Synthesis and crystal structure analyses of tri-substituted guanidine-based copper(II) complexes

2.1 Materials and methods

The chemicals pivaloyl chloride, o-toluidine and o-anisidine were obtained from Fluka whereas mercury(II) chloride, potassium thiocyanate and triethylamine were purchased from local suppliers of Sigma. The solvents used during the syntheses were obtained from local suppliers and distilled twice before use. N-pivaloyl-N′-(2-methoxyphenyl)thiourea was synthesized by a published method [17]. Melting points were determined using an electrothermal melting point apparatus (model MP-D Mitamura Riken Kogyo, Japan) with the capillary tube method. A PerkinElmer series II CHNS/O analyzer 2400 was used for the CHN analysis. Infrared spectra were recorded as KBr disks on Bio-Rad Elmer 16 FPC FT-IR. ¹H and ¹³C NMR spectra were recorded on a Bruker AV300 NMR. ¹H and ¹³C NMR chemical shifts are reported in ppm downfield of TMS and referenced against the residual CDCl₃ peaks. Synthetic procedures as published in our earlier report were adopted as described in detail therein [16].

2.2 Synthesis of ligands

2.3 Synthesis of copper (II) complexes

2.4 Crystal structure determinations

Suitable single crystals of complexes 1, 2 and 3 were mounted on a thin glass fiber coated with paraffin oil. The X-ray data were collected at low temperature (T = 170 K; 1 and 3) or at room temperature (2) using a STOE-IPDS II diffractometer equipped with a normal-focus, 2.4 kW, sealed-tube X-ray source with graphite-monochromatized MoKα radiation (λ = 0.71073 Å). The integration of diffraction profiles was performed with the program X-Area, numerical absorption correction was carried out with the programs X-Shape and X-Red32, all from STOE^© [20].

The structures were solved by Direct Methods (Sir-92, Superflip or Shelxt-2016) [21], [22], [23] and refined against all data by full-matrix least-squares methods on F² (Shelxl-2018) [24]. In the asymmetric unit of complex 2 two halves of the molecular structures were refined and the entire molecules are generated by symmetry (inversion centers). Even though a Platon [25] check suggests the possibility of additional symmetry we can exclude that a higher symmetry was overlooked. The angles between the metal chelate rings and the outer methoxy-benzene rings are distinct in the two molecules (13.05° for Cu1 and 30.51° for Cu2). It might be possible to go to higher symmetry and model a respective “disorder” in this substituent (and all other atoms aside from the metal chelate center as well). However, crystallographically this would not be correct because the two refined molecules behave clearly differently without any apparent disorder in the respective atoms when refined separately.

All nonhydrogen atoms were refined with anisotropic displacement parameters. The C-bound hydrogen atoms were refined isotropically in calculated positions using a riding model with their U_iso values constrained to 1.5 U_eq of their pivot atoms for terminal sp³ carbon and 1.2 U_eq for all other carbon atoms. The H atoms on the nitrogen atoms of complexes 1 and 2 were located and refined isotropically. No restraints or constraints were used for the refinement of the one respective H atom in complex 1 or the two respective H atoms in complex 2. For complex 3, due to the weakly diffracting crystal, the data was very noisy at higher angles/resolution and those fractions of data were cut out from the data set. For treating the disordered groups (two tolyl and one tBu groups), quite strong constraints (EADP, SAME) and some softer constraints (SIMU, DELU) were used. The H atoms on the nitrogen atoms of complex 3 were located and refined with constraints (SADI for the N–H distances and U_iso values set to 1.2 times that of the U_eq value of the parent nitrogen atom).

