Two mononuclear zinc(II) complexes constructed by two types of phenoxyacetic acid ligands: syntheses, crystal structures and fluorescence properties

2.1 Preparation of tetraaqua-bis(3-carboxy phenoxyacetate-κO)-zinc (II), [Zn(3-Hcpa)₂(H₂O)₄] (1)

Zn(CH₃COO)₂·2H₂O (Tianjin Yongda Chemical Reagent Company Limited, Tianjin, P. R. China) (0.5 mmol, 0.110 g), 3-carboxy phenoxy acetic acid (3-H₂cpa, 0.5 mmol, 0.099 g) were dissolved in 20 mL distilled water and the mixture was neutralized with NaOH (Tianjin Damao Chemical Reagent Factory, Tianjin, P. R. China) (0.1 mol L⁻¹) to pH=4.0. The solution was sealed in a 25 mL Teflon reactor (Gongyi zhanjie Credit Instrument Marketing Department, Gongyi, Henan Province, P. R. China, http://81138565701.shop.fengj.com) and kept under autogeneous pressure at T=393 K for 3 days. After cooling to room temperature at a rate of 279 K·h⁻¹, and filtering, colorless block-shaped crystals had grown from the filtrate by slow evaporation 3 weeks later. Yield: 14 mg (11% based on 3-H₂cpa). –C₁₈H₂₂O₁₄Zn (%): calcd. C 40.93, H 4.17; found C 40.99, H 4.11.

2.2 Preparation of tetraaqua-bis(3,5,6- trichloropyridine-2-oxyacetato-κO)- zinc(II) diaqua-bis(3,5,6-trichloropyridine-2-oxyacetato-κO)-zinc(II), [Zn(3,5,6-tcpa)₂(H₂O)₄][Zn(3,5,6-tcpa)₂ (H₂O)₂] (2)

Complex 2 was obtained by a procedure similar to that used for the preparation of 1 except for using Zn(CH₃COO)₂·2H₂O (0.3 mmol, 0.066 g) and 3,5,6-trichloropyridine-2-oxyacetic acid (Hubei Yebang Technology Company Limited, Xiaogan, Hubei Province, P. R. China) (3, 5, 6-Htcpa, 0.3 mmol, 0.077 g) as the starting reactants, and the mixture was adjusted to pH=7.0 with 0.3 mol L⁻¹ KOH (Tianjin Damao Chemical Reagent Factory, Tianjin, P. R. China). After cooling to room temperature, colorless block shaped crystals were obtained and collected. Yield: 33 mg (35%, based on 3,5,6-Htcpa). –C₂₈H₂₄Cl₁₂N₄O₁₈Zn₂ (%): calcd. C 17.13, H 1.90, N 4.44; found C 17.20, H 1.96, N 4.38.

2.3 Single-crystal X-ray structure determinations

Colorless crystals with dimensions of 0.25×0.22×0.20 mm³ (1) and 0.25×0.23×0.22 mm³ (2) were selected for measurement. A total of 3580 (1) and 30935 (2) reflections were collected in the ranges of 6.74≤2θ≤56.69° (1) and 6.51≤2θ≤56.87° (2) using ω-2θ scans, of which 2143 (1) and 5062 (2) were unique with R_int=0.0350 (1) and 0.0681 (2). Using Olex2 (a complete structure solution, refinement and analysis program, http://www.olex2.org) [34], the structures of 1 and 2 were solved with the Shelxt (crystal structure refinement software, http://shelx.uni-ac.gwdg.de/SHELX/) [35] structure solution program using intrinsic phasing and refined with the Shelxl (an ongoing developmental software for Integrated space-group and crystal structure determination) [36] refinement package using least-squares minimization. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were generated geometrically and treated by a mixture of independent and constrained refinement. The crystal data and collection parameters for 1 and 2 are summarized in Table 1. Selected bond lengths and bond angles are given in Table 2. Hydrogen bonds and ClLCl halogen bond parameters are listed in Table 3.

CCDC 1821888 (1) and 1936363 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Description of the structure of [Zn(3-Hcpa)₂(H₂O)₄] (1)

The molecular structure determination revealed that compound 1 is a mononuclear species and the asymmetric unit comprises one Zn^II ion, two monodeprotonated 3-carboxy-phenoxyacetate ligands (3-Hcpa⁻) and four coordinated water molecules.

The Zn^II ion lies on a center of inversion and is six-coordinate with four water molecules [Zn1–O6, 2.148(2) Å; Zn1–O7, 2.048(2) Å; mean 2.098(2) Å] and two trans-related unidentate carboxy oxygen atoms of 3-Hcpa⁻ [Zn1–O1, 2.097(2) Å] in an octahedral geometry (Fig. 1). The mean Zn–O_water bond distance almost equals to those of the Zn–O_carboxy bonds (Table 2). The trans-O–Zn–O bond angles are 180° (by symmetry) and the cis ones range from 87.74(8)° to 92.26(8)°. These parameters support a nearly idealized octahedron for 1.

