Structural Analysis of Powder Complex of Tris (1,10-phenanthroline) Copper(II) Trifluoromethanesulfonate Dihydrate

Introduction

The synthesis and characterisation of complexes containing transition metals with phenanthroline (phen) as ligand and various common counter ions such as NO₃^–, BF₄^–, ClO₄^–, PF₆^– have been reported long time ago, e.g. Ito et al.¹ Triflate salts were already known as highly ionic since the corresponding acid might be considered as super-acid which is much stronger than sulphuric acid, being Hammett acidity to be -12, e.g. Raamata et al., and Howells and McCown^2-3. Triflate ionic liquids are hydrolytically stable and in many cases they are the preferred reaction media in comparison to ionic liquids with the hydrolytically unstable (PF₆)⁻ or (BF₄)⁻ anions⁴.

In the case of coper(II) complex with phenanthroline ligand and triflate anion, the single crystals of six-coordinated site⁵ of [Cu(phen)₃][CF₃SO₃]₂.H₂O and five-coordinated site⁶ of [Cu(phen)₂OH₂][CF₃SO₃]₂ have been analysed. For the two complexes, the triflate anion were found to be uncoordinated to the central atom, Cu(II). The structural cationic complex of the tris-phenanthroline, [Cu(phen)₃]²⁺, with diferent counter anions (X) have also been solved through a number of other corresponding single crystals, these are for  X = (ClO₄)₂⁷, S₄O₆.5H₂O⁸, and (TCNQ)₂⁹. It was not surprisingly found that the corresponding tris (phenanthroline) copper (II) cation adopted different space group, triclinic or monoclinic, with various cell parameters.

No electrical, electronic spectral and magnetic data have been reported at least for the tris(phenantroline)copper(II) triflate. Moreover, the determination of lattice parameters for the powder coordination compounds by XRD has been very little attention up to now, but abundantly for the metal oxides. For those reasons, the powder complex has been prepared, the corresponding cell parameters were analysed following PXRD, and this is the main purpose of this work.

Materials and Methods

Materials

The main chemicals, copper(II) nitrate (Cu(NO₃)₂), 1,10-phenanthroline (C₁₂H₈N₂), potassium triflate (KCF₃SO₃), ammonium chloride (NH₄Cl), calcium chloride (CaCl₂), nickel nitrate (Ni(NO₃)₂), and aluminium chloride (AlCl₃) were purchased from Sigma-Aldrich. All the reagents were used without further purification.

Preparation of tris (phenanthroline) copper (II) Triflate

About 10 mL of an aqueous solution of 0.1 mmol Cu(NO₃)₂ and 5 mL solution of 0.32 mmol phenanthroline in water (with drops of ethanol to disolve) were mixed. The mixture was well stirred and heated to obtain a homogeneous solution. To this solution, an excess of saturated aqueous solution of KCF₃SO₃ (0.4 mmol in 5 mL of water) was added. The mixture was concentrated on warming, and the obtained powder on cooling was then filtered, rinsed with a minimum of cold water, and then dried in aeration.

Instruments

Magnetism. The magnetic data for solid sample were obtained only at room temperature by using Magnetic Susceptibility Balance (MSB) of Auto Sherwood Scientific 10169 model, calibrated with CuSO₄.5H₂O. The molar magnetic susceptibility data were then corrected for diamagnetism calculated using Pascal’s constant¹⁰. The corresponding effective magnetic moment (μ_eff) was then calculated from the corrected molar magnetic susceptibility following the general formula, μ_eff = 2.828 √ (χ_M.T) BM.

Electronic and Infrared Spectra. The electronic spectrum of the solid sample was recorded on Pharmaspec UV 1700 spectrophotometer. Whereas in solution it was recorded on Spektrofotometer UV-Vis Shimadzu 2400 PC Series. The powders were spread on a 2×2 cm particular thin glass adhered with ethanol. The fitting was then placed in the cell holder and the spectrum was recorded at 400-1000 nm. The infrared spectrum of the powdered sample which were pressed on the cell was recorded on an infrared spectrophotometer FTIR Shimadzu Prestige 21 at 500 – 4000 cm^-1.

Metal Content and Ionic Property. The metal content in the sample was calculated based on the data recorded using an Atomic Absorption Spectrophotometer of Shimadzu AA-6650 model. While the ionic property was estimated by recording the electrical (equivalent) conductance by using a conductometer calibrated with an aqueous solution of 1 M potassium chloride at 25⁰. The obtained data of sample was compared to those of known ionic solutions, NH₄Cl, CaCl₂, Ni(NO₃)₂, and AlCl₃, which were also recorded with the same conductometer.

