2-(1,2,3-Triazol-4-yl)-imidazoline, -oxazoline, -thiazoline and -tetrahydropyrimidine as ligands in copper(II) and nickel(II) complexes

2.1 Syntheses

In a preceding paper, we have reported the preparation of the 1,2,3-triazole-carboxamidinium tetraphenylborate 1 and its one-step conversion into the 2-(1,2,3-triazol-4-yl)imidazoline L1 [24]. In an analogous manner, we have now combined salt 1 with enantiomerically pure (R)-propane-1,2-diamine to provide the chiral triazole/imidazoline hybride ligand L2 (Scheme 1). The condensation of salt 1 with 2-aminoethanol, 2-aminoethanethiol and propane-1,3-diamine provided the corresponding triazolyl-substituted 1,3-oxazoline L3, 1,3-thiazoline L4 and tetrahydropyrimidine L5, all of which were formed in high yield (Scheme 1).

Dynamic prototropic equilibria are present in the cases of cyclic amidine ligands L1, L2 and L5, as indicated by broadening of the ¹H and/or ¹³C NMR signals of the NCH₂ (and NCH) nuclei. Only in the case of L2, however, does proton tautomerism lead to structurally different tautomers (L2 and L2′), which are both incorporated in a complex with Cu(OTf)₂ (vide infra).

2.2 Complex formation with copper(II) triflate

Copper(II) trifluoromethanesulfonate (triflate) is sufficiently soluble in several common aprotic-polar organic solvents, so that complexation with organic ligands can take place in a homogeneous medium. Water, which could also act as a ligand, can be excluded a priori. Nevertheless, adventitious water in some cases was incorporated into the complex itself or in the crystal lattice during crystallization attempts or exposure to air of the first formed solid complex. Figure 2 shows the structures of the Cu(OTf)₂ derived complexes, the solid-state structures of which could be established by single-crystal X-ray structure determination. In all complexes derived from Cu(OTf)₂, the organic ligands act as bidentate chelating ones, coordinating to the copper(II) ion at the 1H-triazole ring position N-3 and at the imine nitrogen atom of the cyclic amidine or a related ring system. On the other hand, the coordination of the triflate anion, which is known as being a weakly coordinating ion in metal triflates and triflato complexes [26], is subject to variations depending on the particular complex.

Fig. 2:

Cu(OTf)₂ derived complexes that were structurally characterized in this study (OTf = O₃SCF₃).

By concentration of an ethyl acetate solution of Cu(OTf)₂ and ligand L1, light brown crystals were obtained, the X-ray structure determination of which established the composition [Cu(L1)₂](OTf)₂ (Fig. 3). This complex exhibits crystallographic inversion symmetry; four nitrogen atoms (two of each ligand) surround the copper ion in a distorted square-planar geometry (Table 1). The two triflate anions, which are both slightly disordered by rotation around the C–S bond, are not associated with the metal, but are engaged in an O···H–N hydrogen bond to the imidazoline ring of both organic ligands (d(O···N) = 2.83(3)–2.86(3) Å). Additionally, weak interactions of the O···H–C type between a triflate oxygen atom and CH_triazole (d(O···H) ~2.35 Å) and m-CH_phenyl (~2.42 Å) may be present.

Fig. 3:

Molecular structure of complex [Cu(L1)₂](OTf)₂ in the solid state (Ortep plot).

The blue crystals obtained from Cu(OTf)₂ and ligand (R)-L2/(R)-L2′ by crystallization from ethyl acetate-diethyl ether showed the expected 1:2 metal-to-ligand ratio; surprisingly, however, an X-ray structure determination revealed the composition [CuL2L2′(OTf)₂] · 2H₂O, i.e. a complex molecule containing both imidazoline tautomers, (R)-L2 and (R)-L2′ (Fig. 4, top). The copper(II) ion is surrounded by four nitrogen atoms in a distorted square-planar geometry which is extended into a tetragonally distorted octahedral arrangement by two weakly coordinated triflate anions (Cu1···O1 2.602(3), Cu1···O4 2.620(3) Å)). Notably, the triflate anions do not maintain O···H–N hydrogen bonds with the imidazoline rings of neighboring ligands. Rather, two water molecules, which are present in the asymmetric unit of the crystal, connect two triflate ions via O···H–O bonds (Fig. 4, bottom) and each of them is associated with an imidazoline ring via an O···H–N hydrogen bond.