Fig. 1:

The molecular structure of 1, showing the atom numbering scheme. Displacement ellipsoids are shown at the 30% probability level. The dashed lines show the intra-molecular O–HLO hydrogen bonds.

The packing of the complex 1 is constructed by O–HLO hydrogen bonding interactions (Fig. 2 and Table 2). The complexed H₂O as donor (O7) is involved in intra-molecular hydrogen bonds with uncomplexed carboxy oxygen atoms as acceptors (O2; O7–H7BLO2). The donors and acceptors for inter-molecular hydrogen bonds are as follows, respectively: the protonated carboxy oxygen atom of the 3-Hcpa⁻ group as donor (O5) and the neighboring equivalent as acceptor (O4^d; O5–H5ALO4^d); coordinated water molecule as donors (O6) and the uncoordinated carboxy oxygen atom (O2^b; O6–H6BLO2^b) as acceptor. (See Table 3 for a full list of the symmetry operations.) Through the above hydrogen bondings, 1 is connected into the layer structure in the bc plane, with the ZnLZn separation of 21.959 Å (Fig. 2a). Along the a axis, the coordinated water molecules act as donors (O6, O7), and the coordinated carboxy oxygen (O1^a; O6–H6ALO1^a), ether oxygen (O3^a; O6–H6ALO3^a) as well as coordinated H₂O molecules (O6^c; O7–H7ALO6^c) are acceptors. These hydrogen bonds link the layers of 1 into a 3D network finally (Fig. 2c), with the Zn⋯Zn separation of 4.961 Å between adjacent layers (Fig. 2b).

Fig. 2:

(a) The O–HLO hydrogen bonds projected on the bc plane in 1; (b) another O–HLO hydrogen bonds along the a axis in 1; (c) 3D network formed by hydrogen bondings in 1.

Comparing 1 with its homologues [Ni(3-Hcpa)₂(H₂O)₄] (1-Ni) [11] and [Co(3-Hcpa)₂(H₂O)₄] (1-Co) [12], some important similarities and differences can be found as follows: (i) Among all structures, the average M–O_water distance is almost the same with the M–O_carboxy distance, but the two M–O_water distances differ significantly. This may be attributed to the formation of three inter-molecular hydrogen bonds for one O_water atom but two for the other O_water atom. (ii) The trans O–M–O bond angles are 180° (by symmetry) and the cis ones are close to 90°, suggesting idealized octahedral geometries for all of them. (iii) O–HLO bonds stabilize the 3D networks. (iv) The ZnLZn separation (21.96 Å) in the layer of 1 is a little longer than the NiLNi (21.89 Å) distance in 1-Ni and the CoLCo (18.27 Å) distance in 1-Co, but the ZnLZn separation (4.96 Å) between adjacent layers is shorter than those of NiLNi (5.79 Å) and CoLCo (5.87 Å).

Compound 1 is the first Zn^II complex in which the partly deprotonated 3-Hcpa⁻ anion acts as a monodentate ligand. In the reported Zn^II coordination polymers {[Zn(4,4′-bipy)(H₂O)₄](3-cpa)·H₂O}_n [17], {[Zn(3-cpa)(imidazole)₂]·3H₂O}_n [18] and [Zn(3-cpa)(1H-benzimidazole)₂]_n [19], the fully deprotonated 3-cpa²⁻ anion is a free (un-coordinated) counterion in the first one, and acts as bis-monodentate ligands in the latter two.

3.2 Description of the structure of [Zn(3,5,6-tcpa)₂(H₂O)₄][Zn(3,5,6-tcpa)₂(H₂O)₂] (2)

Compound 2 is an interesting co-crystal of two mononuclear units, featuring two discrete and stereochemically different complexes: one octahedral [Zn(3,5,6-tcpa)₂(H₂O)₄], the other tetrahedral [Zn(3,5,6-tcpa)₂(H₂O)₂].

In the first one, the six-coordinated Zn^II ion lies on a crystallographical inversion center and the coordination sphere consists of four oxygens from water molecules [Zn1–O4, 2.095(2) Å; Zn1–O5, 2.075(2) Å, mean 2.085(2) Å] and two from carboxy groups of unidentate trans-related 3,5,6-tcpa⁻ anions [Zn–O1, 2.129(2) Å] (Fig. 3 and Table 2). The mean Zn–O_water distance [2.085(2) Å] is much shorter than those of the Zn–O_carboxy bond. The trans O–Zn–O bond angles equals to 180° (by symmetry) and the cis ones are in the range of 90.49(7) to 92.15(7)°. These parameters indicate a slightly elongated octahedron.

Fig. 3:

The molecular structure of 2, showing the atom numbering scheme. Displacement ellipsoids are shown at the 30% probability level. The dashed lines show the intra-molecular O–HLO hydrogen bonds.

In turn, the four-coordinated unit contains two water molecules [Zn–O9, 1.976(2) Å] and two unidentate 3,5,6-tcpa⁻ ligands [Zn–O6, 1.952(2) Å] in a distorted tetrahedral geometry, and shows approximate two-fold rotational symmetry (Fig. 3). The O–Zn–O bond angles fall in the range of 99.34(11) to 127.70(1)°.

