Azido and Thiocyanato-Bridged Polymeric Copper(II) Complexes [CuL(μ1,3-N3)]n·2nH2O and [CuL(μ1,3-NCS)]n: Synthesis and Structures

A new azido-bridged polymeric copper(II) complex, [CuL(μ_1,3-N₃)]_n·2nH₂O (1), and a new thiocyanato-bridged polymeric copper(II) complex, [CuL(μ_1,3-NCS)]_n (2) (L = 4-methoxy-2-[(2-morpholin-4-ylethylimino)methyl]phenolate), have been prepared and structurally characterized by elemental analysis, IR spectra, and single-crystal X-ray determination. Each Cu atom in the complexes is coordinated by three donor atoms of Schiff bases and by two atoms of the pseudohalide ligands, forming a square pyramidal geometry. Azide and thiocyanate anions are preferred bridging groups for the construction of polymeric copper complexes with Schiff bases.

Keywords: crystal structure; Cu complex; polymeric structure; pseudohalide; Schiff base

Complexes with polymeric structures bearing bridging groups are currently attracting great attention for their interesting structures and wide applications.[[1]–[3]] Schiff bases derived from salicylaldehyde and its derivatives are a kind of versatile ligands in coordination chemistry. The rational design and construction of polymeric structures of the complexes with Schiff bases are of peculiar interest in structural chemistry. As is well known, azide and thiocyanate are two charming ligands for the construction of polymeric structures.[[4]–[7]] To our knowledge, there are no complexes derived from 4-methoxy-2-[(2-morpholin-4-ylethylimino)methyl]phenol have been reported so far. In this study, a new azido-bridged polymeric copper(II) complex, [CuL(μ1,3-N3)]n·2nH2O (1), and a new thiocyanato-bridged polymeric copper(II) complex, [CuL(μ1,3-NCS)]n(2), were successfully prepared and characterized.

Starting materials, reagents, and solvents with AR grade were purchased from commercial suppliers and were used without further purification. Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer (Nanjing University, China). IR spectra were recorded on a Jasco FT/IR-4000 spectrometer (Liaoning Normal University, China) as KBr pellets. Single crystal structural X-ray diffraction was carried out on a Bruker D8 VENTURE PHOTON diffractometer (Shandong University of Technology, China).

To a methanolic solution (30 mL) of 5-methoxysalicy- laldehyde (1.0 mmol, 0.15 g) was added a methanolic solution (30 mL) of 2-morpholin-4-ylethylamine (1.0 mmol, 0.13 g) with continuous stirring. The mixture was stirred for 30 min at room temperature to give yellow solution. The solvent was evaporated to give yellow gummy product of the Schiff base. Characteristic IR data: 1643 cm−1. Anal. Calcd. for C14H20N2O3: C, 63.6; H, 7.6; N, 10.6. Found (%): C, 63.4; H, 7.7; N, 10.5.

For [CuL(μ1,3-N3)]n·2nH2O (1), to the methanolic solution (5 mL) of HL (0.1 mmol, 0.026 g) were added a methanolic solution (5 mL) of Cu(CH3COO)2·H2O (0.1 mmol, 0.020 g) and an aqueous solution (1 mL) of sodium azide (0.1 mmol, 0.007 g) with continuous stirring. The mixture was stirred for 10 min at room temperature and was filtered. Blue block-shaped crystals of 1, suitable for X-ray crystal structural determination, were formed from the filtrate after a few days. The crystals were isolated by filtration, washed with methanol and dried in air. Yield: 51%. Characteristic IR data (cm−1): 1632 (s), 2045 (s). Anal. Calcd. for C14H23CuN5O5: C, 41.5; H, 5.7; N, 17.3. Found (%): C, 41.6; H, 5.8; N, 17.2.

For [CuL(μ1,3-NCS)]n(2), the complex was synthesized by the similar method as that described for 1, with sodium azide replaced by ammonium thiocyanate (0.1 mmol, 0.008 g). Blue block-shaped single crystals of 2 were isolated, washed with methanol and dried in air. Yield: 45%. Characteristic IR data (cm−1): 1632 (s), 2113 (m). Anal. Calcd. for C15H19CuN3O3S: C, 46.8; H, 5.0; N, 10.9. Found (%): C, 46.6; H, 4.9; N, 11.0.

Diffraction intensities for the complexes were collected at 298(2) K using a Bruker D8 VENTURE PHOTON diffractometer (Shandong University of Technology, China) with MoKα radiation (λ = 0.71073 Å). Collected data were reduced with SAINT,[[8]] and multiscan absorption correction was performed using SADABS.[[9]] Both structures of the complexes were solved by direct methods, and refined against F2 by full-matrix least-squares method using SHELXTL.[[10]] All of the non-hydrogen atoms were refined anisotropically. The water H atoms in 1 were located from difference Fourier maps and refined isotropically, with O‒H and H···H distances restrained to 0.85(1) and 1.37(2) Å, respectively. The remaining hydrogen atoms were placed in calculated positions and constrained to ride on their parent atoms. Crystallographic data for the complexes are summarized in Table 1. Selected bond lengths and angles are given in Table 2.

TABLE 1 Crystallographic and experimental data for complexes and

TABLE 2 Selected bond lengths (Å) and angles (°)

Graph: FIG. 1 A perspective view of the molecular structure of 1 with the atom labeling scheme. The thermal ellipsoids are drawn at the 30% probability level. Unlabeled atoms are at the symmetry position 3/2 – x, –1/2 + y, 3/2 – z.

