Two Organic Cation Salts Containing Tetra(isothiocyanate)cobaltate(II): Synthesis, Crystal Structures, Spectroscopic, Optical and Magnetic Properties

Single crystals of two hybrid organic-inorganic molecular solids, benzyl pyridinium tetra(isothiocyanate)cobalt ([BzPy]2[Co(NCS)4]) (1) and benzyl quinolinium tetra(isothiocyanate)cobalt ([BzQl]2[Co(NCS)4]) (2), were grown using a slow evaporation growth technique at room temperature and their IR, UV-Vis, X-ray crystal structures, luminescence, and magnetism were reported. The crystal structural analysis revealed that two molecular solids crystallize in the monoclinic space group P21/c of 1 and P21/n of 2. The cations form a dimer through weak C–H···π/π···π interactions in 1 and 2, and the adjacent cation (containing N(6) atom) in 2 forms a columnar structure through π···π weak interactions between the quinoline and benzene rings, while the anions in 1 form a layer structure via short S···Co interactions. The anions (A) and cations (C) are arranged alternatively into a column in the sequence of ···A–CC–A–CC–A··· for 1, while the two anions and cationic dimer in 2 form an alliance by the C–H···π, C–H···S and C–H···N hydrogen bonds. A weak S···π interaction was found in 1 and 2. The two molecular solids show a broad fluorescence emission around 400 nm in the solid state at room temperature, and weak antiferromagnetic coupling behavior when the temperature is lowered.


Introduction
During the last two decades, hybrid organic-inorganic molecular materials based on organic cations and inorganic complex anions have played a major role in the development of advanced functional materials.They have attracted the interest of scientists due to their special structural features and physical properties, which include novel catalytic, non-linear optical, conductive, fluorescent and magnetic properties [1][2][3][4][5][6][7].Among these compounds, the transition metal complexes anion (based on the thiocyanate or isothiocyanate ligand) attracts much interest as these ions may coordinate either through the N or S atom as a monodentate ligand, or though N and S atoms as a bridging ligand, forming some complexes with one-, two-or three-dimensional networks [4,[8][9][10].In particular, Crystals 2017, 7, 92 2 of 14 the inherent coordination angle of the isothiocyanate species leads to a wide range of various molecular solids containing [M(NCS) 4 ] 2− (M = Mn 2+ , Co 2+ , Zn 2+ ), which may exhibit interesting magnetic behaviors, electronic, and optical properties [11][12][13][14][15].The selection of the counter organic cation is of key importance as it controls the stacking of the whole molecule, and the non-covalent interactions (such as weak p•••π/π•••π stacking interactions and hydrogen bonds) also play an important role in the arrangement of the structural units of molecular solids [16][17][18].Our previously published studies have been devoted to the syntheses, X-ray single crystal structures, spectroscopic studies, magnetic or fluorescent properties of hybrid materials based on the tetra(isothiocyanate)cobalt/zinc anion and substituted benzyl triphenylphosphonium cations.The examples of [4RBzTPP] 2 [Co(NCS) 4 ] (R = H, F, Br, NO 2 ) [19,20] ] 2, whose crystal structures, spectroscopic, optical and magnetic properties have been investigated.It is of much interest that two solids exhibit dual functionalities such as antiferromagnetic behavior and luminescent activity.

