Synthesis and crystal structure of a new 2D supramolecular Cd(II) complex with 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone ligand

Infrared spectral studies

The IR spectrum of [Cd(PMBP)₂(CH₃OH)₂] (1) was compared with that of the free HPMBP ligand. The strong absorption bands in the range of 3418–3545 cm^-1 can be ascribed to O-H stretching, which indicates the existence of methanol in this complex. The band at 3063 cm^-1 can be assigned to the C-H stretching of phenyl ring. The C=O absorbtion band of free HPMBP is 1645 cm^-1, which is shifted to 1611 cm^-1 in the complex, demonstrating that C=O is coordinated to the central Cd(II) atom. The new band at 550 cm^-1 is close to the peak at 500 cm^-1 of the Mn-O stretching vibration (Yang et al., 2010), which may be attributed to the Cd-O stretching. According to Akama and Tong (1996), the assignments of some other IR bands are as follows: The framework vibrations of phenyl ring are found at 1533 and 1481 cm^-1. The band at 1456 cm^-1 is characteristic peak for the methyl of the pyrazolone ring, but it can also be assigned to the phenyl ring. Methanol methyl absorption shows strong peak at 1435 cm^-1. The rocking of -CH₃ is observed at 1018 cm^-1. The spectra of the pyrazolone ring show two characteristic bands at 1398 and 1362 cm^-1. The characteristic peak of C-H (in-plane bending) is 1229 cm^-1, which overlaps with the peak of ν(C=C) and ν (C-CH₃). The ν (C-N) stretching vibration peak is at 1153 cm^-1. The bands at 999, 947, 916, and 907 cm^-1 are attributable to the stretching vibration of the (C-C₆H₅). The two peaks at 793 and 735 cm^-1 can be verified as the absorptions of C-H (out-of-plane bending).

Structural analysis

X-ray structural investigation reveals that the complex [Cd(PMBP)₂(CH₃OH)₂] (1) crystallizes in the monoclinic system with P21/c space group and exhibits monomeric species. Furthermore, the whole molecule displays central symmetry and the Cd atom acts as the symcenter. The Cd(II) ion is coordinated by six O atoms (Figure 1), four O atoms from two deprotonated HPMBP ligands and another two O atoms from two methanol molecules. Thus, a slightly distorted octahedral structure is formed and the Cd(II) ion occupies the center position. The analogous complex [Cd(PMBP)₂(EtOH)₂] (Sun et al., 2013) has the same coordination mode as complex 1, but there exist obvious differences between the two complexes, as listed in Table 1. As shown in Figure 1, three planes are founded around the Cd atom. The bond angles of O1-Cd-O2, O1ⁱ-Cd-O2, O1-Cd-O2ⁱ, and O1ⁱ-Cd-O2ⁱ (symmetry code: i=1-x, -y, 1-z.) are 83.32(6), 96.68(6), 96.68(6) and 83.32(6)°, respectively, with the sum to be 360°, which shows that the four O atoms (O1, O2, O1ⁱ and O2ⁱ) are coplanar. The same circumstance occurs in two other planes (see Table 2). Given that the Cd(II) ion is chelated by two PMBP ligands, two new six-membered rings (Cd-O1-C9-C8-C11-O2 and Cdⁱ-O1ⁱ-C9ⁱ-C8ⁱ-C11ⁱ-O2ⁱ, symmetry code: i=1-x, -y, 1-z.) are accordingly generated. The bond distances for Cd-O1, Cd-O2 and Cd-O3 are 2.2256(16), 2.2823(16) and 2.3105(16) Å, respectively, which compare well with those found in some other Cd (II) complexes (Nie et al., 2010; Xue et al., 2012; Sharif and Najafi, 2013; Sun et al., 2013), whose steric configuration are also showing octahedral structure. Among the ligand PMBP, the C-O (C9-O1 and C11-O2) bond lengths are 1.267(3) and 1.256(3) Å, respectively, which clearly indicate that a double-bond character is exhibited between the O and C atoms (bond length of C=O is approximately 1.20 Å).

Figure 1:

Coordination environments of the Cd atom.

