Synthesis, structural characterization, and properties of Co(II) and Zn(II) complexes with a mixed N- and O-donor ligand

2.1 Structural description of [Co(L)(H₂O)₅]·4H₂O (1)

Complex 1 crystallizes in the orthorhombic system with space group Pnma and Z=4, showing a discrete mononuclear structure with crystallographic m symmetry. The asymmetric unit of 1 consists of one Co(II) atom with an occupancy of 0.5, 0.5 L²⁻ ligands, 2.5 coordinated water and 2 lattice water molecules. Co(II) is six-coordinated by one pyridyl N atom and five O atoms from coordinated water to exhibit a distorted octahedral coordination geometry [CoNO₅] (Fig. 1a). The distance Co–N is 2.111(2) Å and the distances of Co–O are in the range from 2.0562(17) to 2.128(2) Å, and the bond angles around Co(II) from 85.66(6)° to 179.98(8)° (Table 2). Both carboxylate groups of the L²⁻ units are free of coordination. So the L²⁻ unit just acts as monodentate ligand in complex 1. Apart from the coordinative bonds between ligand and metal centers, there exist abundant hydrogen bonding interactions O–H···O in the crystal structure of complex 1 (Table 3; Fig. 1b), which are important in the stabilization of solid 1.

Fig. 1:

(a) The coordination environment of the Co(II) ions in 1 with displacement ellipsoids drawn at the 30% probability level. The hydrogen atoms are omitted for clarity. Symmetry operations: A x, 1/2−y, z; (b) view of hydrogen bonding interactions in crystals of 1.

2.2 Structural description of [Zn(L)(bpy)]·0.5(DMF)·0.5(H₂O) (2)

Complex 2 crystallizes in the monoclinic system with space group C2/c and Z=8, exhibiting a twofold interpenetrated three-dimensional (3D) framework structure. This Zn complex closely resembles a previously reported Cu complex [6]. The two complexes have very similar crystal data but differ in the co-crystallized solvent molecules.

In the asymmetric unit of 2, there are one Zn(II), one L²⁻ ligands, one bpy, 0.5 lattice water, and 0.5 DMF molecules. Zn(II) is five-coordinated by three pyridyl N atoms (one from L²⁻ and two from bpy) and two carboxylate O atoms from two different L²⁻ ligands to furnish a trigonal bipyramidal coordination geometry [ZnN₃O₂] (Fig. 2a). The bond lengths around the Zn(II) center are in the range from 2.044(2) to 2.182(3) Å; the bond angles around Zn(II) from 87.47(12)° to 178.40(11)°. In 2, both carboxylate groups of the L²⁻ dianion adopt μ₁-η¹:η⁰-monodentate coordination modes. Each bpy links two Zn(II) atoms as an μ₂-connector. Ignoring the coordination of bpy, the interconnection of the L²⁻ ligands and the Zn(II) atoms infinitely extends to form a two-dimensional network (Fig. 2b). Adjacent networks are further linked by bpy ligands to build up a 3D framework (Fig. 2c). From the perspective of topology, each Zn(II) is surrounded by five ligands (L²⁻ and bpy) as a 5-connector, each L²⁻ ligand coordinates to three Zn(II) as a 3-connector, and each bpy is linking two Zn(II) and thus acts as a linear bridge. Finally, the structure of 2 can be simplified as a binodal (3,5)-connected 3D framework with hms (6³)(6⁹,8) topology (Fig. 2d) [7]. Interestingly, there are two interpenetrating 3D nets in the architecture of 2 (Fig. 2e).

Fig. 2:

(a) The coordination environment of the Zn(II) ions in 2 with displacement ellipsoids drawn at the 30% probability level. The hydrogen atoms are omitted for clarity. Symmetry operations: A x, 1−y, −1/2+z; B x, 2 − y, −1/2+z; C −1/2+x, 3/2−y, −1/2+z; (b) view of the network structure in 2; (c) view of the 3D framework architecture in 2; (d) schematic representation of the binodal (3,5)-connected 3D hms framework in 2; (e) schematic representation of the twofold interpenetrating 3D frameworks of 2.

2.3 Thermal stability

Thermogravimetric analyses (TGA) were carried out for complexes 1 and 2, and the result is shown in Fig. 3. Complex 1 shows a continuous weight loss of 35.2% from 95°C to 214°C corresponding to the release of lattice and coordinated water (calcd. 35.0%), and the residue is stable up to about 322°C. For complex 2, there is a continuous weight loss (8.8%) from 102°C to 172°C, attributed to the release of water and DMF (calcd. 9.0%), and the decomposition of the framework of 2 can be observed at 375°C.

