Crystal structure and physical properties of 1-methyl-3-( carboxymethyl ) benzimidazolium betaine CuBr 2 in crystal and water solution †

A new Cu(II) carboxylate coordinating compound [1-methyl-3-carboxymethyl benzimidazolium betaine]2CuBr2 was synthesized and crystallized. The crystal has the triclinic symmetry P% 1, with unit cell dimensions a = 7.9693, b = 8.4129, c = 9.1302 Å, a = 68.058, b = 85.402 and g = 71.258 deg. (Z = 1), and molecules stacked along the a-axis. Cu(II)-complexes are planar and four-coordinated with chromophore CuO2Br2, where two oxygen atoms belong to the carboxylate groups of two betaines acting as unidentate ligands. The compound was characterized by two-dimensional H and C NMR spectroscopy for the determination of the correlation between protons of a ligand molecule. NMR spectra confirm the coordination of Cu(II) ions and allow identification of H(2) proton as easily detached in basic conditions. FT-IR spectra confirm the unidentate coordination of the betaine carboxylate group. UV-Vis spectra show three bands in d–d-transition region. Energies of these transitions were used in the interpretation of the EPR results. From powder and single crystal EPR measurements the g-factors were determined as gx = 2.072, gy = 2.030, gz = 2.241. A non-typical g-factor sequence is a consequence of the orbital mixing in the ground state of Cu(II) complex of D2h symmetry. The g-factors were interpreted in terms of the Molecular Orbital (MO) theory which delivered the Cu(II) unpaired electron density delocalization onto the ligand molecules. A strong delocalization on betaine molecules via in-plane ground-state orbital was found and unexpectedly also via out-of plane orbital directed towards the non-coordinating oxygen of the betaine carboxylate group.


Introduction
Heterocyclic compounds, especially benzimidazole, triazole, pyridine, [1][2][3] and their derivatives are very often used as coordination agents. 4,5Because of having a free electron pair on nitrogen atoms, those compounds can easily form complexes with transition metals and rare earth metals.Their excellent coordination capability opens possibilities of forming new functional materials and potential broad applications in chemistry and biology.Structures they form are very interesting because of their topology and architectureforming MOF's (metal-organic frameworks) or POM's (polyoxometalates).Moreover, they have potential applications in molecular recognition, absorption, fluorescence, catalysis, magnetism and as well as biological properties.An imidazole ring is a part of bioactive molecules (histamine, histidine, urocanic acid) and a number of drugs that stimulate the nervous system as well as being used in the treatment of leukemia (e.g., cimetidine, tizanidine, tazoline). 6Imidazoles attached to RNA affect folding of the molecules providing osmotic stress in hydration of the base pairs. 7][10][11][12][13][14] Imidazole derivatives have been developed as commercial fungicides and herbicides due to their antifungal and antimicrobial activities. 15,16Naturally occurring di-(carboxyalkyl)-imidazole compounds are responsible for the taste, smell and color of food undergoing processes of frying or baking. 179][20] Working-sheets about benzimidazole derivatives containing carboxylate groups coordinated to the different a Faculty of Chemistry, Adam Mickiewicz University in Poznan ´, Umultowska 89b, 61-614 Poznan ´, Poland b Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17,   60-179 Poznan ´, Poland.E-mail: skh@ifmpan.poznan.pl† Electronic supplementary information (ESI) available: Atomic coordinates and equivalent isotropic displacement parameters (Table S1), bond lengths and angles (Table S2), torsional angles (Table S3), FT-IR absorption maxima (Table S4).CCDC 1480526.2][23][24][25][26][27][28][29] Liu and coworkers investigated the structure and DNA-condensing properties of tris(benzimidazolyl)amine-Cu(II) coordination units bridged by carboxylates, which can act as a new type of gene-delivery systems. 21he betaine derivative forms a large molecule with coordinating properties related to COO-group.Free betaine appears in a zwitterionic form Me 3 N + CH 2 COO À .Betaines are involved in the methylation reaction of biomolecules and are used by cells for the protection against osmotic stress.1][32] In most cases the betaine acts as a bidentate ligand although discrete molecules with betaine as unidentate ligand are known as well. 33A characteristic feature of betaine is a larger O-C-O bond angle compared with common carboxylates resulting in better solubility. 33Cu(II) ions are easily coordinated by betaine carboxylate group and halide anions Cl À or Br À .In binuclear Cu(II) complexes with bridging betaines the Cl and Br are located in apical positions. 33,34In monomeric Cu(betaine) 2 Cl 2 two chlorine atoms and two unidentate betaine form compressed tetrahedral CuCl 2 O 2 units, 33 whereas when water molecule is involved, the square-pyramidal complexes are formed with two unidentate betaine and Cl or Br in a basal plane and a water molecule in an apical position. 30n this paper we report the synthesis and crystallization of a new coordination compound of Cu(II)Br 2 and N-alkylcarboxybenzimidazole as a ligand with monodentate carboxylate group of a betaine unit.Perfectly planar CuO 2 Br 2 complexes were found in a crystal structure.
A triclinic crystal structure was determined by X-ray diffraction and correlated to the infrared vibrations.Measurements of powder and single crystal electron paramagnetic resonance (EPR) spectral parameters and their analysis by Molecular Orbital theory gave detailed electronic structure of the Cu(II) complex with delocalization parameters of unpaired electron density via the d-orbitals.

