FENG Yu-Quan ZHONG Zhi-Guo LI Jing

(College of Chemistry and Pharmacy Engineering, Nanyang Normal Uniνersity, Nanyang 473061, China)

ABSTRACT A new binuclear cobalt(II) complex, [Co2(C10H6NO2)2(C12H8N2)2(N3)2]·H2O (1, C10H6NO2-: isoquinoline-1-carboxylate, C12H8N2: 1,10-phenanthroline (phen); N3-: azide), has been synthesized though a mixed-solvothermal method using the pentanol and distilled water as solvent.The complex was characterized by elemental analysis, IR spectrum and single-crystal X-ray diffraction.It crystallizes in triclinic system, space group with Z = 2, a = 1.777(5), b = 11.815(5), c = 17.141(7) Å, α = 74.496(6)°, β= 70.537(6)°, γ = 67.861(6)°, V = 2055.4(14) Å3, Mr= 922.66, Dc= 1.491 g/cm3, λ = 0.71073 Å, μ= 0.870 mm-1, F(000) = 942, R= 0.0455 and wR= 0.1147 for 4960 observed reflections with I> 2σ(I) and S= 1.129.Its molecular structure is constructed by two Co2+ cations, two chelating phen ligands, two chelating isoquinoline-1-carboxylate ligands, two bridging linear azide anions and two uncoordinated half-occupied water molecules.The 3-D supramolecular network results from the multiple hydrogen bonds and π-π stacking interactions among three types of ligands.More importantly, its thermal stability, luminescent properties and magnetic properties have also been studied in detail.

Keywords: binuclear cobalt(II) complex, thermal stability, fluorescence decay, magnetic properties;

1 INTRODUCTION

Currently, the design and syntheses of metal-organic coordination compounds have become one of the hot topics because of their rich interesting structural features and kinds of potential applications, such as functional materials[1-3].Up to the present, great efforts have been made toward the rational preparation of such metal-organic coordination compounds exhibiting excellent physical and chemical properties[4].It has become an effective synthesis strategy to synthesize metal coordination compounds with metal cations and various organic ligands containing multiple coordination sites[5].Containing-N,O ligands like aromatic carboxylate ligands, linear azide anion, phen ligands and so on have been widely used in the construction of coordination compounds[6,7].The aromatic carboxylate ligands and linear azide anion acting as bridging and chelating ligands always exhibit several coordination sites (aromatic carboxylate: N, O and O΄; linear azide anion: N and N΄) and flexible coordination fashions[8,9].The phen ligand can be regarded as a rigid conjugated chelating ligand which easily results in excellent fluorescent properties.Therefore, we expected that the combination of aromatic carboxylate ligands, azide anion, phen ligands and TM (transition metal) cations would generate new complexes with interesting structural features and optical properties.As part of our ongoing studies[10-14], herein, by using three types of ligands (isoquinoline- 1-carboxylate, azide and 1,10-phenanthroline (phen)) we synthesized a new binuclear Co2+coordination complex [Co2(C10H6NO2)2(C12H8N2)2(N3)2]·H2O (1), which enriches the family of Co(II) coordination complexes containing three kinds of ligands.

2 EXPERIMENTAL

2.1 Instruments and reagents

Crystal diffraction data were collected on a Bruker SMART APEX-II CCD diffractometer equipped with a gra- phite-monochromated Mo-Kαradiation (λ= 0.71073 Å) at 296(2) K using anω-υscan mode.Elemental analyses of C, H and N were tested on a Perkin Elmer 240 analyzer.IR spectrum was measured on a Nicolet 5700 FT-IR spec- trometer using KBr pellets between 4000 and 400 cm-1.The TG analysis was tested on a Perkin-Elmer 7 thermal analysis instrument in flowing N2at a heating rate of 10 °C/min in the range of 25～800 °C.Luminescent spectra were investigated on an FLS-980 fluorescence lifetime and steady-state spectrometer.The variable temperature magnetic properties were performed on an MPMS SQUID VSM magnetometer in an applied magnetic field of 1000 Oe in the temperature range of 1.8～300 K.All chemical reagents were obtained from commercial resources and used without further puri- fication.