D−H•
Every unit cell of 2 contains one [Co(NCS) 4 ] 2− anion and two [BzQl] + cations.In the [Co(NCS) 4 ] 2− anion, the average distance of Co-N bonds was 1.963(3), and the average N-Co-N bit angle was 108.86 (15) • .These values are slightly bigger than those in 1.The dihedral angles θ 1 , θ 2 , and θ 3 of the [BzQl] + cation are 166.cations show an orderly stacking structure, for example, the cations containing the N(5) atom (C 1 ) formed a dimer (Figure 3a) through π•••π weak interactions between the quinoline rings in which the centroid of two quinoline rings distance was 3.787(2) Å (d1); while the cations containing the N(6) atom (C 2 ) formed a chain (Figure 3b) through π•••π weak interactions between the quinoline and benzene rings with a distance of 3.518(2) Å (d2).These ordered arrangements between the cations in 2 may be attributed to the bigger delocalized π conjugated system of the quinoline ring which can easily form π•••π weak interactions when the pyridine ring of 1 changed to the quinoline ring of 2 in the cation.An interesting structure was found where a cationic dimer was surrounded by two [Co(NCS) 4 ] 2− anions, and formed an alliance via the C-H•••N and C-H•••S hydrogen bond (Figure 4a and Table 2).In addition, the S•••π interactions [21], (the S(1) to centroid of C(5)-C(10) ring distance of 3.708(2) Å (d3)) and C(33)-H(33)•••S(1) hydrogen bonds with a C(33)℘?℘S(1) distance of 3.701(2) Å were found between the anions and the cations (Figure 4b).These weak interactions (the bond parameters of hydrogen bonds listed in Table 2) improved the stabilization of the unit cell of 2 (Figure S2). were found between the anions and the cations (Figure 4b).These weak interactions (the bond parameters of hydrogen bonds listed in Table 2) improved the stabilization of the unit cell of 2 (Figure S2).By comparing the packing structures of ( 1), ( 2) and [4NO2BzPy]2[Co(SCN)4] [24], it was found that when the anion was identical, the aromatic rings of the cation changed from pyridine to a quinoline ring, or a para-substituted group in the benzyl ring changed from H to NO2 groups, the anion stacking mode, the dihedral angles (θ1, θ2, and θ3), the cations stacking mode and the weak interactions of the cations and anions were significantly different.
The grown crystals of 1 and 2 were finely powered and subjected to powder XRD analysis by employing a Bruker D8 advance power X-ray diffractometer.The powdered samples were scanned over a range of 2θ values 10°-50° in steps of 0.02° at room temperature.As seen in Figure 5, the sharp nature of the peaks in the XRD patterns and the consistency of most of the peak positions in powder XRD and simulated XRD patterns from single crystal XRD using Mercury software from the Cambridge Structural Database System indicates the purity and excellent crystallinity of the grown crystals of 1 and 2. By comparing the packing structures of (1), ( 2) and [4NO 2 BzPy] 2 [Co(SCN) 4 ] [24], it was found that when the anion was identical, the aromatic rings of the cation changed from pyridine to a quinoline ring, or a para-substituted group in the benzyl ring changed from H to NO 2 groups, the anion stacking mode, the dihedral angles (θ 1 , θ 2 , and θ 3 ), the cations stacking mode and the weak interactions of the cations and anions were significantly different.
The grown crystals of 1 and 2 were finely powered and subjected to powder XRD analysis by employing a Bruker D8 advance power X-ray diffractometer.The powdered samples were scanned over a range of 2θ values 10 • -50 • in steps of 0.02 • at room temperature.As seen in Figure 5, the sharp nature of the peaks in the XRD patterns and the consistency of most of the peak positions in powder XRD and simulated XRD patterns from single crystal XRD using Mercury software from the Cambridge Structural Database System indicates the purity and excellent crystallinity of the grown crystals of 1 and 2.

Luminescent Properties
The luminescent properties of the two solids and the intermediate were all investigated in a solid state at room temperature (the intermediates [BzPy]Br (Benzyl pyridinium bromide) of 1 do not obtain solid fluorescence as it is semi-solid state at room temperature).As seen in Figure 7a, upon excitation at 241 nm, compound 1 showed two main luminescent emission peaks at 395 nm and 290 nm, while the main emission bands of both 2 and intermediate [BzQl]Br were found at 289 nm and 394 nm (Figure 7b), which may be attributed to the π*→π or n→π transition [27].In addition, compounds 1 and 2 exhibited a strong emission band at 408 nm and a relatively weak emission band at 448 nm, which may be tentatively assigned to the metal-to-ligand charge transfer band between the Co(II) and the unoccupied π* orbitals of the isothiocyanate groups [28].It is noteworthy that the luminescence intensity of λmax in 2 (186.9 a.u) is much stronger than the intermediate [BzQl]Br (benzyl

