 are observed, corresponding to [Cr(L 1 ) 3 ] + (calculated M + = 925.1708) and Cr(L 2 ) 3 ] + (calculated M + = 892.0987), respectively (see Figure 7). The robustness of 1 and 2 is mainly due to kinetically inert nature of Cr(III), making it quite slow to undergo ligand exchange [34].

The stability of 1 and 2
In order to confirm the crystal structure of 1 and 2 are truly representative of their bulk samples, the PXRD experiments have been carried out on the as-prepared sample. As shown in Figure 5, the experimental diffraction patterns fit perfectly with the patterns simulated from their single-crystal structure, indicating the phase purity of bulk samples of 1 and 2. The thermal stabilities of the two crystalline solids were determined by thermal analysis technique. Thermogravimetric analyses (TGA) of 1 and 2 shown no appreciable weight loss until the temperature reaches around 370 °C for 1 and 395 °C for 2, respectively, this thermal behavior is similar to that of other metal quinolates [9,33] (see Figure 6). The TGA results suggest that the two supramolecular architectures are thermally robust, which are attributed to highly polarized Cr-N and Cr-O bonds. In comparison with compound 1, the higher thermal stability may derive from C-Cl···π interactions and C-H···Cl interactions.     Figure 7). The robustness of 1 and 2 is mainly due to kinetically inert nature of Cr(III), making it quite slow to undergo ligand exchange [34].

The IR spectra of 1 and 2
The FT-IR spectra of the two bidentate ligands and corresponding Cr-based complexes are shown in Figure 8, and the proposed assignment of complexes are presented in Table 2. The characteristic ν (OH) stretching bands (located at ~3360 cm −1 ) and bending vibration bands (located at ~1200 cm −1 ) in the IR spectra of the two free ligand nearly disappears after coordination to Cr ion. The broad absorptions at ~3410 cm −1 in the spectra of two Cr(III) complexes are probably due to the presence of trace water in the KBr discs. The observation of an absorption bands located at ~3059 and 2925 cm −1 in the IR spectra of free ligands are attributed to the stretching vibrations of aromatic and vinylic C-H, which are here scarcely affected after complex formation. The ν (C=C) bands are found at similar energies as in the free ligands (~1536-1578 cm −1 ), respectively, but the ν (C=N) vibrational bands in the two Cr-based compounds are affected after complexation and exhibited at around ~1595 cm −1 . After deprotonation, the band assigned to the ν (C-O) vibration moved to somewhat higher energies in the complexes (from 1205 to 1276 cm −1 ). In addition, the bands related to Cr-O and Cr-N vibrations are exhibited within region 552-497 cm −1 as refereed in Table 3 [35].

The IR spectra of 1 and 2
The FT-IR spectra of the two bidentate ligands and corresponding Cr-based complexes are shown in Figure 8, and the proposed assignment of complexes are presented in Table 2. The characteristic ν (OH) stretching bands (located at~3360 cm −1 ) and bending vibration bands (located at~1200 cm −1 ) in the IR spectra of the two free ligand nearly disappears after coordination to Cr ion. The broad absorptions at~3410 cm −1 in the spectra of two Cr(III) complexes are probably due to the presence of trace water in the KBr discs. The observation of an absorption bands located at~3059 and 2925 cm −1 in the IR spectra of free ligands are attributed to the stretching vibrations of aromatic and vinylic C-H, which are here scarcely affected after complex formation. The ν (C=C) bands are found at similar energies as in the free ligands (~1536-1578 cm −1 ), respectively, but the ν (C=N) vibrational bands in the two Cr-based compounds are affected after complexation and exhibited at around~1595 cm −1 . After deprotonation, the band assigned to the ν (C-O) vibration moved to somewhat higher energies in the complexes (from 1205 to 1276 cm −1 ). In addition, the bands related to Cr-O and Cr-N vibrations are exhibited within region 552-497 cm −1 as refereed in Table 3 [35].