Fig. 4:

Top: molecular structure of complex [CuL2L2′(OTf)₂] · 2 H₂O in the solid state (Ortep plot). Bottom: water molecules involved in O–H···O hydrogen bonds; for clarity, N_imidazoline–H···O bonds involving the water molecules are not shown (Mercury plot). Geometrical data (d(D···A), angle D–H···A; D = donor, A = acceptor) for N5–H···O8: 2.811(7) Å, 157°; for N10–H···O7: 2.768(8) Å, 157°; for O7–H^A···O1: 2.841(7) Å, 155°; for O7–H^B···O6 2.756(7) Å, 163°; for O8–H^A···O3: 2.834(8) Å, 150°; for O8–H^B···O4: 2.872(7) Å, 169°.

Blue crystals of a complex formed from Cu(OTf)₂ and triazolyl-tetrahydropyrimidine L5 were obtained by slow diffusion of diethyl ether into an n-butanol solution of the components; crystallization attempts from other solvents or solvent mixtures were unsuccessful. According to an X-ray analysis, the complex [Cu(L5)₂(OTf)₂] had been formed (Fig. 5, top). Similar to the complex with the triazolyl-imidazoline ligand L2/L2′, a distorted octahedral coordination geometry with the two triflate ions occupying the apical positions is observed (Table 1). The two copper–oxygen distances are rather long again, and additionally, they differ more from each other and the O···Cu···O angle is farther away from linearity. Besides using one oxygen atom for coordination to the copper atom, each triflate group is engaged with a second oxygen atom in an O···H–N hydrogen bond to the amidine NH group of another complex molecule (Fig. 5, bottom).

Fig. 5:

Top: molecular structure of complex [Cu(L5)₂(OTf)₂] in the solid state (Ortep plot). Bottom: N–H···O hydrogen bonding in the crystal structure (Mercury plot). Geometrical data (d(N–H), d(H···O), d(N···O), angle N–H···O) for N5–H5···O5: 0.83, 2.08, 2.895(3), 164(3)°; for N10–H10···O2: 0.83, 2.03, 2.821(4), 159(4)°.

When a solution of Cu(OTf)₂ and ligand L4 in ethyl acetate was slowly concentrated by solvent evaporation with exposure to air, light blue crystals were formed, which analyzed for the composition Cu(L4)₂(OTf)₂(H₂O) but according to an X-ray analysis were in fact 1:1 cocrystals of two different complexes, namely a diaqua and a bis(triflato) copper(II) complex, [Cu(L4)₂(H₂O)₂](OTf)₂ · [Cu(L4)₂(OTf)₂] (Fig. 6). Both complex entities are centrosymmetrical with the copper ion on a crystallographic inversion center. The coordination geometries can be described as distorted octahedral with the oxygen ligands in the apical positions. The Cu···O distances amount to 2.371(3) Å in the diaqua complex and 2.441(2) Å in the bis(triflato) complex. Although the triflate coordination may be rather weak, the contact distance is the shortest one among the triflato complexes presented here. In notable contrast to all other complexes, the Cu–N_triazole and Cu–N_thiazole distances are almost equal in [Cu(L4)₂OTf)₂], and moreover, the Cu–N_triazole bonds are even slightly longer. The two complex entities in the crystals are connected through hydrogen bonds between the water ligands of the diaqua complex and a triflate oxygen atom of the bis(triflato) complex. The two triflate anions, which compensate the positive charge of the diaqua complex, are each involved in one hydrogen bond with a water ligand but remain uncoordinated otherwise.

Fig. 6:

Molecular structure and packing of complex [Cu(L4)₂(H₂O)₂](OTf)₂/[Cu(L4)₂(OTf)₂] in the solid state (Ortep plot). Geometrical data (d(D–H), d(H···A), d(D···A), angle D–H···A; D = donor, A = acceptor) for O1–H^A···O2: 0.82(4), 2.06(4), 2.826(4), 155(4)°; for O1–H^B···O7: 0.86(5), 1.90(5), 2.742(4), 168(4)°.