Synthesis of materials
The final product was obtained in a four-step synthesis route using commercially available starting materials: 1-methylbenzimidazole (Sigma-Aldrich), ethyl bromoacetate (Sigma-Aldrich) and CuBr 2 (Merck).
1-Methyl-3-(ethoxycarbonylomethyl)benzimidazolium bromide -MBImAcOEt Br (1).To the flask, placed on a magnetic stirrer, MBIm (n = 3.88 Â 10 À3 mol) dissolved in anhydrous DMF (V = 2 cm 3 ) was added.The flask was flushed with argon.After this ethyl bromoacetate (n = 4.656 Â 10 À3 mol) was added drop wise.The reaction was carried out for 1.5 h, in 60 1C.After this solvent was evaporated on oil bath and the crude product was crystallized from acetone/MeOH.Yield 93%.Melting point = 156-158 1C. 35he reaction was monitored by the thin layer chromatography-TLC (mobile phase: CHCl 3 : MeOH = 3 : 1 1-Methyl-3-(carboxymethyl)benzimidazolium betaine -MBImAcO (3).To the flask, placed on a magnetic stirrer, (2) (n = 9.2 Â 10 À3 mol) dissolved in 20 ml of methylene chloride was added (hydrobromide did not dissolve completely even after heating).Septa was applied to the neck of the flask and flushed with argon, Et 3 N (n = 9.2 Â 10 À3 mol) was added.Then the balloon was filled with Ar and allowed the contents of the flask stay in the argon atmosphere with stirring on a magnetic stirrer for 12 hours at room temperature.The precipitated solid was filtered off, washed with CHCl 3 and crystallized from EtOH to give 3-(carboxymethyl)-1-methyl-benzimidazolium betaine as a fine crystalline solid.

Physical measurements
Elemental analysis for the percentage of C, N and H in a sample was performed on a Vario EL III Elementar German company.
FT-IR spectra were taken on a Bruker IFS 66 v/S.Samples of solid compounds were prepared as suspensions in Nujol and Fluorolube, and KBr film.The spectra were recorded in the range of mid-infrared 4000-400 cm À1 with resolution 2 cm À1 .Each FT-IR spectrum was measured by acquisition of 64 scans.
UV-Vis spectra were taken on a UV/Vis Thermo Fisher Scientific Evolution 300 Spectrophotometer.The samples were prepared in H 2 O for the same ligand and metal concentration as in samples for potentiometric titrations using a Plastibrand PMMA cell with 1 cm path length.
1 H and 13 C NMR was performed on a Varian 400 MHz and 2D 1 H and 13 C NMR was performed on a Bruker Avance 600 MHz.
The 2D 1 H-1 H (COSY), 1 H-13 C (HETCOR) and HMBC (Heteronuclear Multiple-Bond Connectivity) spectra were obtained with the standard Bruker software.The Cu(MBImAcO) 2 Br 2 crystals selected for single-crystal X-ray diffraction measurements were grown from water as green parallelepipeds.They were stable under normal conditions and the X-ray diffraction measurements were carried out on a Eos X-Calibur diffractometer using MoKa radiation at room temperature: o-scan data collection with Do = 11 frames and 40 s exposures was applied.Data reduction was performed with the CrysAlisPro and CrysAlisRed programs. 37The absorption of crystal was corrected analytically.The max. and min.transmissions were 0.6205 and 0.3033, respectively.The structure was solved by direct methods using SHELXS-97 and refined with full-matrix least-squares on reflections intensities (F 2 ) with SHELXL-97. 38All H-atoms were located from the molecular geometry (C-H 0.93-0.97Å) and their U iso 's were assigned equal to 1.2U eq of their carriers, and U iso = 1.5U eq for the methyl group.
Powder and single crystal EPR spectra were recorded at room temperature and at 77 K using Radiopan SE/X-2547 spectrometer working at X-band with 100 kHz modulation.The spectra were simulated using Bruker SimFonia routine.Angular dependence of the single crystal EPR line was measured in three planes of an orthogonal reference frame 1, 2, 3 related to the crystal plate.Axis 1 is parallel to the [001] direction being the elongation direction of the crystal plane, and axis 3 is perpendicular to the largest crystal face(1% 1% 1).