Powder X-Ray Diffraction. The diffractogram of the complex was recorded on a Rigaku Miniflex Benchtop Diffractometer, CuK_α: λ=1.5406 Å. The powdered sample was spread on the glass plate and then was placed on the cell holder. The reflection data were recorded in scan mode at 2-90 degree of 2θ with interval of 0.02 and rate of 10. The resulting diffractogram was then analysed by Rietica program of Le Bail method at 5-60 degree of 2θ, which was run within 30 cycles.

Results and Discussion

Chemical Formula of the Complex

Phenanthroline is generally known as stable compound when complexation in various solvents, and thus no decomposition to others is expected. Therefore, direct interactions of phenanthroline and copper(II) salt in solution should produce the blue cationic phenanthroline complex which could be precipitated on the addition of triflate salt. The electrical conductance of this complex was recorded with respect to the known ionic simple compounds in aqueous solution, and the result is shown in Table 1. It suggests that the corresponding value is very close to the range of those containing three ions per molecule, and thus the possible empirical formula of Cu(phen)_n(CF₃SO₃)₂.xH₂O was then proposed for this powdered complex.

Table 1:  Electrical equivalent conductance of the complex and some known salts (^*) Sugiyarto et al., 2017)²⁴

Compounds	Equivalent conductance(Λc) Ω-1cm2mol-1	Ratio of cation/anion	Number of ions
NH4Cl	67.04	1 : 1	2
CaCl2	118.8	1 : 2	3
Ni(NO3)2	124.21	1 : 2	3
AlCl3	232.9	1 : 3	4
Mn(phen)n(CF3SO3)2·xH2O*)	101.5	1 : 2	3
Cu(phen)n(CF3SO3)2·xH2O	95.7	1 : 2	3

Metal content obtained from atomic absorption spectral data (6.76%) suggests that the number of phenanthroline molecules, n, and the number of hydrates, x, could then be estimated, and the formula of Cu(phen)₃(CF₃SO₃)₂.2H₂O as in Table 2, might satisfy as expected from the stoichiometric preparation. The conductivity data of this complex confirms the highly ionic property of the triflate salts^2,4,11.

Table 2: The proposed formula of complex following the metal content

Compound	Metal content
	Theoretic calculation (%)	AAS result (%)
Cu(phen)3(CF3SO3)2	7.04	6.76
Cu(phen)3(CF3SO3)2·1H2O	6.90
Cu(phen)3(CF3SO3)2·2H2O	6.77
Cu(phen)3(CF3SO3)2·3H2O	6.64

 

The Magnetic Moment

The magnetic susceptibility data obtained on the measurement (Table 3) were then calculated to the magnetic moment based on the estimated formula as in Table 2, and the result of the three samples separatedly preparared in fact to be paramagnet, 1.85-1.90 BM. It is a slightly higher than the spin-only value for the one unpaired electron in the d⁹ electrons of Cu(II), as also observed in the corresponding hexafluorophosphate, 1.85 BM, which was reported to obey the Curie-Weiss law¹².

Table 3: Magnetic data of Cu(phen)₃(CF₃SO₃)₂.2H₂O at room temperature

Sample	χg   x 10-6 (cgs)	Magnetic Moment (BM)
1	1.031	1.85
2	1.080	1.88
3	1.122	1.90

The Electronic Spectrum

The electronic spectrum of the complex in solution was recorded at 400-1000 nm and shown in Fig. 1 (inset) together with that of starting material, the copper(II) nitrate (1a).  It reveals clearly that the main band was definitely shifted to higher energy (about 14673 cm^-1) in the phenantroline-copper(II) complex (1b) from the lower energy (about 12000 cm^-1) in the starting material. This indicates that the phenanthroline has replaced the aqueous-copper(II) resulting in a stronger ligand field band. The extinction coefficient of about 48,1 L mol^-1 cm^-1 suggests the octahedral environment of central atom, and therefore, the band might be attributed to the spin allowed transition ²E_{g →}²T_2g in copper(II). This band is quite comparable to the corresponding perchlorate¹³, 14815 cm^-1. This reflects the presence of ligand field strength of phenanthroline surrounding Cu(II) metal ion, and thus supports the formula proposed.

Figure 1: Electronic spectra of Cu(NO3)2 (a) and Cu (phen)3 (CF3SO3)2 (b) in solutions (inset) and  powder Cu (phen)3 (CF3SO3)2 (c). Click here to View figure

 

In powder (Fig.1c) the spectrum shows clearly a more asymmetric shoulder band that is likely split into two bands concentrated at about 14085 cm^-1 and at 15380 cm^-1, indicating a strongly octahedral distortion, being associated with Jahn-Teller effect. The distorted octahedral configuration is so common in copper(II) complexes as reported from single crystal sructure of the corresponding perchlorate⁷, the hexafluorophosphate¹², and of the triscyanomethanide¹⁴.