UV/Visible Absorption and Fluorescence Emission Spectra Studies
We obtained UV-vis spectra of HL1, HL2, 1, and 2 in DMF (N,N-dimethylformamide) (see Figure 9). The absorption peaks around 275 nm in HL1 and 305 nm in HL2 can be assigned to the π-π* transition of the conjugated ligands, while the absorption peaks around 365 nm in HL1 and 350 nm in HL2 can be assigned to the n-π* transition of conjugated quinoline rings [35,36]. From the absorption spectra of 1 and 2, we note that main characteristic absorption peaks corresponding to ligands exhibit red-shift. The main absorption bands of 1 and 2 merge into one broad peak centered at 360 nm and 325 nm, respectively, with a shoulder around 295 nm and 365 nm accordingly. Additionally, the emerging absorbance in the visible range around 480 nm in 1 and 460 nm in 2 might be ascribed to metal-toligand charge-transfer transitions (MLCT) [34,35]. The fluorescent properties of both the complexes and related 8-hydroxyquinolate ligands are also investigated in DMF at room temperature. Upon excitation at 365 nm, HL1 and HL2 display maximum emission wavelengths at 525 nm and 488 nm, respectively (see Figure 10). The very low emission of HL1 in DMF solution is probably due to the strong electron withdrawing effect of nitro group on the aromatic ring [37,38]. However, the fluorescent emission of the 2-substituted 8-hydroxyquinoline ligands in DMF solution are completely quenched after the formation of Cr(III) hydroxyquinolinates (see Figure 10), which are similar to the result in the solid state (see below).