Results and discussion
The 1-methyl-3-carboxymethyl benzimidazolium betaine ligand (3) and its complex with CuBr 2 (4) were synthesized.They were characterized by various spectroscopic methods to find correlation between crystal structure and observed chemical and physical properties.
3.1 Ligand and Cu-complex characterization by 13 C and 1

H NMR
The benzimidazole betaine ligand (3) and Cu-complex (4) synthesizes as shown in Schemes 1 and 2, were characterized by 2D NMR spectroscopy in order to assign the exact position of the protons and carbons derived from a benzimidazole ring and possible correlation between atoms through the scalar J-coupling.NMR spectra of benzimidazolium betaine ligand were made in D 2 O. 1 H and 13 C NMR chemical shifts assignments from HMBC experiment are listed in Scheme 3 and 2D spectrum is shown in Fig. 1.The horizontal axis corresponds to 1 H spectrum and the vertical axis to the 13 C spectrum.Proton H(2) peak is not shown in the spectrum, because the acidic H(2) proton was exchanged on the deuterium.Trace amounts of H( 2) are visible at a high zoom.The H (12) signal is coupled to the peaks representing C(2) and C(5) through three bonds.As expected, the H(10) signal is coupled to the peaks C(4), C(2) through three bonds and C(11)OO through two bonds.Signals: H(6) is coupled to the peaks C(8) and C(4); the H( 9) is coupled to the peaks C(7) and C(5) through the three bonds. 1H and 13 C NMR spectra of benzimidazolium betaine ligand (3) and the complex (4) (which was made by mixing betaine (3) with CuBr 2 , in the ration 100 : 1), were made in D 2 O and are compared in Fig. 2 and 3.In 13 C NMR spectrum of complex (4), carboxylate carbon atom C(12) appears as an extended, small peak, which demonstrates the coordination of the copper atom through an oxygen atom bonded directly to a carboxylate carbon atom.Moreover, the peak from carbon atom C(11) almost disappears.At high magnification, it appears to be an expanded signal.It also suggests that there is a coordination of copper atom through carboxylate group which is directly attached to the carbon C (11).All signals of carbons C(4)-C(9) in the benzene ring are distinguishable, because of unsymmetrical substitution of imidazole ring.
The position C(2) has acidic properties, making it easy to detach the H(2) proton at basic conditions.Due to the fact that 13    we can conclude that about 20% of H(2) has been exchanged.
Comparing the signal positions of the ligand and Cu-complex we cannot see any significant differences.