Fig. 3:

TGA curves of complexes 1 and 2.

2.4 Luminescent properties

Previous studies have shown that coordination compounds containing d¹⁰ metal centers such as Zn(II) may exhibit luminescence properties [8], [9]. Therefore, the luminescence of complex 2 and the H₂L ligand has been investigated in the solid state at room temperature. As shown in Fig. 4, an intensive fluorescence can be observed with emission bands at 421 nm (λ_ex=363 nm) for 2 and at 418 nm (λ_ex=359 nm) for the H₂L ligand. This fluorescence may be tentatively assigned to intraligand transitions of the coordinated ligands because a similar emission was observed for the free H₂L [8], [9]. The small red shift of the emission maximum for 2 versus H₂L may be considered to originate from the coordination interactions [10], [11].

Fig. 4:

Emission spectra of 1 and H₂L in the solid state at room temperature.

3.1 Preparation of [Co(L)(H₂O)₅]·4(H₂O) (1)

The reaction mixture of Co(NO₃)₂·6H₂O (29.1 mg, 0.1 mmol), H₂L (24.3 mg, 0.1 mmol), and 2 mL DMF in 10 mL H₂O was sealed in a 16 mL Teflon-lined stainless steel container and heated at T=140°C for 3 days. After cooling at a rate of 10 K h⁻¹ to room temperature, light pink block crystals of 1 were obtained with an approximate yield of 45% based on the H₂L. – C₁₃H₂₅NO₁₃Co (462.27): calcd. C 33.78, H 5.45, N 3.03; found C 34.03, H 5.19, N 2.90%. – IR (KBr pellet, cm⁻¹): ν=3412 (m), 1620 (s), 1603 (s), 1579 (s), 1516 (m), 1438 (m), 1351 (s), 1333 (m), 1088 (m), 826 (m), 765 (m), 735 (m), 718 (m), 655 (m).

3.2 Preparation of [Zn(L)(bpy)]·0.5(DMF)·0.5(H₂O) (2)

The reaction mixture of Zn(NO₃)₂·6H₂O (29.7 mg, 0.1 mmol), H₂L (24.3 mg, 0.1 mmol), 4,4′-bipyridine (15.6 mg, 0.1 mmol), and 2 mL DMF in 10 mL H₂O was sealed in a 16 mL Teflon-lined stainless steel container and heated at 140°C for 3 days. After cooling at a rate of 10°C h⁻¹ to room temperature, colorless block crystals of 2 were obtained with an approximate yield of 40% based on the H₂L. – C_24.50H_19.50N_3.50O₅Zn (508.31): calcd. C 57.89, H 3.87, N 9.64; found C 57.69, H 4.12, N 9.90%. – IR (KBr pellet, cm⁻¹): ν=3430 (m), 1615 (s), 1580 (s), 1552 (s), 1439 (s), 1366 (s), 1285 (m), 1093 (m), 1031 (m), 942 (m), 835 (m), 762 (m), 720 (m), 658 (m).

3.3 X-ray structure determinations

The crystallographic data collections for complexes 1 and 2 were carried out on a Rigaku Rapid II imaging plate area detector with MoKα radiation (λ=0.71073 Å) using a MicroMax-007HF micro-focus rotating anode X-ray generator and VariMax-Mo optics at T=200 K. The diffraction data was integrated using the program Saint [12], which was also used for the intensity corrections for Lorentz and polarization effects. Semiempirical absorption corrections were applied using the program Sadabs [13]. The structures of 1 and 2 were solved by direct methods, and all nonhydrogen atoms were refined anisotropically on F² by the full-matrix least-squares techniques using the program Shelxl-97 [14], [15], [16]. In 1 and 2, all hydrogen atoms at C atoms were generated geometrically. The H atoms at O3, O4, and O5 in 1 could be found at a reasonable position in the difference Fourier maps and located there, whereas the other H atoms of lattice water molecules in 1 and 2 could not be located and thus were not included in the refinement. Some crystal structure problems were depicted in the checkcif reports. Disordered solvent molecules in 1 and 2 may account for many of these alerts. The details of crystal parameters, data collection, and refinements are summarized in Table 1; selected bond lengths and angles are listed in Table 2, hydrogen bond data for 1 in Table 3.

CCDC 1886234 and 1881497 contain the supplementary crystallographic data for this article. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.