UV/Visible Absorption and Fluorescence Emission Spectra Studies
We obtained UV-vis spectra of HL 1 , HL 2 , 1, and 2 in DMF (N,N-dimethylformamide) (see Figure 9). The absorption peaks around 275 nm in HL 1 and 305 nm in HL 2 can be assigned to the π-π* transition of the conjugated ligands, while the absorption peaks around 365 nm in HL 1 and 350 nm in HL 2 can be assigned to the n-π* transition of conjugated quinoline rings [35,36]. From the absorption spectra of 1 and 2, we note that main characteristic absorption peaks corresponding to ligands exhibit Materials 2017, 10, 313 9 of 13 red-shift. The main absorption bands of 1 and 2 merge into one broad peak centered at 360 nm and 325 nm, respectively, with a shoulder around 295 nm and 365 nm accordingly. Additionally, the emerging absorbance in the visible range around 480 nm in 1 and 460 nm in 2 might be ascribed to metal-to-ligand charge-transfer transitions (MLCT) [34,35]. The fluorescent properties of both the complexes and related 8-hydroxyquinolate ligands are also investigated in DMF at room temperature. Upon excitation at 365 nm, HL 1 and HL 2 display maximum emission wavelengths at 525 nm and 488 nm, respectively (see Figure 10). The very low emission of HL 1 in DMF solution is probably due to the strong electron withdrawing effect of nitro group on the aromatic ring [37,38]. However, the fluorescent emission of the 2-substituted 8-hydroxyquinoline ligands in DMF solution are completely quenched after the formation of Cr(III) hydroxyquinolinates (see Figure 10), which are similar to the result in the solid state (see below).  The solid-state diffuse reflectance spectra of HL1, HL2, 1, and 2 present similar bands to those observed in solution, albeit with a slight blue shift. The ligand HL1 shows two broad bands around 269 nm and 350 nm, while HL2 shows two broad bands centered at 262 nm and 340 nm (see Figure 11). All of these absorption bands could be ascribed to the π-π* and n-π* transition of the conjugated system of ligands. Upon binding to the Cr(III) ion, the two main absorption bands of the two free ligands exhibit a red shift by ~15 nm, which is due to the enhancement of rigidity after the formation of complexes. Special absorptions at 480 nm and 460 nm are found for compounds 1 and 2, respectively. We assign these broad absorptions as metal-to-ligand charge transfer transitions (MLCT) between center mental ions and ligands. It should be noted that the broad shoulder at ca. 670 nm for 1 and 635 nm for 2 can be attributed to d-d transition of chromium ion [34]. The emission spectra of the two 8-hydroxyquinoline ligands and their metallocomplexes are examined in the solid state at room  The solid-state diffuse reflectance spectra of HL1, HL2, 1, and 2 present similar bands to those observed in solution, albeit with a slight blue shift. The ligand HL1 shows two broad bands around 269 nm and 350 nm, while HL2 shows two broad bands centered at 262 nm and 340 nm (see Figure 11). All of these absorption bands could be ascribed to the π-π* and n-π* transition of the conjugated system of ligands. Upon binding to the Cr(III) ion, the two main absorption bands of the two free ligands exhibit a red shift by ~15 nm, which is due to the enhancement of rigidity after the formation of complexes. Special absorptions at 480 nm and 460 nm are found for compounds 1 and 2, respectively. We assign these broad absorptions as metal-to-ligand charge transfer transitions (MLCT) between center mental ions and ligands. It should be noted that the broad shoulder at ca. 670 nm for 1 and 635 nm for 2 can be attributed to d-d transition of chromium ion [34]. The emission spectra of the two 8-hydroxyquinoline ligands and their metallocomplexes are examined in the solid state at room temperature as well. Upon excitation at 350 nm, the major emission peaks located at approximately The solid-state diffuse reflectance spectra of HL 1 , HL 2 , 1, and 2 present similar bands to those observed in solution, albeit with a slight blue shift. The ligand HL 1 shows two broad bands around 269 nm and 350 nm, while HL 2 shows two broad bands centered at 262 nm and 340 nm (see Figure 11). All of these absorption bands could be ascribed to the π-π* and n-π* transition of the conjugated system of ligands. Upon binding to the Cr(III) ion, the two main absorption bands of the two free ligands exhibit a red shift by~15 nm, which is due to the enhancement of rigidity after the formation of complexes. Special absorptions at 480 nm and 460 nm are found for compounds 1 and 2, respectively. We assign these broad absorptions as metal-to-ligand charge transfer transitions (MLCT) between center mental ions and ligands. It should be noted that the broad shoulder at ca. 670 nm for 1 and 635 nm for 2 can be attributed to d-d transition of chromium ion [34]. The emission spectra of the two 8-hydroxyquinoline ligands and their metallocomplexes are examined in the solid state at room temperature as well. Upon excitation at 350 nm, the major emission peaks located at approximately 550 and 465 nm for HL 1 and HL 2 , respectively, dominate the fluorescence spectra (see Figure 12). Though the five-membered chelate rings formed between the NO donors and Cr(III) metal centers have increased the π-π* conjugation and conformational coplanarity of 8-hydroxyquinolinate, it is hard to detect the fluorescent signal of the two metallocomplexes. In contrast to other complexes of 8-hydroxyquinoline and its derivatives with metal ions (e.g., Al(III), Ga(III), Zn(II), or Cd(II)), the 3 d 3 Cr(III) ion exhibits striking quenching effect, which might be attributed to the lowest energy transition being metal rather than ligand based [34,39]. The striking fluorescence response of the two 8-hydroxyquinolinate based ligand towards Cr 3+ both in solid and solution indicated they could serve as a good heavy metal chelating agent for Cr 3+ in a range of environmental monitoring and biomedical applications.

Conclusions
In summary, two supramolecular coordination compounds, [Cr(L1)3] (1) and [Cr(L2)3] (2), were prepared by a solvothermal reaction of Cr(OAc) 3 with two 2-substituted-8-hydroxyquinolate ligands, respectively. Structures of compounds 1 and 2 were fully characterized by FTIR, MS, TGA, EPR, and single and powder X-ray diffraction. They also featured a 3D supramolecular architecture constructed from abundant noncovalent inter-and intramolecular forces. The remarkable change in the photoluminescent properties of the two 2-substituted 8-hydroxyquinoline ligands and their corresponding Cr-based complexes, both in solution and solid state, indicate that the two 8-hydroxyquinolate ligands are promising chelating agents in luminescence investigation Cr(III) ion [1,35,40]. Therefore, this study provides an idea to explore new 8-hydroxyquinolinate based ligands and related supramolecular coordination compounds for environmental monitoring and biomedical applications.