Luminescent Properties
The luminescent properties of the two solids and the intermediate were all investigated in a solid state at room temperature (the intermediates [BzPy]Br (Benzyl pyridinium bromide) of 1 do not obtain solid fluorescence as it is semi-solid state at room temperature).As seen in Figure 7a, upon excitation at 241 nm, compound 1 showed two main luminescent emission peaks at 395 nm and 290 nm, while the main emission bands of both 2 and intermediate [BzQl]Br were found at 289 nm and 394 nm (Figure 7b), which may be attributed to the π*→π or n→π transition [27].In addition, compounds 1 and 2 exhibited a strong emission band at 408 nm and a relatively weak emission band at 448 nm, which may be tentatively assigned to the metal-to-ligand charge transfer band between the Co(II) and the unoccupied π* orbitals of the isothiocyanate groups [28].It is noteworthy that the luminescence intensity of λmax in 2 (186.9 a.u) is much stronger than the intermediate [BzQl]Br (benzyl

Luminescent Properties
The luminescent properties of the two solids and the intermediate were all investigated in a solid state at room temperature (the intermediates [BzPy]Br (Benzyl pyridinium bromide) of 1 do not obtain solid fluorescence as it is semi-solid state at room temperature).As seen in Figure 7a, upon excitation at 241 nm, compound 1 showed two main luminescent emission peaks at 395 nm and 290 nm, while the main emission bands of both 2 and intermediate [BzQl]Br were found at 289 nm and 394 nm (Figure 7b), which may be attributed to the π*→π or n→π transition [27].In addition, compounds 1 and 2 exhibited a strong emission band at 408 nm and a relatively weak emission band at 448 nm, which may be tentatively assigned to the metal-to-ligand charge transfer band between the Co(II) and the unoccupied π* orbitals of the isothiocyanate groups [28].It is noteworthy that the luminescence intensity of λ max in 2 (186.9 a.u) is much stronger than the intermediate

Magnetic Properties
The crushed crystals of 1 and 2 were used to collect the variable-temperature (2-300 K) magnetic susceptibility data under a field of 2000 Oe.Plots of χm and χm − 1 versus T are shown in Figure 8a for 1 and Figure 8b for 2, while the plots of χmT versus T are shown in Figure S3a for 1 and Figure S3b for 2. The χMT value of 1 at 300 K was 2.457 emu K mol −1 , which was slightly larger than the spinonly value of high-spin Co(II) (S = 3/2) at 1.875 emu K mol −1 , indicating a contribution of the orbital momentum typical for the 4 T1g ground state [30,31].As the temperature was lowered, first, the χMT product smoothly decreased to 2.255 emu K mol −1 around 34 K, then sharply decreased to a low value (1.132 emu K mol −1 ) at 2.0 K (Figure S3a), indicative of an antiferromagnetic exchange.The data in χM −1 versus T plot is well fitted by the Curie-Weiss law (the solid line in Figure 8a) with the fitting parameters of C = 2.459 emu K mol −1 and θ = −3.641K.The magnetic behavior of 2 also exhibited a very weak antiferromagnetic exchange interaction, and the best fit (the red solid line in Figure 8b) for the data in χM −1 versus T plot in the temperature range 2-300 K using the Curie-Weiss law was C = 2.410 emu K mol −1 , and θ = −1.295K.The magnetic behaviors of 1 and 2 in accordance with their crystal structures, and similar to that in [4NO2BzPy][Co(NCS)4] [24].Due to the large Co-Co distances between the neighboring centers, the magnetic behavior was due to single ion anisotropy with some contribution of antiferromagnetic exchange between the Co(II) centers.The magnetic coupling can be mediated through super-exchange interaction across the isothiocyanate orbitals, as well as the C-H•••S, and C-H•••N hydrogen bonds between the anion and the cation [31,32].