In complex 1, strong intermolecular hydrogen bonds (Figure 2, Table 3) are observed between the nitrogen atom (N2) and the hydrogen atom (H3) of methanol [O3-H3···N2, 2.769(3) Å, 176°], which link the [Cd(PMBP)₂(CH₃OH)₂] unit to form a 2D supramolecular architecture. Intramolecular hydrogen bonds [C2-H2···O1, 2.890(3) Å, 114°] (Figure 2, Table 3) are also observed in the PMBP ligand. In addition, C-H···π interaction [C5-H5···Cg1(C12ⁱⁱ-C17ⁱⁱ), symmetry code: ii = 1-x, 1/2+y, 1/2-z. H···π = 2.69 Å] exists between neighboring PMBP ligands (Figure 3), and another two C-H···π interactions [C18-H18C···Cg2 (Cd1ⁱⁱⁱ-O1ⁱⁱⁱ-C9ⁱⁱⁱ-C8ⁱⁱⁱ-C11ⁱⁱⁱ-O2ⁱⁱⁱ), symmetry code: iii = 1-x, -y, 1-z. H···π = 2.79 Å and C18–H18C···Cg3 (Cd1a^iv–O1a^iv–C9a^iv–C8a^iv–C11a^iv-O2a^iv), symmetry code: iv = x, y, z. H···π = 2.79 Å] are also found in the molecule itself (Figure 3). By virtue of intramolecular hydrogen bonds and the C-H···π interactions, the stability of the 2D supramolecular complex is enhanced.

Figure 2:

Hydrogen bonds (shown as green dashed lines) of the title complex.

Figure 3:

C-H···π interactions of the title complex.

Thermal gravimetric analysis

Thermal gravimetric analysis of this compound was performed under N₂ gas between 25 and 800°C, and the TGA curve is shown in Figure 4, which indicates that the complex decomposes in two stages. The first decomposition was in the range of 106–202°C, with a weight loss of 7.7755%, corresponding to the departure of two methanol molecules (calculated, 8.7651%). At the second step (336–750°C), the PMBP ligands were destroyed gradually.

Figure 4:

TG curve of the complex.

Fluorescence properties

The luminescent properties of complex 1 and HPMBP ligand were investigated in solid-state at room temperature. The emission peaks are 455 nm for complex 1 (λ_ex=214 nm) and 453 nm (λ_ex=212 nm) for the HPMBP ligand. The emission peak of complex 1 is very close to that of the free HPMBP ligand, which shows that the emission shift is neither metal-to-ligand charge transfer nor ligand-to-metal charge transfer in nature, and the emission maybe mainly caused by the PMBP ligand.

General

All reagents were purchased commercially and used without further purification. Elemental analyses of carbon, hydrogen, and nitrogen were performed with a Perkin-Elmer 2400 II element analyzer (PerkinElmer, Waltham, MA, USA). Fourier transform IR (FTIR) spectra were recorded on an IR Affinity-1 FTIR spectrophotometer (Shimadzu Corporation, Kyoto, Japan) (range: 400–4000 cm^-1) as KBr pellets. TGA was conducted with a Mettler5MP/PF7548/MET/400W instrument (Mettler Toledo Company, Zurich, Switzerland) in a N₂ atmosphere in the temperature range of 25–800°C (heating rate=10°C/min). Fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrometer (Varian, Inc., Palo Alto, CA, USA) in the solid-state at room temperature. Crystal structure was determined on a Bruker SMART APEX II CCD X-ray diffractometer (Bruker AXS, Karlsruhe, Germany).

Preparation of [Cd(PMBP)₂(CH₃OH)₂]

A mixture of Cd(NO₃)₂·4H₂O (0.1234 g, 0.4 mmol) and 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (0.3340 g, 1.2 mmol) in an ethanol/methanol (1:1) solution (40 mL) was refluxed for 5 h and filtered. Light-yellow block crystals were obtained after 4 days. Yield: 63.93% based on Cd(NO₃)₂ salt. Elemental analysis calculated (%) for C₃₆H₃₄N₄O₆Cd: C, 59.15; H, 4.69; N, 7.66. Found: C, 59.19; H, 4.56; N, 7.72. IR (KBr, cm^-1): 3545s, 3474s, 3462s, 3447s, 3418s, 3063m, 1611s, 1533m, 1481s, 1456s, 1435s, 1398m, 1362m, 1229w, 1153w, 1132w, 1061m, 1018m, 999w, 947m, 916w, 907w, 843m, 793w, 768m, 735w, 702m, 660m, 611m, 550m.

X-ray crystallography

Single-crystal X-ray diffraction analysis was conducted by a Bruker SMART APEX II CCD X-ray diffractometer at 173(2) K using graphite monochromated MoKα radiation (λ=0.71073 Å). Semi-empirical absorption correction was applied using SADABS program (Sheldrick, 1997a). The structure was solved by direct methods and refined with the full-matrix least-squares technique using the SHELXS-97 and SHELXL-97 programs (Sheldrick, 1997b,c). Selected angles are given in Table 2. Detailed crystal data and structure refinement are listed in Table 4. Crystallographic data has been deposited with the Cambridge Crystallographic Centre as supplementary publication number CCDC-977202.