In the IR spectra of the complexes, absorption bands in the 1540–1650 cm⁻¹ region, attributable to vibrations in the triazole and the second heterocyclic ring, are significantly displaced compared to the free ligands. In detail, the C=N stretching vibrations of the imidazoline, oxazoline, thiazoline and tetrahydropyrimidine rings appear between 1625 and 1640 cm⁻¹ in the free ligands and are shifted to higher wavenumbers in the complexes containing the cyclic amidine ligands and to lower ones in the oxazoline and thiazoline complexes. Absorption bands of the N-benzyl-1,2,3-triazole part of the ligands are found in the range 1531–1573 cm⁻¹ for the free ligands, but at higher wavenumbers (by 12–40 cm⁻¹) in the complexes, except for Cu(L3)₂Cl₂, where the triazole part remains uncoordinated (vide infra). Of particular interest is the region between 1300 and 1000 cm⁻¹, where strong absorptions associated with stretching modes of the CF₃SO₃ group dominate. In the past, the assignment of the vibration modes of the triflate group has found much attention, mainly in order to distinguish between a free and a coordinated triflate ion. When a triflate ion becomes mono- or di-coordinated – either to a metal atom or by a strong hydrogen bond – the C_3v symmetry of the free triflate ion is reduced to C_s, resulting in a splitting of the E type vibrations. Thus, the asymmetric stretching vibration of the SO₃ group, which appears around 1270 cm⁻¹ for the uncoordinated triflate [27], is expected to be split. This splitting was observed indeed in a number of triflate-containing copper complexes, the solid-state structures of which have been determined. Following the established assignment of the relevant vibration modes of the triflate ion (for examples, see lit. [27–32]), the assignments shown in Table 2 were made for the Cu(OTf)₂ complexes under study. It can be seen, that splitting of the ν_as(SO₃) vibration is observed in all cases, including complex [Cu(L1)₂](OTf)₂, where the triflate ions are involved in N–H···O hydrogen bonding according to the crystal structure analysis, but not in metal coordination. As we have indicated in Table 2, some of the absorption bands arising from the triflate ions appear at more or less the same wavenumber as absorptions also observed for the free ligand, but due to the high intensity of the triflate absorption bands, the assignment is unequivocal.

Electronic spectra of some of the Cu(OTf)₂ complexes were recorded. For solubility reasons, acetonitrile had to be used as the solvent; since no visible color change was noted between the complexes in the solid state and in solution, we can assume that acetonitrile molecules did not replace the triflate ions as ligands. The spectra display two absorption maxima above 350 nm, which are not present in the spectra of the free ligands: one around 370–380 nm, belonging to a broadened absorption that extends into the visible region and in some cases includes a shoulder at the longer wavelength side, and the other one as a very broad, low-intensity absorption in the visible region with maxima in the range of 608–664 nm (Table 3 ). The latter absorption band is unsymmetrical, with a long tailing up to about 950 nm. Absorptions in this range are commonly assigned to ligand-field transitions within tetragonally distorted octahedral Cu(II) complexes. [28, 33]. The absorption at higher energy is likely to arise from a π(ligand)-d(Cu(II)) charge transfer transition [28].

2.3 Complexes with CuCl₂ and NiCl₂

Similar to Cu(OTf)₂, CuCl₂ is also soluble in a variety of aprotic-polar organic solvents and in the lower aliphatic alcohols. The complexation of solvatochromic CuCl₂ by ligand L1 in several solvents was indicated by a characteristic color change to dark green, but the very thin needle-shaped crystals or amorphous solids obtained were not suited for single-crystal X-ray structure determination.

With the triazole/oxazoline ligand L3, on the other hand, copper(II) chloride in tetrahydrofuran formed crystals of the dark blue complex [Cu(L3)₂Cl₂] · THF. In the crystal structure, copper resides on an inversion center and has a square-planar coordination with the symmetry-related ligands in trans position (Figs. 7 and 8). The organic ligands coordinate solely at the imine nitrogen atom of the oxazoline ring, but not at the less nucleophilic N1 position of the triazole ring. The structure of the complex is analogous to that of Cu(1-ethyltetrazole-N⁴)₂Br₂] [34] and also to the complex comprising CuCl₂ and ligand 2 (Fig. 1, X = CH₂O), [Cu(2)Cl₂], where geometrical constraints obviously do not allow the oxygen atom in the side chain to coordinate to the metal atom [16]. On the other hand, ligand 3 (Fig. 1, R¹ = Me, R² = tert-Bu) and CuCl₂ form a 1:1 chelate complex with a distorted tetrahedral geometry around the copper atom [35]. Notably, the long-wavelength absorption of our CuCl₂/bis(oxazoline) complex appears at λ = 748 nm, i.e. with a large batochromic shift compared to the complexes derived from Cu(OTf)₂ (Table 2).

Fig. 7:

Structural formulae of complexes derived from CuCl₂ and NiCl₂.

Fig. 8:

Molecular structure of complex [Cu(L3)₂Cl₂] · THF in the solid state (Ortep plot). The positionally disordered tetrahydrofuran molecule is not shown. Selected bond lengths and angles: Cu–N4 = Cu–N4′ 1.957(2), Cu–Cl = Cu–Cl′ 2.2957(6), C3–N4 1.277(3) Å; N4–Cu–N4′ 180.00(4), Cl–Cu–Cl′ 180.00(3), N4–Cu–Cl 89.40(5), N4′–Cu–Cl 90.60(5), C3–N4–Cu 128.51(15)°.