Magnetic Properties
The crushed crystals of 1 and 2 were used to collect the variable-temperature (2-300 K) magnetic susceptibility data under a field of 2000 Oe.Plots of χ m and χ m − 1 versus T are shown in Figure 8a for 1 and Figure 8b for 2, while the plots of χ m T versus T are shown in Figure S3a for 1 and Figure S3b for 2. The χ M T value of 1 at 300 K was 2.457 emu K mol −1 , which was slightly larger than the spin-only value of high-spin Co(II) (S = 3/2) at 1.875 emu K mol −1 , indicating a contribution of the orbital momentum typical for the 4 T 1g ground state [30,31].As the temperature was lowered, first, the χ M T product smoothly decreased to 2.255 emu K mol −1 around 34 K, then sharply decreased to a low value (1.132 emu K mol −1 ) at 2.0 K (Figure S3a), indicative of an antiferromagnetic exchange.The data in χ M −1 versus T plot is well fitted by the Curie-Weiss law (the solid line in Figure 8a) with the fitting parameters of C = 2.459 emu K mol −1 and θ = −3.641K.The magnetic behavior of 2 also exhibited a very weak antiferromagnetic exchange interaction, and the best fit (the red solid line in Figure 8b) for the data in χ M −1 versus T plot in the temperature range 2-300 K using the Curie-Weiss law was C = 2.410 emu K mol −1 , and θ = −1.295K.The magnetic behaviors of 1 and 2 are in accordance with their crystal structures, and similar to that in [4NO2BzPy][Co(NCS) 4 ] [24].Due to the large Co-Co distances between the neighboring centers, the magnetic behavior was due to single ion anisotropy with some contribution of antiferromagnetic exchange between the Co(II) centers.The magnetic coupling can be mediated through super-exchange interaction across the isothiocyanate orbitals, as well as the C-H•••S, and C-H•••N hydrogen bonds between the anion and the cation [31,32].

Magnetic Properties
The crushed crystals of 1 and 2 were used to collect the variable-temperature (2-300 K) magnetic susceptibility data under a field of 2000 Oe.Plots of χm and χm − 1 versus T are shown in Figure 8a for 1 and Figure 8b for 2, while the plots of χmT versus T are shown in Figure S3a for 1 and Figure S3b for 2. The χMT value of 1 at 300 K was 2.457 emu K mol −1 , which was slightly larger than the spinonly value of high-spin Co(II) (S = 3/2) at 1.875 emu K mol −1 , indicating a contribution of the orbital momentum typical for the 4 T1g ground state [30,31].As the temperature was lowered, first, the χMT product smoothly decreased to 2.255 emu K mol −1 around 34 K, then sharply decreased to a low value (1.132 emu K mol −1 ) at 2.0 K (Figure S3a), indicative of an antiferromagnetic exchange.The data in χM −1 versus T plot is well fitted by the Curie-Weiss law (the solid line in Figure 8a) with the fitting parameters of C = 2.459 emu K mol −1 and θ = −3.641K.The magnetic behavior of 2 also exhibited a very weak antiferromagnetic exchange interaction, and the best fit (the red solid line in Figure 8b) for the data in χM −1 versus T plot in the temperature range 2-300 K using the Curie-Weiss law was C = 2.410 emu K mol −1 , and θ = −1.295K.The magnetic behaviors of 1 and 2 are in accordance with their crystal structures, and similar to that in [4NO2BzPy][Co(NCS)4] [24].Due to the large Co-Co distances between the neighboring centers, the magnetic behavior was due to single ion anisotropy with some contribution of antiferromagnetic exchange between the Co(II) centers.The magnetic coupling can be mediated through super-exchange interaction across the isothiocyanate orbitals, as well as the C-H•••S, and C-H•••N hydrogen bonds between the anion and the cation [31,32].