Crystal structure
The crystal data and details of the data collection and structure refinement of [1-methyl-3-carboxymethyl benzimidazolium betaine] 2 CuBr 2 are given in Table 1, and the atomic coordinates are listed in ESI † (Table S1).CCDC 1480526.Structural drawings were prepared using ORTEPII and XP programs. 39,40he complex (4) is formed by the Cu(II) cation coordinated by two Br anions and two zwitterionic MBImAcO molecules.Each MBImAcO molecule forms a unidentate type of coordination bond through its carboxylate oxygen O(2) to the Cu(II) cation (Fig. 4).
The Cu(II) cation is located at the inversion centre.Consequently, the benzimidazole rings of the ligands are mutually parallel as required by the crystal symmetry as it is visible in autostereographic projection of Fig. 5. 41 Thus, also the all carboxylate groups in the crystal are mutually parallel.The carboxylate group is twisted by 77.5(2)1 from the plane of the benzimidazole ring.There is one [Cu(MBImAcO) 2 Br 2 ] complex molecule per the unit cell and half of the complex is symmetry independent.The molecules form chains along the crystal a-axis with stacked benzimidazole rings.CuO 2 Br 2 -complex is planar with Cu(1)-O(2) distance equal to 1.9293 Å and Cu(1)-Br(1) bond 2.4125 Å.The second oxygen atom of the betaine is at longer distance Cu(1)-O(1) = 2.9492 Å and does not disturb the square-planar geometry of CuO 2 Br 2complex.The perfect planarity of CuO 2 Br 2 -complex is an effect of a weak coupling of Br-atom to the lattice allowing the electrostatic forces to form the linear Br-Cu-Br bond.However, a planar structure is not an intrinsic property of four coordinated    Cu-complexes with identical ligand atoms.A free CuX 4 tetrahedron has distorted geometry being a result of a balance between crystal field stabilization favoring square-planar geometry and Br-Br electrostatic repulsion favoring tetrahedral geometry.This balance is reached for Br-Cu-Br angle of about 1201.Hydrogen bonds tend to remove the charge from bromine atoms reducing electrostatic repulsion and thus enhancing a tendency to square-planar coordination.Crystal packing effect resulting from alignment of large organic molecules and hydrogen bonds can dominate the intrinsic effect leading to the planar geometry even for CuX 4 complexes.Two of the shortest intermolecular contacts with respect to the van der Waals radii have the forms of a weak CHÁÁÁBr hydrogen bonds: Br(1)ÁÁÁH(112)-C(11) (symmetry code: 1 À x, 2 À y, 2 À z) and Br( 1)ÁÁÁH(111)-C(11) (symmetry code: x, y À 1, z).Their HÁÁÁBr distances of 2.720 and 2.988 Å are by 0.31 and 0.04 Å, respectively, shorter than the sum of the van der Waals radii of H and Br (1.2 and 1.83 Å according to ref. 42).The next shortest contacts between the complex aggregates are all longer than the sums of van der Waals radii.The bond lengths and bond angles are presented in Tables S2 and S3 of the ESI.†

Vibrations of benzimidazole betaine (3) and Cu-complex (4) studied by FT-IR
FT-IR spectra of free ligand (3) and Cu(II)-complex (4) are compared in Fig. 6.Band assignment is given in the Experimental section and compared for benzimidazolium hydrobromide (2), benzimidazolium betaine ligand (3) and Cu(II)-complex (4) in Table S4 at ESI. † Broad band with resolved peaks at 3466 cm À1 and 3396 cm À1 in ligand spectrum can be assigned as stretching vibrations of OH-groups indicating the existence of a single water molecule attached to the betaine ligand.This band does not exist in the Cu(II) complex (4) confirming that the final green crystal does not contain water molecules.The antisymmetric O-C-O stretching mode 1616 cm À1 for ligand (3) is shifted to 1626 cm À1 indicating the coordination of Cu(II) ion by the carboxylate group.The coordination with the bromine atom is not detectable in the presented spectrum.Cu-Br antisymmetric and symmetric stretching vibrations are expected in 240-250 cm À1 and 200-210 cm À1 region, respectively, 43 which are out of range of our measured spectrum.
A coordination type can be established by a comparison of the symmetric (n s -COO) and antisymmetric (n as -COO) stretching modes frequencies.5][46] If the difference D is lower than the 203 cm À1 (observed value for sodium acrylate), then we are dealing with the type of bidentate coordination.In contrast, when this value is greater than 203 cm À1 , this shows unidentate coordination.Frequencies n s (COO) and n as (COO) of the symmetric and antisymmetric O-C-O stretching modes of the coordinated formate ion in ( 4) are assigned at 1375 and 1626 cm À1 , respectively.Value D = 251 cm À1 suggests a unidentate coordination mode between Cu ion and carboxylate group of a betaine ligand.