The formation of nickel complexes was thwarted by the insufficient solubility of simple nickel salts in common organic solvents. However, crystals of the light green complex [Ni(L1)₂Cl₂] could be obtained in modest yield from nickel(II) chloride and L1 in n-propanol. The X-ray structure determination revealed a distorted octahedral coordination geometry with the planes of the two ligands L1 being perpendicular to each other (Figs. 7 and 9). The complex has crystallographic C₂ symmetry; the twofold rotation axis bisects the Cl–Ni–Cl′ angle. As in most of the copper complexes, the Ni–N(imine) distance is distinctly shorter than the Ni–N(triazole) distance (2.046 vs. 2.172 Å). The crystal packing is stabilized by N–H···Cl hydrogen bonds which connect each chlorine atom of a complex molecule with the imidazoline rings of another molecule.

Fig. 9:

Left: molecular structure of complex [Ni(L1)₂Cl₂] in the solid state (Ortep plot). Selected bond lengths and angles: Ni–N1 = Ni–N1′ 2.1723(14), Ni–N4 = Ni–N4′ 2.0460(13), Ni–Cl = Ni–Cl′ 2.4186(5), C3–N4 1.294(2) Å; N1–Ni–N1′ 88.03(7), N1–Ni–Cl 88.51(4), N1–Ni–Cl′ 168.82(4), Cl–Ni–Cl′ 96.78(2), N1–Ni–N4 95.71(5), N4–Ni–N4′ 171.63(8), Ni–N1–C2 110.92(10), Ni–N4–C3 116.57(11)°. Right: Details of the crystal structure, featuring N–H···Cl hydrogen bonds (Mercury plot). Data for N5–H5N···Cl (x–0.5, y–0.5, –z+1.5): d(N–H) 0.88(3) Å, d(H···Cl) 2.33(3) Å, d(N···Cl) 3.195(2) Å, (N–H···Cl) 167(2)°.

4.1 General information

¹H and ¹³C NMR spectra were recorded on a Bruker DRX 400 spectrometer (¹H: 400.13 MHz; ¹³C: 100.61 MHz); δ values are reported in ppm and coupling constants in Hertz (Hz). The signal of the solvent was used as an internal standard: δ(CHCl₃) = 7.26 ppm, δ(CDCl₃) = 77.0 ppm; CD₃OD: 3.31 and 49.0 ppm; [D₆]DMSO: 2.50 and 39.5 ppm; CD₃CN: 1.94 and 1.30 ppm. – IR spectra: Bruker Vector 22; wavenumbers (cm⁻¹) and intensities (vs = very strong, s = strong, m = medium, w = weak, br = broad) are given, spectra were recorded from KBr pellets. – Mass spectra: MALDI-TOF spectra were recorded with a Bruker Daltonics REFLEX III instrument, trans-2-(3-(4-tert-butylphenyl)-2-methyl-2-propenylidene)malononitrile (DCTB) was used as matrix. Elemental analyses: elementar vario MICRO cube and elementar Vario EL; values are given in percent. Melting points were determined on a Büchi Melting Point B-540 apparatus. Solvents were distilled, dried by standard methods and stored under argon. Column chromatography was performed on silica gel 60 (Macherey–Nagel, 0.063–0.2 mm) or basic activated alumina 90 (Merck).

4.2 Syntheses of ligands L1–L5

1-Benzyl-N,N,N′,N′-tetramethyl-1,2,3-triazole-4-carboxamidinium tetraphenylborate (1) and 1-benzyl-4-(4,5-dihydro-1H-imidazol-2-yl)-1H-1,2,3-triazole (L1) were prepared according to the published procedures [24].

4.3 Syntheses of complexes

4.4 X-ray structure determinations

Suitable single crystals of the investigated complexes were obtained as described above in the synthetic procedures. Data collection was performed on an Oxford SuperNova diffractometer (dual source, Atlas CCD) at the University of Ulm and a Bruker Kappa APEX II Duo diffractometer at the University of Stuttgart. Software for structure solution and refinement: Shelxs/l-97 [38] and Shelxl-2014/6 [39]; molecule plots: Ortep-iii [40, 41] and Mercury [42]. In all Ortep plots, the size of the displacement ellipsoids represents 30% probability. In the refinement procedure, hydrogen atoms were usually placed in geometrically calculated positions and treated as riding on their bond neighbors in the refinement. NH and OH hydrogen atom positions were taken from a difference-Fourier electron density map and were refined. Further details are provided in Tables 4 and 5. For [CuL2L2′(OTf)₂] · 2H₂O, which crystallizes in the noncentrosymmetric space group P2₁, the correct absolute structure is supported by the value of the Flack parameter.

CCDC 1446994, 1446997 and 1447013–1447016 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.