CCDC 1997840 (complex 1), 1997841 (complex 2) and 1997842 (complex 3) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Elemental analyses and IR-spectroscopic characterization

The tri-substituted guanidine ligands were synthesized by condensing N-pivaloyl-N′-(2-methoxyphenyl)thiourea with o-anisidine (L¹), aniline (L²) and o-toluidine (L³) [16]. All three guanidine ligands were reacted with the copper(II) acetate to yield the blue complexes [16]. The elemental composition for each of the synthesized ligands and Cu(II) derivatives was in accordance with the theoretically calculated values. As this type of ligand has been described in detail previously [16], the following discussion will be predominantly confined to the Cu(II) complexes of these ligands. The infrared spectroscopic data of the ligands support the presence of intramolecular amide-imidic acid tautomerism (Scheme 1) which is evident from the appearance of the weak vibrational band in the region around 3400 cm^‒1. The tautomerism is observed in all three synthesized ligands and can be seen in their Cu(II) derivatives as well.

Scheme 1:

Amide-imidic acid tautomerism in guanidines. R¹ = 2-methoxyphenyl, R² = 2-methoxyphenyl (L¹), phenyl (L²) and o-tolyl (L³).

The IR spectral data also reveal that the guanidine ligands coordinated to the metal center are in their imidic acid tautomeric forms to produce the tetragonal planar N₂O₂ type of arrangement around Cu(II) as shown in Scheme 2.

Scheme 2:

Cu(II) complexes of L¹–L³. R¹ = 2-methoxyphenyl in all three cases, R² = 2-methoxyphenyl (L¹), phenyl (L²) and o-tolyl (L³).

3.2 Molecular structures

The copper complexes of the three guanidine ligands were crystallized from concentrated solutions in dichloromethane and diethyl ether (Table 1). The complexes bis(N-pivaloyl-N′-(2-methoxyphenyl)-N″-phenylguanidinato)copper(II) (2) and bis(N-pivaloyl-N′-(2-methoxyphenyl)-N″-(2-tolyl)guanidinato)-copper(II) (3) crystallize in (slightly) distorted square planar geometries whereas bis(N-pivaloyl-N′,N″-bis(2-methoxyphenyl)guanidinato)-copper(II) (1) exhibits a heavily distorted square pyramidal geometry around the copper center. In complex 1, the penta-coordinated Cu(II) center is bonded by nitrogen donors of the two guanidine moieties, by the hydroxyl oxygen donors of the pivaloyl moieties and by one of the methoxy oxygen atoms attached to the phenyl ring of the R² substituent on guanidine N" (Figure 1a). The other methoxy group is in a longer distance to the Cu(II) center allowing only a much weaker interaction between O and Cu or rather none at all. The different behavior of the methoxy moieties may be deduced from their bond lengths viz. Cu1‒O3 = 2.743 Å and Cu1‒O6 = 2.907 Å summarized in Table 2. The C1‒O1 distance of the pivaloyl moiety is 1.263(4) Å and comparable to similar bond lengths in aqua[µ-(N¹-carboxylatomethylguanidino)oxidoacetato](µ-guanidinoacetic acid)dicopper(II) nitrate dihydrate, [Cu₂(C₅H₆N₃O₅)(C₃H₇N₃O₂)(H₂O)]NO₃·2H₂O {1.276(3) Å} [26]. The two guanidine nitrogen and the two oxygen donor atoms are located trans to each other with bond lengths Cu1–O1 = 1.892(3) Å, Cu1–O4 = 1.903(2) Å, Cu1–N3 = 1.947(3) Å and Cu1–N6 = 1.940(3) Å. The Cu–O bond lengths are shorter than those reported in [Cu(ClO₄)₂(L^3m)(CH₃OH)] {where L^3m = 1-(methoxymethanimidoyl)-2-(pyridin-2-ylmethyl)guanidine} and in [Cu₂(µ-OH)₂(DPipG₂p)₂](PF₆)₂ {where DPipG₂p = bis(dipiperidinomethylene)propane-1,3-diamine} with Cu–O {2.0132(16) Å} [27] Cu–O {1.935(2) Å}[28]. Similarly, the Cu–N bond lengths follow the same trend in bond lengths as found in [Cu(ClO₄)₂(L^3m)(CH₃OH)] with Cu–N = 1.9711(17) Å despite the similar nature of bonding [27]. The differences may arise from the fact that the guanidine N-pivaloyl-N′,N″-bis(2-methoxyphenyl)guanidine is more sterically demanding in nature. The N6–Cu1–N3 = 158.44(12)° and O1–Cu1–O4 = 147.44(12)° angles are nonlinear in nature, and the O1–Cu1–N3 = 91.59(12)° and O4–Cu1–N6 = 91.46(11)° angles are almost identical. However, the O3_apical–Cu1–N3_basal arrangement with 66.39° is deviating substantially from the ideal bond angles. The geometry for five coordinated compounds can be obtained from the τ value (τ = β–α/60, where β and α are the two largest basal angles). The τ value for complex 1 is 0.001 revealing a distorted square pyramidal geometry around the Cu(II) center. The two planes Cu1–N3–C6–N1–C1–O1 and Cu1–N6–C26–N4–C21–O4 are lying at 43.50° suggesting a twist due to coordination of methoxy moiety.