The Schiff base HL was synthesized by reaction of equimolar quantities of 5-methoxysalicylaldehyde with 2-morpholin-4-ylethylamine in methanol. Both copper complexes were synthesized by reactions of the Schiff base with copper acetate and sodium azide or ammonium thiocyanate in methanol under ambient condition. Both complexes are stable in air at room temperature for at least two months.

Graph: FIG. 2 A perspective view of the molecular structure of 2 with the atom labeling scheme. The thermal ellipsoids are drawn at the 30% probability level. Unlabeled atoms are at the symmetry position 1/2 + x, 1/2 – y, 1/2 + z.

Graph: FIG. 3 Packing structure of 1.

Single-crystal X-ray diffraction shows that the complexes crystallize as similar azido- or thiocyanato-bridged polymeric structures (Figure 1 for 1, Figure 2 for 2). The adjacent Cu···Cu distances are 5.674(2) Å for 1, and 5.851(3) Å for 2. The smallest repeat unit of 1 contains one [CuL(N3)] and two water molecules of crystallization, while of 2 contains one [CuL(NCS)]. The Cu atom in each complex is in square-pyramidal coordination environment and is five-coordinated by one phenolic O and two N atoms of the Schiff base ligand and by two atoms of the pseudohalide ligands (azide for 1 and thiocyanate for 2). The significant distortion of the square pyramids is revealed by the bond angles between the apical and basal donor atoms (Table 1). The bond angles N1‒Cu1‒N2 in complexes 1 and 2 deviate from 90° by 5.5(5)° and 5.9(1)°, which are due to the strain created by the five-membered chelate rings Cu1/N1/C9/C10/N2 for 1 and Cu1/N1/C8/C9/N2 for 2. The deviation of Cu atoms from the best-fit square planes are 0.117(5) Å for 1 and 0.160(2) Å for 2.

The apical bonds [Cu1–N5A for 1 and Cu1–S1B for 2; Symmetry codes: A: 3/2 – x, –1/2 + y, 3/2 – z; B: 1/2 + x, 1/2 – y, 1/2 + z] are much longer than the basal bonds, indicating they are not very strong. The Cu–O and Cu–N bond lengths in both complexes are comparable to each other, and also comparable to those observed in other Schiff base copper(II) complexes.[[11]–[13]] The azide and thiocyanate bridging groups are nearly linear and show bent coordination mode with the Cu atoms [for 1: N3–N4–N5/Cu1–N3–N4/Cu1A–N5–N4 = 177.3(4)/124.9(3)/122.6(3)°; for 2: N3–C15–S1/Cu1– N3–C15/Cu1B–S1–C15 = 178.7(2)/168.0(1)/100.6(2)°].

In the crystal structure of 1, [CuL] units are linked by end-to-end azide bridges, to form chains running along the b-axis (Figure 3). The chains are further linked by water molecules through hydrogen bonds [O5–H5B···O1: O5–H5B = 0.85(1) Å, H5B···O1 = 2.05(2) Å, O5···O1 = 2.887(4) Å, O5–H5B···O1 = 169(5)°; O4–H4A···O3i: O4–H4A = 0.85(1) Å, H4A···O3i = 2.40(3) Å, O4···O3i = 3.170(4) Å, O4–H4A···O3i = 152(4)°; O4–H4B···O4ii: O4–H4B = 0.85(1) Å, H4B···O4ii = 2.09(2) Å, O4···O4ii = 2.854(7) Å, O4–H4B···O4ii = 153(5)°; O5–H5A···O4iii: O5–H5A = 0.85(1) Å, H5A···O4iii = 2.01(2) Å, O5···O4iii = 2.842(4) Å, O5–H5A···O4iii = 168(5)°; symmetry operators: i) 1 + x, y, z; ii) 2 – x, – y, 1 – z; iii) 3/2 – x, 1/2 + y, 3/2 – z]. In the crystal structure of 2, [CuL] units are linked by end-to-end thiocyanate bridges, to form chains running along c-axis (Figure 4).

Graph: FIG. 4 Packing structure of 2.

The coordination number 5 for copper(II) complexes is very common. The question arises as to whether the coordination polyhedra around the Cu atoms can be described as distorted square pyramids or trigonal bipyramids. Further information can be obtained by determining the structural index τ,[[14]] which represents the relative amount of trigonality (square pyramid, τ = 0; trigonal bipyramid, τ = 1); τ = (β – α)/60°, α and β being the two largest angles around the metal atom. The values of τ are 0.032 for 1 and 0.167 for 2. From the τ values, it can be concluded that all the Cu atoms in the complexes adopt distorted square pyramidal coordination.

In the IR spectra of the complexes, the strong absorption bands at 1632 cm−1 can be assigned to the azomethine stretching frequencies of the Schiff base ligands, whereas for the free Schiff base the corresponding band is observed at 1643 cm−1. The shift of these bands towards lower frequencies on complexation suggests coordination to the Cu atoms through the imine N atoms. The ν(C–O) mode is present as middle bands at about 1180 cm−1 for the complexes. The intense band at 2045 cm−1 for 1 and 2113 cm−1 for 2 are assigned to the stretching vibration of the azide ligand in 1 and thiocyanate ligand in 2, respectively. The weak bands indicative of the Cu‒O and Cu‒N bonds are located in the region 600–350 cm−1.

The present article reports the synthesis and crystal structures of two new azido- and thiocyanato-bridged polymeric copper(II) complexes prepared from 4-methoxy-2-[(2-morpholin-4-ylethy limino)methyl]phenol. The azide and thiocyanate anions are preferred bridging groups for the construction of polymeric structures of complexes. The present copper complexes may possess interesting magnetic property, which deserve further investigation.

CCDC 879014 (1) and 879015 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge viahttp://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.