2.2 Synthesis of [Co2(C10H6NO2)2(C12H8N2)2(N3)2]·H2O1

1 was obtained under mixed-solvothermal conditions using pentanol and distilled water as solvents.A mixture of CoCl2.6H2O (0.12 g, 0.5 mmol), isoquinoline-1-carboxylic acid (0.09 g, 0.5 mmol), 1,10-phenanthroline (phen, 0.05 g, 0.25 mmol), NaN3(0.07 g, 1.0 mmol), H2O (2.0 mL) and pentanol (3.0 mL) was stirred for half an hour, transferred into a Teflon-lined stainless-steel autoclave (50 mL) and heated at 423 K for 120 h.When the mixture was cooled down to RT at a rate of 5 K/h, red block crystals were yield.The crystals were filtered off and washed with deionized water (yieldca.39%, based on CoCl2·6H2O).We once tried to synthesize 1 using H2O as a single solvent.Unfortunately, complex 1 could not been obtained under the same conditions.We could only obtain the known complex [Co(N3)2(phen)2] which has been reported in previous studies[15].This illustrates that the pentanol plays a crucial role in forming 1.At the same time, it also shows that the mixed-solvothermal method is a very effective way for the synthesis of 1.Elemental analyses (C, H and N) were measured on a Perkin-Elmer 240 analyzer.Analysis for C44H30Co2N12O5, calculated: C, 57.10; H, 3.24; N, 18.17%.Found: C, 57.39; H, 3.58; N, 18.01%.

2.3 X-ray crystallographic determination

A suitable crystal of 1 (0.26mm × 0.21mm × 0.17mm) was applied for structural analysis.Intensity data were collected on a Bruker APEX-II CCD detector at 296(2) K using anω-υscan mode and Mo-Kαradiation (λ= 0.71073 Å) in a range of 2.0≤θ≤25.0° with -15≤h≤14, -15≤k≤13 and -22≤l≤12.Of the 12557 reflections collected, 7110 were unique (Rint= 0.019).Absorption correction was used by the SADABS[16].The structure was solved by direct methods and refined by full-matrix least-squares onF2through Olex 2 program package[17].All of the non-hydrogen atoms were refined anisotropically.The H atoms of the isoquino- line-1-carboxylic acid and 1,10-phenanthroline were placed in the geo- metrically idealized positions and treated using a riding- model approximation, with C-H = 0.93 Å andUiso(H) = 1.2Ueq(C).Owing to the largerUeqvalues of two molecular sites, the two uncoordinated water molecules (O(1W) and O(2W)) were refined with S.O.F.= 0.5.Selected bond lengths and bond angles of 1 are given in Table 1.

Table 1.Selected Bond Lengths (Å) and Bond Angles (°) for 1

3 RESULTS AND DISCUSSION

3.1 Crystal structure

Fig.1.Molecular structural unit of compound 1.Displacement ellipsoids are drawn at the 30% probability level

In addition, two types of hydrogen bonds (C-H···O and C-H···N) andπ-πstacking interactions can be observed in its packing structure of 1 (Fig.2).The detailed hydrogen- bonding parameters are provided in Table 2.The related information ofπ-πstacking interactions is as follows: (i) adjacent pyridine rings (N(11)/C(28)～C(34)) are parallel to each other, with a centroid-to-centroid distance of 3.3511(18) Å; (ii) adjacent benzene rings (C(15)/C(16)/C(19)～C(22)) are parallel to each other with the centroid-to-centroid distance of 3.4179 (19) Å.These hydrogen bonds andπ-πstacking interactions in 1 may enhance its crystal structural stability.By means of the multiple hydrogen bonds andπ-πstacking interactions, the packing structure of 1 exhibits a 3-D supramolecular structure (Fig.2).

Fig.2.Three-dimensional supramolecular structure of 1.The dashed lines represent hydrogen bonds

Table 2.Hydrogen Bond Lengths (Å) and Bond Angles (°)

3.2 Infrared spectroscopy

In the IR spectrum of 1 (Fig.3-left, 4000～400 cm-1), the peak at 3436 cm-1is ascribed to uncoordinated water molecules.The absorption peaks of 2053 and 1340 cm-1are assigned to N3-anions.The absorption peaks of azide in NaN3are 2031 and 1327 cm-1, respectively.Compared with the absorption peaks of azide in 1 and NaN3, the two absorption peaks in 1 have red-shifts of 22 and 13 cm-1, respectively.This not only reveals the existence of azide anions, but also indicates that azide anions are coordinated by the terminal N atoms[20].The peaks at 1596 and 1271 cm-1belong to the carboxylate ligands.A series of peaks in the scope of 1132～635 cm-1(1132, 1028, 832, 726 and 635 cm-1) belong to the C-H vibration of aromatic rings derived from the chelating phen ligands and isoquinoline-1- carboxylate ligands.