The 3D supramolecular architecture of the complex 2 arises from two weak non-covalent interactions, i.e. O–H⋯O hydrogen bonding and weak ClLCl halogen bonding (Fig. 4 and Table 2).

Fig. 4:

(a) The O–HLO hydrogen bonds along the a axis in 2; (b) The O9–H9BLO7^e hydrogen bond and Cl2LCl6^g halogen bond interactions projected on the bc plane in 2 (for symmetry operations see Table 3); (c) 3D framework connected by hydrogen bonds and halogen bond interactions of 2.

The intra-molecular hydrogen bonding is established between the complexed water molecules as donors (O4, O5, O9) with uncomplexed (O2; O4–H4BLO2) and complexed carboxy oxygen atoms (O6, O1; O5–H5ALO6, O9–H9ALO1) as acceptors (Fig. 3). For the inter-molecular ones, the water molecules serve as donors (O4, O9) for uncomplexed carboxy oxygen atoms as acceptors (O7^e, O7^f; O9–H9B⋯O7^e, O4–H4A⋯O7^f) (Fig. 4). In other words, all aqua ligands are involved in hydrogen bondings with either adjacent octahedral (two) or tetrahedral units (three) (Table 2).

The halogen⋯halogen interactions are found between C4–Cl2 and Cl6^g–C12^g of neighboring octahedral and tetrahedral units with a Cl2⋯Cl6^g separation of 3.246 Å (Fig. 4b). In general, inter-molecular C–X1⋯X2–C interactions (X=F, Cl, Br, I) are classified into two types according to the values of the angles θ₁=∠C–X1⋯X2 and θ₂=∠X1⋯X2–C. The interactions with similar angles (θ₁=θ₂) are called Type I. The condition of θ₁=180° and θ₂=90° is defined as the interactions of Type II. In 2, the geometry of halogen⋯halogen interactions is characterized by ∠C4–Cl2⋯Cl6^g (θ₁=157.89°) and ∠Cl2⋯Cl6^g–C12^g (θ₂=150.91°), corresponding to halogen⋯halogen interactions of Type I [38].

Complex 2 is the first crystal structure with two mononuclear Zn^II units with different coordination numbers (tetrahedral four and octahedral six) comprising the ligand 3,5,6-trichloro-pyridine-2-oxyacetate (3,5,6-tcpa). A similar co-crystal with tetra- and hexa-coordinated mononuclear Zn^II centers has been described before: [Zn(2,4-dcpa)₂(H₂O)₄][Zn(2,4-dcpa)₂(H₂O)₂] (2A) (2,4-dcpa=2,4-dichloro-phenoxyacetate) [37]. Their structures can be compared, and some interesting similarities and differences are obvious. (i) The corresponding equivalent Zn–O_carboxy and Zn–O_water bond lengths equal to each other in 2, but differ significantly in 2A. (ii) For the octahedral complex unit, the trans-O–Zn–O bond angles all equal to 180° in 2 (by symmetry), whereas they range from 176. 8° to 178.7° in 2A. (iii) Because of the similar steric hindrance of three or two chloro substituents, none of the phenoxy oxygen atoms are involved in coordination, but they take part in extensive inter- and intra-molecular hydrogen bonds. (iv) 3,5,6-tcpa has one more chlorine atom in contrast to 2,4-dcpa, giving rise to the formation of weak Cl⋯Cl halogen bonds in the crystal structure of 2, which are absent in compound 2A.

3.2.1 Fluorescence spectra

The luminescence properties of compounds 1 and 2 in the solid state were investigated at room temperature (Figs. 5 and 6). The maximal emission peak appears at 421 nm (λ_ex=366 nm) for 1 and at 407 nm (λ_ex=305 nm) for 2. In contrast, free 3-H₂cpa does not display any luminescence [15] and free 3,5,6-Htcpa has the emission peak maximum at 405 nm (λ_ex=321 nm) [20], probably attributable to π*→n or π*→π transitions. The emission of 1 is assigned to ligand-to-metal charge transfer (LMCT) as was previously proposed for a related 1D polymer [15].

Fig. 5:

The excitation and emission spectra of complex 1 in the solid state at room temperature.

Fig. 6:

The excitation and emission spectra of complex 2 in the solid state at room temperature.

Compared with the spectra of 3,5,6-Htcpa, the co-crystal 2 has a similar emission band shape and location, which indicates that the intra-ligand transition is responsible for the emission of 2. The 2-nm red shift and enhancement of the luminescence emissions of 2 may be attributed to the increased rigidity of the ligands due to their complexation with Zn^II, resulting in a decrease in energy loss by radiationless decay [39]. The result is consistent with those for the related Zn^II complexes with the 2,4-dichloro-phenoxyacetic acid ligand [28], [29], [30]. These results suggest that complexes 1 and 2 have potential as new purple luminescence materials.