The Infrared Spectrum

In general, the spectral shape of the phen and bipy complexes is quite similar¹⁴. Thus, the effect of chelation has been reported to shift the absorption bands of the functional group in the two ligands^15-16.  In this work as shown in Fig. 2, the infrared spectrum of this complex recorded at 500-4000 cm^-1 reveals the ring modes (ν_C-C and ν_C-N) of phen at about 3067, 2299-1925, 1510, 1427, 1350 and 1215 cm^-1, and the ring deformation near 725 cm^-1, being quite similar to those observed in chelation of another Cu(II)-phen complex¹⁶.

Figure 2: Infrared spectrum of [Cu(phen)3] (CF3SO3)2.2H2O Click here to View figure

 

In the case of triflate ion, the infrared of some metallic triflate salts are used as comparison^15-18, and therefore, the corresponding infrared spectrum can be readily assigned. The modes of vibrations observed at 1280 and 1149 cm^-1 are to be  SO₃ and CF₃ asymmetric stretching modes, respectively, while those at 1030 and 1225 cm^-1 are SO₃ and CF₃ symmetric stretching modes, respectively. The bands at 780 and 636 cm^-1 are likely deformation symmetry of CF₃ and SO₃, respectively, and the bands at 580 and 517 cm^-1 are deformation asymmetry of CF₃ and of SO₃, repectively. Therefore, the infrared data strongly suggest that the complex sample should contain the chelation of phen to copper(II) and free triflate anion.

The occurence of lattice water is likely indicated in the spectrum¹⁹. Thus, the mode observed at about 1624 cm^-1 in the spectrum might be due to free molecule of (bending) H₂O⁵ as also proposed by Sinha¹⁵. Moreover, broad absorption at about 3500-3600 cm^-1 should be clearly due to symmetric-antisymetric streching mode of -OH of H₂O^5,19.

The Powder X-Ray Diffaction and Structural Analysis

The single crystals of cation [Cu(phen)₃]²⁺ with some particular anions have been reported^5,7-9,12,14. The tetrathionate pentadydrate⁸ adpots triclinic symmetry of the space group, P1, and similarly for the tetracyanoquinodimethanide monoethanol⁹. Both of them, however, show much different in the cell parameters (Table 3). While for the tricyanomethanide¹⁴ and the perchlorate⁷, they are monoclinic symmetry but with different space group of P₂₁/n for the former and of C2/c(15) for the latter. Again the cell parameters are quite different between the two. For the hexafluorophosphate¹², it is also monoclinic of space group C2/c but with much longer in a direction compared to the perchlorate, resulting in much bigger in volume of the cell. Another single crystal of the triflate monohydrate⁵,[Cu(phen)₃](CF₃SO₃)₂. H₂O, in fact has also been reported and it was found to be triclinic system of the space group, P1. The cell parameters are quite similar to the corresponding tetrathionate pentahydrate. The cationic complex with those various counter anions confirm that the tris-chelate phenantroline ligands were coordinated to the metal atom. It has been reported that two single crystals of  another complexes, [Cu(bipy)₃](ClO₄)₂ have been separately produced^13,20 to have significantly difference in cell parameters, though they are the same symmetry and space group.

Figure 3: Diffractogram of [Cu(phen)3](CF3SO3)2·2H2O (black sign +, a), and of refined triclinic space group of P1 model (red full line, b) Click here to View figure

 

with it’s posision of 2 theta (blue bars, c), and the difference between the black observed and the red refined model (green, d).

From those facts, the powder X-ray diffraction of [Cu(phen)₃](CF₃SO₃)₂·2H₂O was then recorded, and the diffractogram together with its refinement following Le Bail method of Rietica program is shown in Figure 3. The black signs (+) represent the observed experimental data, the red full line is the refinement at 5-55 degree of 2 theta for the expected model of triclinic symmetry of space group P1 which are the blue bar-lines, and the green curve indicates the difference between the observed diffractogram and the analysis result. It is clear that the red full line does almost pass through the black observed data, and it is demonstrated by the almost flat green curve, indicating that the corresponding analysis might be considered very good with the Goodness of Fitting (GOF) to be 28.58 and derived Bragg R-Factor of 0.03. Detailed results of cell parameters of the crystal structure are presented in Table 4 together with the known single crystal data of the same cation of those anions discussed.