Figure 1:

Molecular structures of complex 1 (a), of complex 2 (b; only one of two refined molecules is shown) and of complex 3 (c) in the crystal and atom numbering scheme adopted. Displacement ellipsoids are drawn at the 50% probability level, H atoms as spheres with arbitrary radius.

Complexes 2 and 3 have similar coordination modes to that of bis(N-pivaloyl-N′-(2,3-dimethylphenyl)-N″-phenylguanidinato)copper(II) [16]. A closer look on the geometrical properties of both complexes reveals square planar arrangement of the guanidyl ligands around the Cu(II) center with small differences. Therefore, we are refraining from an in-depth study until and unless the differences are sufficient to warrant discussion. The symmetry of the complex follows from a crystallographic inversion center with planar Cu₂O₂ and Cu₂N₂ cores. The Cu(II) centers in both complexes are coordinated in square planar manner with a deviation of Cu(II) from the N₂O₂ core in complex 3. In complex 3, the N–Cu–O unit is twisted by 31.11° between the planes produced Cu1–O3–C21–N4–C26–N6 and Cu1–O1–C1–N1–C6–N3. Whereas, in complex 2, the same angles (between Cu1–O2–C9–N2–C8–N3 and Cu1–N3–C8–N2–C9–O2) are 0.0° revealing the square planar arrangement. The Ortep plots of complexes 2 and 3 can be seen in Figure 1b and c. In complex 2, the Cu1–O2–C9–N2–C8–N3 planes form an angle of 11.9° to the anisol groups (C2 to C7), i.e. they are almost coplanar, whereas the torsion angle between Cu2–04–C28–N5–C27–N6 and the anisole ring (C21 to C26) is 30.5°. It can be deduced that one complex in compound 2 crystalizes with higher angles only and one complex with smaller angle.

The representative bond lengths and angles for complexes 2 and 3 are shown in Tables 3 and 4, respectively. The difference in bond lengths in complexes 2 and 3 are insignificant in terms of Cu–O distances {Cu1–O2 = 1.891(3) Å in complex 2, Cu1–O3 = 1.893(5) Å and Cu1–O1 = 1.905(5) Å, in complex 3}. However, in terms of Cu–N bond distances the two complexes are surprisingly and significantly different {Cu1–N3 = 1.970(3) Å in complex 2, Cu1–N6 = 1.929(7) Å and Cu1–N3 = 1.941(7) Å in complex 3}. These Cu–N distances can be compared to respective bonds in bis(N-pivaloyl-N′-(2,3-dimethylphenyl)-N″-phenylguanidinato)copper(II) [16]. Notable is the close similarity of the Cu–N bond lengths in complex 2 and in bis(N-pivaloyl-N′-(2,3-dimethylphenyl)-N″-phenylguanidinato)copper(II) with a difference of only 0.012 Å. The differences of the Cu–N distances in all three new copper complexes and in bis(N-pivaloyl-N′-(2,3-dimethylphenyl)-N″-phenylguanidinato)copper(II) may be attributed to the substituent on the N″ donor substituent.