Orbital d-d transitions of Cu-complex (4) in UV-Vis spectrum
UV-Vis spectrum of the complex ( 4) is shown in Fig. 7 with an inset presenting enhanced spectrum in the Cu(II) d-d transition region.The d-d region can be decomposed on the three bands as shown by the dashed lines and can be assigned as transitions from the ground state |x 2 À y 2 i of symmetry A g to excited orbitals in D 2h complex symmetry as: |xyi (B 1g-symmetry) at 12 594 cm À1 , |xyi (B 2g -symmetry) and |yzi (B 3g -symmetry) at 15 198 cm À1 , and |z 2 i (A g -symmetry) at 17 065 cm À1 .It is a typical sequence of orbital energies for the planar or strongly elongated octahedral Cu(II) complexes.
The spectrum at UV-region can be decomposed on four bands located at 253 nm, 277 nm, 339 nm and 397 nm resulting from intramolecular transitions.
Fig. 6 FT-IR spectra of benzimidazolium betaine (3) and the Cu-complex (4).Fig. 7 UV-Vis spectrum of Cu(II)-complex (4).The inset shows region of d-d transitions decomposed on three bands (short dashed lines) located at 586, 658 and 794 nm, corresponding energies in cm À1 are shown, respectively.The sum of the bands (dashed line) is perfectly superimposed on the experimental spectrum.
3.5 Electronic structure of Cu-complex (4) crystal from EPR spectroscopy In the case of triclinic crystals of magnetically condensed paramagnets, EPR allows a determination of the local magnetic properties of individual Cu(II) complex.It is possible since for a single complex in the crystal unit cell (Z = 1) a single resonance line is observed with parameters not averaged by exchange interaction between differently oriented (magnetically nonequivalent) complexes.Such a situation appears in Cu(MBImAcO) 2 Br 2 crystal where a single nearly Lorentzian line is observed in all crystal orientations.Lorentzian lineshape suggests exchange coupling between Cu(II) ions.This coupling is rather weak but strong enough for smearing out of any expected hyperfine structure.One can expect maximal hyperfine splitting order of 20 mT from 63 Cu nuclei (I = 3/2) and 5 mT from 79,81 Br nuclei (I = 3/2). 47This allows evaluation of exchange coupling as larger than 0.1 cm À1 .Thus, only the g-factor is an available EPR parameter since the linewidth varies only in the narrow range around 0.8 mT at room temperature and around 1.5 mT at 77 K.
EPR spectrum was recorded for powder samples and for single crystals.Powder spectrum recorded at room temperature and at liquid nitrogen temperature is presented in Fig. 8.The spectral parameters obtained from computer simulations of the powder spectra (dashed lines) are collected in Table 2.This is a typical spectrum described by non-axial g-tensor.The shift of the low-field line, corresponding to the g-factor along the main complex symmetry axis (z-axis) and line broadening on cooling are non-typical behavior due to exchange and dipolar coupling competition.
More precise information on the g-tensor and its orientation in the crystal one can obtain from the analysis of angular dependence of the resonance line position.Such measurements were performed by crystal rotation around three orthogonal axis of the reference frame related to the largest crystal plane(1% 1% 1) as it is shown in the inset of Fig. 9.The g 2 -tensor components were calculated (see Table 2) and the tensor diagonalization gave the principal value and principal axes direction cosines (see Table 2).Principal values are in good agreement with the powder data.Principal g 2 -tensor directions show that the main symmetry axis of the CuO 2 Br 2 complex (normal to the coordination plane) well coincides with local crystal z-axis (compare the results in Table 2) from EPR measurements.The Br À -ligand is a source of relatively weak crystal field (only I À ion gives weaker) as shown by its localization in the spectrochemical series, and its contribution to crystal field at Cu(II) site is smaller than that from oxygen atom.Thus, the g-factor along O-Cu-O direction is expected to be the lowest giving g x -value, whereas along Br-Cu-Br ( y-axis) the medium value g-factor appears.Orbital mixing, discussed below, produces reversing the g x and g y values.The local crystal field axes x, y, z lie close to the planes of the reference frame as marked in Fig. 9 and the complex localization with local axes is shown in Fig. 10.
The minimal g-factor is lower than 2.04.It cannot result from the d-orbital splitting but it is an effect of the orbital mixing in the orbital ground state.Local geometrical and crystal field symmetry of the CuO 2 Br 2 complex is D 2h .In this relatively low symmetry the orbitals having the same symmetry can be mixed.In D 2h the orbitals |x 2 À y 2 i and |z 2 i have A g -symmetry and are mixed in the ground state leading to the anti-bonding molecular orbitals where L i are a linear combination of ligand orbitals s and p of appropriate symmetry and the mixing coefficients fulfill normalization condition a 2 + b 2 = 1.
For the A g ground state symmetry the principal g-tensor components are: 48 where l = 829 cm À1 is spin-orbit coupling constant of Cu(II) and E ij are orbital splittings.The first, but mostly not obvious consequence of the mixing, hidden in eqn (2), is reversing the g-factors sequence with g x 4 g y .This appears even for a very small mixing effect. 48g. 8 Powder EPR spectrum of Cu(II) in Cu(MBImAcO) 2 Br 2 recorded at room and liquid nitrogen temperature at frequency 9.380 GHz.Dashed lines are the simulated spectra with parameters collected in Table 2.
From the analysis of eqn (1) using orbital energies from UV-Vis spectrum one can calculate molecular orbitals coefficients a, a, b, g 1 and g 2 .These coefficients characterize electronic structure of the studied paramagnetic center, i.e. a degree of unpaired electron spin density localization onto central ion and delocalization via coordination bands.The coefficients cannot be exactly calculated from the three eqn (2) and the normalization condition.Thus, we propose approximate approach for the coefficient evaluation, with sufficiently good accuracy, taking into account that squared MO-coefficient cannot be lower than 0.5 (unpaired electron cannot be strongly delocalized onto ligands).
Mixing coefficient a can be evaluated from g y value plotting a 2 vs. a using eqn (2).Since a 2 4 0.5 thus from the plot a = 0.964-0.988.It is a relatively small orbital mixing in the ground state.From the equations for g x and g y the g-values can be evaluated by plotting the ratio g 1 /g 2 vs. a from the equation where Dg i = g i À 2.0023.From such the plot we concluded that g 1 2 and g The mixing coefficient a value shows a relatively small admixture of |z 2 i orbital in the ground state |x 2 À y 2 i.Thus, a delocalization of spin densities via the admixture state is negligible confirming planar structure of the Cu(II) complex.Large delocalization takes place via |x 2 À y 2 i indicating a strong coordination bonding of Cu(II) by betaine oxygen atom O(2).The g 1 2 E 1 indicates that the delocalization to Br À ligand via out-of plane orbital |xzi is negligible and the Cu-Br bond has mostly ionic character with the two bromine anions ensuring charge compensation for Cu(II).A large spin delocalization appears unexpectedly via    2).|yzi orbital directed towards the betaine O(1) in COO-group.Thus, the main coordination bond is Cu(1)-O(2) while the Cu(1)-O(1) can be considered as a weak coordination bond.It seems to suggest that although betaine acts here as a unidentate ligand, its essentially bidentate character is perceptible as a coupling to the central ion via its out-of plane orbital.