It has been stated that the application of Rietica program has not been performed to many powdered complexes. The first fitting might come from Zhu, Wu, and Le Bail himself²¹ (1999) who demonstrated the fitting for powdered of cobalt complex containing monodentate of ammine. It resulted in very low R factors, being 0.053 for R_Bragg, 0.1 for R_p, 0.119 for R_wp and 0.04 for R_exp leading to 8.85 for χ² (GOF). The next applicationwas performed on powdered complexes containing bipy partially²² in 2008 with reasonably good fittings resulted in derived Bragg R-factors of 0.0048-0.0128 and  c² of 1.9-9.75. The application of Le Bail method was also performed on oxcalate complexes²³ in 2009, and in fact resulting in much high values of R factors, being 16.4-17.9 for R_p, 13.3-13.58 for R_wp and high values of 279.36-290 for (R_wp/R_exp)² or  χ². Recently²⁴, Sugiyarto et al. (2017) reported the powder XRD for the complex of [Mn(phen)₃](CF₃SO₃)₂.6.5H₂O, with the R values of 3.67 (R_p), 7.34 (R_wp) and 3.61(R_exp) leading to χ² value of 4.13, and 0.16 (R_Bragg). Concerning how low of R factors, “How good is good enough” of the fitting²⁵ is in fact still not so easy to conclude as discussed by Toby (2006). Quite recently²⁶, Kusumawardani et al. (2017) reported the powder XRD for the complex of Cu(bipy)₃(CF₃SO₃)₂.(0.5-1) H₂O, with the R values of 5.50 (R_p), 9.94 (R_wp) and 2.68 (R_exp) leading to χ² of 13.72, and  0.02 (R_Bragg). From those reasons, there is no doubt that the complex in this work might be considered to adopt triclinic symmetry of the space group, P1, with the R’s values of being reasonably low.

In principle, it is possible and promising to draw the molecular structure of the (cationic) complex based on the hkl produced by the fitting, however, it is still very hard struggling.

Table 4: Detailed cell parameters of [Cu(phen)₃]X, where X= (CF₃SO₃)₂·2H₂O (a)*, (CF₃SO₃)₂·H₂O (b)⁵, (S₄O₆)·5H₂O (c) ⁸, (TCNQ)₂·0.5 C₂H₅OH (d) ⁹, (C(CN)₃)₂ (e)¹⁴, (ClO₄)₂ (f)⁷, and (PF₆)₂ (g)¹²; (a)*This work due to Le Bail method of Rietica program.

(X)	(a)*	(b)5	(c)8	(d)9	(e)14	(f)7	(g)12
Symmetry	Triclinic	Triclinic	Triclinic	Triclinic	Monoclinic	Monoclinic	Monoclinic
Space Grup	P1	P1	P1	PI	P21/ n	C2/c (15)	C2/c
α (Å)	9.96201	10.7448(3)	11.232(4)	21.945(5)	9.3854	35.98(5)	54.836(8)
b (Å)	39.89891	13.4992(3)	13.569(6)	11.106(3)	31.179	15.97(3	15.9230(1)
c (Å)	17.19187	14.8755(4)	14.654(6)	10.186(3)	12.7972	12.20(3)	12.2990(1)
V (Å)	6776.3286	1897.10(8)	2012.6(14)	2450.4	3744.2	6864	10708(2)
α (°)	95.92929	104.1650(10)	99.67(1)	86.50(5)	90	90	90
β (°)	86.18185	104.2840(10)	99.44(1)	84.41(6)	91.084	101.7(3)	94.382(2)
γ (°)	92.65168	105.7230(10)	109.75(1)	83.21(4)	90	90	90
Rp	5.19
Rwp	10.65
Rexp	1.99
Z	4	2	2	2	4	8	12
GOF: χ2	28.58	1.046	n.a	n.a	n.a	n.a	1.05
RBragg	0.03

Conclusion

The powder complex of [Cu(phen)₃](CF₃SO₃)₂·2H₂O has been synthesized. The magnetic moment was found to be 1.85-1.90 BM, being normal paramagnet for an unpaired electron of Cu(II). In solution the electronic spectral property show a slight asymmetry band concentrated at about 14673 cm^-1, but in the powder it reveals an asymmetry shoulder band concentrated at about 14085 cm^-1 and 15380 cm^-1 which is attributed to the spin allowed transition, ²E→²T₂. The infrared spectrum of the complex shows the typical vibrations of the functional groups of phen and the corresponding uncoordinated triflate anion. The Powder X-Ray Diffraction analysed by Rietica program of Le Bail method suggests that the complex adopts triclinic symmetry of space group P1 with the derived Bragg R-factor, 0.03.

Acknowlegment

The authors thank to the Faculty of Mathematics and Natural Science, YSU, due to the funding of DIPA-2015 for this work.

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