Fig.3.Left: IR spectrum of 1; right: thermal stability of 1

3.3 Thermal stability

The thermal stability of 1 was tested under N2flowing between 25 and 800 °C at a heating rate of 10 K·min-1.The TG curve of 1 displays two steps of weight loss (Fig.3-right).The first weight loss is 2.03% (Δmcalcd/m= 1.95%) in the range of 69～146 °C, corresponding to the release of water molecules.The weight loss in the second step is 81.33% (Δmcalcd/m= 81.82%) between 170 and 651 °C, which is assigned to the removal of the three kinds of organic ligands.According to these results, we can conclude that the final residue should be cobalt oxide, which is in good agreement with those from structural analysis.

3.4 Fluorescent properties

The fact that the complex contains benzene ring and pyridine which are potential luminescent groups promotes us to study its fluorescent properties.The luminescent pro- perties of 1 have been measured in the solid state at room temperature.The emission spectrum is provided in Fig.4-left.Within the emission spectrum, the peaks at 456 nm (weak), 466 nm (strong, maximum emission) and 475 nm (moderate) can be assigned toπ*-πtransition of the coordinated phen ligand when the excitation wavelength is 374 nm[21].Moreover, the CIE chromaticity coordinate (x,y) is obtained according to the emission peaks and it is illustrated in a CIE (1931) chromaticity diagram (Fig.4-right).Thexandyvalues are as follows: 0.1437 and 0.0345, respectively, which reveals that 1 owns a blue emission.In order to fully learn its luminescent properties, the fluorescence decay has also been investigated underλex= 374 nm (maximum excitation wavelength) andλem= 466 nm (maximum emission wavelength) at room temperature.The plot of counts versus decay (Fig.5-left) can be well fitted to the double-exponential equation (I=A1exp(-t/τ1)+A2exp(-t/τ2)) with theτtotalvalue of 0.8428 ns (whereI,A, andtare the relative fluorescence intensity, constant and fluorescence lifetime, respectively).The IRF (Instrument Response Function) curve and luminescent decay curve of 1 are given in Fig.6.

Fig.4.Left: emission spectrum of 1; right: CIE (1931) chromaticity diagram of 1

Fig.5.Left: luminescent decay and IRF curve of 1; right: magnetic behaviours of 1

3.5 Magnetic behaviors

The molecular structure of 1 contains two Co2+cations, so we are interested in its magnetic behaviors.Its variable temperature magnetic properties were performed on an MPMS SQUID VSM magnetometer in an applied magnetic field of 1000 Oe in the temperature range of 1.8～300 K.The measured curve ofχMTversusTof 1 is given in Fig.5-right.The experimental result reveals that theχMTvalue of 1 is 4.78 cm3·K·mol-1at 300 K.This is very close to the expected calculated value of 4.50 cm3·K·mol-1for two Co2+cations.TheχMTvalue increases with the increase of temperature, indicating the compound exhibits an anti-ferromagnetic interaction.Furthermore, in order to further confirm the presence of anti-ferromagnetic interaction in 1, the Curie-Weiss lawχM=C/(T-θ) is employed.We have well fitted the curve ofχMTversusTbetween 90 and 300 K.Meanwhile, the Curie and Weiss constants were also calculated (C: 5.06 cm3·K·mol-1,θ: -17.78 K).The negative Weiss constant (θ =-17.78 K) further illustrates an anti-ferromagnetic interaction between Co2+cations[22].Complex 1 and the two known complexes {Co3(mimda3-)2(bpe)3·3H2O}nand {Co3(eimda3-)2(bipy)3·8H2O}n(H3mimda = 1H-2-methyl- 4,5-imidazole-dicarboxylic acid, H3eimda = 1H-2-ethyl-4,5- imidazole-dicarboxylic acid, bpe = 1,2-di(4-pyridyl) ethylene and bipy = 4,4΄-bipyridine) exhibited anti-ferromagnetic interactions.Differently, the anti-ferromagnetic interactions are mediated by azide anions in 1, while by carboxylic groups in the two complexes reported above[23].

4 CONCLUSION

In summary, a new binuclear cobalt(II) complex con- taining three types of ligands has been isolated by using a mixed-solvothermal method.The thermal stability and luminescent and magnetic properties have also been studied in detail.The magnetic properties revealed an anti-ferro- magnetic interaction between Co2+cations.The luminescent properties indicated that the complex exhibits a blue emission with the short luminescent decay of 0.8428 ns under 374 nm excitation, indicating that it can be used as a potential optical functional material.The compound provides an effective synthesis method for binuclear TM metal complexes with multiple ligands and potential applications.This indicates that more new polynuclear complexes with various ligands and functions will be synthesized by using similar mixed- solvothermal method.