Conclusion
The molecules in [1-methyl-3-carboxymethyl benzimidazolium betaine] 2 CuBr 2 crystal are stacked with Cu(II) ions forming a chain along the a-axis.However, the coupling between neighboring copper ions along the chain is very weak and 1-dimensional magnetic character does not appear.Cu(II) complex has two uncommon properties.The complex is planar with chromophore CuO 2 Br 2 (most such complexes have distorted tetrahedral structure) and with unidentate coordination of betaine carboxylate group (in most cases carboxylate group acts as a strong bidentate ligand).Local magnetic properties of Cu(II) complex determined from EPR measurements confirm perfectly planar complex geometry and an analysis of EPR g-factors in terms of Molecular Orbital theory shows strong delocalization of the spin density onto ligands in the coordination plane and allow to detect coupling via out-of -plane orbital to the more distant oxygen atom of the coordinating betaine.

Scheme 3 1 H
Scheme 3 Numbering of atoms in ligand molecule and chemical shift for 1 H (upper number) and 13 C (lower number).

Fig. 5
Fig. 5 Autostereographic projection 41 of the molecular packing in crystal structure of Cu(MBImAcO) 2 Br 2 viewed along [001].The shortest HÁÁÁBr contacts are indicated by the dashed lines.

Fig. 9
Fig. 9 Angular dependence of the resonance field in three planes of 1, 2, 3 orthogonal frame (see inset).Solid lines are theoretical plots with principal g-tensor values and direction cosines given in Table 2.The z-axis and x-axis (close to the Cu-Br bonds) marked in the figure do not lie in the 12 and 23 planes but are close to the indicated directions (see corresponding principal direction cosines in Table2).

Fig. 10
Fig. 10 Crystal structure view in the crystal rotation around the c-axis with Cu(II) complex localized in the unit cell corner with marked local crystal field axes x, y, z.The picture visualizes a stacked-type crystal packing with chain of Cu(II)O 2 Br 2 complexes along the crystal a-axis.

Table 1
Crystal data and structure refinement

Table 2
Powder and single crystal EPR parameters