Characterization Technique
Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Perkin-Elmer (Massachusetts, MA, USA) Model 240C elemental analyzer.IR spectra were recorded on a Nicolet (Wisconsin, WI, USA) FT-IR spectrophotometer in 4000-400 cm −1 regions with a KBr pellet.UV-Vis spectra were recorded (in acetonitrile) on a Shimadzu (Tokyo, Japan) UV-2500 spectrophotometer in the region of 250-800 nm.The power XRD pattern was analyzed by a Bruker D8 Advance X-ray diffractometer (λ = 1.5406Å).Single crystal XRD data were collected at room temperature using a Bruker SMART APEX instrument (Mo Kα radiation, λ = 0.71073 Å) for 1 and 2. Cell parameters were retrieved using SMART software [34] and refined using SAINTPlus (Göttingen, Germany) software [35] on all observed reflections.Data reductions were also performed using the SAINTPlus software.The structure was solved by direct method and refined by the least-squares methods on F 2 using the SHELXTL (Göttingen, Germany) program package [36].All non-hydrogen atoms were refined anisotropically.Hydrogen atoms were located in the Fourier map and their positions refined with fixed isotropic thermal parameters.Details of the data collection, refinement and crystallographic data are summarized in Table 3.

Fluorescence Emission Spectra Measurements
Solid-state emission spectra upon excitation at 241 nm were recorded for the solid samples loaded into a sample cell (1 cm diameter) which was then fixed on a bracket at room temperature with a Hitachi F-7000 (Tokyo, Japan) fluorescence spectrophotometer.The excitation and emission slits used for the measurement of the solid state of the crystals were 2.5 and 5.0 nm wide, the scan speed was 240 nm/min −1 , and the scan voltage was 400 V.

Magnetic Properties Measurements
Variable-temperature magnetic susceptibility measurements were carried out for 1 and 2 in the 2-300 K temperature range at a magnetic field of 0.2 T on ground polycrystalline samples with a Quantum Design MPMS-XL-7 (San Diego, CA, USA) super-conducting quantum interference device (SQUID) magnetometer.Samples were prepared by finely grinding single crystals into a powder and packing the powder into a gelatin capsule with weights of 34.36 mg and 60.40 mg for 1 and 2, respectively.The susceptibility data were corrected for the diamagnetic contributions deduced by the use of Pascal's constant tables.

Figure 5 .
Figure 5. Simulated and experimental powder XRD patterns of 1 (a) and 2 (b): (i) calculated from single crystal structural analysis (blue line) and (ii) experimental (red line).

Figure 5 .
Figure 5. Simulated and experimental powder XRD patterns of 1 (a) and 2 (b): (i) calculated from single crystal structural analysis (blue line) and (ii) experimental (red line).

Figure 5 .
Figure 5. Simulated and experimental powder XRD patterns of 1 (a) and 2 (b): (i) calculated from single crystal structural analysis (blue line) and (ii) experimental (red line).

Figure 7 .
Figure 7. (a) Emission spectrum of 1 in a solid state at room temperature.(b) Emission spectra of 2 (black) and [BzQl]Br (red) in a solid state at room temperature.

6 χ M (emu mol - 1 ) 7 .
(a) Emission spectrum of 1 in a solid state at room temperature.(b) Emission spectra of 2 (black) and [BzQl]Br (red) in a solid state at room temperature.

Figure 7 .
Figure 7. (a) Emission spectrum of 1 in a solid state at room temperature.(b) Emission spectra of 2 (black) and [BzQl]Br (red) in a solid state at room temperature.

Figure 8 .
Figure 8.(a) Plots of χm and χm −1 versus T for 1.(b) Plots of χm and χm −1 versus T for 2. The solid lines are reproduced from the theoretical calculations and detailed fitting procedure described in the text.

Figure 9 .
Figure 9. Synthetic route of the two complexes.

Figure 9 .
Figure 9. Synthetic route of the two complexes.

Table 1 .
Selected bond lengths and bond angles for 1 and 2.

Table 3 .
Crystal data and structure refinement for 1 and 2.